paleomagnetic study of some cretaceous and tertiary sedimentary rocks of the klamath mountains province california paleomagnetic investigation of cretaceous outliers and tertiary sedimentary strata of the klamath mountains province and of onlapping cretaceous strata has shown the rocks to be largely remagnetized samples studied are from the upper jurassic to upper cretaceous great valley sequence upper cretaceous hornbrook formation eocene montgomery creek formation and oligocene weaverville formation cretaceous samples that survived the remagnetization have a mean remanence direction that is very close to the expected direction of the cretaceous magnetic field at the locality of the klamath mountains data from both primary and remagnetized samples suggest the possibility of 11 5 15 8 of post cretaceous clockwise rotation of the klamath mountains province none of the data from either the primary or remagnetized samples shows evidence of the large amounts 70 of clockwise rotation that other workers have measured for the lower tertiary of the oregon coast range our data indicate that the oregon coast range and klamath mountains province did not behave as a single rigid block during the early tertiary they also suggest that any post oligocene rotation of the klamath mountains province is less than the approximately 30 post oligocene rotation recently proposed for a combined oregon coast range klamath mountains cascade range block

neogene tectonics and kinematics of western north america recent quantitative studies of deformation in western north america including pacific and north american relative plate motion estimates of neogene displacement in the southern basin and range and anomalous paleomagnetic declinations of the pacific northwest are self consistent and together determine a simple kinematic model of the deformation that is an extension of the 1970 plate tectonic model of atwater a large continental lithospheric block called here the sierra nevada klamath block was displaced about 300 km to the w nw relative to the north american craton during the neogene the southern end of the overriding plate in the subduction zone along the pacific northwest has been shifted the same amount resulting in the subduction zone and overlying accretionary complex rotating clockwise about 30 the pivot is located near seattle and the several tens of kilometers of n s shortening required to accommodate the rotation is taken up in the major thrusts of the juan de fuca strait and olympic peninsula

stress strain paradox plate coupling and forearc seismicity at the cascadia and nankai subduction zones abstract in the continental forearcs of the cascadia and nankai subduction zones geodetic strain measurements indicate crustal contraction in the direction of plate convergence nearly margin normal but earthquake focal mechanisms and other stress indicators show maximum compression along strike margin parallel it is evident that the geodetic strain signals reflect the temporal fluctuations of elastic stress associated with subduction earthquake cycles not the absolute stresses the absolute margin normal stress is not only much less than the margin parallel stress but also no greater than the lithostatic value indicating a very weak subduction thrust fault weak subduction faults are also consistent with geothermal data which require very low frictional heating along the faults great subduction earthquakes occur at very low shear stresses and cause small perturbations to the forearc stress regime because these small elastic stress perturbations are relatively fast they give large strain rates that are detected by geodetic measurements the present nearly margin normal contraction in both places is due to the locking of the subduction fault and the consequent increase in elastic stress in the direction of plate convergence with the margin parallel compression being dominant the small increase in margin normal stress may change the forearc seismicity in cascadia from a mixture of thrust and strike slip types into mainly thrust before the next great subduction earthquake in nankai the change should be from strike slip to quiescence as observed in the nankai forearc prior to the 1944 1946 great subduction earthquakes

finite element study of uplift and strain across vancouver island geological evidence for sudden coastal subsidence along the west coast of southern vancouver island points to the occurrence of great prehistorical subduction earthquakes contemporary uplift and crustal shortening patterns in southern vancouver island appear to indicate that the subduction megathrust fault is currently locked to understand better the dynamics of the observed surface deformation we develop a finite element model of earthquake cycles for the northern cascadia subduction zone across southern vancouver island using a linear viscoelastic rheology the model consists of the continental and oceanic lithospheres the asthenospheric mantle with a viscosity of 5 1019 pa s and a low viscosity 1018 pa s mantle wedge between the subducted oceanic plate and the overlying continental plate the shallow geometry of the subducted juan de fuca plate is well defined by the results of various geophysical surveys and the deep geometry is constrained by the results of seismic tomography the model me

unroofing of active metamorphic core complexes in the d entrecasteaux islands papua new guinea metamorphic core complexes in the d9entrecasteaux islands are forming as a result of active extension at the western end of the woodlark basin spreading center high grade metamorphic rocks exist in the cores of gneissic domes including eclogites formed at minimum depths of 45 75 km i e 13 21 kbar high temperatures were maintained during uplift of basement rocks from 13 21 kbar t 730 900 c through 7 11 kbar t 570 730 c to 5 6 kbar t 730 c subsequent rapid cooling 100 c m y of the basement occurred between 1 and 2 ma on the basis of 40 ar 39 ar geochronology the core zone rocks were transected by kilometre scale amphibolite facies ductile shear zones active at depths of 25 35 km extensive intrusion of granodiorite plutons occurred and the age of granodiorite intrusions constrains late stage shear zone activity to 2 ma and younger unroofing was accomplished by faults and shear zones bounding the gneiss domes juxtaposing them against ultramafic rocks and sedimentary deposits surface uplift continues today and has led to topographic elevations of up to 2 5 km

refrigeration of the western cordilleran lithosphere during laramide shallow angle subduction the laramide orogeny has generally been attributed to a shift from normal angle to shallow angle subduction beneath the western margin of the north american plate in addition to important mechanical effects this shift may have had important thermal effects on the lithosphere in the western cordillera before the laramide geothermal gradients in the western cordillera were probably normal to high because of the presence below of a hot asthenospheric wedge with the shift to shallow subduction the wedge was probably expelled and replaced by a cold subducting slab that extended just below the cordilleran lithosphere this would shift the western cordillera into a cold forearc like thermal setting dominated by the refrigeration effects of the subducting slab thermochronologic data from the sierra nevada great basin mojave desert western arizona and perhaps the colorado plateau record latest cretaceous early tertiary cooling that may be evidence of such refrigeration if regional refrigeration occurred it would have several important implications 1 metamorphism in the western cordillera would have waned with the start of the laramide orogeny 2 the crust in the western cordillera would have strengthened and become much more resistant to deformation e g gravitational collapse and extension of thickened crust in the sevier hinterland would have been impeded and 3 latest cretaceous early tertiary isotopic cooling ages may not be valid indicators of a period of major regional uplift and unroofng

