early miocene change in direction of least principal stress southwestern united states conflicting inferences from dikes and metamorphic core detachment fault terranes previous investigations have suggested that the direction of least principal stress in the western united states has shifted from northerly to easterly between 50 to 30 ma an effort is made to more tightly constrain the time of this shift using igneous dike host rock systems in which the isotopic age of both host and dike is known and differs by less than 10 ma in such systems the possibility that magma intruded along old fractures unrelated to the state of stress at the time of intrusion is minimized and the axis of least principal stress is considered to be normal to the strike of the subvertical dike during the time interval between the ages of the dike and host rock these systems indicate that a northerly least principal stress orientation persisted more recently than previously believed until probably after 30 ma possibly about 26 ma and conceivably as recently as 18 ma in some areas the data are insufficient and too loosely bracketed in time to decide how fast the change in state of stress occurred in any one place and if by systematic rotation or abrupt shift however the change could be diachronous earlier in the southeast and later in the northwest major easterly trending magmatic and tectonic elements of tertiary age in the great basin of utah and nevada are compatible with the dike data however numerous metamorphic core complexes and detachment fault terranes seem to indicate more or less easterly extension throughout the time period represented by the dikes this inconsistency is not clearly resolvable but may reflect differences between the regional plate stress field and gravitational stresses in a crust overthickened by mesozoic compression

dyke intrusion into a pre existing joint network the aiguablava lamprophyre dyke swarm catalan coastal ranges abstract a structural analysis has been performed in the upper permian lamprophyric dyke swarm of aiguablava costa brava ne iberia emplacement of sub vertical dykes is related to the presence of a widespread joint network mostly developed during the cooling and decompression of the late variscan leucogranitic host rocks the joint pattern consists on multiple sub vertical joint sets among which two orthogonal sets at n23 and n113 are predominant a sub horizontal set is also prominent the sub vertical dykes have a mean n100 n125 trend which corresponds to the trend of one of the main joint sets however dyke segmentation is noticeable at the dm to cm scale and this is inferred to be related to dyke propagation and emplacement along the variably oriented pre existing joints a mean robust sub horizontal nne ssw net dilation direction was measured from matching dyke jogs markers in the host rock and analysis of maximum dyke thicknesses and this is in line with the minimum principal stress axis 3 derived from a three dimensional paleostress analysis from dyke orientations the inferred maximum principal stress axis 1 is sub vertical indicating that dykes intruded under conditions of tectonic extension furthermore a mohr construction allowed calculation of the stress ratio 0 29 close to a prolate stress ellipsoid and a driving pressure ratio r 0 27 which corresponds to a magmatic pressure almost equal to the intermediate principal stress axis 2 it is inferred that many of the pre existing joint sets were exploited by the magma the wnw ese joint set normal to 3 being the most favorable for dyke emplacement the present study highlights the structural control of a pre existing fracture network on emplacement of the aiguablava lamprophyres in the upper crust during late permian nne ssw brittle extension

the determination of true shear senses from the deflection of passive markers in shear zones the sense of movement in shear zones can often be determined from the reorientation of pre existing planar or linear features within the zone this cannot always be done simply by examining the change in strike or trend of these features sometimes these will show the opposite sense of rotation to the true sense of shear in the zone this is illustrated for a generalized oblique slip zone using the deflection method9 described here the true movement sense can be deduced from the apparent rotation the emphasis being on a three dimensional approach consideration of the movement on a large scale shear zone the laxford front underlines the importance of being aware of this aspect of shear zone geometry the method may be applied to brittle fault offsets as well as to deflections in ductile shear zones

strain and strain rate in a synkinematic trondhjemitic dike evidence for melt induced strain softening during shearing bohemian massif czech republic abstract emplacement related deformation of a cambrian trondhjemitic dike located at the western border of the tepla barrandian unit central european variscides is studied in detail the trondhjemitic melt was emplaced at t 750 c into an active low temperature ca 350 c east northeast trending dextral transcurrent shear zone thermal modelling indicates that the rheologically critical melt fraction and the solidus of the dike were achieved after 1 and 3 days of cooling respectively the major part of the shearing occurred within 8 days after the melt emplaced the zonation of the dike in strain magnitude mineralogy and geochemistry can be explained by the laterally varying rheology from margin to centre during cooling and shearing the deformation was volume constant and the strain data plot in the apparent constrictional field the unusually high value of the calculated longitudinal strain rate 2 8 10 6 s 1 equivalent to a displacement rate of 2 cm h 1 is probably related to the intruding melt that lubricated the shear zone and thus enhanced its displacement velocity consequently melts that intrude as dikes into active transcurrent shear zones may significantly weaken the strength of the middle and upper continental crust

