timing of arc construction and metamorphism in the slate creek complex northern sierra nevada california late paleozoic to early mesozoic rocks in the sierra nevada and klamath mountains western united states preserve a record of lateral growth of continental crust by incorporation of ophiolites volcanic arcs and associated sedimentary rocks deciphering the timing of arc construction and metamorphism is critical for elucidating tectonic and thermal evolution during this type of crustal growth but is difficult in complex accretionary terranes in this study we address the timing of arc construction and regional metamorphism in the slate creek complex a volcano plutonic terrane in the central part of the northern sierra nevada new 40 ar 39 ar ages from relict volcanic hornblende demonstrate that the youngest volcanic unit in the slate creek complex is ca 170 ma at least 30 m y younger than previous estimates of ca 200 ma for metaplutonic rocks in the lower part of the complex consequently the slate creek complex is polygenetic with two distinct episodes of arc magmatism the distribution of mineral assemblages and textures indicates that greenschist and epidote amphibolite facies metamorphism is younger than 170 ma and is associated with crosscutting plutons that yield cooling ages of 150 ma and younger amphiboles from foliated rocks in the slate creek complex yield plateau ages of 156 1 0 6 and 152 0 0 7 ma that date cooling subsequent to or dynamic recrystallization during the metamorphic event age and lithologic similarities suggest that the lower parts of the slate creek complex correlate with plutons ca 200 ma and the volcanic cover sequence of the rattlesnake creek terrane in the klamath mountains the uppermost volcanic unit correlates with the ca 170 ma western hayfork terrane that structurally overlies the rattlesnake creek terrane metamorphism in the slate creek complex correlates broadly with a similar late metamorphic event in the klamath mountains

evolution of oceanic lithosphere a driving force study of the nazca plate the role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modeling of the nazca plate results indicate that ridge push forces when distributed over that portion of the oceanic lithosphere giving rise to the force predict a stress state that varies more spatially than does previous modeling with ridge forces concentrated at the ridge crest in particular the component of stress normal to the ridge vanishes near the ridge for the new modeling predicted stresses for distributed ridge force models are still in qualitative agreement with the limited data on nazca intraplate stress stress states have also been calculated for a variety of slab pull models which include possible dependence on age rate and angle of subduction the predicted stresses vary substantially near the trench among these models with less variation farther from the trench combinations of ridge push and slab pull models are consistent with the earlier result of ah upper limit of about a factor of 2 for net slab pull over ridge push simple viscous modeling with only distributed ridge forces predicts relative motion between the nazca and south american plates that is in fairly poor agreement with the observed direction the fit to the observed direction could probably be improved by either including slab forces or the effect of the largely transform boundary with the antarctic plate

petrotectonic evolution and melt modeling of the penon blanco arc central sierra nevada foothills california the penon blanco arc of the jurassic triassic arc belt in central california is composed of the jasper point formation penon blanco formation and coeval don pedro intrusive suite all exposed in the core of the cotton creek anticline the jasper point formation consists of 900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts it passes upward into the penon blanco formation which is made up of 700 m of crystal lithic basaltic tuff 1 3 5 km of augite rich volcaniclastic rocks and up to 3 5 km of massive to brecciated flows of augite phyric basalt the penon blanco formation is paraconformably overlain by the oxfordian kimmeridgian mariposa formation which provides a minimum age of juxtaposition for the penon blanco arc against the inboard calaveras complex new geochemical data from the penon blanco arc show that the two volcanic suites are geochemically distinct jasper point basalts are tholeiitic and are characterized by high large ion lithophile element lile abundances moderate high field strength element hfse and heavy rare earth element hree abundances and low ti v ratios penon blanco basalts are calc alkaline have higher lile abundances lower hfse and hree abundances and lower ti v ratios geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel bearing mantle source at 30 60 km depth by low to moderate amounts of partial melting 3 5 for jasper point and 5 7 5 for penon blanco the geochemical modeling and field data suggest that the jasper point basalts are similar to normal mid ocean ridge basalt n morb and were associated with forearc rifting while the penon blanco basalts represent the transition to arc volcanism and head on subduction this model is consistent with interpretations for a single ensimatic arc on the jurassic margin of north america

mesozoic transpression transtension subduction and metallogenesis in northern and central california middle paleozoic to middle jurassic terrane assemblies in the klamaths and sierran foothills consist of mafic ultramafic complexes fine grained terrigenous strata derived from previously accreted continental margin belts sutured oceanic terranes reflect c 230 myr of margin parallel slip involving chiefly transtension and transpression quartzofeldspathic clastic rocks and blueschists eclogites are very rare little devolatilization occurred at magmagenic depths hence coeval hydrothermal ore deposits and granitoids are uncommon in contrast nearly head on cretaceous subduction of the farallon plate generated the massive klamath sierra nevada volcanic plutonic arc reflecting dewatering of the eastward descending oceanic lithosphere in the magmagenic zone immature great valley forearc and franciscan trench deposits shed from the arc record c 70 myr of rapid crustal growth au bearing solutions rising from magmagenic depths exsolved from plutons and expelled from heated wall rocks were mobilized attending arc construction precipitation of gold bearing quartz veins occurred where h2o co2 bearing fluids encountered major geochemical discontinuities in the wall rocks

travel time of accreted igneous assemblages in western pacific orogenic belts and their associated sedimentary rocks abstract accreted igneous assemblages in orogenic belts maybe divided into three types depending on whether they derive from seamounts ocean ridges or subduction related ophiolites seamount type basalts are associated with shallow water sediments mostly reefoidal limestones ocean ridge type basalts are generally overlain by pelagic cherts subduction related ophiolitic eruptives often underlain by gabbroic and ultramafic rocks are associated with hemipelagic mudstones the age of such diverse eruptive lithologic assemblages reflects the time taken for them to have traveled from their locus of generation to their place of accretion at a continental margin this relationship has been established for each type of accretionary complex examples being taken mostly from japan and the western pacific rim in order to represent evolutionary processes at a typical active plate margin in general the seamount types are older ridge types are of intermediate age and the ophiolitic types are by far the youngest usually close to zero age seamount type basalts are accreted by shallower scraping of the seamount s sediment apron together with fragments of seamount basalt ridge type by peeling due to permeability contrast and the ophiolitic types by deeper scraping as a consequence of an inflected temperature gradient accordingly it is concluded that the ophiolitic rocks are generated close to the trench and may be accreted as a result of ridge subduction