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

geochronological evidence for continuous exhumation through the ductile brittle transition along a crustal scale low angle normal fault simplon fault zone central alps 1 major low angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation field relationships alone cannot establish whether these different responses to deformation represent 1 parts of a single process of exhumation along the detachment or 2 two separate events with the later more discrete brittle detachment exhuming a fossil ductile shear zone from depth these two general models are critically assessed for the low angle normal simplon fault zone sfz in the central alps the sfz shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics the age of the ductile shear zone and ductile brittle transition is controversial we present a detailed geochronological study based on fission track 40ar 39ar and rb sr microsampling dating coupled with structural petrological and chemical analyses that provides tight constraints on sfz timing discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 ma on the basis of synkinematic white mica in low temperature shear zones and necks of foliation boudinage the brittle ductile transition in the footwall could be dated at 14 5 10 ma overall the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework over a period of 15 ma the sfz is therefore an example of a telescoped crustal section within a single major low angle fault involving a continuous period of exhumation rather than a two stage structure

miocene crustal extension and uplift in southeastern nevada constraints from fission track analysis the gold butte block of southeastern nevada appears to be a cross section through the upper approximately 17 km of colorado plateau crust tilted eastward and tectonically denuded via top to the west normal faulting in tertiary time the east side of the block exposes a steeply dipping cratonic paleozoic section 2 to 3 km thick proterozoic basement to the west is continuously exposed for an apparent thickness of approximately 15 km structurally below the basal cambrian unconformity apatite fission track dating and confirmed track length distributions have been used to determine the time of unroofing of the gold butte block and hence the timing of one episode of extension in this part of the basin and range province samples collected in as east west transect along the block indicate an abrupt transition from a period of relative thermal and tectonic stability to the onset of rapid cooling the fission track data indicate that the unroofing of the gold butte block was rapid and that the crustal section was brought to within a few kilometers of the surface at ca 15 ma

continental extension of the pacific plate at the southern termination of the hikurangi subduction zone the north mernoo fault zone offshore new zealand a complex transition from subduction at the hikurangi margin to transpressive continental collision across the southern alps occurs in the ne south island region of new zealand offshore the southern limit of oceanic subduction is constrained by the chatham rise a submerged continental plateau on the pacific plate which is presumably too buoyant for normal subduction in the nw corner of the rise is the north mernoo fault zone nmfz a 100 by 300 km region of basement involved active normal faulting of late miocene circa 8 6 ma to recent age the nmfz evolved contemporaneously with the rotation and development of the adjacent plate boundary zone growth faulting is characteristic within the sedimentary cover which is generally less than 2 km thick but the distribution of faulting has varied temporally the amount of mid pliocene recent extension is estimated to be not more than a few percent the nmfz consists of an array of overlapping south dipping normal faults which are typically 2 5 km apart and which trend roughly e w at a high angle to the plate boundary zone late quaternary surface traces are widely distributed on the mid to upper continental slope but many surface scarps are poorly preserved due to extensive erosion of the seafloor the western end of the fault zone crosses submarine canyons at the southern end of the hikurangi trough and almost abuts two fold and thrust fault systems on the ne south island continental margin although few data constrain the kinematics of extension a single mw 6 1 earth quake at the western end of the nmfz suggests planar rotating faults with a component of left lateral oblique slip two possible models of regional extension are considered 1 lateral buckling of the upper crust in which faulted slivers of crust are peeled off as the chatham rise slides past the transpressive marlborough fault system toward the southern alps and 2 flexure of the edge of continental pacific plate as the chatham rise is bent downward into the southern end of the hikurangi subduction zone improved seismological data and deep crustal seismic reflection profiles may help to constrain the kinematics of deformation

moment tensor inversion of explosive long period events recorded on arenal volcano costa rica constrained by synthetic tests in order to constrain the moment tensor solution of an explosive seismic event recorded on arenal volcano costa rica we perform tests using synthetic data these data are generated using a 3d model including the topography of the volcano and the best estimation of the velocity model available for arenal solutions for i the moment tensor components and ii the moment tensor plus single forces are analyzed when noisy data and mislocated sources are used in the inversion spurious single forces are easily generated in the solution for the moment tensor components plus single forces forces also appear when the inversion is performed using an explosive event recorded on arenal in 2005 synthetic tests indicate that these forces might be spurious however the mechanism is correctly retrieved by the inversion in both solutions the ability to recover the explosive mechanism for the 2005 event combined with the interpretative aids from the synthetics tests will enable us to invert for the large variation in events observed on arenal

