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

crustal structure of the diablo and gabilan ranges central california a reinterpretation of existing data the compressional wave velocity structure of the crust of the coast ranges of central california was modeled from seismic refraction data reported previously for both the diablo range east of the san andreas fault and the gabilan range west of the fault two alternative velocity models were derived for each range by iterative two dimensional ray tracing each pair of models shows a sedimentary layer and an underlying crust composed of three or four layers for the diablo range the velocities in the sediments range from 2 9 to 4 8 km sec and the depth to basement varies from 0 2 km within the central diablo range to a maximum of 3 4 km north of the livermore valley in the gabilan range the velocities in the sediments overlying the granitic basement are higher north of the gabilan range 3 6 to 4 6 km sec than to the south 2 2 to 3 8 km sec and the depth to basement both north and south of the range is less than 2 km below the sediments and fractured near surface material a resolvable difference in the crustal velocity structure on opposite sides of the san andreas fault indicates compositional differences at depth the upper crust has an average velocity of 5 7 km sec at depths between 3 and 16 3 km in the diablo range and an average velocity of 6 1 km sec at depths between 2 and 9 1 km in the gabilan range the lower crust has an average velocity of 6 9 km sec at depths between 16 3 and 30 km in the diablo range and an average velocity of 6 5 km sec at depths between 10 and 24 km in the gabilan range the models also show that the depth to moho differs by several kilometers between the two ranges in the diablo range models the crust mantle boundary becomes shallower from south to north rising from a depth of 30 to 26 km in the gabilan range models the moho is at a depth of 24 to 26 km depending on the velocity assumed at the base of the crust laboratory measurements of rock velocities and the mapped surface geology allow us to interpret the velocity models in terms of crustal composition we conclude that the diablo range probably consists largely of metagraywacke to a depth of 16 3 km and gabbroic material below this depth the crust of the gabilan range probably consists of granitic material to a depth of 9 1 km and gneissic material below this depth franciscan rocks are not regionally present in the crust of the gabilan range if the gneissic lower crust consists of the same rocks as are found at the surface in the gabilan range then the presumed large scale lateral motion of the salinian block has taken place at or below the crust mantle boundary rather than along a mid crustal slip plane

seismic potential revealed by surface folding 1983 coalinga california earthquake the 2 may 1983 coalinga california earthquake magnitude 6 5 failed to rupture through surface deposits and instead elastically folded the top few kilometers of the crust the subsurface rate of fault slip and the earthquake repeat time are estimated from seismic geodetic and geologic data three larger earthquakes up to magnitude 7 5 during the past 20 years are also shown to have struck on reverse faults concealed beneath active folds

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

cocorp seismic profiles near coalinga california subsurface structure of the western great valley cocorp seismic reflection profiles collected in 1977 on the west side of the san joaquin valley near coalinga in southern california provide information on the cenozoic and mesozoic structures in the subsurface of the western great valley the data show evidence of significant normal faulting during the cretaceous when this part of california was the site of a forearc basin neogene compression probably associated with transform motion on the nearby san andreas fault system may be reactivating these pre existing faults in a reverse sense and causing the active folding of the southern coast ranges the recent coalinga earthquake sequence that began on may 2 1983 appears to correspond to movement on the high angle reverse fault within the basement that is inferred from the cocorp lines

ophiolitic basement to the great valley forearc basin california from seismic and gravity data implications for crustal growth at the north american continental margin the nature of the great valley basement whether oceanic or continental has long been a source of controversy a velocity model derived from a 200 km long east west reflection refraction profile collected south of the mendocino triple junction northern california in 1993 further constrained by density and magnetic models reveals an ophiolite underlying the great valley great valley ophiolite which in turn is underlain by a westward extension of lower density continental crust sierran affinity material we used an integrated modeling philosophy first modeling the seismic refraction data to obtain a final velocity model and then modeling the long wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model the crustal section of great valley ophiolite is 7 8 km thick and the great valley ophiolite relict oceanic moho is at 11 16 km depth the great valley ophiolite does not extend west beneath the coast ranges but only as far as the western margin of the great valley where the 5 7 km thick great valley ophiolite mantle section dips west into the present day mantle there are 16 18 km of lower density sierran affinity material beneath the great valley ophiolite mantle section such that a second deeper present day continental moho is at about 34 km depth at mid crustal depths the boundary between the eastern extent of the great valley ophiolite and the western extent of sierran affinity material is a near vertical velocity and density discontinuity about 80 km east of the western margin of the great valley our model has important implications for crustal growth at the north american continental margin we suggest that a thick ophiolite sequence was obducted onto continental material probably during the jurassic nevadan orogeny so that the great valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle

