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

quaternary blind thrusting in the southwestern sacramento valley california patterns of microearthquakes and quaternary surface deformation suggest that the tectonic setting of the sw sacramento valley is similar to areas of the western san joaquin valley known to be underlain by seismogenic blind thrust faults on the basis of previous work and analysis of geologic and seismic reflection data the following late cenozoic tectonic features and processes are identified 1 uplift of the northern coast ranges beginning approximately 3 4 ma and eastward propagation of uplift into the southwestern sacramento valley by 1 0 ma 2 uplift and homoclinal flexure of plio pleistocene strata at the eastern coast ranges mountain front 3 uplift and folding above blind thrusts approximately 15 km east of the mountain front in the southwestern sacramento valley similar associations of structures and processes have been observed in thrust belts in pakistan the peruvian andes and the canadian cordillera and are commonly attributed to thrusting within an intercutaneous wedge or triangle zone by using other thrust belts as analogs the propagation of an eastward tapering triangle zone is interpreted to be the principal mechanism for uplift and homoclinal flexure at the eastern coast ranges mountain front seismic reflection profiles reveal that 1 the triangle zone consists primarily of east vergent blind thrusts and 2 west vergent backthrusts exposed in the the eastern coast ranges and southwestern sacramento valley are rooted in the east vergent thrusts transfer of slip from the east vergent blind thrusts to the west vergent backthrusts occurs locally beneath the southwestern sacramento valley fault bend folding in the hanging walls of the backthrusts has created a north northwest striking chain of low hills approximately 15 km east of the mountain front the folds deform 3 4 1 0 ma fluvial sediments and thus are middle pleistocene in age or younger local variations in strike suggest that the fold chain is segmented like the new idria coalinga kettleman hills segmented fold chain in the southwestern san joaquin valley stein and ekstrom 1989 these data have implications for seismic hazard assessment anecdotal accounts indicate that two m 6 0 events of the 1892 winters vacaville earthquake sequence probably occurred beneath the eastern coast ranges dale 1977 toppozada et al 1981 ground cracking was observed following the main shocks along the mountain front in the southwestern sacramento valley we propose that the earthquakes were generated by slip on a blind thrust beneath the coast ranges and that the ground cracking in the valley represents propagation of the eastward tapering triangle zone the 1892 earthquake sequence suggests that blind thrusts beneath the southwestern sacramento valley are active and capable of generating moderate to large magnitude earthquakes

seismically active fold and thrust belt in the san joaquin valley central california structural analysis of late cenozoic folds along the western and southern margins of the san joaquin basin suggests that the folds are related to development of a fold and thrust belt rather than to wrench tectonics the folds have formed by the processes of fault bend folding and fault propagation folding which commonly occur in fold and thrust belts our structural interpretation attributes the seismically active coalinga and kettleman hills north dome anticlines to fault bend folding above a thrust fault s that steps up from a detachment within the franciscan assemblage to a detachment at the base of the great valley group this thrust does not reach the surface blind thrust and its slip is consumed in backthrusting and formation of subsurface folds under the san joaquin valley movement on the postulated thrust s would explain the cause of the may 2 1983 coalinga earthquake and the august 15 1985 avenal earthquake and would account for the lack of significant surface rupture or shallow subsurface faulting during both earthquakes well documented examples of fault bend and fault propagation folding also occur at wheeler ridge in the san emigdio mountains and at kettleman hills south dome deformation associated with fold and thrust belts can be extremely complicated considerable fault movement can occur without accompanying surface rupture or shallow level faulting conversely surface rupture can occur that has no direct relationship to fault slip at depth an example would be flexural slip folding wherein the slip planes reach the surface but do not root into any significant faults at depth consequently traditional geologic approaches to seismic risk evaluation which rely largely on surface data are subject to numerous pitfalls when applied to fold and thrust belts our interpretation of the structural style that developed in central california during the late cenozoic requires that the strain associated with the transpressive motion between the pacific and north american plates be resolved into normal and tangential components thrust faulting and folding normal to the plate boundary and strike slip faulting parallel to the plate boundary san andreas fault the thrust faults root in a deecollement at the brittle ductile transition zone above which shortening is associated with folding and thrust faulting and beneath which shortening in the lower crust is accommodated by ductile processes of tectonic thickening or incipient subduction