subsurface geometry and evolution of the seattle fault zone and the seattle basin washington the seattle fault a large seismically active east west striking fault zone under seattle is the best studied fault within the tectonically active puget lowland in western washington yet its subsurface geometry and evolution are not well constrained we combine several analysis and modeling approaches to study the fault geometry and evolution including depth converted deep seismic reflection images p wave velocity field gravity data elastic modeling of shoreline uplift from a late holocene earthquake and kinematic fault restoration we propose that the seattle thrust or reverse fault is accompanied by a shallow antithetic reverse fault that emerges south of the main fault the wedge enclosed by the two faults is subject to an enhanced uplift as indicated by the boxcar shape of the shoreline uplift from the last major earthquake on the fault zone the seattle basin is interpreted as a flexural basin at the footwall of the seattle fault zone basin stratigraphy and the regional tectonic history lead us to suggest that the seattle fault zone initiated as a reverse fault during the middle miocene concurrently with changes in the regional stress field to absorb some of the north south shortening of the cascadia forearc kingston arch 30 km north of the seattle fault zone is interpreted as a more recent disruption arising within the basin probably due to the development of a blind reverse fault manuscript received 23 august 2001

shortening in the upper plate of the buckskin rawhide extensional detachment fault southwestern u s and implications for stress conditions during extension commonwealth post graduate research award u s geological survey national cooperative geologic mapping program under statemap g11ac20455

tertiary structure and thermal history of the harquahala and buckskin mountains west central arizona implications for denudation by a major detachment fault system the harquahala and buckskin mountains lie in the footwall of the whipple buckskin bullard detachment system in the harquahala mountains mesozoic fabric and structure are progressively more intensely overprinted by penetrative tertiary deformation toward the northeastern pan of the range tertiary mylonitic deformation is recognized by the presence of deformed miocene mafic dikes and characteristic textural features lineations in the mylonite trend 040 060 and megascopic kinematic indicators mostly indicate top to the northeast sense of shear the crest of an antiform defined by tertiary mylonitic fabric does not coincide with the topographic crest of the range reorientation of mesozoic foliation in the vicinity of this antiform suggests that it originated as two shear zones as opposed to a single zone that was then bent the northeastern part of the brown s canyon granite acts as a large low strain lozenge in the southeast limb of the foliation arch mylonitic foliation along the southeast side of the range above this lozenge is strongly oblique to the trend of the detachment fault but the relative orientation is consistent with left oblique normal shear evidence of tertiary plastic deformation is absent southwest of sunset pass minor low angle normal faults in the southwestern harquahala mountains and little harquahala mountains dip northeast the low angle normal faults are cut by nw trending high angle right oblique faults in the central and southwestern harquahala mountains 40ar 39ar age spectra from k feldspar muscovite and hornblende and total gas ages from biotite indicate late cretaceous to early tertiary cooling to argon closure temperatures biotite and k feldspar from the area northeast of sunset pass record rapid early to middle miocene cooling reconstruction of the original geometry of the detachment system based on thermal differences indicated by contrasting cooling histories and the orientation of an early miocene dike swarm indicate that the initial dip of the detachment fault was most probably between 30 and 40 thus the harquahala mountains are a lilted block exposing of the order of a 10 km section through the pre tertiary crust heterogeneous proterozoic gneiss sparse paleozoic and mesozoic metasedimentary rocks and an oligocene plutonic complex are extensively overprinted by tertiary mylonitic fabrics in the buckskin mountains hornblende 40ar 39ar cooling ages suggest that most of these rocks were below hornblende closure temperature by early tertiary time except in the vicinity of the oligocene plutonic complex feldspar and biotite 40ar 39ar cooling ages suggest that the footwall of the whipple detachment system experienced a more uniform cooling history in the buckskin mountains than in the harquahala mountains cooling ages between about 13 and 20 ma are recorded throughout the range with no consistent spatial pattern of ages the eagle eye detachment fault southeasternmost extension of the whipple buckskin bullard fault system becomes a transfer fault along the southeastern side of the harquahala mountains this fault separates two distinct extensional terranes on the northwest approximately 50 km of crustal extension has been accommodated on the whipple fault system while on the southeast a similar total magnitude of crustal extension is accommodated by a series of smaller normal faults in the adjacent big horn lake pleasant extended terrane as the deformation becomes more distributed maximum slip on any one normal fault decreases to the point where no single fault has enough slip to denude crustal levels at which tertiary plastic deformation has occurred

tillamook volcanic series further evidence for tectonic rotation of the oregon coast range paleomagnetic directions from the eocene tillamook volcanic series of the oregon coast range point 46 o clockwise from the expected eocene field direction potassium argon dating of six dikes and flows from this formation yields a mean age of 44 3 0 6 m y these results establish that the oregon coastal block of simpson and cox 1977 extends north to the oregon washington border and that this block has rotated clockwise 46 o 13 o during the past 44 m y the block has undergone no detectable north south translation a two phase model is developed to explain the tectonic history and anomalous magnetic field directions of the pacific northwest region phase i of the rotation active between 50 and 42 m y b p results from fragmentation of the farallon plate with rotation of a fragment during its accretion to north america phase ii of the rotation active between 20 m y b p and the present occurs in association with extension in the basin and range province