roof and walls of the red mountain creek pluton eastern sierra nevada california usa implications for process zones during pluton emplacement abstract the red mountain creek pluton has a sub circular outline in map section generally flat roof and steep sides the pluton roof sharply truncates host rock markers has a highly irregular geometry with rectangular steps and passes continuously into a steep wall the roof wall transition is not offset by faults and shows no evidence of extensive diking or synemplacement ductile strain which requires that the pluton roof and its wall remained attached to each other and were not faulted or ductilely deformed during emplacement of the pluton we propose that the exposed section through the red mountain creek pluton may represent the crestal portion of a vertically extensive piston shaped plutonic system the upper part of which was largely emplaced by magmatic stoping although other material transfer processes may have previously operated during emplacement characteristics of the pluton roof presented in this paper as well as in other papers on pluton roofs directly rule out some emplacement models commonly applied to plutons in the sierra nevada place limitations on other models and fit well the expected characteristics of visco elastic diapirs around which variable material transfer processes largely displace host rock downwards in the upper crustal plutons the most widespread preserved process of downward transport of host rock was magmatic stoping

the link between volcanism and tectonics in the southern volcanic zone of the chilean andes a review abstract the nature of the interplay between tectonics and volcanism is a major question in continental margin tectonics the southern andes volcanic zone svz located at the obliquely convergent nazca south america plate margin between 33 s and 46 s offers a unique opportunity to address this question because of along strike changes in crustal thickness tectonic style and well constrained long term and short term kinematic history the complex interaction between tectonic and magmatic processes is evidenced by both the architecture and geochemical signature of volcanic systems main first order factors accounting for the along strike variations in the nature and composition of volcanism are crustal thickness and the existence of a major intra arc fault system the liquine ofqui fault zone lofz second order factors include the local nature of the volcanic arc basement two main categories of volcano tectonic associations have been identified according to the spatial distribution and internal organization of individual volcanoes and clusters of volcanoes with respect to both the overall strike of the volcanic arc and the first and second order active inactive basement faults kinematically coupled associations those that are directly related to the current dextral transpressional tectonic regime include ne trending volcanic alignments of stratovolcanoes and or monogenetic cones kinematically uncoupled associations in which coupling with the current regime is not required include stratovolcanoes built on top of ancient reverse and strike slip faults of the volcanic arc basement and monogenetic cones lying along master faults of the lofz whereas mostly primitive magmas are found in the first category more evolved terms can occur genetically associated with the second category we propose a simplified three dimensional model for the magma plumbing system along and across the svz that integrates kinematically coupled and kinematically uncoupled volcano tectonic associations within the framework of first and second order geological and structural constraints

water flow across the interface of contrasting materials pressure discontinuity and its implications water flow along or across the interfaces of contrasting materials is ubiquitous in hydrology and how to solve them in macroscopic models derived from volumetric average of the pore scale processes remains elusive while the change in the average velocity and pressure at water sediment interface has been well established for channel flow over porous beds whether a volumetric average alerts the pressure continuity when water flows across the interface of two porous materials is poorly understood despite its imperative implications in hydrological modelling the primary purpose of this paper is to provide evidences via pore scale simulations that volumetrically averaging the pore scale processes indeed yields a discontinuous pressure when water flows across a material interface we simulated two columns numerically reconstructed by filling them with stratified media one is an idealised two layer system and the other one is a 3d column filled by fine glass beads over coarse glass beads with their pore geometry acquired using x ray computed tomography the pore scale simulation is to mimic the column experiment by driving fluid to flow through the void space under an externally imposed pressure gradient once fluid flow reaches steady state its velocity and pressure in all voxels are sampled and they are then spatially averaged over each section perpendicular to the average flow direction the results show that the average pressure drops abruptly at the material interface no matter which direction the fluid flows compared with the effective permeability estimated from the homogenization methods well established in the literature the emerged discontinuous pressure at the interface reduces the combined ability of the two strata to conduct water it is also found that under certain circumstances fluid flow is direction dependant moving faster when flowing in the fine coarse direction than in the coarse to fine direction under the same pressure gradient although significant efforts are needed to incorporate these findings into practical models we do elicit the emergence of discontinuous pressure at material interface due to volumetric average as well as its consequent implications in modelling of flow in heterogeneous and stratified media