the crustal structure of the axis of the great valley california from seismic refraction measurements abstract in 1982 the u s geological survey collected six seismic refraction profiles in the great valley of california three axial profiles with a maximum shot to receiver offset of 160 km and three shorter profiles perpendicular to the valley axis this paper presents the results of two dimensional raytracing and synthetic seismogram modeling of the central axial profile the crust of the central great valley is laterally heterogeneous along its axis but generally consists of a sedimentary section overlying distinct upper middle and lower crustal units the sedimentary rocks are 3 5 km thick along the profile with velocities increasing with depth from 1 6 to 4 0 km s the basement upper crust consists of four units 1 1 a 1 0 1 5 km thick layer of velocity 5 4 5 8 km s 2 2 a 3 4 km thick layer of velocity 6 0 6 3 km s 3 3 a 1 5 3 0 km thick layer of velocity 6 5 6 6 km s and 4 4 a laterally discontinuous 1 5 km thick layer of velocity 6 8 7 0 km s the mid crust lies at 11 14 km depth is 5 8 km thick and has a velocity of 6 6 6 7 km s on the northwest side of our profile the mid crust is a low velocity zone beneath the 6 8 7 0 km s lid the lower crust lies at 16 19 km depth is 7 13 km thick and has a velocity of 6 9 7 2 km s crustal thickness increases from 26 to 29 km from nw to se in the model although an unequivocal determination of crustal composition is not possible from p wave velocities alone our model has several geological and tectonic implications we interpret the upper 7 km of basement on the northwest side of the profile as an ophiolitic fragment since its thickness and velocity structure are consistent with that of oceanic crust this fragment which is not present 10 15 km to the west of the refraction profile is probably at least partially responsible for the great valley gravity and magnetic anomalies whose peaks lie about 10 km east of our profile the middle and lower crust are probably gabbroic and the product of magmatic or tectonic underplating or both the crustal structure of the great valley is dissimilar to that of the adjacent diablo range suggesting the existence of a fault or suture zone throughout the crust between these provinces

seismicity and geometry of a 110 km long blind thrust fault 1 the 1985 kettleman hills california earthquake the august 4 1985 kettleman hills earthquake was the third in a sequence of moderate shocks to occur beneath the northern half of a 110 km long fold chain bounding the eastern california coast ranges the 1982 mw 5 4 new idria 1983 mw 6 5 coalinga and 1985 mw 6 1 kettleman hills events define a southward progression of seismic activity beneath the fold we use teleseismic waveforms geodetic modeling hypocenters relocated in a three dimensional velocity model and subsurface structural data to investigate the kettleman hills earthquake the main shock results from motion on a shallowly dipping thrust fault buried at 10 km depth aftershocks and coseismic fault slip extend 20 km along the fold axis nearly the full extent of the kettleman hills north dome anticline aftershocks occur primarily several kilometers in front of the fault tip and in the core of the anticline the main shock and several foreshocks occurred at a 2 km right step in the quaternary fold axis which also corresponds to the southern end of the 1983 coalinga and northern end of the 1985 kettleman hills aftershock zones from this we infer that the step in the fold is caused by an offset or tear in the underlying fault the scalar seismic moment is 1 6 1018 n m consistent with the geodetic deformation and the duration of rupture is 16 s 3 4 times greater than for the average earthquake with this scalar moment the slow rate of moment release provides an explanation for the low level of ground shaking and low local magnitude reported for the event the peak of the geodetic uplift is located 5 km perpendicular to the quaternary fold axis we argue that the fault is propagating northeast into the undeformed san joaquin valley sediments and that the overlying fold is growing at about 0 5 mm yr