palaeoclimates in central and south america since 18 000 bp based on pollen and lake level records abstract the last 18 ka of palaeoclimates are discussed at 3 ka intervals based on palaeoenvironmental changes shown in pollen and lake level records from central and south america a total of 61 pollen records fulfill the requirements of continuity and chronology to be used in this synthesis 8 records from tropical and subtropical lowlands in central and south america 7 records from the central american highlands 26 records from the high andes north and south of the equator 2 records from the southern subtropics and 18 from the southern andes and subantarctic islands palaeoclimatic interpretation of the pollen records is semiquantitative based on bioclimatic parameters such as e g elevational temperature gradients or latitudinal precipitation gradients that can be related to vegetation zonation additional palaeoclimatic information is obtained from lake level records derived either from dated shorelines or from the limnological history of lake sediment sections large scale palaeoclimatic comparison of the proxy data records indicates that full and late glacial climates throughout central and south america were cooler than today by about 4 to 5 c but that moisture patterns showed regional differentiation thus in both the southern subtropics and high latitudes in southern chile conditions were substantially moister than today during full and late glacial times whereas everywhere else conditions were far drier than today in terms of palaeocirculation this scenario can be interpreted as a poleward shift of the subtropical as well as the westerly circulation a scenario that has been suggested by palaeoclimatic simulations from general circulation models gcm another major palaeoclimatic aspect predicted by the gcm simulations is the suppressed early holocene summer monsoon in the southern subtropics although proxy records are few from this zone their palaeoenvironmental history during that time shows a period of aridity that would support the model one of the most controversial aspects of south american palaeoclimates concerns the existence of a younger dryas equivalent a cold period between 11 and 10 ka bp based on the existing data from south america that show a great diversity of palaeoclimatic trends the concept of an interval that is characterized by the same climatic signal as documented for the north atlantic seems too simplistic moreover at this point only very few records have been analysed in sufficient detail to yield unambiguous climatic information for the interval in question the principal outcome of this palaeoenvironmental and palaeoclimatic synthesis is the better understanding of the type and quality of data available at present and the realization of regions and periods where much more and concerted effort is needed to address problems of intra and interhemispheric palaeoclimatic correlations only then can questions concerning the mechanism of climatic change be tackled successfully

40ar 39ar thermochronology and thermobarometry of metamorphism plutonism and tectonic denudation in the old woman mountains area california discrimination of individual tectonometamorphic events in polymetamorphosed terranes requires a comprehensive understanding of the relative timing and conditions of metamorphism and plutonism we have applied a combination of 40 ar 39 ar thermochronology petrology and thermobarometry to reconstruct the complex early proterozoic through early cenozoic tectonic and metamorphic evolution of continental crust in the old woman mountains area south eastern california strong mesozoic thermal events obscure the earlier history in much of the old woman mountains area in those areas where early proterozoic rocks underwent only lower greenschist facies metamorphism during the mesozoic thermobarometry of pelitic schists indicates that proterozoic metamorphism occurred at 9 to 11 kbar and 700 c 40 ar 39 ar ages of hornblende from samples of interbedded proterozoic amphibolite indicate that this high grade metamorphism took place before 1600 ma the relatively high pressure conditions of early proterozoic metamorphism in the old woman mountains area contrast with the low pressure granulite facies metamorphism that occurred elsewhere in the mojave desert at this time 40 ar 39 ar analyses of hornblende from proterozoic rocks within mesozoic shear zones and hornblende barometry from jurassic intrusive rocks suggest that tectonism and burial of paleozoic strata to 10 km began between 170 and 150 ma this tectonism resulted in regional greenschist facies metamorphism late stage mineral assemblages in proterozoic and paleozoic pelitic rocks in the old woman mountains area indicate an increase in metamorphic grade from greenschist to upper amphibolite facies toward late cretaceous plutons of the 73 ma old woman piute batholith metamorphic temperatures from garnet biotite thermometry increase from 650 c and 40 ar 39 ar ages of hornblende from proterozoic rocks decrease from 1600 to 73 ma toward these plutons barometric calculations from garnet bearing metamorphic rocks suggest that this cretaceous metamorphism took place at 3 5 to 5 0 kbar in the old woman mountains similar pressures are obtained from hornblende barometry of late cretaceous granodiorite 40 ar 39 ar and apatite fission track data from the 73 ma batholith indicate that the rocks exposed in the old woman mountains cooled rapidly to below 350 to 300 c by 70 1 ma and that moderately rapid cooling at rates of 10 to 30 c m y continued until after 60 ma the presence of maximum mesozoic temperatures on the borders of plutons hornblende ages of ca 73 ma and similar pressure estimates for plutons and country rocks indicate that the high temperatures needed to cause ductile deformation and amphibolite facies metamorphism were related to heating from the intrusion of the 73 ma old woman piute batholith amphibolite facies metamorphism along pluton contacts was superimposed on the regional greenschist facies metamorphism related to thrusting and crustal thickening the extremely rapid cooling in the old woman mountains between 73 and 71 ma was due largely to conduction of heat into the country rocks whereas protracted rapid cooling to temperatures below ambient temperatures estimated for the depth of emplacement was due to tectonic denudation at 1 to 2 km m y until about 68 to 65 ma

