detrital zircon evidence for progressive underthrusting in franciscan metagraywackes west central california we present new u pb ages for detrital zircons separated from six quartzose metagraywackes collected from different franciscan complex imbricate nappes around san francisco bay all six rocks contain a broad spread of late jurassic cretaceous grains originating from the klamath sierra nevada volcanic plutonic arc units young structurally downward consistent with models of progressive underplating and offscraping within a subduction complex the youngest specimen is from the structurally lowest san bruno mountain sheet at 52 ma it evidently was deposited during the eocene none of the other metagraywackes yielded zircon ages younger than 83 ma zircons from both el cerrito units are dominated by ca 100 160 ma grains the upper el cerrito also contains several grains in the 1200 1800 ma interval these samples are nearly identical to 97 ma metasedimentary rock from the hunters point shear zone zircon ages from this melange block exhibit a broad distribution ranging from 97 to 200 ma with only a single pre mesozoic age the albany hill specimen has a distribution of pre mesozoic grains from 1300 to 1800 ma generally similar to that of the upper el cerrito sheet however it contains zircons as young as 83 ma suggesting that it is significantly younger than the upper el cerrito unit the skaggs spring schist is the oldest studied unit its youngest analyzed grains were ca 144 ma and it is the only investigated specimen to display a significant paleozoic detrital component sedimentation and subduction accretion of this tract of the trench complex took place along the continental margin during early to early late cretaceous time and perhaps into eocene time franciscan and great valley deposition attests to erosion of an andean arc that was active over the entire span from ca 145 to 80 ma with an associated accretionary prism built by progressive underthrusting we use these new data to demonstrate that the eastern franciscan complex in the northern and central coast ranges is a classic accretionary prism where younger structurally lower allochthons are exposed on the west and older structurally higher allochthons occur to the east in the heavily studied san francisco bay area

u pb ages of detrital zircons in pacheco pass metagraywackes sierran klamath source of mid cretaceous and late cretaceous franciscan deposition and underplating 1 using laser ablation icp ms techniques u pb ages are reported for 338 detrital zircons separated from four franciscan metagraywacke samples from imbricate thrust sheets cropping out in the pacheco pass area east central california coast ranges structurally higher slabs are clastic quartz albite bearing rocks typified by neoblastic lawsonite jadeitic pyroxene the lowest exposed slab lacks jadeitic pyroxene but contains traces of newly grown pumpellyite and lawsonite in addition to abundant quartz albite studied specimens contain moderate amounts of phengite titanite chlorite stilpnomelane carbonate iron oxides rock fragments and carbonaceous matter these diablo range clastic sediments were deposited in mid cretaceous and late cretaceous time the highest allochthon unit i was deposited after 102 ma unit iv was deposited after 90 102 ma and unit v was deposited after 86 ma metagraywacke depositional accretionary ages within slab iv young upward but ages of the slabs decrease progressively downward in the four sample aggregate most zircons have igneous ages falling in the 135 170 ma range with a smaller population at 85 120 ma the rocks contain rare zircons of middle proterozoic late archean ages a minor source in northern california and nw nevada seems likely for the pacheco pass metagraywackes but the sierran klamath calcalkaline arc provided most of the clastic debris judging by the volcanic nature of lithic clasts in these eastern belt metagraywackes and because massive sierran plutonism occurred at 85 120 ma the abundant 135 170 ma zircons probably were derived chiefly from the eroding comagmatic volcanic arc rather than from the less voluminous jurassic arc plutons