accurate source depths and focal mechanisms of shallow earthquakes in western south america and in the new hebrides island arc synthetic seismograms are matched to long period p waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms accurate depths are obtained for a large sample of moderate sized 6 ms 7 events which produced relatively simple p waveforms and do not require intensive analysis the estimated error in the determination of depth is about 5 km or less for most of the events considered the procedure is applied to two suites of events one in south america 83 events and one in the new hebrides 61 events in these two areas of contrasting styles of subduction the accurate depths provide new information on the bending of descending plates near trenches the seismicity and tectonics of the upper plate and the geometry and structure of the zone of contact between the upper and descending plates depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with chapple and forsyth s 1979 preferred model of a bending elastic perfectly plastic plate however an unusually deep event occurred beneath the fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late miocene time the calculated depth of the event 48 km places it below the lithosphere asthenosphere boundary derived from thermal models vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin 10 km thick curved zone of contact dipping arcward or landward from the trench axis however a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact these events would be accommodated by an interplate zone 15 km thick in both subduction zones interplate events occur at depths between about 15 and 50 km however the downdip width of the south american interplate zone is 2 to 3 times larger than that in the new hebrides most of the intraplate events near the interplate boundary are located in the descending plate in the upper plate seismicity is more pronounced in the back arc regions of the two subduction zones thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc depths of shallow earthquakes in the eastern andes and sub andean regions of south america range between about 9 and 36 km indicating that both the upper and middle crust are seismically active about 75 of the events occurred in the upper 25 km furthermore the focal mechanisms indicate that from ecuador to argentina the crust of the eastern side of the andean cordilleran is in a state of east west compression oriented nearly parallel to the direction of convergence between the nazca and south american plates the depths the orientations of the earthquake nodal planes and the close relationships to quaternary tectonics imply basement involved crustal shortening in much of the eastern andes and sub andean zones of peru and argentina

stress modeling in the andes constraints on the south american intraplate stress magnitudes regions of high topography which exhibit both compressional and extensional deformation provide a natural laboratory to place constraints on horizontal intraplate stress magnitudes an elastic finite element analysis of the lithospheric stress field in the cordillera bianca region of peru has been used to evaluate the lithospheric stress state and constrain the south american intraplate stress magnitudes the orthogonal orientation of the andean high topography to the east west regional stress field in conjunction with the well documented change in deformation style from compression to extension across the cordillera provides a unique setting for the stress modeling a two dimensional finite element grid consisting of an assembly of isotropic elastic quadrilateral elements in a state of plane strain was used to represent the lithosphere an isostatically balanced crustal density model was used to calculate the stresses associated with the lateral density variations between the andean topography and the brazilian shield in some models horizontal stress was concentrated at midcrustal depths by including vertical variation in young s modulus the modeling results indicate bounds of 10 and 75 mpa for the magnitude of the average horizontal stress averaged over a 100 km thick lithosphere a preferred value of 25 mpa is obtained on the basis of fitting all of the data for the rheological models considered and predicting a transition from extension to compression near the 3000 m elevation contour such a far field average stress is consistent with a ridge push origin because the modeling performed in this study assumes that the topographic load is entirely compensated by the buoyancy of the deflected moho these estimates represent upper bounds the effect of incorporating thermal support for the topographic load of the cordillera bianca would be to further reduce our estimate of the intraplate stress magnitudes analysis of the distribution of calculated stress with depth indicates that previous assumptions about the vertical plane shear stress within the lithosphere being negligible are acceptable for the geometry of the andes

tectonic setting of the jurassic smartville and slate creek complexes northern sierra nevada california jurassic ophiolitic complexes in california and oregon are widely distributed outboard of a jurassic and older continental margin whether these outboard complexes were exotic and collided with the jurassic continental margin or whether they were simply part of a wide forearc to a jurassic continental magmatic arc has long been at issue we have studied two such complexes and crosscutting intrusions in the fault bounded central and western belts of the northern sierra nevada we report new and revised u pb zircon dates for the early jurassic and younger slate creek complex ca 203 171 ma in the central belt the late jurassic smartville complex 159 3 ma in the western belt and crosscutting intrusions ranging from middle jurassic to early cretaceous age the new ages imply that the central and western belts have been part of the same terrane since middle jurassic time and that the fault separating them has accommodated only minor displacements since ca 160 ma isotopic compositions of common pb indicate derivation from mantle sources but evidence for inheritance of pre cambrian zircon is common in plutons within the early and late jurassic complexes and in middle jurassic late jurassic and early cretaceous crosscutting intrusions although there are several potential sources for such contamination the ages of the inherited zircon are consistent with a north american origin this contamination would have been accomplished most plausibly near the north american margin

why does near ridge extensional seismicity occur primarily in the indian ocean abstract the possibility that thermoelastic stresses due to plate cooling contribute significantly to the stress field and seismicity in young oceanic lithosphere has been a subject of considerable recent interest this effect is suggested by three key observations a decrease in seismicity with lithospheric age the fact that focal mechanisms show extension perpendicular to the spreading direction and a depth stratification of mechanism types a difficulty with this idea is that although thermoelastic stresses should be comparable in different regions the intraplate seismicity seems to occur in local concentrations in particular the ridge parallel extensional seismicity occurs preferentially in the central indian ocean region we explore the possibility that much of the data favoring thermoelastic stresses can be interpreted in terms of stresses resulting from individual plate geometry and local boundary effects in particular the dramatic concentration of extensional seismicity in the central indian ocean region is consistent with finite element results for the intraplate stress incorporating the effects of the himalayan collision and the various subduction zones the ridge parallel extensional stresses show a decrease with age similar to that of the seismicity as earthquakes in this area provide a major portion of the data for both ridge parallel extension and depth stratification these effects may be due more to the regional stress we thus propose that thermoelastic stresses provide a low level background in all plates but that the dominant effect is that of individual plate geometry and local boundary effects

tectonic controls on mudrock geochemisry mesozoic rocks of eastern oregon and western idaho u s a implications for cordilleran tectonics abstract tectonic models for the mesozoic evolution of accreted island arcs and subduction complexes in the blue mountain province bmp of eastern oregon and western idaho vary widely the bmp is situated between coeval accreted terranes of southern british columbia and the western u s and is critical for interpreting mesozoic paleogeography of the u s cordillera in this study we interpret the triassic jurassic evolution of the bmp using geochemistry as a tool to evaluate the provenance of mudrocks in fine grained turbidites temporal and spatial variations in mudrock geochemistry indicate that during late triassic to early jurassic time the wallowa terrane was an intra oceanic island arc and the olds ferry terrane was a pericratonic continental fringing island arc subsequently a regionally extensive middle to late jurassic marine basin received input from continental sources and sediment may also have been derived from coeval terranes to the south comparison of provenance data from the bmp with similar data from accreted terranes to the north and south suggests that triassic jurassic basins of the western u s share a long term provenance history within a complex system of continent fringing and intra oceanic island arcs in contrast it appears that the quesnel terrane of southern british columbia had a distinct provenance with a more direct link to laurentia during triassic time early to middle jurassic sedimentary rocks from southern british columbia and the western u s show a high degree of geochemical provenance similarity this may indicate reorganization of formerly discrete tectonic regions into a nascent integrated western laurentian margin prior to complete development of a cretaceous andean type margin the results of this study demonstrate the utility of mudrock geochemistry in tectonic analyses and show that major and trace element geochemistry of mudrocks is useful for developing a comprehensive understanding of regional tectonic evolution