petrofacies and provenance of late mesozoic forearc basin northern and central california data from the great valley group sequence represent the most complete information regarding sandstone petrology of sediment derived from a magmatic arc this information is useful in documenting tectonic and magmatic events within the arc and related terranes and forms the basis for the establishment of petrostratigraphic units for mapping and correlation sandstone and conglomerate compositions are controlled by changes in provenance many of which were basinwide and synchronous clay mineral composition is controlled primarily by burial metamorphism careful attention to sample collection sample preparation and petrographic techniques is essential for uniform results seven petrographic parameters p f lv l m qp q q f and l listed in decreasing importance to petrofacies discrimination define eight petrofacies stony creek platina lodoga grabast boxer cortina los gatos and rumsey listed in approximate order of decreasing age the upper jurassic lower cretaceous petrofacies stony creek platina and lodoga contain higher lithic contents supracrustal sources whereas the upper cretaceous petrofacies especially the rumsey contain higher proportions of plutoniclastic components quartz feldspar and micas the proportion of potassium feldspar increases from near zero in the upper jurassic to nearly 50 of all feldspars in the uppermost cretaceous the lower part of the great valley group upper jurassic and lower cretaceous contains significant quantities of sedimentaclastic and metamorphiclastic material eroded from accreted and deformed terranes tectonic highlands formed by the arc arc collision nevadan orogeny that occurred prior to initiation of the franciscan great valley sierra nevada arc trench system the klamath mountains area provided a major proportion of this detritus ophiolite and serpentinite detritus was deposited locally near the base of the great valley group as a result of deformation along the east side of the growing franciscan subduction complex volcaniclastic detritus was fed into the entire forearc basin as magmatism increased in the sierra nevada area during the cretaceous as the volcanic cover was stripped off plutoniclastic and metamorphiclastic detritus from the underlying batholithic terranes was provided in abundance to the forearc basin crustal components were more continental in the southern sierra nevada and more oceanic in the northern sierra nevada as demonstrated by the higher proportions of metamorphiclastic detritus and by the more felsic nature of volcaniclastic detritus to the south by the middle of the late cretaceous extensive batholithic terranes provided potassium feldspar rich arkosic detritus to the entire forearc basin by the paleogene arc magmatism had migrated eastward sufficiently that deeper levels of the california part of the arc were exposed by erosion tectonic activity decreased in the forearc basin and the basin was filled to sea leve in most parts

seismicity and geometry of a 110 km long blind thrust fault 2 synthesis of the 1982 1985 california earthquake sequence during 1982 1985 three 5 4 mw 6 5 earthquakes migrated 65 km along the north half of a string of quaternary folds at the east front of the california coast ranges there is no surface fault associated with these earthquakes but the fold chain oriented parallel to the san andreas fault 30 km to the west takes up contraction normal to the san andreas fault identification of active blind thrust faults is hindered by the absence of a fault trace but may be revealed by the presence of growing folds at the surface and by earthquakes at depth because of excellent seismic geodetic and geologic data this earthquake sequence illuminates the geometry of the blind faults and enables us to probe their mechanics we relocate seismicity and compute focal mechanisms in a three dimensional velocity space and model the vertical deformation associated with the 1983 m 6 5 coalinga earthquake the aftershock zones abut at echelon offsets in the fold axes and the mainshocks display reverse slip perpendicular to the axes suggesting that the folds conceal a contiguous segmented thrust fault background seismicity concentrates at bends and breaks in the fold chain sites that may correspond to tears and ramps in the thrust fault at depth seismic reflection profiles reveal thrust and reverse faults dipping toward the san andreas fault at depths of 5 10 km with several kilometers of cumulative slip and high angle reverse faults in the anticlines with several hundred meters cumulative slip coseismic fold uplift accompanied the coalinga and kettleman hills north dome earthquakes suggesting that fold growth is episodic and coupled closely to repeated earthquakes on the underlying thrusts the north half of the fold chain has been the site of several 6 m 6 5 earthquakes since 1885 thus kettleman hills middle dome the next fold segment to the south has an elevated seismic potential the aftershock zones of the three main shocks are diffuse and occupy a region much larger than the site of seismic slip aftershocks occur 5 7 km from the fault regions where on the basis of a boundary element model the shear strain increase caused by the fault slip exceeds 20 ppm equivalent to about 0 7 mpa we argue that the broad aftershock zone is a product of high sustained off fault stress caused by repeated displacement on faults that do not cut the earth s surface the fault tip stresses lead to the formation of secondary faults which can become sites of aftershocks and postseismic creep