structural stratigraphic and petrologic aspects of the arenal chato volcanic system costa rica evolution of a young stratovolcanic complex geologic mapping on a scale of 1 10000 and detailed stratigraphic studies of lava flows and tephra deposits of the arenal chato volcanic system reveal a complex and cyclic volcanic history this cyclicity provides insight into the evolution of magma batches during the growth of the andesitic volcanic system the arenal and chato volcanoes have a central zone comprised of a lava armor and a distal zone comprised of a tephra apron during arenal s last two eruptive periods major craters formed near intersections of regional fractures at the lava armortephra apron transition we suggest that such intersections are potential sites for future major explosions the earliest rocks i e the chato lava flows range in composition from basaltic andesite to andesite these rocks except for the andesitic domes of chatito and la espina appear to have evolved from a common parental magma the last active period of chato volcano occurred 3550 b p the earliest known activity of arenal volcano is 2900 b p arenal lava flows have 54 56 wt sio2 and may be subdivided into a high alumina group hag al2o3 20 wt and a low alumina group lag al2o3 19 wt compared to the hag the lag also has smaller amounts of incompatible elements and higher amounts of feo and mgo arenal tephra deposits were emplaced by plinian sub plinian explosions occurring at 300 150 yr intervals these deposits are compositionally zoned and alternate between dacite and basalt the stratigraphy reveals an apparent magmatic cycle consisting of a dacitic andesitic tephra b hag lava flows c lag lava flows and d andesitic basaltic tephra this magmatic cycle is repeated four times during arenal s history and is interpreted to have developed by the crystal fractionation and crystal redistribution of a single magma batch the period of this cycle and consequently the life of a magma batch is about 800 years if the cyclic pattern continues a basaltic explosive phase may occur in the next 250 years

strain distribution and fabric development modeled in active and ancient transpressive zones 1 a model based on kinematics of transpression allows the measurable internal parameters simultaneous strain fabric and vertical elevation to be simulated in relation to the macroscopically determined external parameters of transpressive zones i e zone width velocity angle of convergence and zone depth we present a diagram of convergence angle against time for homogeneous transpression where isolines of strain intensity d strain symmetry k and vertical elevation of rock samples are superposed however the distribution of internal strain parameters is sensitive to three types of strain partitioning 1 discrete partitioning results in general decrease of finite strain accumulations and in increase of pure shear component 2 ductile partitioning splits the transpressional domain into a pure shear zone where strain accumulations decreases and in a wrench dominated zone where strain accumulations increases and 3 viscosity partitioning is marked by different strain rates in zones of different viscosity and therefore by different strain parameters

the 1968 andesitic lateral blast eruption at arenal volcano costa rica the magmatic andesitic eruption of arenal volcano on july 29 31 1968 after centuries of dormancy produced three new fissural craters a b and c on its western flank and a multilayered pyroclastic deposit emplaced by complex transport mechanisms the explosions were initially triggered by a volatile oversaturated 4 7 wt h2o magma several lines of evidences suggest a small blast surge where a wood rich pyroclastic deposit was emplaced as a ground layer followed by several units of coarse grained md between 0 65 and 5 40 tephra deposits lu lapilli units dau double ash units lu 1 2 3 dau 1 and 2 consist of unconsolidated and well to poorly sorted vesiculated bombs and lapilli of andesite some blocks ash and shredded wood the individual units are possibly correlated with the major explosions of july 29 the thickness of the deposits decreases with the distance from the volcano from 5 6 m to a few centimeters on average 90 of the components are juvenile 10 dense andesite and 90 vesicular these coarse grained beds were deposited in rapid succession by a complex transport process involving normal fallout strong ballistic trajectories with a lateral hot 400 c blast surge lu equivalent to a1 ballistic and coarse tephra sprayed in a narrow 85 area within about 5 5 km from the lowest crater and a high ca 10 km eruption column dispersed airfall fine lapilli ash 100 km from the volcano ashcloud forming explosions producing thin pyroclastic surge and muddy phreatomagmatic fallout deposits flau equivalent to a2 and a3 closed the blast surge sequence the successive explosions on july 30 31 mainly produced block and ash flows and widely dispersed ash fall the total volume of pyroclastic material is calculated as 25 8 5 5 10 6 m 3 9 4 2 0 10 6 m 3 dre a model is proposed to explain the peculiarities of the formation transportation and emplacement of the blast deposits the intrusion of the presumed andesitic cryptodome possibly happened through an active thrust fault favoring not only the formation of the lowest crater a but also the low angle explosive events prior to the eruption several minerals were settling to the bottom of the magma chamber as is suggested by the increase of incompatible elements towards the bottom of the stratigraphic section the major elements indicate that some crystal redistribution occurred and the maximum concentration of al2o3 and eu and sr support plagioclase enrichment in early phases of the eruption top of lu 1 and dau 1 from the about 20 recognized prehistoric and historic blast deposits in the world approximately half were produced by sector collapse of the volcano and the other half by sudden decompression of cryptodomes or lava dome collapses the recent blasts 1888 1990s elsewhere have an apparent recurrence of one event decade compared to just a

the relative contribution of accretion shear and extension to cenozoic tectonic rotation in the pacific northwest large cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the pacific northwest and many tectonic models have been proposed to explain them including 1 rotation of accreted oceanic microplates during docking 2 dextral shear between north america and northward moving oceanic plates to the west and 3 microplate rotation in front of an expanding basin and range province stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations coast range structures basin and range extension and paleomagnetic data from middle miocene 15 ma coast range rocks indicate that dextral shear is responsible for at least 40 of the post 15 ma rotation of the coast range and that basin and range extension is responsible for the remainder reconstructions based on extrapolation of this ratio back to 37 and 50 ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has been a significant contributor to rotation during most of tertiary time changes in the dextral shear rotation rate over the past 50 m y correlate directly with changes in the velocity of the farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost basin and range region and suggest 17 extension since 15 ma 39 since 37 ma and 72 since 50 ma near latitude 42 n