full frequency range transient solution for compressional waves in a fluid saturated viscoacoustic porous medium1 an analytical transient solution is obtained for propagation of compressional waves in a homogeneous porous dissipative medium the solution based on a generalization of biot s poroelastic equations holds for the low and high frequency ranges and includes viscoelastic phenomena of a very general nature besides the biot relaxation mechanism the viscodynamic operator is used to model the dynamic behaviour associated with the relative motion of the fluid in the pores at all frequency ranges viscoelasticity is introduced through the standard linear solid which allows the modelling of a general relaxation spectrum the solution is used to study the influence of the material properties such as bulk moduli porosity viscosity permeability and intrinsic attenuation on the kinematic and dynamic characteristics of the two compressional waves supported by the medium we also obtain snapshots of the static mode arising from the diffusive behaviour of the slow wave at low frequencies lntroduction the acoustics of porous media within the framework of petroleum geophysics have been receiving more attention in recent years regional exploration seismology aimed at the discovery of hydrocarbon reservoirs is based on simplified rheological models and ray representations of the wavefield in the past these approximations were sufficient since the degree of resolution required was not high in the present time when most reservoirs are delimited the need to increase the resolution and use not only the traveltimes but also the information contained in the amplitude and phase of the seismic wavefield has gradually emerged the new exploration scenario is confined to reservoir areas and involves the presence of oil wells and data from seismic logs and well seismics which have enough resolution to see the effects of bulk properties porosity permeability fluid content and fluid solid interaction on the seismic pulse in this case the correct

a crustal scale cross section of the south western alps combining geophysical and geological imagery a geotransect across the south western alps from the pelvoux massif external french alps to the dora maira massif internal italian alps through the monviso ophiolitic complex was investigated in the framework of the geo france 3d alpes programme a new interpretative crustal scale section across the south western alps is proposed combining geological and geophysical 2d 3d data the apulian mantle i e the ivrea body might be divided into two rigid pieces separated by the downward prolongation of the penninic frontal thrust these mantle indenters drive the decoupling of the european crust beneath the high to ultra high pressure metamorphic nappes the deep structure results from the stacking of crustal slices extracted from the european lithosphere the proposed struc tural model provides a basis for discussing the evidence of the crustal scale partitioning of the current strain pattern as well as the location of the seismicity

discussion on structural evolution of the central part of the krusne hory erzgebirge mountains in the czech republic evidence for changing stress regime during the variscan by j konopasek k schulmann and o lexa

unsaturated flow through a fracture matrix network dynamic preferential pathways in mesoscale laboratory experiments 1 we conducted two laboratory experiments at the meter scale in which water was applied to the top of an initially dry uncemented wall composed of porous bricks one experiment experiment 1 encouraged evaporation and resulting mineral precipitation while the other experiment 2 was designed to minimize these processes in both cases processes acting within the fracture network controlled early time behavior forming discrete pathways and demonstrating fractures to act as both flow conductors and capillary barriers at a later time evaporation mineral precipitation in experiment 1 constrained flow strengthening some pathways and starving others in experiment 2 the wetted structure took on the appearance of a diffuse plume however individual pathways persisted within the wetted structure and interacted displaying erratic outflow over a wide range of timescales including switching between pathways thus under conditions of constant supply and both with and without evaporation mineral precipitation unsaturated flow through fractured rock can create dynamic preferential pathways

deep subduction and rapid exhumation role of crustal strength and strain weakening in continental subduction and ultrahigh pressure rock exhumation 1 the exhumation of crustal ultra high pressure uhp material depends on temporal and spatial variations in its detachment within the subduction channel this dependence is investigated using numerical models with variable initial crustal strengths representing a range of initial crustal compositions and parameterized strain weakening representing a range of processes that reduce effective crustal viscosity during deformation competition between down channel shear traction favoring subduction and up channel buoyancy favoring exhumation is expressed as the exhumation number e which can vary with time and position along the channel exhumed lower strength crust which resists subduction owing to weak down channel traction records peak conditions 38 kbar given sufficient strain weakening exhumation proceeds at 60 km ma 1 indicating that buoyancy e 1 drives exhumation in these models in all models exhuming uhp material forms a deforming ductile plume with a range of possible structural relationships predicted between exhumed uhp and hp materials

