glaucophane schists and ophiolites of the northern california coast ranges isotopic ages and their tectonic implications the geology of a strip of 15 quadrangles across the franciscan complex in the coast ranges of northern california is the basis for interpretation of the significance of potassium argon and fossil ages of glaucophane schists ophiolitic materials and associated sedimentary rocks whole rock and mineral separates of franciscan rocks were analyzed and 69 potassium argon ages were obtained white micas and actinolites from blueschist blocks in melange give apparent ages clustering about 142 and 153 m y but they probably represent a continuum blue amphibole ages from blocks range from 98 to 151 m y the younger ages are from the eastern portion of the central melange belt published u pb ages from the ophiolite belt give crystallization ages from 153 to 165 m y k ar ages extend from 143 to 166 m y whole rock ages from the south fork mountain schist range from 113 to 158 m y but cluster around 124 m y the association of older k ar ages with more coarsely crystalline schist suggests that metamorphism occurred prior to 124 m y ago fossil evidence indicates that the basal sediments of the great valley sequence were deposited upon the disrupted coast range ophiolite within 5 to 10 m y of the oldest crystallization ages for ophiolite fossils within melange units broken formations and unsubducted trench slope deposits provide evidence for melange development by the early cretaceous with youngest melange units generally lying to the west available chronologic evidence is compatible with a tectonic model involving melange formation in a restricted flow channel beneath the accretionary wedge of the subduction complex upflow of melange returned blocks of coarsely crystalline metamorphic rocks that were accreted during the initial stages of subduction when the hanging wall was still hot the viability of this and other proposed tectonic models can be assessed by continued careful field documentation of the nature of contacts between key lithotectonic units and by detailed application of temperature sensitive geochronologic tools

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

nappes tectonics of oblique plate convergence and metamorphic evolution related to 140 million years of continuous subduction franciscan complex california this paper presents a new synthesis of franciscan complex tectonics with the emphasis on the pre san andreas fault history of these rocks field relations suggest that the franciscan is characterized by nappe structures that formed during sequential accretion at the trench the presence of these structures along with other field relations including the lack of evidence for large offset of conglomerate suites indicates that strike slip fault systems of large displacement 500 km did not cut the franciscan complex during subduction regional geology and comparisons to modern arc trench systems suggest that strike slip faulting associated with oblique subduction took place inboard east of the franciscan in the vicinity of the magmatic arc the franciscan varies along strike because individual accreted elements packets of trench sediment seamounts etc did not extend the full length of the trench different depths of underplating distribution of post metamorphic faulting and level of erosion pro

counterclockwise p t t paths from amphibolites franciscan complex california relics from the early stages of subduction zone metamorphism overprinting mineral relationships in amphibolites from the franciscan complex california suggest metamorphic evolution from high temperature amphibolite facies to the blueschist facies with increasing p t ratio a counterclockwise p t t path early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are in turn cut or rimmed by sodic amphiboles some amphibolites are overprinted successively by eclogite and blueschist facies assemblages clinopyroxenes are zoned with increasing jadeite component from core to rim multiple generations of phengites are present in the same sample and are more si rich with each successive generation of growth garnets in eclogite overprinted amphibolites are zoned with an initial mgo increase away from the core to a maximum mgo zone followed by mgo decrease toward the rim garnets in other amphibolites show continually increasing mgo from core to rim several samples representative of the variety of franciscan amphibolites are describ