the 40ar 39ar thermochronology of the eastern mojave desert california and adjacent western arizona with implications for the evolution of metamorphic core complexes mesozoic thickening and cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern mojave desert southeastern california and western arizona the application of 40ar 39ar thermochronology to rocks in this region provides information about the timing and nature of thrusting plutonism metamorphism denudation and detachment faulting the 40ar 39ar ages of 175 to 125 ma from the clipper piute turtle mohave bill williams and hualapai mountains are interpreted to be the result of a middle mesozoic thermal event s caused by crustal thickening the 40ar 39ar data from the clipper and piute mountains suggest that this thermal event was followed by a period of cooling at rates of 1 5 c m y orogenesis culminated during the late cretaceous when rocks exposed in the old woman piute chemehuevi and sacramento mountains attained temperatures 500 c which reset the k ar systems of minerals from proterozoic rocks high grade metamorphism in the old woman mountains area was caused by the intrusion of the old woman piute batholith at 73 1 ma cooling rates following batholith emplacement in the old woman mountains were 100 c m y between 73 and 70 ma and 5 10 c m y from 70 to 30 ma between 65 and 25 ma the entire eastern mojave desert underwent a period of cooling at a rate of 2 10 c m y by 30 ma rocks exposed in the old woman piute marble ship clipper and turtle mountains were below 100 c the 40ar 39ar ages from the sacramento mountains suggest that mylonitization caused by the onset of regional extension occurred at 23 1 ma when extension started in the chemehuevi mountains rocks exposed in the southwestern and northeastern portions of footwall to the chemehuevi detachment fault were at 180 c and 350 c respectively this suggests that the exposed part of the chemehuevi detachment fault initiated at a dip of 5 30 or as a series of higher angle faults that cut to a depth of 10 12 km and were later rotated to their present dip unroofing of the footwalls to detachment faults in the sacramento and chemehuevi mountains resulted in average cooling rates of 10 50 c m y between 22 and 15 ma

current tectonics of northern cascadia from a decade of gps measurements 1 global positioning system gps measurements to study regional deformation were initiated in northern cascadia in the late 1980s and early 1990s on the basis of a decade of gps data we derive a crustal velocity field for nw washington sw british columbia the permanent and campaign gps velocities are defined with respect to north america in the itrf2000 reference frame velocity uncertainties are estimated using a model of time series noise spectra this new velocity field is the basis for interpretation of the tectonics of the northern cascadia subduction system gps velocities are interpreted in terms of interseismic loading of the megathrust using different coupling models our data confirm that the upper part of the megathrust is nearly fully locked an exponential model for the downdip transition zone gives slightly better agreement with the data compared to the common linear transition the landward decrease of forearc strain loading is smaller than predicted by any of the current subduction interseismic models this could be a consequence of a small 0 3 mm yr long term motion of the southern vancouver island forearc with respect to north america or of a concentration of interseismic strain across the elastically weaker cascadia volcanic arc in northern vancouver island our velocity field supports the existence of an independent explorer microplate currently underthrusting underneath north america at least up to brooks peninsula further north gps velocities indicate transient and or permanent deformation of northernmost vancouver island related to the interaction with the explorer microplate and possibly with the queen charlotte transform margin

forces acting on the sierra nevada block and implications for the strength of the san andreas fault system and the dynamics of continental deformation in the western united states 1 we present an analysis of the forces acting on the sierra nevada block in the western united states vertically integrated loads acting on the block are quantified through numerical modeling of the state of stress in the interior of the block constrained by stress observations the sierra nevada block is located adjacent to the pacific north american plate boundary allowing determination of 1 the vertical distribution of shear stress at the plate boundary and the strength of the san andreas fault system 2 the horizontal distribution of shear stress coupling the pacific and north american plates and 3 the magnitude of stresses transmitted between rigid crustal blocks we find that the san andreas fault system supports a relatively large vertically integrated shear stress of 2 4 0 4 tn m to the northwest of san francisco and only 0 3 tn m to the southeast the majority of this load is likely supported in the frictional portion of the crust and therefore the san andreas fault system is both frictionally strong and weak shear stress coupling the pacific and north american plates diminishes away from the plate boundary from 1 5 tn m on the western margin of the sierra nevada block to 1 1 tn m on the eastern side a south directed load of 2 4 tn m on the northern end of the block balances this reduction in shear stress across the sierra nevada block and indicates that the sierra nevada block is compressing the rigid cascade fore arc from the south contributing to the forces causing n s compression of the pacific northwest

multistage exhumation of the anjiayingzi gold deposit northern north china block geodynamic settings and exploration implications abstract post mineralisation exhumation is pivotal because of its strong effects on the preservation of ore deposit the jiguanzi quartz monzonite hosts the anjiayingzi au deposit at the centre of the kalaqin antiform or kalaqin metamorphic core complex in the northern north china block ncb we can link the exhumation of the deposit with regionally geodynamic events and evaluate its exploration implications using low temperature thermochronology from the quartz monzonite host sericite 40ar 39ar dating of auriferous quartz sulphide veins indicates that gold mineralisation timing is 130 1 ma zircon fission track ages of fresh samples from the host quartz monzonite vary from 118 6 to 99 4 ma and apatite fission track ages vary from 99 6 to 66 5 ma a four stage post mineralisation exhumation history is revealed by combining the binomial peak fitting of single grain ages and low temperature thermal history inverse modelling the first exhumation event is dated at ca 130 100 ma and characterised by the most rapid exhumation rate of 167 m ma and ca 5 km rise this movement is related to the top to nw and a se ductile brittle extensional deformation of the kalaqin antiform regionally it is also contemporaneous with the formation of extensional metamorphic core complexes development of basins and increasing heat flow and decreasing thermal lithospheric thickness in the eastern ncb all of these synchronous events can be linked to the back arc setting of the subducting izanagi oceanic plate and resulted geodynamic changes beneath the ncb the relatively slow stage 2 exhumation dated at ca 100 65 ma with an exhumation rate of 20 m ma was triggered by the collision between the izanagi oceanic plate and eastern china at ca 100 ma and following change in the drift direction of the izanagi oceanic plate sedimentation of basins in the ncb was continuous during this period resumed stage 3 exhumation ca 65 45 ma 27 m ma and synchronous sedimentary inversion are products of regional compression event related to the collision between india and the eurasian plates change of the pacific oceanic plate drifting direction at ca 50 ma finally induced the monotonic cooling and exhumation of stage 4 19 m ma the estimated post mineralisation exhumation is 7 1 km and the ore forming depth of the anjiayingzi au deposit is between 5 1 and 10 1 km in combination with previously published data the differential exhumation history exhumation degree and preservation potential of major gold provinces in ncb are confirmed which further indicate that their gold prospectivity must be assessed individually