self organized spatial temporal structure within the fractured vadose zone influence of fracture intersections under conditions of unsaturated flow others have shown experimentally that fracture intersections can direct flow to a single exiting fracture in addition they have been found to gather water from above to release as a pulse below we formulate a simple model where these two behaviors are embedded within a network with slow steady inflow distributed randomly along the top of the network the system self organizes to form avalanches of water that can penetrate to great depths when all intersections split their outflow flow diverges with depth and develops into a self organized dynamical state where the distribution of avalanche sizes follows a power law over many decades as the fraction of intersections that direct outflow singly is increased spatial structure passes from divergent through braided to a fully convergent hierarchical flow regime where avalanche size is minimized along one dimensional slender pathways

the p t t history of blocks in serpentinite matrix melange west central baja california thermochronologic petrologic and geochemical analyses of epidote amphibolite amphilbolite eclogite and blueschist facies blocks in serpentinite matrix melanges from east san benito island and cedros island west central baja california provide constraints on the p t t history of this disrupted terrane results of sup 40 ar sup 39 ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different p t t histories mid jurassic 160 170 ma epidote amphibolite and amphibolite facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction coarse grained blueschist blocks were likely metamorphosed during continued subduction in late early cretaceous time that is 115 100 ma some epidote amphibolite blocks are partially overprinted by blueschist facies mineral assemblages and apparently record both metamorphic events fission track analyses of apatites indicate that the blocks underwent significantly different post metamorphic cooling histories epidote amphilbolite and amphibolite blocks cooled below approximately 100 degrees c form mid jurassic to paleocene time blueschist blocks cooled below approximately 100 degrees c in oligocene miocene time in general mafic blocks have trace element concentrations and ree patterns characteristic of ocean flood basalts this study demonstrates that samples with very different p t t histories can evidently occur over relatively small length scales 1 km in serpentinite matrix melanges 67 refs 16 figs 4 tabs

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

unsaturated flow through fracture networks evolution of liquid phase structure dynamics and the critical importance of fracture intersections 1 experiments and analyses are presented to elucidate the critical control of fracture intersections on the evolution and dynamics of the liquid phase structure within unsaturated fracture networks in impermeable media phase structure was visualized within a thick vertical sheet of broken glass where the breaks constituted the fracture network the critical system parameters applied flow rate viscous forces and initial condition were varied in a series of experiments when initially dry individual fracture intersections acted as capillary barriers and created a repeated dynamic from which a network scale slender ladder phase structure emerges that is composed of pools above each intersection joined by fingers or tendrils below at low flow rates pulsation is found at intersections within fingers and along horizontal fractures in some cases pulsation extends to larger volume cascade events where several intersections act in concert at higher flow rates viscous forces remove pulsation reinvasion upon drainage demonstrates that when initially wet the capillary barrier behavior of the individual fracture intersections vanishes and intersections are rapidly spanned this marked hysteretic response tends to guide flow and cause pathway persistence from one event to the next

convergent flow observed in a laboratory scale unsaturated fracture system 1 an experiment was designed to observe water flow in a simple analogue fractured rock network the fracture network was modeled by the gaps between an array of limestone blocks whose permutations generated different realizations of the network three out of five of these realizations displayed flow convergence in contrast to that predicted by porous continuum models such convergence occurs because fracture intersections act as capillary barriers that integrate or focus flow within the network