tectonic evolution of the tobago trough forearc basin the tobago trough is a modern marine forearc basin of the lesser antilles arc it lies above an undrilled mainly conformable sedimentary succession at least 10 km thick and probable oceanic basement the strata occur in a forearc basin belt between the crystalline arc platform and the accretionary forearc for hundreds of kilometers north and southwest of the tobago trough seismic data permit division of tobago trough strata into four sequences a ab bc and c a is quaternary ab early pleistocene to late miocene bc miocene and c includes miocene probably through middle eocene and perhaps rocks as old as mid cretaceous or older thickness maps of the four sequences indicate major configurational changes in basins that preceded the modern one a southern c basin extended southwest to the carupano basin north of venezuela the northern c basin extended and perhaps thickened northeast of its current limit of recognition at the crosscutting inner forearc deformation belt ifdb the southern c basin continued unchanged through bc time whereas the northern bc basin was confined to the region west of the ifdb the depoaxis of the bc basin lay close to the western basin flank in contrast sequences ab and a accumulated in an asymmetric basin with maximum subsidence at its eastern margin at the foot of the ifdb a stratigraphic model of eocene and younger layers in the tobago trough is derived from seismic attributes cenozoic rocks on nearby islands well data in the carupano basin and piston cores the model proposes that 1 sequences c and lower bc are volcanogenic hemipelagite including ashfall and fine turbidite to the north and possibly continent derived turbidite to the south 2 upper bc and ab are mainly proximal basin flank derived turbidite to the north perhaps grading south to mainly continent derived hemipelagite and 3 a is mainly continent derived hemipelagite and volcanogenic ashfall vertical velocity distributions in tobago trough strata suggest mainly compaction related increases at an undercompacted rate for the upper 3 4 km local increases in velocity gradient at these depths could represent defluidization possibly at layer parallel detachments extending from the ifdb depositional rates in the tobago trough may have increased monotonically through later cenozoic time they have always greatly exceeded deep oceanic rates the northwestern margin of the c basin was a preexisting irregular edifice that included the arc platform and the st lucia ridge the flank edifice and deep basin floor were covered by onlap of forearc basin strata in a water filled basin and at the same time underwent southeast down rotation the c tilting was without evident local deformation and was perhaps due to intrusion at a currently incompletely defined locus to the northwest the initiation of the bc depoaxis at the western edge of the northern basin records the miocene development of a clastic wedge of arc platform provenance perhaps due to the onset of neogene volcanism and or emergence of the arc platform the shift of the depoaxis in ab and a times to the eastern basin margin reflects loading of the lithosphere by the tectonically thickened ifdb the miocene and younger features of the tobago trough probably reflect a superposition of tectonism associated with the development of the neogene lesser antilles arc on an older arc system the neogene structures crosscut the older ones and are not simply the result of progressive growth of the accretionary forearc

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

geothermal energy release at the solfatara of pozzuoli phlegraean fields phreatic and phreatomagmatic explosion risk implications the h2o co2 and h2s outputs at the solfatara of pozzuoli have been measured and a map of the exhaling areas has also been made the energy released at the surface by the fluids has been estimated to be 1019 ergs day

petrofacies and provenance of the great valley group southern klamath mountains and northern sacramento valley lower cretaceous sandstone and conglomerate of the great valley group rest depositionally on the southern klamath mountains at the north end of the sacramento valley exposures include nonmarine shallow marine and deep marine sediments which are part of the platina formation petrofacies approximately 150 sandstone point counts 500 points per section using gazzi dickinson method and 32 conglomerate clast counts indicate a klamath provenance of mixed magmatic arc and recycled orogenic character standard triangular plots and multivariate analyses combined with paleocurrent and paleogeographic data suggest discrete source areas within the klamath terrane however the general homogeneity of both sandstone and conglomerate petrology is consistent with all of the units being included within the platina petrofacies

reappraisal of hypothetical franciscan thrust wedging at coalinga implications for tectonic relations along the great valley flank of the california coast ranges 1 correlation of the character and orientation of seismic reflectors imaged on profiles across the coalinga anticline with known surface geology indicates that local franciscan thrust wedging favored for past geodynamic interpretations of the 1983 coalinga earthquake is an untenable hypothesis the inference that a subhorizontal detachment surface of simple shear is present beneath the anticline is consequently unnecessary and unlikely in view of the coherent areal pattern of wrench folding within the southern diablo range elsewhere along the structural front of the california coast ranges at the western flank of the great valley interpretations of similar thrust wedging based in part on analogy with supposed structural relations at coalinga are equally questionable in a wider context the seismogenic behavior of blind thrust and reverse faults in california cannot be interpreted with the invalid concept of thrust wedging at coalinga as a guiding template for geotectonic interpretations