a formulation for vertically integrated groundwater flow in a stratified coastal aquifer we present the comprehensive discharge potential for steady three dimensional flow in horizontally stratified coastal aquifers with a horizontal base and a vertical coastline the gradient of this comprehensive potential gives the vertically integrated discharge throughout the aquifer i e the specific discharge vector as a function of three dimensional space integrated over the saturated portion of the aquifer the boundary values of the comprehensive potential along the coast can be computed precisely given the geometry of the aquifer the hydraulic conductivities of the strata the elevations of the horizontal planes that separate the strata and the elevation of the impermeable base of the aquifer relative to sea level boundary conditions of the comprehensive potential may either be given in terms of its gradient or computed from given heads along the boundaries the governing equation of the comprehensive potential is the poisson equation in areas of infiltration and the laplace equation elsewhere the computation of interface elevations piezometric heads and the vertical distribution of flow requires that an assumption be made regarding the relation between the comprehensive potential and piezometric heads we adopt the dupuit forchheimer approximation for this purpose and make use of the ghyben herzberg equation we present several applications of the approach and find that the stratification may have a significant effect on the boundary value of the comprehensive potential and thus on the flow rates in the aquifer this article is protected by copyright all rights reserved

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

a plate tectonic model for late jurassic ophiolite genesis nevadan orogeny and forearc initiation northern california recently published age and structural data allow the reconciliation of previously conflicting models for late jurassic genesis of the josephine smartville and coast range ophiolites and the nevadan orogeny in the klamath mountains and sierra nevada the resulting model is consistent with the mode of initiation location and geometry of the great valley forearc basin and with the lack of a significant forearc basin west of the klamath mountains the coast range ophiolite formed by backarc spreading west of an east facing intraoceanic arc soon thereafter a remnant arc was calved off the west side of this arc and the smartville ophiolite formed by backarc interarc spreading during this time the sierran phase of the nevadan orogeny began as the intraoceanic arc encountered the west facing continental margin arc of north america an east west trending calcalkaline dike swarm in the sierra nevada foothills may mark the trajectory of the colliding arcs at the initiation of the collision simultaneously a new subduction zone was initiated west of the collision suture zone and this new trench propagated southward thus trapping the coast range ophiolite in the new forearc area south of the klamath area intense deformation in the sierran region resulted from this collision and both magmatic arcs became inactive as the last remnant of intervening oceanic crust was subducted continued westward relative movement of the north american arc was permitted north of the sierra nevada owing to the lack of a colliding intraoceanic arc the result was the westward rifting of the continental margin arc by intraarc spreading which formed the josephine ophiolite in the klamath area the klamath phase of the nevadan orogeny resulted from contraction of the west facing intraoceanic arc and josephine backarc basin beneath the continental margin basal sediments of the great valley forearc basin were derived primarily from the sutured arc ophiolite terranes and were deposited on top of the coast range ophiolite the southern edge of the klamaths and the western side of the sierra nevada a new late mesozoic magmatic arc was superposed across the previously accreted terranes and formed the primary sediment source for the cretaceous forearc basin

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

cenozoic noncoaxial transtension along the western shoulder of the ross sea antarctica and the emplacement of mcmurdo dyke arrays field data on cenozoic faults and the mcmurdo dyke arrays in the reeves glacier mawson glacier area victoria land antarctica allow us to support noncoaxial transtensional tectonics along the n s trending western shoulder of the ross sea dyke injection within a crustal scale right lateral strike slip shear zone is testified by magma filled tension gash like arrangements within some master fault zones and by the left stepping arrangements of dykes in the intrafault zones the noncoaxiality of deformation is shown by the re activation of many dyke walls as right lateral strike slip faults this suggests an increase in the strike slip component over time along the western shoulder of the ross sea our data support the relevance of transtensional to strike slip tectonics for triggering melting and controlling the geometry and modes of magma emplacement