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

mineral parageneses regional architecture and tectonic evolution of franciscan metagraywackes cape mendocino garberville covelo 30 60 quadrangles northwest california 1 the franciscan complex is a classic subduction zone assemblage in northwest california it comprises a stack of west vergent thrust sheets westernmost eastern belt outliers central belt melange coastal belt yager terrane coastal belt coastal terrane coastal belt king range false cape terranes we collected samples and determined p t conditions of recrystallization for 88 medium fine grained metasandstones to assess their subduction exhumation histories and assembly of the host allochthons feebly recrystallized yager coastal and king range strata retain clear detrital features scattered neoblastic prehnite occurs in several coastal terrane metasandstones traces of possible pumpellyite are present in three yager metaclastic rocks pumpellyite lawsonite aragonite bearing central belt metasandstones are moderately deformed and reconstituted intensely contorted thoroughly recrystallized eastern belt affinity quartzose metagraywackes contain lawsonite jadeitic pyroxene aragonite glaucophane we microprobed neoblastic phases in 23 rocks documenting mineral parageneses that constrain the tectonic accretion and metamorphic p t evolution of these sheets quasi stable mineral assemblages typify eastern belt metasandstones but mm sized domains in the central and coastal belt rocks failed to achieve chemical equilibrium eastern belt slabs rose from subduction depths approaching 25 30 km whereas structurally lower central belt melanges returned from 15 18 km coastal belt assemblages suggest burial depths less than 5 8 km eastern and central belt allochthons sequentially decoupled from the downgoing oceanic lithosphere and ascended into the accretionary margin k feldspar rich coastal belt rocks were stranded along the continental edge without undergoing appreciable subduction probably during paleogene unroofing of the older deeply subducted units of the franciscan complex in east vergent crustal wedges

influence of fracture intersections under unsaturated low flow conditions recent experimental evidence suggests that the capillary heterogeneity associated with fracture intersections can act to impose temporal and spatial structure on network scale flows a simple intersection between orthogonal fractures one horizontal and the other vertical has been shown to integrate unsaturated flows at low flows the intersection forms a capillary barrier that accumulates water in a growing pool eventually the retaining meniscus snaps discharging a pulse of water here we develop a mechanistic explanation for this observed behavior and experimentally consider three perturbations to the geometry of the simple orthogonal intersection two of the perturbations also act as capillary barriers while the third formed a capillary bridge across the intersection at low flow all of our experimental intersections imposed a temporal signal with the nature of that signal dependent on intersection geometry and participation by the horizontal fractures in dynamic storage at high flow a continuous fluid tendril spanned the system from inlet to outlet with water pooled above the intersection caused by a narrow fluid connection that restricted flow across the intersection results from all experiments suggest that pulsation is critically sensitive to small variations in the geometry of fracture intersections and storage in the horizontal fractures when combinedmore with dependency on supply rate this sensitivity can generate pulsation of flow across a wide range of time periods and discharge volumes less

continental rifting and metamorphic core complex formation ahead of the woodlark spreading ridge d entrecasteaux islands papua new guinea 1 we evaluate the role of a metamorphic core complex mcc on normanby island in the woodlark rift located 1 km thickness of blueschist derived mylonites formed in a midcrustal shear zone during the pliocene at 400 500 c this top to the north zone appears to have reactivated the gently dipping base of the papuan ophiolite papuan ultramafic body pub and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the mcc and rapid subsidence of the woodlark rise mylonites in the mcc s lower plate have been exhumed along a detachment as a result of 50 km of slip at rates of 12 mm yr the inactive back tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the plio pleistocene plate motion a second se vergent detachment has been established on the opposite flank of this rolling hinge style mcc probably since 0 8 at depth and provide a sufficient mechanism for activating low angle normal faults in the rift mcc inception was not localized to the tip of the woodlark mor instead extreme crustal thinning near the mcc preconditioned later continental breakup the lower crust appears to be weak thickening beneath unloaded footwalls to uplift mccs above sea level and flowing laterally to even out regional crustal thickness contrasts on a 1 6 m y timescale deep seated transforms separate rheologically distinct domains in which extension has been localized along the weak pub to cause mcc formation vs those in which slip is distributed across an imbricate zone of more uniform strength normal faults the trobriand fault connects in the eastern woodlark rift to the owen stanley fault in the papuan ranges which is probably moving at nearly the full plate velocity

fracture intersections as integrators for unsaturated flow 1 a simple experiment finds that fracture intersections can act to integrate unsaturated flows such that regular low flows entering the intersection from above are transformed into large less frequent pulses below at low flows our simple intersection forms two capillary barriers water from above pools at the intersection until sufficient pressure builds to breach the barriers and discharge stored fluid the barriers then reform and the process is repeated at low flows the volume discharged from the intersection remains relatively uniform across a range of flow rates at higher flows discharge volume is highly variable and a viscous stabilized non pulsating regime occurs at the highest flow rate that we considered