active faulting and deformation of the coalinga anticline as interpreted from three dimensional velocity structure and seismicity this work gives a clear picture of the geometry of aftershock seismicity in a large thrust earthquake interpretation of hypocenters and fault plane solutions from the 1983 coalinga coast range california earthquake sequence in combination with the three dimensional velocity structure shows that the active faulting beneath the fold primarily consists of a set of southwest dipping thrusts uplifting blocks of higher velocity material above the main listric blind thrust there is a conjugate fault steeply northeast dipping that provides the western limit of the aftershocks within the coalinga anticline and that corresponds in location and spatial extent with the adjacent pleasant valley syncline the character of the seismicity varies with the degree of previous deformation on each section of the anticline where the previous uplift was largest the shallow seismicity shows secondary faulting on either side of the fold with orientations that correspond to the preexisting geologic structure diffuse seismicity characterizes the area with the least previous deformation the mainshock rupture terminated where the fold trend was no longer uniform but had competing north and west trending features the upward extent of the mainshock rupture ended at the approximate boundary between franciscan and great valley sequence rocks above that depth the main thrust appears to splay into a steeper segment and a near horizontal segment thus the extent of rupture area is limited by the area of uniform structural orientation and by the variation in the type of material with the three dimensional velocity model each individual hypocenter moved slightly 0 2 km in accord with the details of the surrounding velocity structure so that secondary features in the seismicity pattern are more detailed than with a local one dimensional model and station corrections the overall character of the fault plane solutions was not altered by the three dimensional model but the more accurate ray paths did result in distinct changes in particular the mainshock has a fault plane dipping 30 southwest instead of the 23 obtained with the one dimensional model

distribution and formation of rock varnish in southern tunisia field observations chemical analysis electron and light microscopy and radiolabelling experiments provide information on the characteristics formation and distribution of fe and mn rock varnishes in southern tunisia unstable rocks are not surface coated but when broken they commonly reveal internal microbial solution fronts stable rocks are coated in fe and more rarely mn rock varnishes radiolabelling indicates the presence of many respiring microbes on both fe and mn varnishes microscopy shows fungi and bacteria both on the fe varnishes and within the rock only rare microcolonial fungi occur on mn varnishes suggesting that the microbes are within or beneath the varnish the abundance of microorganisms supports a biogenic origin for rock varnishes different microbial communities appear to be involved in fe and mn varnish formation microbes appear to live beneath the surface of unstable rocks but coat stable rocks which cannot be penetrated apparently to avoid u v light and desiccation this explains the concentration of elements that form u v opaque minerals and clays the origin of these elements is external to the substrate these results suggest that rock varnish dating and paleoenvironmental techniques can be applied in southern tunisia

two dimensional velocity structure along the synclinal axis of the great valley california abstract in 1981 and 1982 the u s geological survey conducted a seismic refraction study of the great valley central california in order to obtian a better understanding of the deep crustal structure and tectonic evolution of the region we present the interpretation of a 147 km long seismic refraction profile recorded along the synclinal axis of the valley the northern 98 km of the profile are reversed whereas the remaining southern portion is unreversed observed travel times and amplitudes of primary and secondary arrivals from three shot points have been modeled using two dimensional ray tracing techniques the resultant p wave velocity cross section consists of a 27 km thick crust whose isovelocity contours generally rise gently to the southeast the crust is described in six layers with average velocities average layer thickness in parentheses of 2 8 km sec 3 km 4 25 km sec 3 km 5 5 km sec 2 km 6 1 km sec 6 km 6 75 km sec 6 km and 7 2 km sec 7 km the second through fifth layers are separated by 1 km thick high gradient transition zone the lower crustal boundary 6 75 7 2 km sec and the moho 7 2 8 1 km sec are modeled with first order discontinuities the first layer cenozoic sedimentary deposits is characterized by a velocity gradient of 0 7 km sec km starting with a velocity of 1 7 km sec at the surface the second layer 4 25 km sec consists of the cretaceous great valley sequence and shows a very modest velocity gradient of 0 2 km sec km the third layer 5 5 km sec basement may consist of metasedimentary rocks of the franciscan assemblage fractured crystalline rocks or possibly serpentinized ophiolite the fourth layer 6 1 km sec apparently has no velocity equivalent 30 km to the west in the diablo range it and the layer beneath it 6 75 km sec may be correlated with the metamorphic and igneous rocks of the eastern foothills of the sierra nevada the basal crustal layer 7 2 km sec is evidently mafic igneous rock an independent interpretation of this profile using the reflectivity method produced a similar model an interpretation of a parallel profile 10 to 15 km east of our line shows a structure similar to our model below 15 km but a thinner sedimentary section and velocities as much as 0 9 km sec faster 6 to 15 km deep the crustal velocity structure of the central great valley is atypical of either continental or oceanic crust but is consistent with the geologic hypotheses that the valley is a thickened marginal basin formed in either a forearc or backarc setting located west of the mesozoic sierra nevadan arc