provenance of franciscan graywackes in coastal california a systematic comparison of available detrital modes for graywacke sandstones of the franciscan subduction complex and for coeval sandstones of the great valley sequence in the california coast ranges indicates that both were apparently derived from the same general sources the inferred provenance terrane was the ancestral sierran klamath magmatic arc from which mixed volcanic and plutonic detritus readily entered the adjacent great valley forearc basin at intervals along the trend of the arc trench system arc derived detritus also bypassed the forearc region through submarine canyons that fed the franciscan trench longitudinal flow along both the great valley trough and the franciscan trench achieved wide dispersal of the turbidite sediment suites of both franciscan and great valley samples include an array of subquartzose compositions ranging from feldspatholithic to lithofeldspathic mean framework modes of 17 franciscan suites comprising 203 individual samples and of 23 great valley suites comprising 410 individual samples range from 14 to 44 quartz grains 15 to 54 feldspar grains and 7 to 71 total lithic fragments the ratio of quartz to feldspar remains relatively constant as the proportion of lithic fragments changes the compositional variations reflect differences mainly in the admixture of lithic fragments derived principally from volcanic cover with quartz and feldspar derived principally from erosion of underlying plutons despite major overlap in the compositions of the two sets of samples some franciscan sandstones are somewhat more feldspathic and less lithic than any known great valley counterparts and were probably derived from segments of the arc terrane where exposures of plutons were more extensive than within typical great valley sources higher proportions of non volcanic to volcanic lithic fragments in some franciscan sandstones probably reflect complex recycling processes on the trench slope diagenetic effects in many franciscan suites include apparent wholesale replacement of k feldspar by albite present age control is inadequate to test fully for time dependent trends in the compositions of franciscan sandstones analogous to the known stratigraphic variations in the composition of great valley sandstones this question ought to be investigated in future studies

large pressure gap between the coastal and central franciscan belts northern and central california abstract the nature of the metamorphism of the central belt of the franciscan complex has long been debated did the whole central belt or its melange matrix undergo high p t metamorphism to solve this problem we have investigated the regional distribution of aragonite in these rocks in northern and central california we have discovered more than 300 new localities of aragonite using the staining method by friedman 1959 aragonite occurs as network veins or pods in both metagraywacke and matrix forming metashale and is invariably partially replaced by calcite on its rim thus we conclude that aragonite is stable in all parts of the franciscan central belt from the lower boundary between the coastal and central belts to the topmost part of the central belt and that both the matrix forming melange shale and exotic blocks of the central belt suffered high p t metamorphism we traced the boundary between the central and coastal belts in northern california using the staining method along the gently eastward dipping thrust plane between the underlying coastal and overlying central belts more than 2 5 kbar pressure break is obtained through the lawsonite laumontite aragonite quartz relations such an observed large pressure gap was formed during the exhumation process of the central belt of the franciscan complex

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

provenance and thermal history of the franciscan accretionary complex constraints from zircon fission track thermochronology the franciscan accretionary complex of california contains large belts of sediments that were subducted to depths of 10 to 30 km metamorphosed under high pressure and low temperature conditions and later returned to the surface reconnaissance zircon fission track ft data 21 samples place constraints on the sedimentary provenance thermal history and or exhumation history of various franciscan units in a transect across the northern coast ranges through mendocino pass metagraywacke samples from the valantine springs formation of the franciscan eastern belt experienced maximum temperatures tmax ranging from 250 to 310 c zircon ft and previously reported apatite ft data indicate that parts of the western eastern and eastern central belts cooled at about 100 70 ma from 300 c to 100 c considering the low paleogeothermal gradient of roughly 9 c km during franciscan subduction these rocks were apparently uplifted and exhumed from 30 to 10 km depth at this time one sample from the eyler mountain sequence in the northwestern diablo range similarly shows tmax 310 c and major cooling at 100 70 ma this uplift and exhumation may have reflected accelerated accretion at the base as well as the front of the franciscan wedge accompanied by extensional thinning at shallower levels as a result of massive sediment influx into the franciscan trench from the sierra nevada magmatic arc in the northern coast ranges the western part of the central belt and the coastal belt experienced lower tmax probably 200 c and retain source area detrital zircon ft ages burnt hills terrane samples in the diablo range also experienced tmax 230 c and retain detrital ages the detrital single grain age patterns from these areas are compatible with those expected from the sierra nevada magmatic arc and its country rocks this is consistent with derivation of franciscan sediments from arc terranes in california and provides no particular suggestion that these parts of the franciscan are highly allochthonous paleogene coastal belt sandstones also yield detrital ages compatible with derivation from the sierra nevada but also include early tertiary zircon populations compatible with reconstructions suggesting these sandstones were sourced partly from the idaho batholith