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

early jurassic deformation in the pine forest range northwest nevada and implications for cordilleran tectonics triassic and jurassic rocks in the black rock desert of northwest nevada comprise part of the mesozoic volcanic arc successions found throughout the westernmost u s cordillera detailed structural studies in the pine forest range located in the western black rock desert provide new insight into the jurassic structural history of this part of the mesozoic arc system regional jurassic deformation and metamorphism in the pine forest range affected a thick 10 5 km section of lower paleozoic through triassic strata that were tilted after deformation and are now exposed in approximately cross sectional view at high stratigraphic levels deformation produced a slaty cleavage and relatively open folds both of which are concentrated in rheologically incompetent rock types and was accompanied by metamorphism at low grade at deeper stratigraphic levels the regional foliation intensifies and becomes more pervasive folds tighten and metamorphic grade increases through greenschist into amphibolite grade at the deepest stratigraphic levels the rocks are amphibolite grade l s tectonites with a pronounced mylonitic fabric the 2 km thick zone of mylonites is interpreted as a ductile shear zone that formed when the stratigraphic section was still upright whereas the less deformed younger rocks are interpreted to occupy the upper plate of the shear zone and to record a strain gradient away from the shear zone a variety of structural and regional relations indicate that the shear zone is a ductile thrust fault along which younger rocks moved to the northwest in present day coordinates with respect to older rocks the timing of regional jurassic deformation and metamorphism is constrained by relations in and around two plutons dated by the u pb zircon method the 201 1 ma big creek pluton was intruded as a sill like body along the upper levels of the shear zone and is associated with a variety of features indicating syntectonic intrusion including internal textural features in plutonic rocks and metamorphic changes in wall rocks with proximity to the pluton the 185 1 ma theodore pluton which intrudes across part of the shear zone has a thermal aureole which statically overprints the shear zone fabric deformation was thus ongoing at 201 ma and over by 185 ma the early jurassic age of regional shortening deformation in the pine forest range contrasts with the middle to late jurassic 170 150 ma age of regional shortening documented in many other parts of the mesozoic arc system including the klamath mountains sierra nevada and mojave desert regions of california it also contrasts with the record of early jurassic extensional tectonism documented in the sierra nevada mojave desert and western arizona segments of the mesozoic arc system these conflicting relations indicate a complex tectonic evolution for the jurassic u s cordillera plate margin and lend support to the idea that disparate parts of this plate margin were juxtaposed along younger strike slip faults

subduction induced fragmentation of the nazca plate off peru mendana fracture zone and trujillo trough revisited the mendana fracture zone and the trujillo trough are two major intra oceanic features on the nazca plate off peru during the seaperc cruise of the r v jean charcot in 1986 sea beam bathymetric data single channel seismic records geomagnetic and gravity measurements and heat flow data have been obtained over these two areas in this paper we confirm that the mendana fracture zone is actively opening perpendicular to its trend resulting in the formation of new oceanic crust since about 3 5 ma we calculate that this new rift is propagating westward along the fracture zone at a velocity of about 10 cm yr 1 or 2 cm yr 1 with respect to the trench this extensional feature appears to be genetically linked with the trujillo trough located 200 km to the north this trough is bounded to the east by a left lateral transpressive fault which became active probably at the time when the mendana rift started we then discuss a possible mechanism to explain the origin of these two areas of intraoceanic deformation

satellite radar interferometry for assessing coseismic liquefaction in portoviejo city induced by the mw 7 8 2016 pedernales ecuador earthquake the portoviejo city located in the central part of the ecuadorian pacific coast province of manabi was severely affected during the pedernales mw 7 8 earthquake of april 16 2016 accompanied by coseismic liquefaction phenomena that induced processes of ground subsidence lateral spreading sinkhole and sand boils the present study proposes the detection and delimitation of the areas affected by the relief deformation associated liquefaction processes which occurred in the urban area of portoviejo city and especially in the zero zone of greater destruction after the earthquake through the application of differential interferometry synthetic aperture radar insar methodology using sentinel 1a satellite images plus the support of geological geotechnical data obtained from boreholes and standard penetration tests to evaluate the liquefaction potential of the layers of the subsoil based on the potential liquefaction index considering the seismic scenario of amax 0 5 g for the urban area and the zero zone this procedure allowed the delimitation of areas of high seismic risk for the proper coordination and management of the construction and reconstruction processes in the city of portoviejo and specifically in the ground zero

on the origin of the cocos nazca spreading center to investigate the mechanism underlying the break up of the farallon plate into the cocos plate and nazca plate we analyze the state of stress in the farallon plate at about 30 m y b p just prior to the fragmentation to this purpose we use finite element methods and a reconstruction of regional plate boundaries appropriate for 30 m y b p a key role in the model is played by the dependence of two important plate tectonics forces slab pull and ridge push on the age of oceanic lithosphere the results show a highly tensional stress field with maximum principal stresses of 5 to 6 kbar north south tension in the vicinity of present day panama is proposed to have been the cause for the fragmentation of the farallon plate and the inception of spreading along the new cocos nazca plate boundary because the kind of plate interaction that gave rise to the high level of tensional stresses in the farallon plate is not restricted to the area of this study the proposed mechanism seems to shed light on the problem of fragmentation of oceanic plates in general

stress observations and driving force models for the south american plate stress observations are compared to predictions of driving force models for the south american plate stress observations have the potential to give detailed information about the forces driving the plates in that sets of forces with the same net torque can produce quite different stress patterns for a two dimensional vertical section model of a plate there are three conditions for dynamical equilibrium the net forces and the net torque about any point must all be zero these conditions are used to estimate the magnitudes of forces as a function of age with ridge push and slab pull as active forces and plate drag and slab resistance as passive forces the results are consistent with earlier published models and indicate that the primary balance for an oceanic plate is between slab pull and the resisting drag forces we generalize the model for the possible forces driving the south american plate and compare with observed stress measurements and kinematics the south american plate has a small slab pull component amounting to about one fifth the ridge push force with the other components reasonably isolated geographically thus the plate balance is largely between ridge push and plate drag this gives an opportunity of resolving the effects of trench suction or collision at the subduction zones most remarkable is the nearly uniform e w orientation of the regional stress field shmax directions which is extensional in the high andes a second feature is the n s orientations in the amazon basin the two dimensional horizontal stress patterns are compared for successive models of combinations of driving forces models with ridge push slab pull at the scotia and caribbean arcs and trench suction near the west coast balanced by plate drag produce similar stress patterns to those observed the magnitude of a possible trench suction force is estimated using a model with an eddy between the nazca slab and the overlying south american plate

timescales of orogeny jurassic construction of the klamath mountains an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 94ycj2454 timescales of orogeny jurassic construction of the klamath mountains b r hacker m m donato c g barnes m o mcwilliams and w g ernst diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts forty five new 40ar 39ar ages from the klamath mountains illuminate the deformation metamorphism magmatism and sedimentation involved in the jurassic construction of that orogen leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes the new geochronologic relationships show that many jurassic units of the klamath mountains had 200 ma or older volcanoplutonic basement subsequent formation of a large 170 ma arc was followed by contractional collapse of the arc collision with a spreading ridge may have led to large scale nw se extension in the central and northern klamaths from 167 to 155 ma coincident with the crystallization of voluminous plutonic and volcanic suites marked cooling of a large region of the central klamath mountains to below 350 c at 150 ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels these data demonstrate that the klamath mountains and perhaps other similar orogens were constructed during areally and temporally variant episodes of contraction extension and magmatism that do not fit classical definitions of orogeny