stress triggering in thrust and subduction earthquakes and stress interaction between the southern san andreas and nearby thrust and strike slip faults 1 we argue that key features of thrust earthquake triggering inhibition and clustering can be explained by coulomb stress changes which we illustrate by a suite of representative models and by detailed examples whereas slip on surface cutting thrust faults drops the stress in most of the adjacent crust slip on blind thrust faults increases the stress on some nearby zones particularly above the source fault blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults we calculate that during a progressive thrust sequence in central california the 1983 mw 6 7 coalinga earthquake brought the subsequent 1983 mw 6 0 nunez and 1985 mw 6 0 kettleman hills ruptures 10 bars and 1 bar closer to coulomb failure the idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events in agreement with findings of prior investigations subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust faulting events on the periphery of the seismic rupture and its downdip extension these features are evident in aftershocks of the 1957 mw 9 1 aleutian and other large subduction earthquakes we further examine stress changes on the rupture surface imparted by the 1960 mw 9 5 and 1995 mw 8 1 chile earthquakes for which detailed slip models are available calculated coulomb stress increases of 2 20 bars correspond closely to sites of aftershocks and postseismic slip whereas aftershocks are absent where the stress drops by more than 10 bars we also argue that slip on major strike slip systems modulates the stress acting on nearby thrust and strike slip faults we calculate that the 1857 mw 7 9 fort tejon earthquake on the san andreas fault and subsequent interseismic slip brought the coalinga fault 1 bar closer to failure but inhibited failure elsewhere on the coast ranges thrust faults the 1857 earthquake also promoted failure on the white wolf reverse fault by 8 bars which ruptured in the 1952 mw 7 3 kern county shock but inhibited slip on the left lateral garlock fault which has not ruptured since 1857 we thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales

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

style and magnitude of tectonic shortening normal to the san andreas fault across pyramid hills and kettleman hills south dome california a blind thrust zone and the overlying fold forming the kettleman hills south dome were produced by late cenozoic shortening normal to the san andreas fault west of kettleman hills toward the san andreas fault the region of pyramid hills experienced structural growth as early as late cretaceous time and growth accelerated in pliocene to holocene time these structures were imaged to a depth of 7 km by reprocessing a seismic reflection record section restored structural cross sections indicate a three stage growth history for kettleman hills south dome 1 late miocene reverse faulting 2 pliocene fault propagation type folding and 3 late pliocene to holocene tectonic wedge emplacement late pliocene to holocene structural growth at pyramid hills probably resulted from thrust emplacement of the franciscan complex a forward model demonstrates that tectonic wedging could account for post 2 5 ma deformation of pyramid hills and kettleman hills south dome horizontal shortening across pyramid hills and kettle man hills south dome normal to the san andreas fault since the cretaceous period is 3 7 km this figure does not include shortening within the basement adding shortening of our tectonic wedge model yields 7 9 km of shortening since cretaceous time post 2 5 ma shortening determined by our tectonic wedge model is 6 3 km the rate of post 2 5 ma shortening 2 5 mm yr when added to that on the west side of the san andreas fault is consistent with the values derived from plate motion models that describe the san andreas discrepancy