magnitude and significance of miocene crustal extension in the central mojave desert california the newly recognized waterman hills detachment fault whdf of the central mojave desert california is significant because it provides the first unambiguous evidence for large scale core complex style crustal extension in the central mojave desert and because it has significantly rearranged the pre miocene paleogeography of the mojave desert the whdf places steeply dipping to overturned miocene volcanic and sedimentary rocks upon mylonitic pre tertiary basement the mylonites which apparently formed during extension are predominantly l tectonites which manifest top to northeast shear the whdf dips to the northeast beneath dominofaulted ranges of the central mojave desert and detachment faults of the colorado river trough forming an imbricated early miocene system of detachment faults extension continued in the colorado river trough after extension had ceased in the central mojave desert tentative correlations of mesozoic intrusions suggest about 40 km of slip across the whdf which carries eugeoclinal paleozoic rocks in its hanging wall and cratonal miogeoclinal paleozoic rocks in its footwall restoration of 40 km of slip 1 removes a prominent kink in the boundary between eugeoclinal and cratonal miogeoclinal facies 2 aligns cratonal miogeoclinal strata near victorville more closely with the late paleozoic continental margin farther north 3 places cratonal miogeoclinal rocks structurally beneath eugeoclinal rocks implying that the facies were stacked by thrusting and 4 straightens the western margin of the late jurassic independence dike swarm

plutonism oblique subduction and continental growth an example from the mesozoic of california major episodes of mesozoic plutonism in california correlate with periods of oblique subduction and trench parallel transport of western california along intrabatholithic faults major episodes of plutonism occurred in the late jurassic during left oblique convergence and in the mid cretaceous during right oblique convergence in contrast a conspicuous lull in plutonism but continuation of volcanism in the earliest cretaceous coincides with a time when the north america farallon convergence vector although large in magnitude was oriented perpendicular to the trench this correlation suggests that plutonism is facilitated by strike slip faulting within the batholithic belt one explanation which helps to solve the plutonic room problem is that plutons are passively emplaced at releasing bends in the strike slip faults and volume is conserved by thrusting at the trench if this correlation is generally applicable then it implies that mid crustal plutonism is limited beneath areas in which the convergence vector is subperpendicular to the trench continental growth in such areas may occur dominantly by volcanism

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

southern extension of the independence dike swarm of eastern california new geochronologic data show that jurassic dikes in the western mojave desert and eastern transverse ranges are part of the independence dike swarm of eastern california these occurrences extend the length of the dike swarm to more than 500 km u pb zircon concordia intercept ages for dikes near stoddard well in the western mojave desert and big wash in the eagle mountains are identical within analytical uncertainties to 148 ma zircon ages for independence dikes north of the garlock fault recognition of this widespread magmatic and tectonic event with a restricted age has important ramifications 1 crosscutting relations between the dikes and pre upper jurassic and cretaceous rocks help establish relative ages 2 persistence of dikes across several inferred terrane boundaries constrains terrane accretion ages 3 the dike rocks represent magmatic processes in a wide range of wall rock settings 4 the dikes are structural markers that record translations and rotationsof crustal flakes 5 regional persistence and dilational emplacement of the independence dike swarm suggest that it is the last extension in a series of alternating extensional and compressional events during the jurassic the tectonic regime responsible for the independence dikes may be related to arc normal extension changes in plate motions or oblique subduction with left lateral shear

propagation effects of an underground excavation abstract induced seismicity associated with the horizontal excavation of a simple cylindrical tunnel within a homogeneous unfractured rock mass has been used to investigate the propagation effects associated with an underground excavation the tunnel was found to significantly delay p wave travel times due to diffraction effects which were accounted for using a finite difference transmission technique underground excavations will also result in amplitude changes associated with reflection mode conversion and diffraction effects which were qualitatively illustrated using an example of a finite difference solution of the wave equation velocity inversion of the induced seismicity data indicated significant velocity changes in various regions around the tunnel which could be validated with the known rock mass deformation in the roof of the tunnel where notches formed analogous to borehole breakouts inversion results were able to resolve relatively high velocities in the highly stressed zone and decreased velocities delineating the excavation induced fractured zone in the region at the end of the tunnel decreased velocities were found corresponding to where 3d numerical stress models predicted a tensile regime this controlled case study illustrates how an underground excavation can significantly affect seismic propagation and how images of the p wave propagation velocity can be used to delineate induced changes in the rock mass properties

vertical axis rotations in the mojave evidence from the independence dike swarm the 500 km long late jurassic independence dike swarm in california extends from the central sierra nevada through the mojave desert to the eastern transverse range the dikes are mostly vertical and their azimuth in the sierra nevada is 315 however in the mojave region their azimuth varies significantly 300 320 in the argus range the spangler hills and the stoddard well and soda mountain areas and 340 350 in the granite mountain alvord mountain and eagle mountain areas apparently the dikes in the granite alvord and eagle mountains areas have rotated 50 clockwise about vertical axis relative to the sierra nevada and the stoddard well area these rotations are remarkably consistent with known post early miocene paleomagnetically derived rotations the clockwise rotations are associated with the east west trending left lateral strike slip faults in the northeastern mojave and the eastern transverse range regions in contrast the data show that little or no rotation occurred in the central mojave domain which is characterized by northwest trending faults