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

origin of conglomerate stratigraphy in the franciscan assemblage and great valley sequence northern california in the northern california coast ranges the forearc basin great valley sequence of late jurassic and cretaceous age contains three distinctive types of conglomerate 1 chert rich conglomerate of tithonian to valanginian age 2 volcanic rich conglomerate of mainly valanginian age and 3 mixed clast conglomerate of valanginian and younger age all three types are found in stratigraphic sequence along the west side of the sacramento valley but in three belts of outliers that overlie the franciscan assemblage to the west the oldest conglomerate present in each belt is progressively younger southwestward this may reflect the widening of the forearc basin over time as progressively younger conglomerate types lapped onto the elevated western margin of the basin the same three conglomerate types occur in coeval rocks of the accretionarywedge franciscan assemblage and show a similar southwestward progression in composition possibly due to trench deposition of conglomerate with the same sources as the great valley sequence each conglomerate type being accreted successively to the west side of the accretionary wedge the franciscan must have underthrust the forearc basin become deeply buried been metamorphosed and returned to the surface without major disruption of the distribution of conglomerate types

diverse sources for igneous blocks in franciscan melanges california coast ranges igneous blocks in franciscan melanges are of three chemical petrologic types 1 tholeiitic basalts of both arc and spreading center origin with depletions in light relative to heavy rare earth elements 3 tio 2 1 high y zr and y ti ratios and relict augites that generally have low al and ti and well defined iron enrichment trends 2 basalts of probable seamount origin with marked enrichments in light relative to heavy rare earth elements 5 tio 2 1 lower y zr and y ti than 1 and ti al rich augites showing little if any iron enrichment trends and 3 hypabyssal intrusives having sio 2 52 tio 2 1 flat or only slightly fractionated rare earth abundance patterns and diopsidic augites that are very low in ti and al and show no iron enrichment trends all of the blocks are metamorphosed most are undeformed pumpellyite bearing greenstones and a few contain sodic amphibole lawsonite sodic pyroxene the melanges are probably olistostromal in origin deriving the

primary fabric ellipsoids in sandstones implications for depositional processes and strain analysis abstract when measuring strains in deformed clastic rocks geologists usually assume one or more of the following 1 objects were initially circular 2 objects were initially non circular but had uniform orientations 3 object populations initially had a fabric but that this fabric had a symmetrical relationship to bedding or that 4 initial fabrics are recognizable even after straining to evaluate these assumptions we measured 43 fabric ellipsoids in non strained poorly sorted sandstones from four depositional settings dsdp core 174 fan and abyssal plain deposits off the coast of oregon the cretaceous great valley sequence california fore arc deposits the cretaceous pigeon point formation california accreted and slumped turbidites and crossbedded sandstones from intracontinental basins in california and australia our results indicate the following 1 in two dimensional cuts individual grains have variable but usually small axial ratios these data and microstructures in grains and matrix cements also argue against significant compaction during burial in these clast supported sand rich units and instead suggest that the primary fabrics largely reflect grain packing processes during deposition and or slumping and packing in the case of the pigeon point samples

density changes of fluid inclusions in high pressure low temperature metamorphic rocks from crete a thermobarometric approach based on the creep strength of the host minerals abstract the densities of fluid inclusions in quartz from high pressure low temperature metamorphic rocks are generally inconsistent with the p t conditions derived from solid phase equilibria therefore some process of change must regularly have occurred during exhumation however evidence of decrepitation is frequently lacking consideration of the rates of change of p and t in metamorphic rocks shows that very low strain rates are sufficient for volume adaptation dislocation creep appears to be a feasible deformation mechanism allowing for continuous ballooning driven by a small differential pressure during decompression down to temperatures of about 300 c in quartz decrepitation requires a higher differential pressure in natural rocks it is therefore restricted to relatively low temperatures this concept was tested on inclusions in the phyllite quartzite unit of crete in these rocks cooling to below ca 300 c at confining pressures of 3 to 4 kbar is indicated corresponding to depths of 10 to 13 km it is anticipated that in metamorphic rocks the isochores of all fluid inclusions in quartz that have been formed at higher temperatures and remain unaffected by decrepitation should intersect close to the 300 c isotherm thereby defining a point on the p t paths rarely accessible by other methods extension of this concept to other minerals with known mechanical properties is straightforward and can under favourable circumstances be used to fix several points on p t paths