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

growth fault bend folding at southeast lost hills san joaquin valley california synsedimentary deformation has created complex structural and stratigraphic relationships along the west side of the san joaquin valley california this case study of lost hills anticline documents how a simple structural process fault bend folding results in complex growth stratigraphy lost hills anticline is a fault bend fold caused by a step in the lost hills thrust the thrust steps up to the northeast at 27 from a detachment at the base of the tertiary to an upper detachment in the reef ridge formation the lower and upper detachments are at 20 000 ft 6 km and 13 000 ft 4 km displacement on the lower detachment is approximately 4 mi 6 5 km folding consumes 1 2 mi 2 km and the remaining 2 8 mi 4 5 km continues northeast beneath the san joaquin va ley forward and back limb dips of 42 and 27 respectively are consistent with fault bend fold theory pliocene pleistocene sediments onlap a time transgressive unconformity on the northeast front limb but have uniform dip and thickness on the southwest back limb this characteristic growth fault bend fold geometry indicates basinal sediments rolled through an axial surface onto the back limb whereas contemporaneous sediments deposited above the front limb are undeformed basinal sediments onlapping the fold crest indicates fault slip is greater than ramp length onlapping growth sediments indicate folding began during deposition of the etchegoin formation 8 ma and continued to the late pleistocene or holocene corresponding to an average displacement rate on the lost hills thrust of 2 700 ft m y 820 m m y continuous burial of the fold during growth caused each material e physical point in the fold to be at its maximum depth of burial contours of equal density and seismic velocity are relatively flat so that the anticline causes little or no gravity anomaly and less than expected pullup in seismic time sections the geometry of growth sediments imaged on seismic lines indicates lost hills and adjacent anticlines grew by uniform displacement to the northeast with no significant rotation oblique convergence between the pacific and north american plates is manifested as synchronous but spatially separate transcurrent and compressive displacements thus lost hills anticline and perhaps other folds in the area are more appropriately described as compressive folds in a transpressive region than as transpressive folds duplicated lower tertiary strata beneath the lost hills kettleman hills trend are potential targets for petroleum exploration

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

recognition of a santonian submarine canyon great valley group sacramento basin california implications for petroleum exploration and sequence stratigraphy of deep marine strata seismic stratigraphic and outcrop interpretation of santonian campanian portions of the great valley group in the north central sacramento basin california reveal the presence of a large north south oriented submarine canyon named after an overlying town williams canyon appears to have been cut during the middle santonian and filled during the late santonian and early campanian approximately 85 80 ma this fossil canyon is more than 100 km in length ranges from 12 to 22 km in width and contains compacted sedimentary fill with a maximum thickness of 1 5 km over its mapped extent williams canyon is comparable in scale to the paleogene gorges of the sacramento basin stratigraphic relations indicate active folding locally during the turonian santonian and probable structural control on the location of the canyon regional correlations showing uplift and regression along the northern basin margin synchronous with transgression of the eastern basin margin during canyon incision suggest that tectonic tilting of the basin initiated cutting of williams canyon whereas the role of eustatic sea level change appears to have been negligible delineation of williams canyon clarifies geometries of outcropping and subsurface strata because some of the canyon fill previously has been correlated to older strata and canyon boundaries locally have been misinterpreted as faults over most of its areal extent the sequence boundary associated with canyon incision is characterized by facies associations that do not conform to widely cited sequence stratigraphic models of deep marine deposits suggesting that such models are oversimplified combination trapping geometries created by williams canyon cut and fill represent an untested gas exploration play

regional extent of great valley basement west of the great valley california implications for extensive tectonic wedging in the california coast ranges interpretation and modeling of the magnetic field of central california indicate that the magnetic basement of the forearc deposits of the great valley sequence extends westward beneath the coeval subduction related rocks of the franciscan complex the basement surface slopes gently to the west reaching midcrustal depths 15 19 km at distances of 50 100 km west of the great valley this magnetic basement is disrupted by the hayward rodgers creek fault system and is cut by the san andreas fault at the south end of the great valley and possibly throughout much of central california the widespread presence of the great valley basement beneath rocks of the franciscan complex implies that the basement is more extensive than proposed in earlier interpretations based on seismic studies near the franciscan complex great valley sequence contact this result forces major modifications to ideas concerning this fossil subduction complex and other subduction zones the eastern boundary fault of the franciscan complex coast range fault is not and never was a subduction zone thrust fault but rather was originally a roof thrust wedge roof fault formed above the eastward wedging mass of franciscan complex intruded along the top of the basement beneath the great valley deposits this tectonic interpretation offers a solution for the question of how high pressure metamorphic rocks of the franciscan complex were juxtaposed at the coast range fault against low pressure metamorphic rocks of the great valley sequence this interpretation also implies an older flat lying thrust fault wedge floor fault that forms the top of magnetic basement between the active san andreas and hayward faults at depths of 15 17 km this older thrust fault may today transfer strain between the two young strike slip faults possibly explaining the apparent coupling of major nineteenth century earthquakes on these two faults the former east dipping subduction zone along which the rocks of the franciscan complex accumulated must lie west of the western limit of the great valley magnetic basement