oblique opening and noncoaxial emplacement of the jurassic independence dike swarm california abstract several features of the independence dike swarm ids indicate that dikes opened obliquely to their walls in a sinistral sense the ids is largely latest jurassic in age 148 ma and has been traced for more than 600 km along its nw se strike from east central to southern california field and petrographic observations from the ids in the sierra nevada indicate that i some and perhaps all of the dikes initially opened perpendicular to their margins and then were sinistrally sheared and ii sinistral displacement occurred during and shortly after dike injection dike emplacement overlapped in time with the formation of wall rock mylonite zones which accommodated shear varying in sense from sinistral to west side down reverse we suggest that the ids intruded dilatant fractures within a regional sinistral shear system the swarm is located within or along the eastern flank of the cordilleran subduction related magmatic arc the ids and associated wall rock deformation may record partitioning into the magmatic arc of the sinistral component of strongly oblique late jurassic subduction

kinematics of plate convergence deduced from mesozoic structures in the western cordillera mesozoic structures in the western cordillera of the united states yield important constraints for kinematic reconstruction of plate motions relative to north america structures in the mesozoic arc s of the northern sierra nevada and of northeastern oregon are remarkably uniform in timing style and with late cretaceous and or cenozoic rotations removed orientation rocks of both regions experienced three major episodes of contraction occurring in the triassic early jurassic the late jurassic and the early cretaceous cross folds were developed after the first two deformations but were not associated with significant shortening the three major deformations are nearly homoaxial and have shortening axes oriented subperpendicular ne sw to the regional trend of the arc system in the back arc region of the northwestern great basin deposition was continuous during the triassic and early jurassic and the region was shielded from tectonism in the arc to the west development of a regional fold thrust belt associated with major nw se shortening was initiated in the back arc region during the middle or late jurassic and continued through the early cretaceous corresponding deformation did not occur in the arc and the arc and back arc were decoupled by a major left lateral fault system that subparallelled the arc axis in the mid cretaceous approximately 100 m y b p regional compression in the back arc abruptly changed to ne sw and both the arc and back arc experienced major shortening development of homoaxial structures in the sierran arc is probably analogous to shortening in present day arc systems undergoing oblique subduction in which the shortening axis is parallel to the normal component of convergence thus the dominant structures in the sierran arc simply may be a reflection of the orientation of the trench and may not constrain the sense of obliquity of convergence strike slip faults and related compressional structures such as the back arc left lateral fault and associated fold thrust belt are probably the response to the strike slip component of convergence and indicate a period of left oblique subduction relative plate motions constrained by paleomagnetic data are in agreement with regional structures and indicate strong right lateral components of motion for the last 100 m y before 100 m y b p relative plate motions are inadequately constrained by paleomagnetic data and regional structures indicate a period of left oblique convergence from the middle or late jurassic to approximately 100 m y b p during the triassic and early jurassic the relative sense of convergence is equivocal and the possibility exists that continuous left oblique convergence occurred throughout the late paleozoic and early mesozoic until 100 m y b p based on the available data we cannot discount the possibility that a complex convergence history existed however with changes from right to left convergence in the middle or late jurassic and from left to right convergence in the mid cretaceous