early proterozoic oceanic crust in the northern colorado front range implications for crustal growth and initiation of basement faults 1 early proterozoic oceanic crust exposed in a shear zone in the northern colorado front range provides insight into the tectonic evolution of the colorado province tectonically intermingled mylonitic amphibolite locally pillowed metagabbro clinopyroxenite metachert and melange are preserved within the buckhorn creek shear zone west of fort collins whole rock geochemical data support formation of the igneous protoliths in an oceanic environment major and trace element data overlap samples from modern oceanic ridges and are distinct from modern arc trends metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes core and mantle microstructures indicate initial deformation at temperatures 900 c secondary amphiboles define both subhorizontal and subvertical lineations kinematic analysis indicates that the rocks experienced e w strike slip deformation prior to and during amphibolite facies metamorphism followed by n s shortening 40ar 39ar data from secondary hornblende yield age steps as old as 1 9 ga based on the chemical data and kinematic evidence for strike slip deformation we propose that the buckhorn creek rocks originated at an oceanic ridge transform intersection the high t pyroxene fabrics formed during ocean floor deformation prior to 1 7 ga regional deformation of the colorado province the buckhorn creek fragment likely represents original oceanic basement for 1 8 1 7 ga arc magmatism and sedimentation that ultimately formed the colorado province continental crust reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in colorado once formed subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long lived crustal discontinuities

origin of the amphibolite sole of the josephine ophiolite emplacement of a cold ophiolite over a hot arc the amphibolite sole that occurs along the basal thrust of the josephine ophiolite madstone cabin thrust is distinct from those described for ophiolites such as the bay of islands and oman antigorite bearing serpentinite mylonites occur directly above the amphibolite and the presence of antigorite brucite indicates formation between 300 and 400 c the antigorite mylonites also contain relict lizardite chrysotile suggesting formation of antigorite by prograde metamorphism along the base of the ophiolite further evidence for prethrusting low temperature serpentinization comes from a lizardite chrysotile shear zone within fresh peridotite directly above the thrust that is cut by undeformed diabase dikes having geochemical affinities to the upper pillow lavas of the josephine ophiolite the prograde metamorphism of the serpentinite at the base of the ophiolite was apparently synchronous with greenschist facies retrogressive metamorphism of the underlying amphibolite the amphibolite is chemically diverse and has been divided into two groups group 1 amphibolite appears to be derived from mafic plutonic rocks and has immobile trace element abundances indicating affinities to volcanic arc magmas group 2 amphibolite is metavolcanic contains some quartzite and has magmatic affinities to mid ocean ridge basalt and within plate basalt an association that is distinctive of older basement rocks elsewhere in the western klamath terrane we suggest the amphibolite was derived from the footwall of the madstone cabin thrust including country rocks for the chetco complex group 2 and possibly older intrusive phases of the chetco complex itself group 1 previous workers have shown that the base of the amphibolite is intruded by gneissic gabbro of the chetco complex that intrusion of the chetco complex overlapped in time with deformation of the amphibolite and that during and after emplacement the josephine ophiolite and overlying flysch were intruded by abundant arc related dikes and plutons emplacement of the josephine ophiolite appears to have occurred by underthrusting of a cold ophiolite by an active magmatic arc

analysis of the south american intraplate stress field the first order south american intraplate stress field was modeled through a finite element analysis to evaluate the relative contribution of plate boundary forces and intraplate stress sources the finite element mesh consisted of 3100 nodes in a network of 5993 equal area triangular elements which provided a spatial resolution of about 1 at the equator an important aspect of our modeling is the inclusion of topographic forces due to the cooling oceanic lithosphere along the mid atlantic ridge e g ridge push the continental margins along the east coast of brazil and argentina and the elevated continental crust e g the andean cordillera predicted intraplate stresses for two representations of the western collisional boundary forces are evaluated pinned collisional boundaries and applied collisional boundary forces constraint for the modeling was provided by information about the orientation of the maximum horizontal compressive stress shmax provided by 217 stress indicators from the world stress map project as well as by shmax magnitude estimates and torque information from previous investigations our modeling results demonstrate that the first order features of the observed stress field can be explained with simple tectonic models which balance the torque acting on the plate either with a fixed western margin or drag forces applied along the base of the plate the predicted intraplate stress field is characterized by a nearly uniform e w shmax orientation throughout most regions of the plate with stress magnitudes generally less than 20 mpa averaged over a 100 km thick lithosphere significant perturbation of this regional stress field occurs in the western part of the plate in response to forces associated with the high topography of the andes although the magnitude of the collisional boundary forces acting along the western margin remains poorly constrained we estimate a plausible upper bound on the force per unit length acting along the peru chile trench to be about 2 5 1012 n m 1 while some of our models are consistent with a driving basal drag to balance the torques acting on the plate the magnitude of the drag torque is small compared to the contribution from other sources of stress such as the ridge push force

the driving mechanism of plate tectonics relation to age of the lithosphere at trenches sea floor age depth and age trench depth relations suggest that the oceanic lithosphere continues to thicken and subside with age beyond 80 ma if so slab pull fs and ridge push fr forces can be calculated from the age of the sea floor at trenches t and if the shear stress acting on the base of the plate increases with absolute velocity a the approximate force balance equation is dva2 svat32 rt where d s and r are constants related to drag slab pull and ridge push respectively the relation between velocity and age is va2 s vat12 r we have tested this model by linear regression using rate and age data for 15 trenches va mm a 2 9 20 32 vat12 26 250 the strength of this correlation r 0 98 is strong evidence in favor of the validity of this simple model these results suggest that ridge push is just sufficient to overcome drag at the base of the plate and does not contribute significantly to the motions of oceanic plates though the value of r is consistent with the motions of plates not attached to subducted slabs

kinematic significance of l tectonites in the footwall of a major terrane bounding thrust fault klamath mountains california usa abstract detailed geologic mapping cross section reconstructions strain analyses and kinematic analyses enable the reconstruction of a one kilometer wide domain of l tectonites in the east west striking subhorizontal to gently south dipping pigeon point high strain zone pphsz associated with a major thrust fault separating oceanic and arc affinity terranes in the klamath mountains california l tectonites are associated with 1 a convex upward warp of the upper high strain zone boundary 2 a transition from mafic metavolcaniclastic rocks to micaceous quartzites 3 folds subparallel with mineral lineations 4 emplacement of synkinematic ultramafic mafic intrusive bodies and 5 a local temperature increase from greenschist to amphibolite facies conditions pure shear dominated deformation accommodated zone normal shortening and transport parallel elongation coupled with subordinate top to the west directed thrust style simple shear l tectonite formation was controlled by the shape of the high strain zone boundary driving lateral flow into the apex of the lens shaped zone in response to a favorable kinematic geometry and bulk strain in the constrictional field localized magmatic heating best explains the shape of the high strain zone boundary but l tectonites are not partitioned into a single rheological domain during terrane amalgamation strain path partitioning occurred with localized top to the west directed simple shear partitioned into a structurally overlying thrust zone and pure shear dominated subvertical shortening and transport parallel elongation partitioned into the pphsz