progressive arcward contraction of a mesozoic tertiary fore arc basin southwestern sacramento valley california the modern sacramento valley intermontane basin is the successor to a deformed mesozoic tertiary fore arc basin based on analysis of seismic reflection profiles drill hole data and surface mapping we propose a model for progressive arcward contraction of the fore arc basin in the rumsey hills region southwestern sacramento valley beginning in cretaceous and continuing episodically through cenozoic time we present our interpretation in the form of a kinematically restorable forward model the major tectonic and depositional events include 1 mid cretaceous or older shortening of fore arc basin strata along imbricate west dipping thrust faults 2 erosion of the deformed fore arc strata 3 onlap of cenomanian and younger fore arc strata onto deformed older strata and crystalline sierran basement 4 renewed shortening in latest cretaceous to early tertiary accommodated by delamination of upper cretaceous strata from the mid cretaceous angular unconformity tectonic wedging and west directed backthrusting 5 erosion of the deformed upper cretaceous strata and deposition of the eocene capay formation 6 uplift and east directed shortening from eocene to late neogene with local erosion of the capay formation over the ancestral rumsey hills 7 uplift of the northern coast ranges beginning ca 3 4 ma and deposition of the pliocene pleistocene tehama formation in the western sacramento valley as a syntectonic molasse and 8 fault propagation folding of the tehama formation beginning ca 0 5 1 0 ma with the exception of quaternary shortening progressive arcward contraction of the western basin margin occurred during east dipping subduction and plate convergence beneath western california the deformation of the ancestral sacramento valley fore arc basin is analogous to arcward contraction of the tobago trough in the lesser antilles arc trench system and suggests that tectonic wedging may be a relatively common process in the evolution of fore arc regions late quaternary shortening has occurred in a transpressional setting and suggests a continuity in structural style despite a change in plate boundary kinematics from convergence to transpression during the past 3 5 m y

contribution of ductile flow to exhumation of low temperature high pressure metamorphic rocks san juan cascade nappes nw washington state the san juan cascade sjc nappes were subducted to a depth of 18 km metamorphosed under low temperature high pressure conditions and then exhumed all within 16 m y during exhumation penetrative deformation by solution mass transfer smt resulted in a widespread spaced cleavage strain directions were determined for 27 sandstone samples and absolute strain measurements for a subset of 19 samples z directions generally plunge moderately to the ne and x and y directions are scattered in the plane perpendicular to z smt deformation is constrictional at the local scale but the tensor average indicates plane strain uniaxial shortening at the regional scale with sx sy and sz equaling 1 01 0 91 and 0 58 respectively the average flattening plane xy dips 30 to the ne and the average x direction plunges 20 to the north the large shortening in z was compensated by a mass loss volume strain of 47 we present a simple one dimensional model that illustrates the relationship between finite strain and ductile exhumation for a steady state convergent wedge assuming depth dependent ductile flow and no reversal of principal strain rates with depth this model indicates that ductile thinning of the sjc nappes accomplished only 13 of the total exhumation despite a vertical shortening strain of 36 there is no evidence that normal faulting contributed significantly to exhumation we conclude that erosion operating at an average rate of 1 1 km m y 1 was the dominant exhumation process

tectonic implications of radiogenic noble gases in planetary atmospheres abstract studies of the radiogenic isotopes 4 he and 40 ar in planetary atmospheres provide important constraints on tectonic processes within planetary interiors for mantle k u and th u mass ratios r k u 12 700 and r th u 3 8 and a urey number ur 0 7 the 40 ar in the earth s atmosphere can be obtained by complete outgassing of the crust and upper mantle and a 7 outgassing of the lower mantle or by a 23 to 33 outgassing of the whole mantle and crust the flux of 4 he from the mantle can be derived by complete steady state outgassing of the upper mantle or by partial outgassing 9 of the entire mantle the 40 ar in the venus atmosphere can originate from the outgassing of only the upper 6 km of an enriched crust 8 5 of the entire mantle or 28 of the upper mantle the low concentration of 36 ar in the earth s atmosphere relative to venus strongly supports the collision hypothesis for the origin of the moon such a major collision would have eroded a large fraction of the earth s atmosphere the 40 ar on mars requires that only the upper 640 m of the crust be outgassed however the presence of large volcanoes on mars indicates greater degassing thus supporting the hypothesis of past erosion of the martian atmosphere by collisions with planetesimals and large meteorites the 40 ar flux through the lunar atmosphere requires the outgassing of the upper 9 km of the lunar crust