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

crustal structure beneath the eastern part of the coast ranges diablo range of central california from explosion seismic and near earthquake data abstract seismic refraction and near earthquake data of the u s geological survey for central california have been compiled into record sections along profiles and interpreted in terms of crustal structure the profiles are located northeast of the san andreas fault of central california and run parallel to the general structures for the explosion seismic line through the centre of the diablo range an uppermost layer franciscan formation with p velocities of 3 6 5 0 km s 1 decreases in thickness towards the northwest the lower boundaries of layers with constant velocities of 5 75 and 6 8 km s 1 are found at almost constant depths of 12 and 21 km respectively between 21 and 26 km depth a well defined low velocity zone appears whose velocity is estimated as 5 3 km s 1 with the aid of a hedgehog inversion and the calculation of amplitudes this zone is underlain by a layer 3 5 km thick with a velocity of 7 6 km s 1 the upper mantle velocity beneath the moho at 29 30 km depth is 8 2 km s 1 the near earthquake profiles located 20 km southwest and parallel to the explosion seismic line follow more or less the hayward and calaveras fault systems the velocity depth distribution derived for the earthquake data is very similar to that found beneath the diablo range however the low velocity zone at 21 26 km depth does not seem to exist everywhere along the line the moho is not disturbed beneath the calaveras hayward and silver creek faults it rises slightly from the diablo range towards the southwest

distributed shear rotation and partitioned strain along the san andreas fault central california folds commonly form along transform fault systems in the san andreas strike slip fault two mechanisms of folding have been described that are regarded as incompatible 1 wrench tectonics or distributed shear and 2 strain partitioning shortening being perpendicular to the fault the compatibility and relative importance of these two mechanisms are not well understood in part because folds are the result of lengthy deformation this paper synthesizes paleomagnetic data sedimentary thickness maps fault patterns and seismic reflection profiling data along the san andreas transform fault system that indicate a 20 30 clockwise map view reorientation of pliocene en echelon folds to their present position subparallel to the san andreas fault the folds continue to develop today as a result of shortening perpendicular to the fault folds apparently rotated with continuing strain in the wrench zone and with increasing shortening across the shear zone rotation likely occurred when changing pacific plate motions increased the component of convergence across the fault system this field example documents deformation by these two different mechanisms in the same fold belt which has broad implications for interpreting other fold belts formed in similar settings the geologic record suggests that the opposing theories of wrench deformation and strain partitioned shortening perpendicular to the fault are both important and compatible processes that act at different times and places

passive roof duplexes under the rocky mountain foreland basin alberta seismic reflection data in the central alberta foothills near edson reveal the presence of small passive roof duplexes in upper cretaceous rocks within the otherwise undeformed foreland basin as much as 40 km northeast of the mountain front monocline the tops and bottoms of the duplexes are defined by backthrusts and sole thrusts which follow bedding planes within upper cretaceous strata overlying the structures is an essentially uncontracted 1 8 km thick section of upper cretaceous and tertiary rocks which is passively uplifted over the thickened duplexes the underlying autochthonous sequence of mesozoic and paleozoic rocks exhibits some minor folding but is also uncontracted our interpretation extends both the upper and lower detachments of the widely accepted triangle zone model more than 30 km farther under the foreland basin than has previously been supposed the seismic data illustrate relatively clearly the form of the leading edge of the last phase of rocky mountain thrusting we expect that similar features will be observed elsewhere in the rocky mountain foothills and probably at other mountain fronts worldwide

a transect across the mesozoic accretionary margin of central california subductive accretion formed along the pacific margin of north america in the mesozoic we are exploring deep structure across this accreted margin using coordinated reflection and detailed refraction profiling gravity and magnetic interpretation together with surface and well geology although none of these techniques alone is sufficient to define the critical elements of structure in complement they place important constraints on geologic interpretation for both the foothill rocks and particularly the franciscan assemblage we find evidence of obductive emplacement overthrusting from the west in contrast to the eastward subduction of most previous views these surprises emphasize the importance of such geophysical studies to geologic analysis