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

self organized spatio temporal structure within the fractured vadose zone the influence of dynamic overloading at fracture intersections under low flow conditions where gravity and capillary forces dominate within an unsaturated fracture network fracture intersections act as capillary barriers to integrate flow from above and then release it as a pulse below water exiting a fracture intersection is often thought to enter the single connected fracture with the lowest invasion pressure when the accumulated volume varies between intersections the smaller volume intersections can be overloaded to cause all of the available fractures exiting an intersection to flow we included the dynamic overloading process at fracture intersections within our previously discussed model where intersections were modeled as tipping buckets connected within a two dimensional diamond lattice with dynamic overloading the flow behavior transitioned smoothly from diverging to converging flow with increasing overload parameter as a consequence of a heterogeneous field and they impose a dynamic structure where additional pathways activate or deactivate in time

pre collisional high pressure metamorphism and nappe tectonics at active continental margins a numerical simulation the evolution of a subduction channel and orogenic wedge is simulated in 2d for an active continental margin with p t paths being displayed for selected markers in our simulation subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years with four zones from the trench into the forearc i an accretionary complex of low grade metamorphic sedimentary material ii a wedge of nappes with alternating upper and lower crustal provenance and minor interleaving of oceanic or hydrated mantle material iii a megascale melange composed of high pressure hp and ultra high pressure uhp metamorphic rocks extruded from the subduction channel and iv the upward tilted frontal part of the remaining lid the p t paths and time scales correspond to those typically recorded in orogenic belts the simulation shows that hp uhp metamorphism of continental crust does not necessarily indicate collision but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction

early cretaceous transition from nonaccretionary behavior to strongly accretionary behavior within the franciscan subduction complex 1 during subduction at the franciscan trench beginning at 170 160 ma and continuing to the present marine sedimentary and lesser volcanic rocks have been underthrust accreted and metamorphosed to form the franciscan accretionary wedge the south fork mountain schist sfms forms the eastern margin and structural top of the wedge and so was apparently the first unit of substantial size to accrete into the franciscan new u pb ages of detrital zircons and 40ar 39ar ages of metamorphic white micas indicate that the sfms accreted surprisingly late in the time span of franciscan subduction probably at ca 123 ma at least parts of the valentine spring formation and yolla bolly terrane accreted soon thereafter therefore accretion of the sfms apparently marks a fundamental transition from virtually nonaccretionary behavior to strongly accretionary behavior in the franciscan forearc about 75 of modern forearcs are nonaccretionary so recognition of an early nonaccretionary period is consistent with modern examples the franciscan mafic high grade exotic blocks and related very small slabs which yield ca 132 to 169 ma metamorphic ages are the only well documented rock record of the early 40 m y long nonaccretionary period and represent very sparse accretion during that interval the transition into an accretionary regime at ca 123 ma was essentially synchronous with the end of the late jurassic early cretaceous magmatic lull in the sierra nevada magmatic arc with the waning of early cretaceous faulting and warping within the great valley forearc basin and possibly with changes in motion vectors of the pacific and farallon plates

development of slender transport pathways in unsaturated fractured rock simulation with modified invasion percolation 1 slender transport pathways have been found in laboratory and field experiments within unsaturated fractured rock here we consider the simulation of such structures with a modified form of invasion percolation mip results show that slender pathways form in fracture networks for a wide range of expected conditions can be maintained when subsequent matrix imbibition is imposed and may arise even in the context of primarily matrix flow due to the action of fractures as barriers to inter matrix block transport