denudation of metamorphic core complexes and the reconstruction of the transition zone west central arizona constraints from apatite fission track thermochronology apatite fission track thermochronologic results from transects across the basin and range and transition zone provinces in west central arizona provide constraints on the denudational history and structural framework of the region apatite fission track ages decrease from 21 to 14 ma in the harcuvar mountains and from 16 to 13 ma in the buckskin rawhide mountains in the slip direction sw ne of detachment faults in the lower plates of the metamorphic core complexes mean lengths of confined fission tracks from the core complexes are all 14 m indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone 40 c m y for lower plate rocks apparent apatite ages in the transition zone province generally increase from 25 ma to 100 ma away from the basin and range province this trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the weaver mountains and the colorado plateau gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the phillips kirkland drill hole reveal parts of denuded mesozoic and cenozoic apatite partial annealing zones these paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks the reconstructions indicate that the major faults in the transition zone province have relative displacements of 1 km and that offset on them occurred mostly after 25 ma these data also indicate that most of the rocks now exposed in the transition zone of west central arizona were not exposed until miocene time

helium and argon thermochronometry of the gold butte block south virgin mountains nevada one of the largest exposures of precambrian crystalline rock in the basin and range province of the southwestern usa is the gold butte block of the south virgin mountains about 15 km west of the colorado plateau it has been interpreted as a largely continuous crustal cross section about 15 20 km thick that was exhumed by a deeply penetrating normal fault during miocene extension to test this interpretation as well as the use of the newly developed titanite u th he thermochronometer we examined the low temperature thermal history of the gold butte block with the apatite and titanite u th he and muscovite 40 ar 39 ar thermochronometers apatite he ages average 15 2 1 0 2 ma throughout the block indicating that the entire section was warmer than 70 c prior to miocene exhumation titanite he ages increase from 18 6 1 5 ma near the paleobottom west end of the block to 195 15 ma near the paleotop east end a rapid change from mid tertiary to increasingly older titanite he ages to the east is observed at about 9 3 km paleodepth and is interpreted as a fossil he partial retention zone for titanite assuming a pre exhumation geotherm of 20 c km consistent with earlier apatite fission track work this depth would have corresponded to 196 c prior to exhumation indicating that laboratory derived he diffusion characteristics for titanite that yield a closure temperature of about 200 c are applicable and correct muscovite 40 ar 39 ar ages are 1 0 1 4 ga near the paleotop of the block and 90 ma near the paleobottom together with 207 pb 206 pb ages on apatite and titanite and an earlier apatite fission track transect across the gold butte block our data indicate that the continental crust at the western edge of the colorado plateau resided at moderate geothermal gradients and slowly declined in temperature from 1 4 ga to about 100 200 ma a 90 ma cooling event clearly affected the mid crust deepest portions of gold butte which may reflect accelerated cooling or a brief heating and cooling cycle at this time after which gradients returned to about 20 c km prior to rapid exhumation in the miocene this work thus supports previous structural and thermochronologic studies that suggest that the gold butte block is the thickest largely continuous cross section of crust exposed in the southwestern usa

the extensional messaria shear zone and associated brittle detachment faults aegean sea greece structural thermochronological and metamorphic data are used to elucidate the tectonic nature and evolution of the ductile extensional messaria shear zone and the associated brittle messaria and fanari detachment faults which exhumed their footwall from mid crustal depths on the island of ikaria in the aegean thermobarometric data indicate that the messaria shear zone formed at 350 400 c and 3 4 kbar i e at a depth of c 15 km normal faulting was accompanied by the intrusion of two granites which together with the thermobarometric data indicate a relatively high thermal field gradient of 25 35 c km 1 zircon and apatite fission track and apatite u th he ages demonstrate rapid cooling in the footwall of the messaria detachment from c 400 c to c 40 c between 11 and 3 ma age distance relationships of the data suggest that the messaria shear zone and the messaria detachment slipped at apparent rates of c 6 9 km ma 1 kinematic indicators show a consistent top to the nne shear sense for the extensional faults however at the southern part of the messaria detachment some late stage shear sense indicators are top to the ssw and are assumed to be associated with updoming of the footwall numerous deformed pegmatite veins in the messaria shear zone allow the reconstruction of deformation and flow parameters the mean kinematic vorticity number ranges from 0 13 to 0 80 indicating that shearing deviated significantly from simple shear that is extensional shearing was associated with vertical ductile thinning which contributed to tectonic exhumation finite strain shows oblate geometries and axial ratios of the finite strain ellipse in sections parallel to tectonic transport and normal to the mylonitic foliation range from 1 8 to 19 9 we calculate using a 1d numerical model that vertical ductile thinning contributed c 20 to exhumation during extensional shearing normal faulting was the major agent exhuming the footwall from c 15 km depth