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

intraplate stresses and plate driving forces in the philippine sea plate we use a spherical shell elastic finite element analysis to characterize intraplate stresses and plate driving forces in the philippine sea plate our finite element mesh is comprised of 489 nodes and 914 elements and provide a spatial resolution of about 1 the philippine sea plate is unusual in that it does not have large numbers of seismic or tectonic stress indicators and there are no midplate borehole stress data which can be used as constraints in this finite element study however it does possess a unique boundary configuration which potentially can lend more insight to plate forces and stresses than can similar studies on plates with more seismically active interiors because the plate s boundary consists primarily of subduction zones with roughly equal proportions of boundary on the overlying and downgoing sides of trench the resultant plate stress field is very sensitive to subduction related forces as a result borehole stress measurements in certain parts of the plate would be useful in further constraining these forces we find that trench pull forces can explain a large component of the velocity of the philippine sea plate however trench pull has somewhat less overall effect on the stresses across the plate than do the lateral density and topographic variations within the plate i e gravitational or potential energy force and trench suction forces are probably about an order of magnitude smaller than the net trench pull forces collisional forces may dominate the stress fields near taiwan and the izu peninsula but they are of only secondary importance in determining the motion and deformation of the philippine sea plate the forces that generate stress orientations most closely matching the observed stress indicators produce a lithospheric stress field across the plate that averages about 15 mpa over a 70 km thick lithosphere scaling inversely with lithosphere thickness values up to four times as great are produced locally

the fixed hotspot hypothesis and origin of the easter sala y gomez nazca trace reconstructions of the pacific and nazca plates suggest that both the nazca and tuamotu ridges originated from a melting anomaly which existed beneath the pacific farallon ridge during the time interval between anomalies 19 and 11 as would be predicted from the fixed hotspot hypothesis the hotspot hypothesis interpreted according to morphology of the easter sala y gomez nazca trace would further suggest that the hotspot was located entirely beneath the nazca plate during the time interval between anomalies 11 and 2 at the end of this interval the eastern boundary of the easter plate formed centered above the hotspot according to this scheme the present location of the hotspot is about 200 km west of easter island however the easter model is incompatible with the hotspot model for the hawiian emperior chain if the hotspots are fixed relative to one another as the age of the point on the tuamotu ridge which corresponds with the hawaiian emperor bend 53 to 55 m y is significantly greater than the age of the bend 43 m y the fixed hostpot hypothesis can be satisfied by the two traces only if one or a combination of the following conditions is met the kinematic age of the hawaiian emperor bend is substantially greater than the isotopically dated age the magnetic anomaly time scale is in significant error and or magnetic anomaly identifications in the vicinity of the tuamotu ridge are erroneous it might appear that other mechanisms for the origin of hotspot traces are preferable to the hotspot hypothesis the most promising of these involves propagating fractures resulting from intraplate stress this hypothesis has the advantage of accommodating anomalies in some hotspot traces such as persistence of igneous activity long after passage of the trace over the hotspot

paleomagnetic contributions to the klamath mountains terrane puzzle a new piece from the ironside mountain batholith northern california abstract we obtained paleomagnetic samples from six sites within the middle jurassic ironside mountain batholith 170 ma which constitutes the structurally lowest part of the western hayfork terrane in the klamath mountains province of northern california and southern oregon structural attitudes measured in the coeval hayfork bally meta andesite were used to correct paleomagnetic data from the batholith comparing the corrected paleomagnetic pole with a 170 ma reference pole for north america indicates 73 5 10 6 of clockwise rotation relative to the craton nearly one half of this rotation may have occurred before the terrane accreted to the composite klamath province at 168 ma no latitudinal displacement of the batholith was detected

ridge forces absolute plate motions and the intraplate stress field torque poles are calculated for a variety of possible forces acting on the plates including ridge push slab pull and collisional resistance these torque poles are then compared to the directions of absolute plate motions there is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the north american south american pacific cocos and eurasian plates simple slab pull torques correlate well with absolute motion azimuths for the pacific nazca and cocos plates and moderately well with the absolute motion azimuth of the indoaustralian plate collisional resistance torque poles correlate with the absolute motion azimuth of the eurasian plate only the correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves torque poles for various forces are also compared with several long wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions in general ridge torque directions agree well with the orientations of maximum horizontal stresses for stable north america western europe and south america and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag collisional resistance forces can also explain the alignment of stresses in western europe for the indoaustralian plate the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate other processes in addition to collisional resistance must be important for the indo australian plate ridge push forces may account for a significant portion of the long wavelength features of the intraplate stress field especially away from continental collisions such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism as previously suggested slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates

new evidence for cretaceous strike slip faulting in the united states cordillera and implications for terrane displacement deformation patterns and plutonism early to mid cretaceous dextral strike slip faulting involving up to 400 km displacement has been documented in the southern sierra nevada of california in western idaho the salmon river suture juxtaposes unrelated oceanic and continen tal rocks along a major near vertical shear zone that was active in the early to mid cretaceous these two structural zones have been treated as separate entities however new data indicate that they likely connect via a structural discontinuity in northwest nevada and southeast oregon herein called the western nevada shear zone wns current structures within the wns record dip slip contractional deformation however regional relations structural and stratigraphic indicate a mismatch in the early mesozoic geology of rocks across the wns that is best explained by dextral strike slip displacement of several 100 km these relations argue that an early strike slip structure along the wns was reactivated and obscured during later regional shortening deformation most likely related to the cretaceous sevier orogeny the wns is along strike and broadly coeval with the salmon river suture zone and strike slip boundaries in the sierra nevada we hypothesize that all these features are related elements of a major structural boundary in the western united states cordil lera this boundary is interpreted to have accommodated significant dextral strike slip displacement of outboard arc terranes of the cordillera in the early cretaceous followed by reactivation as a localized contractional zone in the mid cretaceous during the sevier orogeny cretaceous batholiths of the united states cordillera closely parallel the boundary which we suggest formed a major crustal flaw that localized and facilitated emplacement of arc magmas

geochemistry and tectonic setting of the ophiolitic ingalls complex north cascades washington implications for correlations of jurassic cordilleran ophiolites abstract this study synthesizes new and existing chemical data to interpret the tectonic setting of the late jurassic ingalls complex of the northwest cascades washington and to compare it with other cordilleran ophiolites of similar age mafic rocks of the ingalls complex represent a spectrum of magma types including normal type and enriched mid ocean ridge basalt within plate basalt and island arc basalt based on mafic magma chemistry compositions of cr spinels in associated peridotite and field relations the ingalls complex is a suprasubduction zone ophiolite formed in a back arc basin cut by an oceanic fracture zone regional tectonic relations also support this hypothesis our synthesis of available chronologic geochemical and stratigraphic data for all jurassic cordilleran ophiolites shows that the ingalls complex has similarities and differences with each the most compelling likeness is between the ingalls complex and josephine ophiolite both of which exhibit a wide range of magmatic af