jurassic magmatism in the central mojave desert implications for arc paleogeography and preservation of continental volcanic sequences exposures of jurassic magmatic rocks in the west central mojave desert provide insight into the changing character of volcanism throughout jurassic time and the paleogeography of the continental arc middle jurassic explosive volcanism lower sidewinder volcanic series resulted in collapse of multiple calderas this was followed by north south extension broadly coeval with batholith emplacement late jurassic effusive volcanism upper sidewinder volcanic series appears to reflect transtension over a larger region during intrusion of the independence dike swarm the lower sidewinder volcanic series consists of a subaerial nested caldera complex with an aggregate thickness of 4 km the first caldera formed during the eruption of crystal poor rhyolite ignimbrite outflow and intracaldera facies are intercalated with quartzose sandstone of probable cratonal provenance the second caldera formed during the eruption of crystal rich rhyolite to dacite ignimbrite this ignimbrite exhibits complex mineralogical zoning and contains two units of probable collapse related mesobreccia within the 1 400 m thick intracaldera sequence a third caldera nested within the second is filled with crystal rich biotite dacite ignimbrite and tuff breccia a fourth caldera largely coincident with the second formed during eruption of lithic and pumice lapilli dacite ignimbrite eruption and collapse appear to have been multistage as several units of reworked tuff and fallout tuff occur within the 1 750 m thick intracaldera sequence and caldera collapse breccias occur at the base and near the middle of the sequence the top of this sequence contains reworked tuffs and epiclastic rocks suggesting that the fourth caldera provided a posteruptive depocenter for accumulation of sediment normal faulting tilting and erosion followed explosive volcanism and was broadly contemporaneous with intrusion of middle jurassic porphyritic quartz monzonite plutons plutons and tilted lower sidewinder volcanic series are intruded and unconformably over lain by latest jurassic volcanic rocks upper sidewinder volcanic series the upper sidewinder volcanic series is characterized by alkalic basalt to basaltic andesite and rhyolite lavas hypabyssal intrusions and dikes including the 148 ma independence dike swarm regional northeast southwest extension is suggested by the presence of the northwest striking dike swarm and by bimodal volcanism the absence of debris flow deposits and epiclastic rocks suggests an intra arc region of low relief although local caldera related subsidence was the principal control on accumulation of lower sidewinder volcanic rocks preservation was likely enhanced by middle jurassic extension late jurassic extension was of broader regional extent but lesser magnitude because it did not create basins or cause significant tilting of strata the geometry timing and regional setting of middle and late jurassic extension and magmatism suggest a sinistral oblique subduction regime for the jurassic arc of the southern u s cordillera

north american apparent polar wander plate motion and left oblique convergence late jurassic early cretaceous orogenic consequences the north american apparent polar wander apw path indicates an episode of unusually rapid absolute northward motion of western north america between 150 and 135 ma during this time the northward component of absolute motion of points along the washington oregon california coast was in excess of 150 km m y and perhaps as high as 230 km m y we believe that such high absolute northward velocity for north america probably ensures that relative motions of oceanic plates and terranes influenced by them were to the south at this time the inception of rapid northward motion and left oblique convergence was abrupt and should be recorded in the geology of the western cordillera it is tempting to correlate this period of unusual pacific basin north american interaction with the nevadan orogeny in the klamath mountains as well as with left lateral strike slip structures such as the pine nut fault and bear mountains fault zone significant differences exist between north american plate motion recorded by the late jurassic cretaceous apw path and that predicted by a fixed hotspot model we believe that this discrepancy reflects uncertainty associated with pre late cretaceous hotspot tracks and poorly constrained relative plate motions during the cretaceous normal polarity superchron

vertically integrated flow in stratified aquifers abstract we present a set of continuous discharge potentials that can be used to determine the vertically integrated flow in stratified aquifers the method applies to cases where the boundaries are vertical and either the hydraulic head is given or the boundary is a seepage face or the integrated discharge is given the approach is valid for cases of given recharge through the upper and or lower boundaries of the aquifer the method is valid for any values of hydraulic conductivity there are no limitations of the contrast for the method to be valid the flows in the strata may be either confined or unconfined and locally perched conditions may exist but the effect of capillarity is not included the hydraulic head is determined by applying the dupuit forchheimer approximation the main advantage of the approach is that very complex conditions in stratified aquifer systems including locally perched conditions and extremely complex flow systems can be treated in a relatively straight forward approach by considering only the vertically integrated flow rates the approach is particularly useful for assessing groundwater sustainability as a model to be constructed prior to developing a fully three dimensional numerical model

magmatic flow direction indicators in a giant diabase feeder dike connecticut recent studies have suggested that many large diabase dikes have been emplaced laterally rather than vertically these studies report only unidirectional flow which is usually based on measurements of anisotropy of magnetic susceptibility however in the 250 km long higganum dike of southern new england united states seven independent flow direction indicators record a complex history of forward and backward moving magma we would expect other similar size dikes to have had equally complex histories measurements within one en echelon segment of the higganum dike show that as the magma rose it spread laterally toward the ends of the segment and that backflow was in the reverse direction of the forward flow the higganum dike is interpreted to have formed from a linear array of intrusive fingers each finger spreading laterally to form an en echelon dike segment as it rose through the lithosphere

interconnected sills and inclined sheet intrusions control shallow magma transport in the ferrar large igneous province antarctica field observations and structural data from intrusive complexes at allan hills and terra cotta mountain south victoria land antarctica demonstrate that interconnected sills and inclined sheets transported magma through the shallow subsurface these sills and sheets represent the upper crustal top 4 km plumbing system of the 183 ma ferrar large igneous province the sheets are short in length