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

a multitracer approach to the study of erosion in the northeast equatorial pacific abstract erosion of sediment at two sites within the siliceous clay province of the northeast equatorial pacific is investigated by examining radionuclide radiolarian and percent smectite profiles of three cores a numerical model of the sediment is presented that examines the effect of various particle rain rates erosion rates and biological mixing parameters on these profiles 230 th because of its longer half life is a more sensitive tracer of erosion than 231 pa because the input of these radionuclides into the sediment is not well defined however conservative tracers such as the percent smectite of the clay fraction and fossil radiolarians are used to constrain the rate of sediment recycling dissolution of radiolarians though can greatly affect the interpretation of the observed biostratigraphy

metamorphism of franciscan tectonostratigraphic assemblage pacheco pass area east central diablo range california coast ranges the pacheco pass area well known for the relatively high pressure low temperature mineral assemblages developed in a well ordered section of chiefly metaclastic strata was restudied in order to elucidate the geologic paragenetic and textural relationships among the various lithologic units the mapped 7 minute quadrangle extends from 4 km west of the range crest eastward to the ortigalita fault tectonic contact of the franciscan assemblage with coeval great valley strata semicontinuous metachert beds underlain by scraps of na cpx and or na amphibole bearing metabasaltic rock mark the base of terrigenous tectonostratigraphic units evidently decoupled from the downgoing oceanic crust capped plate underplating along newly recognized bedding plane faults has juxtaposed members of an apparently conformable franciscan sedimentary sequence four depositional units each consisting of monotonous metagraywacke and interlayered metashale are stacked within the accretionary section from top down these are tectonostratigraphic members a b c and d unit d differs from overlying members in possessing a greater abundance of bluestone greenstone pods minor serpentinite bodies and rare metaconglomerate lenses varying degrees of stratal disruption indicate that it has been arrested in the transformation to broken formation the ortigalita fault is nearly vertical strikes north northwest and exhibits apparent dextral offset it truncates the east west striking gonzaga fault and other franciscan structures the topographic low occupied by san luis reservoir is a cenozoic tectonic depression sited at a releasing bend on the ortigalita fault newly acquired textural chemical mineralogic and areal relations of albitic and jadeitic pyroxene bearing franciscan metagraywacke near pacheco pass demonstrate that 1 na cpx and na amphibole are metamorphic minerals not clastic materials 2 pumpellyite is a minor neoblastic phase in albite bearing metagraywacke west of pacheco pass and in the north western corner of the quadrangle but it does not occur in jadeitic pyroxene bearing metaclastic rocks 3 higher textural grade metagraywacke specimens generally contain higher modal proportions of jd 4 abundance textural grade and chemistry of na cpx are not related to tectonostratigraphic units and 5 because of sluggish reaction rates albite persisted metastably into the higher pressure jadeitic pyroxene quartz p t field production of na cpx resulted from a reaction of the sort ab chl jd iws qtz gln h 2 o irregular oscillatory zoning within single jadeitic pyroxene prisms and variable jd ab modes for rocks of the same bulk composition reflect growth governed by diffusion rather than recrystallization within a p t transition zone high pressure overstepping of the phase boundary explains variable modal proportions of ab and jd as well as chemical heterogeneity of the jd elevated p attended metamorphism of na cpx bearing rocks exposed in the pacheco pass area as indicated by the wide spread coexistence of quartz and naalsi 2 o 6 rich pyroxene inferred physical conditions of prograde metamorphism were 150 50 c at 7 8 kbar or more franciscan rocks in the east central diablo range underwent subduction zone metamorphism accompanying paleo pacific lithospheric plate descent during mid cretaceous convergence underplating contraction and sequestering of the tectonostratigraphic assembly at depths of 25 30 km during underflow followed by gradual rise took place under conditions of refrigeration due to protracted descent of the oceanic slab paleogene diapiric uplift and extension followed indicating more complete decoupling of the buoyant trench complex from the downgoing plate finally passage of the east pacific rise triple junction in neogene time initiated the present dextral slip regime in the california coast ranges