late quaternary slip on the santa cruz island fault california the style timing and pattern of slip on the santa cruz island fault were investigated by trenching the fault and by analysis of offset late quaternary landforms a trench excavated across the fault at christi beach on the western coast of the island exposed deformation of latest pleistocene to holocene sediments and pre quaternary rocks recording repeated large magnitude rupture events the most recent earthquake at this site occurred ca 5 ka coastal terraces preserved on western santa cruz island have been dated using the uranium series technique and by extrapolation using terrace elevations and the eustatic record offset of terraces and other landforms indicates that the santa cruz island fault is predominantly left lateral having a horizontal slip rate of not more than 1 1 mm yr and probably about 0 8 mm yr the fault also has a smaller reverse component slipping at a rate of between 0 1 and 0 2 mm yr combined with measurements of slip per event this information suggests a long term average recurrence interval of at least 2 7 k y and probably 4 5 k y and average earthquake magnitudes of mw 7 2 7 5 sense of slip recurrence interval and earthquake magnitudes calculated here for the santa cruz island fault are very similar to recent results for other faults along the southern margin of the western transverse range including the malibu coast fault the santa monica fault the hollywood fault and the raymond fault supporting the contention that these faults constitute a continuous and linked fault system which is characterized by large but relatively infrequent earthquakes

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

stickney forming impact on phobos crater shape and induced stress distribution abstract the size and rim shape of the crater stickney on phobos and the impact generated stress patterns on the surface are modeled by a simplified method the results explain the general features of stickney some of the surface stress pattern of the p waves suggests that a region of higher tensile stress may have occurred around longitude 270 w and lattitude 0 correlation of this pattern with the groove pattern focusing on the trailing side of phobos implies a connection of the stickney impact with the grooves

mineralogy and phase relations in the blueschist facies of the black butte and ball rock areas northern california coast ranges microprobe study of minerals in rocks of the blueschist facies from the black butte and ball rock areas of the south fork mountain schist northern california coast ranges is the basis in this report for analysis of reactions and phase relations among lawsonite pumpellyite epidote and associated minerals observed assemblages are intermediate in a progression from those observed elsewhere in high p low t schists of the franciscan e 9 panoche pass where lawsonite is present in nearly all lithologies to the relatively low phigh t assemblages of the shuksan blueschist washington where epidote is ubiquitous two successive reactions in this sequence in order of increasing temperature are l lawsonite na amphibole epidote quartz albite chlorite hzo 2 na amphibole pumpellyite quartz epidote ca amphibole albite chlorite h2o the pressure of metamorphism for the black butte and ball rock areas is estimated from the jadeite content of pyroxene and from the stability of aragonite to have been approximately 7 kbar temperature estimates are not tightly constrained by mineral equilibria but are suggested to be in the range of 250 to 300 c

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

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

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

tectonic wedging along the rear of the offshore taiwan accretionary prism abstract the structural geometry kinematics and density structure along the rear of the offshore taiwan accretionary prism were studied using seismic reflection profiling and gravity modeling deformation between the offshore prism and forearc basin at the point of incipient collision and southward into the region of subduction has been interpreted as a tectonic wedge similar to those observed along the front of mountain ranges this tectonic wedge is bounded by an east dipping roof thrust and a blind west dipping floor thrust an east dipping sequence of forearc basin strata in the hanging wall of the roof thrust reaches a thickness in excess of 4 km near the tip of the interpreted tectonic wedge section restoration of the roof sequence yields an estimate of 4 km of shortening which is small compared with that inferred in the collision area to the north based on the variation in distance between the apex of the prism and the island arc previous studies propose that either high angle normal faulting or backfolding has exhumed the metamorphic rocks along the eastern flank of the central range in the collision zone on land to better constrain the initial crustal configuration we tested 350 crustal models to fit the free air gravity anomaly data in the offshore region to study the density structure along the rear of the accretionary prism in the subduction and initial collision zones before the structures become more complex in the collision zone on land the gravity anomaly observed in the region of subduction 20 2 n can be modeled with the arc basement forming a trenchward dipping backstop that is overlain by materials with densities in the range of sedimentary rocks near the point of incipient collision 20 9 n however the free air gravity anomaly over the rear of the prism is approximately 40 mgal higher compared with the region of subduction and requires a significant component of high density crustal rocks within the tectonic wedge these results suggest that the forearc basement may be deformed along the rear of the prism associated with the onset of collision but not in the subduction region further to the south

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

structural anatomy of the early paleozoic laohushan ophiolite and subduction complex implications for accretionary tectonics of the proto tethyan north qilian orogenic belt northeastern tibet

the style of late cenozoic deformation at the eastern front of the california coast ranges the 1983 coalinga earthquake occurred at the eastern boundary of the california coast ranges in response to northeast directed thrusting such movements over the past 2 ma have produced coalinga anticline by folding above the blind eastern tip of the coalinga thrust zone the 600 km length of the coast ranges boundary shares a common structural setting that involves westward upturn of cenozoic and cretaceous strata at the eastern front of the coast ranges and a major southwest facing step in the basement surface beneath the western great valley like coalinga anticline pliocene and quaternary folding and faulting along the rest of the boundary also result from northeast southwest compression acting nearly perpendicular to the strike of the san andreas fault we suggest that much of this deformation is related to active thrusts beneath the eastern coast ranges the step in the basement surface beneath the great valley seems to have controlled the distribution of this deformation and the shape of the coast ranges boundary