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

joint inversion of the kinematics and dynamics of the north american plate abstract finite element models are presented in this paper to study the kinematics and dynamics of the north american plate the plate is represented by a portion of thin spherical shell modeled with newtonian fluid the external actions acting at the plate boundary as well as on the plate bottom are expressed by a set of undetermined parameters in terms of the results of the nuvel 1 model the plate viscosity is considered as regionally uniform constrained by the observed data of principal stress orientation and vlbi velocity the tectonic deformations and stresses in the north american plate as well as the driving forces are jointly inverted by the method of least squares and the principle of linear superposition the inversion is posed as a standard nonlinear optimization problem which is solved numerically by the genetic algorithms combined with a kind of quasi newtonian method the computed results show that some of our simple dynamic models can well and consistently fit the observed kinematic and dynamic data even though the data have very different time scales the data fitting indicates that continental lithosphere is an order of magnitude more viscous than oceanic lithosphere which in turn is an order of magnitude more viscous than lithosphere in plate boundaries including the diffuse plate boundary in the western united states in the middle part of the plate the predicted tectonic stresses are uniform both in orientation and magnitude with a typical regime of strike slip type the modeling results suggest that the plate motion is driven by forces acting along the atlantic and arctic ridges the south american boundary the caribbean boundary and the san andreas fault while resisted by forces acting along the other boundary segments and by the viscous drag forces at the base of the plate

thermal origin of the hawaiian swell heat flow evidence and thermal models the lithospheric reheating hypothesis for the origin of the hawaiian swell has been tested by heat flow measurements along the swell between hawaii and midway ninety five measurements at eight carefully selected sites show that the heat flux on the swell increases systematically from near normal values 52 2 mw m 2 near hawaii to values near midway 59 2 mw m 2 about 20 25 higher than normal for lithosphere of that age 110 m y this heat flow pattern is qualitatively consistent with a deep seated thermal anomaly created in a lithospheric plate moving across a fixed heat source modeling of the heat flow and bathymetry data on the hawaiian swell strongly suggest that the swell is caused by a rapid heating within 40 to 50 km depth of normal 90 m y old lithosphere over a relatively broad 500 1000 km region near hawaii this model is also consistent with gravity and geoid anomalies associated with the swell which require a low density region in the lower lithosphere and with flexure studies of seamounts in the hawaiian chain which indicate that the rigidity of the lithosphere is not significantly reduced by the reheating appendices a c are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 005 1 00 payment must accompany order

earthquake ground effects and intensity of the 16 april 2016 mw 7 8 pedernales ecuador earthquake implications for the source characterization of large subduction earthquakes

the rattlesnake creek terrane klamath mountains california an early mesozoic volcanic arc and its basement of tectonically disrupted oceanic crust the rattlesnake creek terrane in the south western klamath mountains consists of a serpentinite matrix melange basement unconformably overlain by a coherent cover sequence of upper triassic and lower jurassic volcanic hemipelagic and clastic sedimentary rocks both the meelange and cover sequence are intruded by a suite of ca 193 207 ma u pb zircon gabbroic to quartz dioritic plutons these rocks collectively record a complex multistage evolution through several distinct tectonic environments blocks in the meelange basement include peridotite greenstone amphibolite pillow basalt various generally mafic plutonic rocks metachert and limestone all suspended in a matrix of sheared serpentinite trace element chemistry indicates that basalt amphibolite and greenstone blocks were derived from normal to enriched mid oceanic ridge basalt n morb to e morb and within plate basalt wpb protoliths there is no geochemical evidence that any blocks were derived from a magmatic arc nor do any of the sedimentary rocks contained within the meelange reflect deposition near a terrigenous sediment source collectively these relations indicate that the basement meelange was derived by disruption of oceanic crust and upper mantle in a setting far removed from either an arc or a terrigenous landmass and are most consistent with an interpretation that meelange formation oc curred in an oceanic fracture zone the overlying cover sequence is divisible into a stratigraphically lower salt creek assemblage and a stratigraphically higher dubakella mountain assemblage both deposited in a marine setting volcanic rocks in the lower assemblage are basaltic pillow lava and pillow breccia with the trace element characteristics of a primitive island arc tholeiite suite whereas those in the upper assemblage are predominantly chnopyroxene phyric basalt to basaltic andesite and compositionally similar volcaniclastic rocks with the geochemical characteristics of a more evolved cale alkaline to shoshonitic volcanic arc suite the geochemical character of these volcanic rocks is indistinguishable from that of the coeval early mesozoic plutons leading to the conclusion that these plutons represent the intrusive roots of the volcanic succession collectively these rocks reflect construction of a late triassic to early jurassic island arc and indicate that subduction had initiated within or near the under lying fracture zone assemblage by at least the late triassic the latest stages of crustal disruption in the basement thus probably occurred in a subduction zone environment both cover sequence units also contain inter layered chert and argillite as well as coarse epiclastic rocks with an abundance of quartzose sedimentary and metasedimentary grains these elastic rocks provide the first evidence for terrigenous input into the rattle snake creek terrane and indicate that the early mesozoic arc developed in proximity to a terrigenous sediment source probably paleozoic rocks of the western north american cordillera several lines of evidence suggest that early mesozoic arc magmatism was accompanied by an episode of extensional tectonism these include structural evidence for an early mesozoic phase of high angle faulting that appears to have localized cover sequence deposition in fault bounded grabens sedimentologic evidence indicating that both the basement and terrigenous sediment sources experienced episodic pulses of uplift and erosion during cover sequence deposition and widespread intrusion of arc plutons into the upper crustal levels of the are carapace both the basement complex and the early mesozoic arc assemblage of the rattlesnake creek terrane were then deformed by thrust faulting folding and penetrative foliation development during over thrusting from the east by the western hayfork terrane in the middle jurassic regional de formation was in response to northeast south west directed shortening and was accompanied by metamorphism to subgreenschist grade insofar as the western hayfork terrane represents part of a middle jurassic arc whose plutonic roots intrude most klamath mountain terranes east of and including the rattle snake creek terrane it is clear that the rattlesnake creek terrane formed part of the klamath mountain province by middle jurassic time the paleogeographic and tectonic relation of the rattlesnake creek terrane to the rest of the klamath mountains prior to the middle jurassic however is unclear

topographic and soil differences from peridotite to serpentinite pedologists commonly lump peridotite and serpentinite soils together without reporting differences between them peridotite and serpentinite are chemically similar but mineralogically quite different a detailed soil survey in which soils with peridotite and serpentinite parent materials were separated in mapping revealed appreciable differences in geomorphic and pedologic features between these two types of ultramafic rocks in mountainous terrain of the klamath mountains california and oregon slopes tend to be steeper on peridotite and the soils redder more luvisols alfisols were found on peridotite and more phaeozems mollisols on serpentinite very shallow soils 7 of the area which are leptosols mostly mollisols and entisols few alfisols are more common on serpentinite sparsely vegetated barrens are commonly fragmental colluvium talus on peridotite and erodible slightly to moderately stony summits and sideslopes on serpentinite large landslides are much more extensive in serpentinite terrain vegetative cover differences from peridotite to serpentinite are much less pronounced than the topographic and soil differences the plant differences are obvious only on shallow soils