kinematic data for the coast range fault and implications for exhumation of the franciscan subduction complex kinematic data from two segments of the coast range fault are used to test the hypothesis that the franciscan subduction complex was exhumed by extensional faulting the data reveal a consistent geometry for the principal directions of brittle strain the maximum extension direction has an east west orientation and lies subperpendicular to the present attitude of the coast range fault which dips steeply to the east the maximum shortening direction is subvertical and lies at a low angle to the present down dip direction of the fault given that the coast range fault probably formed with a gentle dip our results indicate subhorizontal crustal shortening in a northeast direction which is at variance with interpretations that invoke regional scale extension between the franciscan subduction complex and the overlying coast range ophiolite and great valley group we favor a model involving out of sequence thrust faults such a model is consistent with fault parallel contraction and the pronounced discontinuity in metamorphic grade across the coast range fault as well as substantial thinning of the coast range ophiolite this interpretation implies that erosion not extensional faulting was the major process that exhumed the franciscan subduction wedge during the late cretaceous

low grade blueschist facies metamorphism of metagreywackes franciscan complex northern california metagreywackes in the eastern belt of the franciscan complex contain the assemblage qtz ab lws chl ph pmp fgl hem cal arg or compatible subassemblages blue amphibole first appears in the westernmost part of the belt and pumpellyite is absent in the eastern part the compositions of the coexisting minerals and the nature of the continuous reactions in these low grade blueschists suggest that the distribution of blue amphibole and pumpellyite in the eastern belt of the franciscan complex reflects differences of effective bulk composition rather than differences in physical conditions of metamorphism in rocks lacking pumpellyite white mica may be essential to the growth of blue amphibole but carbonate plays only a limited role the continuous reaction that limits the appearance of blue amphibole and the disappearance of coexisting pumpellyite has the general form pmp chl ab qtz hem h2o femg 1 fgl lws this reaction requires significant hydration as pressure increases in order to produce blue amphibole most of the eastern belt of the franciscan complex formed in limited ranges of temperature and pressure which are estimated to be 240 280 c 6 5 7 5 kbar pressures in the westernmost part of the area were about 1 kbar lower than in the east pressures of about 8 5 10 kbar are estimated for tectonic blocks that contain sodic clinopyroxene

franciscan complex calera limestones accreted remnants of farallon plate oceanic plateaus the calera limestone part of the franciscan complex of northern california may have formed in a palaeoenvironment similar to hess and shatsky rises of the present north west pacific1 we report here new palaeomagnetic results palaeontological data and recent plate motion models that reinforce this assertion the calera limestone may have formed on farallon plate plateaus north of the pacific farallon spreading centre as a counterpart to hess or shatsky rises in one model2 the plateaus were formed by hotspots close to the farallon pacific ridge axis on accretion to north america plateau dissection in the late cretaceous to eocene 50 70 myr could explain the occurrence of large volumes of pillow basalt and exotic blocks of limestone in the franciscan complex partial subduction of the plateaus could have contributed to laramide 70 40 myr compressional events3

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

contrasts in thermal maturity within terranes and across terrane boundaries of the franciscan complex northern california the franciscan subduction complex of northern california includes several tectonostratigraphic units we have used the technique of vitrinite reflectance to document levels of thermal maturity for the central belt and the yager coastal and king range terranes spatial variations in thermal maturity help define the thermal structure of each franciscan terrane and each terrane displays a different geometry of thermal structure these differences are controlled by the style of structural deformation the overprinting effects of deformation following metamorphism and thermal overprinting caused by hydrothermal circulation anomalies along and contrasts across terrane boundaries help establish the relative timing and the structural mechanisms of terrane amalgamation for example localized anomalies are evident along the eel river fault central yager boundary and these increases in thermal maturity may be due to shear heating hydrothermal discharge or conductive heat transfer across the fault cross c