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

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

implications of franciscan complex graywacke geochemistry for sediment transport provenance determination burial exposure duration and fluid exchange with cosubducted metabasites 1 interpretation of graywacke provenance has long been used to evaluate the record of tectonic process in orogenic belts our geochemical data from graywackes of the franciscan subduction complex california show that the connection between sedimentary record and geologic processes may be more complex than previously believed trace elements and nd sr pb isotopes of franciscan graywackes indicate two sources types one group lacking negative eu anomaly eu eu 0 9 shows slightly concave up heavy rare earth elements arc like trace element patterns and western pacific island arc like pb isotopes reflecting derivation from older accreted oceanic arc terranes in the sierra nevada klamath mountains the other group displays small negative eu anomalies with trace element patterns resembling post archean australian shale and pb isotopes similar to jurassic cretaceous sierran batholith there is no systematic separation of these two groups by depositional ages thus geochemistry of the graywackes may partly reflect variation in location of sediment delivery systems rather than solely reflecting evolution of the neighboring arc variation of nd sr isotopes with stratigraphic age for the graywackes mimics the trends of the coeval great valley group clastic rocks suggesting that 1 they share the same sediment sources 2 there are no exotic sediment sources that fed the franciscan trench and 3 burial exposure cycles for franciscan clastic rocks were comparatively brief comparison of franciscan graywacke and metabasite geochemistry corroborates earlier conclusions that metabasites had little or no chemical exchange with fluids from cosubducted graywacke detrital zircon age populations major element chemistry and detrital framework modes when compared to our data suggest that the former three parameters underrepresent the mafic component of clastic sediment provenance

subsurface geometry of the san andreas calaveras fault junction influence of serpentinite and the coast range ophiolite while an enormous amount of research has been focused on trying to understand the geologic history and neotectonics of the san andreas calaveras fault saf cf junction fundamental questions concerning fault geometry and mechanisms for slip transfer through the junction remain we use potential field geologic geodetic and seismicity data to investigate the 3 d geologic framework of the saf cf junction and identify potential slip transferring structures within the junction geophysical evidence suggests that the san andreas and calaveras fault zones dip away from each other within the northern portion of the junction bounding a triangular shaped wedge of crust in cross section this wedge changes shape to the south as fault geometries change and fault activity shifts between fault strands particularly along the calaveras fault zone cfz potential field modeling and relocated seismicity suggest that the paicines and san benito strands of the cfz dip 65 to 70 ne and form the southwest boundary of a folded 1 to 3 km thick tabular body of coast range ophiolite cro within the vallecitos syncline we identify and characterize two steeply dipping seismically active cross structures within the junction that are associated with serpentinite in the subsurface the architecture of the saf cf junction presented in this study may help explain fault normal motions currently observed in geodetic data and help constrain the seismic hazard the abundance of serpentinite and related cro in the subsurface is a significant discovery that not only helps constrain the geometry of structures but may also help explain fault behavior and the tectonic evolution of the saf cf junction

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

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

subnormal geothermal gradients in the great valley forearc basin california during franciscan subduction a fission track study outboard parts of forearcs commonly have subnormal geothermal gradients because relatively cold lithosphere subducts beneath them the strata of the 10 to 15 km thick late jurassic tertiary great valley forearc basin accumulated in the forearc of the franciscan subduction zone of coastal california in this study paleogeothermal gradients in this basin were reconstructed to constrain the tectonic history of the convergent plate margin backstripping was used to reconstruct the burial histories of outcropping basin strata and apatite fission track analysis was used to determine the maximum burial temperatures experienced by 64 rock samples plots of maximum burial temperatures versus maximum burial depths indicate earliest tertiary time of deepest burial forearc basin geothermal gradients of about 9 c km backstripping shows that strata from the lower parts of two sections were deeply buried 6 5 km for the period 90 65 ma without experiencing high temperatures 105 c indicating that gradients were at least moderately subnormal throughout that period the data permit gradients no greater than about 15 c km at 90 ma decreasing to no greater than about 9 c km at 65 ma lower strata in another section were probably deeply buried without experiencing high temperatures from 130 or 120 ma until the latest cretaceous or early tertiary indicating that subnormal gradients probably existed throughout that period k ar ages of franciscan complex blueschists indicate subnormal gradients in the california forearc in the late jurassic and early cretaceous fission track data from the sierra nevada indicate subnormal gradients in the early and middle cenozoic and modern heat flow patterns indicate subnormal gradients in the late cenozoic together these indicators suggest that thermal gradients in the forearc were continuously subnormal from the initiation of franciscan subduction in the late jurassic until the termination of subduction in the late cenozoic gradients in the range that would permit blueschist facies metamorphism in the franciscan complex were thus long lived the broad range of k ar ages noted for franciscan blueschists need not reflect multiple discrete metamorphic events but are also consistent with a continuous extended period of high p t conditions blueschists need not have been uplifted soon after metamorphism to avoid overprinting by normal p t conditions but rather may have remained at depth for extended periods preliminary modeling indicates that an interruption in subduction of longer than 5 20 m y should cause detectable heating of forearc basin strata while a much shorter interruption should heat accretionary wedge rocks since there is no evidence of such heating it appears that convergence at the franciscan trench was essentially continuous if allochthonous terranes traveled northward parallel to the california margin during the late jurassic to late cenozoic period of franciscan subduction they apparently did so during oblique subduction not during extended interludes of pure transform motion