late jurassic magmatism metamorphism and deformation in the blue mountains province northeast oregon an early to mid mesozoic record of sedimentation magmatism and metamorphism is well developed in the blue mountains province of northeast oregon detailed studies both north and south of the blue mountains province e g terranes of the intermontane belt klamath mountains and western sierra nevada have documented a complex middle to late jurassic orogenic evolution however the timing of magmatic metamorphic and deformational events in the blue mountains and the significance of these events in relationship to other terranes in the western north american cordillera remain poorly understood in this study we investigate the structural magmatic and metamorphic histories of brittle to semibrittle deformation zones that indicate widespread late jurassic orogenesis in the blue mountains province folding and faulting associated with contractional deformation are primarily localized along terrane boundaries e g baker wallowa and baker izee olds ferry boundaries and within the composite baker oceanic melange terrane e g bourne greenhorn subterrane boundary these brittle to semibrittle deformation zones are broadly characterized by the development of e w oriented slaty to spaced cleavage in fine grained metasedimentary rocks of the baker terrane e g elkhorn ridge argillite approximately n s bivergent folding and n and s dipping reverse and thrust faulting on opposite flanks of the baker terrane similarly oriented contractional features are also present in late middle triassic to early late jurassic i e oxfordian stage ca 159 ma sedimentary rocks of the john day and huntington areas of northeast oregon radiometric age constraints from youngest detrital zircons in deformed sedimentary rocks and crystallization ages of postkinematic plutons which intrude the deformation zones limit deformation to between ca 159 and ca 154 ma we suggest that the widespread approximately n s directed contractional features in the blue mountains province record a short lived intense early late jurassic deformational event and preserve an example of upper crustal strain localization associated with terminal arc arc collision between the olds ferry and wallowa island arc terranes the age interval of deformation in the blue mountains province is younger than middle jurassic deformation in the canadian cordillera and klamath mountains siskiyou orogeny and predates classic nevadan orogenesis

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

contrasting early and late mesozoic petrotectonic evolution of northern california devonian middle jurassic terrane assemblies in the klamath mountains and sierra nevada foothills consist chiefly of ophiolite chert argillite sequences mafic ultramafic complexes are oceanic whereas associated fine grained deep water terrigenous sediments were derived mainly from adjacent previously docked klamath sierran terranes coeval calc alkaline arc rocks are volumetrically rare geologic and petrochemical relations suggest a rifted arc origin for klamath mafic metavolcanic units inter layered with distal turbidites in the 170 200 ma north fork terrane detrital zircon u pb ages indicate that the clastic debris had a regional eastern klamath source the eastern hayfork cherty melange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby eastern klamath antelope mountain quartzite the seaward 200 ma rattlesnake creek terrane is an ophiolitic melange with north fork petrotectonic affinities the north fork eastern hayfork rattlesnake creek amalgam correlates with the calaveras complex and the outboard jura triassic arc belt in the sierran foothills geochemical bulk rock and zircon u pb age data support interpretation of the 200 ma jura triassic arc as an adjacent offshore mafic belt overlying a 300 ma ophiolitic basement these oceanic complexes were sutured against the central metamorphic belt eastern klamath feather river northern sierra terrane backstop before deposition and deformation of the outboard upper jurassic galice and mariposa formations klamath sierran terrane assemblies reflect 230 m y of transpression transtension involving only minor episodes of subduction producing ubiquitous ophiolite chert argillite lithologies and rare felsic arc rocks in contrast the late jurassic to largely cretaceous klamath sierra nevada quartzo feldspathic volcanic plutonic arc attests to massive calc alkaline magmatism attending a strong eastward component of underflow by the farallon plate the coeval galice mariposa formations followed by the cretaceous great valley forearc and franciscan trench deposits are first cycle felsic debris shed mainly from the klamath sierran arc these units record 70 m y of rapid sialic crustal growth attending major periods of approximately margin normal convergence this profound transition in northern california included devonian middle jurassic rifting drifting and stranding of ophiolite chert argillite terranes along an adjacent curvilinear continental margin then nearly head on cretaceous subduction that resulted in massive calc alkaline igneous activity the erosion of which generated the felsic great valley group forearc basin and franciscan complex trench clastic sedimentary units

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

mechanical problems in geodynamics and work done in china the subject of geodynamics concerns the dynamics of the global motion of the earth of the motion in the earth s interior and its interaction with surface features together with mechanical processes in the deformation and rupture of geological structures a brief historical review is given from sir i newton maclauwin jacobi poincare poisson lame darwin rayleigh and love who dealt with the homogeneous sphere to leibenson takeuchi and meinesz concentrating on layered spheres and after the advent of plate tectonics on the use of numerical simulation to the analyses of tectonic features earthquake mechanism the application of nonlinear dynamics by discussing the forward and inverse mechanical problems and the questions facing the inverse problems of searching for the structural parameters driving forces etc are raised in more detail thereafter some works accomplished in china on the global and regional stress fields analyses tectonic features analyses mantle flow studies experimental studies of rocks and their constitutive relations are presented finally the interdisciplinary nature of the subject is emphasized and the main mechanical problems that need special attention are then proposed

statistical trends in the intraplate stress field the world stress map wsm database contains thousands of intraplate stress indicators with the potential to provide important constraint for arguments about the relationship between tectonic stresses and both the kinematics and dynamics of plate motion previous studies which relied almost exclusively on visual inspection of the data established the existence of broad regions of uniform maximum horizontal compressive stress orientation shmax and stress regimes on a regional scale in the present study we present a statistical analysis of the wsm stress indicators with the aim of quantifying trends in both the shmax orientations and stress regimes the analysis was carried out within 5 5 bins which provide a resolution of several hundred kilometers only the 4537 high quality wsm indicators with rating of a to c were used in the analysis we present results for two types of analysis on the information contained within the bins first we evaluate the spatial distribution of the average stress regime normal strike slip or thrust second we apply the rayleigh test a standard statistical method in the analysis of directional data to the distribution of shmax orientations to test the null hypothesis that the orientations are random an important aspect of our study is the quantification of the conclusions drawn from visual inspection of the world stress map our results indicate that broad regions of uniform shmax orientations exist in most continental regions at high confidence levels 90 and 95 and are less robust in the slowest moving continental plates we also quantify the predominance of strike slip and compressional stress regimes in continental regions importantly our analysis provides information about trends in the shmax orientations in regions where large amounts of scatter in the directional data prevented conclusions being drawn from visual inspection of the data for example in western north america and continental australia furthermore we find a strong correlation between average shmax orientations and both the ridge push torque and the absolute plate velocity azimuths our observation that a greater number of shmax orientations correlate with the ridge push torque directions is further evidence that the intraplate stress field is strongly influenced by the ridge push force

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

fe ti basalts and propagating rift tectonics in the josephine ophiolite the jurassic josephine ophiolite of northwest california and southwest oregon is one of the largest and most complete ophiolites in north america and clearly formed in a suprasubduction zone setting the geochemistry of lavas and dikes is highly variable because of a range in magma types and different degrees of fractionation basalts rich in fe and ti occur in the uppermost part of the extrusive sequence and as late dikes cutting oceanic serpentinite they have morb mid oceanic ridge basalt affinities and appear to be unrelated to the rest of the ophiolite that has affinities dominantly transitional between iat and morb iat island arc tholeiite this late fe ti morb suite was erupted while the ophiolite was undergoing the last half of 50 of tilting of the entire crustal sequence and after widespread serpentinization of ultramafic cumulates and uppermantle peridotite related to freezing of the axial magma chamber fe ti basalts are characteristic of propagating spreading centers on mid ocean ridges and in at least one backarc basin geochemical structural stratigraphic and regional geologic constraints suggest formation of the josephine ophiolite by propagation of a spreading center into rifted island arc lithosphere that is preserved along the margin of the ophiolite another possible indication of propagating rift tectonics is the presence of off axis metalliferous sedimentary rocks that may have been deposited as hydrothermal plume fall out from a second propagating spreading center the lau basin provides modern analogues including spreading centers propagating into rifted arc lithosphere into older backarc ocean crust and across the arc into the forearc

on the gravitational potential of the earth s lithosphere the mean potential energy of the lithosphere is useful for defining the tectonic reference state trs of the earth and can be used to constrain the ambient state of stress in the plates in the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the trs would remain undeformed thus the difference between the potential energy of a lithospheric column and the trs determines whether the column is in an extensional joeutral or compressional state of stress we evaluate and intraplate variations about this mean using a simple first order lithospheric density model this model assumed that the continental geotherm is linear and density variations below a depth of 125 km have negligible influence on and is consistent with observed geoid anomalies across continental margins is estimated to be 2 379 1014 n m 1 which is equivalent to the potential energy of both near sea level continental lithosphere 160 to 220 m for an assumed crustal density c in the range 2800 2700 kg m 3 and cooling oceanic lithosphere at a depth of 4 3 km with the exception of eurasia which has anomalously high mean potential energy 2 383 1014 n m 1 the mean potential energies of the continental plates are nearly identical to the global mean the mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere both the global and plate mean potential energies are relatively insensitive to a wide range in c the potential of the mid ocean ridges 2 391 1014 n m 1 is greater than the global mean which is consistent with the divergent nature of the ridges elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to suggesting that in the absence of external forces continental regions will be in a slightly extensional state of stress the importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the south american nazca indo australian and pacific plates

on the relation between transform fault resistance and plate motion a simple model for power dissipation due to friction along transform faults is used to examine the relations between relative velocity and power dissipation across transforms transform lengths and plate motions comparison is made between ridge transforms i e those that are part of spreading systems and major boundary transforms such as the san andreas fault power dissipation is calculated using velocity across the transform area above the 400 c oceanic or 350 c continental isotherm transform thermal structure is modeled by averaging the temperatures on either side of the transform whereas ridge transforms across which the relative plate velocity is slow can be of any length fast relative velocities only appear across short transforms when all transforms are used in this comparison a maximum speed as a function of transform length is suggested a minimum speed of 13 5 mm yr across ridge transforms was found in a case study of the pacific plate the transform lengths were found to have varied with time which has minimized the total power dissipation across the plate s transforms

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

sources of recent tectonic stress in the pannonian region inferences from finite element modelling summary we present the results of finite element modelling of the recent stress field in the pannonian basin and surrounding alpine orogenic belt our results show that the recent predominantly compressive stress regime in the alpine pannonian carpathian dinaric system is governed by distinct tectonic factors of great importance is the deformation of crustal blocks with diverent geometries and rigidities in an overall convergent setting associated with the africa europe collision the most important stress source appears to be the counterclockwise rotation of the adriatic microplate at the southwest boundary of the pannonian basin this plate tectonic unit has been interpreted as moving independently of both the european plate and the african plate additional boundary conditions active shortening and compression in the vrancea zone and the bohemian massif and the evect of the immobile moesian platform also significantly influence the modelling results the incorporation of additional stress sources such as crustal thickness variation and the presence of two main fault zones separating the primary tectonic units in the study area have only locally important evects but improve the fit between the calculated results and the observed stress pattern

transform push oblique subduction resistance and intraplate stress of the juan de fuca plate the juan de fuca plate is a small oceanic plate between the pacific and north america plates in the southernmost region referred to as the gorda deformation zone the maximum compressive stress 1 constrained by earthquake focal mechanisms is n s off oregon and possibly off washington nw trending left lateral faults cutting the juan de fuca plate indicate a 1 in a ne sw to e w direction the magnitude of differential stress increases from north to south this is inferred from the plastic yielding and distribution of earthquakes throughout the gorda deformation zone to understand how tectonic forces determine the stress field of the juan de fuca plate we have modeled the intraplate stress using both elastic and elastic perfectly plastic plane stress finite element models we conclude that the right lateral shear motion of the pacific and north america plates is primarily responsible for the stress pattern of the juan de fuca plate the most important roles are played by a compressional force normal to the mendocino transform fault a result of the northward push by the pacific plate and a horizontal resistance operating against the northward or margin parallel component of oblique subduction margin parallel subduction resistance results in large n s compression in the gorda deformation zone because the force is integrated over the full length of the cascadia subduction zone the mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike normal compressive stresses internal failure of the gorda deformation zone potentially places limits on the magnitude of the fault normal stresses being transmitted and correspondingly on the magnitude of strike parallel subduction resistance transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner

stress inversion and basement cover stress transmission across weak layers in the paris basin france we investigate the source of non purely gravitational horizontal stresses in the paris basin a nowadays tectonically quiet intracratonic basin in its eastern border of which outstandingly dense stress measurements are available based on a synthesis of published data the stress state in the basin is first shown to be very close to the one that may be extrapolated for the underlying basement in terms of principal stress orientations and horizontal to vertical stress ratios this is in favour of a mechanical coupling between the basement and its sedimentary cover which may seem contradictory to the presence of several weak rock layers in the basin fill e g an argillite layer that was shown to bear low deviatoric stresses and salt layers that are implicated in a major decollement elsewhere to unravel this apparent contradiction a 3d numerical modelling is performed following a rigorous inverse problem approach to determine the long term elastic properties of both the basement and the basin rocks the objective is to find the set of elastic constants that provides the best fit between the calculated stress state in the basin and the in situ data by assuming that the stress state in the basement is known this methodology provides a realistic set of mechanical parameters in agreement with previous studies which leads to the conclusion that the horizontal stresses in the basin constitute its mechanical response to the stresses that developed in the underlying basement during and since the last tectonic event alpine phase the fact that horizontal stresses could be transmitted across the weak horizons contrary to what may be expected at first glance is explained both by the geometry of the basin and the fact that over the long term the stiffnesses of the various sedimentary rocks are only slightly different from each other

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

north american plate dynamics deformation within the north american plate in response to various tectonic processes is modeled using an elastic finite element analysis a coarse grid contains 328 elements and 190 nodes while a fine grid contains 718 elements and 396 nodes for a spatial resolution of 2 3 abundant information about the present day state of intraplate stress constrains the modeling the dominant pattern that must be fit by all acceptable models is an ene trend for the maximum compressive stress for most of the plate east of the rocky mountains the tectonic processes considered in the modeling include ridge forces associated with the normal thermal evolution of oceanic lithosphere shear and normal stresses transmitted across transforms normal stresses transmitted across convergent boundaries stresses due to horizontal density contrasts within the continent and shear tractions applied along the base of the plate model stresses are calculated with respect to a lithostatic reference stress state distributed ridge forces of magnitude 2 1012 n m predict deviatoric stresses of the order of 20 40 mpa that are capable of accounting for the dominant observed ene stress trend driving drag models also fit the trend but are not preferred because of a predicted tenfold increase in compressive stress magnitudes from east to west across the plate assuming that ridge forces account for the dominant ene stress trend bounds may be placed on other possible forces for example shear stresses transmitted across transform boundaries along the san andreas and caribbean are small of the order of 5 10 mpa also compressive stresses of the order of 5 10 mpa transmitted across the major transforms improve the fit to the data compressive stresses across convergent margins along the aleutians and the middle america trench are important