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

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

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

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

biostratigraphic constraints on formation and timing of accretion in a subduction complex an example from the franciscan complex of northern california the determination of the total age coverage of pelagic bedded chert is particularly important in studies of ancient accretionary complexes because the time span represents the minimum travel time of an oceanic plate before accretion at an island arc or continental margin the yolla bolly terrane of the franciscan complex consists of rare metabasalt overlain by bedded radiolarian chert which in turn is overlain by metagraywacke and subordinate slaty mudstone and conglomerate the bedded chert ranges in age from aaleian early middle jurassic to tithonian latest jurassic a lithologic gradation from pelagic bedded chert through hemipelagic siliceous mudstone occurred during the tithonian and thus marks the arrival of the oceanic sediments at the continental margin the tithonian age also agrees with previously reported tithonian to valanginian early cretaceous fossils in the overlying terrigenous metagraywacke both chert and metagraywacke are intruded by gabbroic sills suggesting off ridge volcanism

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

mixed carbonate siliciclastic sedimentation in forebulge grabens an example from the ediacaran bambui group sao francisco basin brazil abstract mixed carbonate siliciclastic successions have been described in multiple phanerozoic sedimentary settings recording the dynamic interplay of tectonics eustasy climate in situ carbonate production and variations in siliciclastic sediment supply the ediacaran bambui 1st order sequence i e bambui group covers most of the intracratonic sao francisco basin southeast brazil and encompasses thick packages of carbonate and fine to coarse grained siliciclastic strata recording a marine foreland basin stage that developed in the sao francisco plate during the neoproterozoic early paleozoic west gondwana assembly the ediacaran deposits unconformably overlie archean paleoproterozoic metamorphic assemblages of the sete lagoas basement high and fill a series of kilometer long grabens in the southern sao francisco basin seismic data reveal that these troughs developed through the extensional reactivation of ancient basement structures along with the tectonically driven forebulge uplift of the sete lagoas high in the early evolutionary stages of the bambui basin cycle based on the detailed description of continuous drill cores of a well recently drilled during hydrocarbon exploration campaigns we recognized two transgressive regressive 2nd order sequences preserved within one of the focused grabens i sequence 1 includes the glaciogenic deposits of the basal carrancas formation that grade upward into the carbonate ramp successions of the sete lagoas formation ii sequence 2 contains the siliciclastic dominated and deep water to deltaic strata of the serra de santa helena formation and passes upward into peritidal carbonates of the lagoa do jacare formation these sedimentary successions encompass suites of retrogradational aggradational and progradational lower rank cycles and are bounded by erosional surfaces regional seismic interpretation well data and the available literature indicate that most of these deposits and their correlatives are continuous over most of the sete lagoas basement high showing remarkable variations within the studied forebulge grabens our analysis demonstrates that the depositional architecture of sequence 1 and sequence 2 marks the interplay of successive episodes of tectonically driven forebulge uplift and regional eustatic and climatic changes the latter are mainly recorded by the lower second order sequence sequence 1 which contains typical sedimentary features deposited in the aftermath of an important glaciation affecting lower latitudes during the late neoproterozoic an additional local accommodation control is restricted to the forebulge grabens and is associated with episodic and local subsidence pulses as indicated by lower to higher rank cycles it has been responsible for the deposition of thicker transgressive to early regressive successions a few gravitational flow deposits and the development of the deltaic to shallow marine carbonate cycles of sequence 2 the overall stratigraphic architecture of the studied successions suggests that several basin mechanisms operating during the phanerozoic were also present in the late neoproterozoic the close association between organic rich post glacial shales potential reservoir facies and rift related migration paths and hydrocarbon traps indicate that forebulge grabens represent important elements to consider in hydrocarbon exploration campaigns