k metasomatism and detachment related mineralization harcuvar mountains arizona the bullard detachment fault a regional low angle normal fault exposed in the harcuvar mountains of west central arizona separates lower plate mylonitic rocks and chloritic breccia from upper plate volcanic and sedimentary rocks areally extensive k metasomatism has converted upper plate mafic flows and felsic ash flow tuffs into rocks with 8 to 12 wt k 2 o 2 o and a simple k feldspar hematite quartz mineralogy the secondary k feldspar is very pure or 95 to 99 5 monoclinic and structurally similar to orthoclase differences in 18 o values between secondary k feldspar replacing sanidine phenocrysts 9 to 11 and k feldspar replacing groundmass 11 to 14 in the tuffs imply differential o exchange with migrating fluids whole rock 18 o values for tuffs 10 to 14 and mafic flows 6 to 9 do not therefore represent primary igneous values the rocks apparently became k metasomatized and 18 o enriched while interacting with low to moderate temperature neutral to alkaline oxidizing brines that accumulated in an extensional basin above the detachment fault cu au ag mineralization is concentrated in faults and fissures in metasomatized mafic flows above the detachment fault fluid inclusion studies show that mineralizing fluids had minimum temperatures of 290 to 330 c along the detachment fault 240 to 290 c in mafic flows above the detachment fault and 100 to 130 c in barite calcite mn oxide veins farther from the detachment fault the predominant mineralizing fluids near the detachment fault and in the mafic flows were saline brines with 13 to 17 equivalent wt naci more dilute brines with 6 to 12 equivalent wt naci formed the barite calcite mn oxide veins inferred 18 o values of the mineralizing fluids range from 3 for high temperature quartz sulfide veins to 5 for lower temperature barite calcite mn oxide veins the high salinities oxygen isotope compositions and geologic setting indicate that the mineralizing fluids were basinal brines the mineralizing fluids apparently evolved from early deep level reducing basin brines to a later stage marked by the influx of higher level oxidizing basin brines relatively minor amounts of less evolved lower 18 o meteoric water entered the system during the very late stages of mineralization paragenetic relations and geochemical and isotopic data indicate that mineralization was superimposed on previously k metasomatized rocks mineralization and k metasomatism may be indirectly linked however because both occurred during detachment faulting and both involved basinal brines specifically k metasomatism of upper plate units liberated elements such as cu pb zn mn sr and na that were incorporated into the mineralizing basin brines multiple fluid regimes during detachment faulting are indicated because basin brine dominated mineralization overprinted lower plate mylonitic rocks and breccia that had probably previously equilibrated with igneous or metamorphic fluids at deeper levels of the detachment system

paleomagnetism of the tertiary clarno formation of central oregon and its significance for the tectonic history of the pacific northwest the clarno formation a mostly eocene and partly early oligocene sequence of andesitic lavas and volcaniclastic rocks is the oldest tertiary formation exposed in north central oregon remanent magnetization directions at 46 sites in the lavas provide a paleomagnetic pole at 84 n 278 e with a 95 confidence cone of 7 comparison of this pole with the north american reference pole for eocene time indicates that the clarno formation has rotated 16 clockwise with an uncertainty of 10 but has undergone no significant latitudinal displacement further comparison with paleomagnetic data from the miocene columbia river basalt group and steens basalt shows that large scale block rotations in the eastern columbia embayment ceased sometime between 38 m y and 15 m y a paleogeographic reconstruction for 38 m y is offered which is consistent with the observed rotation of the clarno and with the other paleomagnetic data from the pacific northwest which are briefly reviewed this reconstruction shows that it is possible to account for virtually all of the paleomagnetically indicated rotations in pre miocene tertiary rocks of the pacific northwest by an extensional tectonic model as a consequence only part of the rotations in the pre tertiary rocks of the eastern columbia embayment need to be the result of accretionary tectonics the initiation of magmatism in the eastern columbia embayment that is represented by the clarno formation itself may have been contemporaneous with the beginning of crustal extension in the pacific northwest

tertiary extension in the old woman mountains area california evidence from apatite fission track analysis apatite fission track ages and confined track length distributions from the old woman mountains area southeastern california record evidence for periods of rapid cooling during the late cretaceous following mesozoic orogenesis and during miocene crustal extension apatites from the clipper marble and turtle mountains give fission track ages between 62 and 69 ma as a result of cooling of these upper crustal rocks after regional late cretaceous plutonism and metamorphism in the piute mountains apatite fission track ages decrease from 70 ma with mean track lengths of 13 5 m to 14 m from west to east intermediate aged samples from the piute mountains have mean track lengths between 11 and 13 m due to their extended residence in the apatite annealing zone during the early tertiary this pattern is the result of 20 30 of westward tilting of the piute mountains block as indicated by the dip of tertiary strata the apatite data indicate that tilting occurred after 18 ma and caused rapid cooling of rocks exposed on the eastern side of the mountain range knowing the amount of tilting and the apparent paleotemperatures across the piute mountains allows us to estimate that the synextensional geothermal gradient was 50 20 c km apatites from the northeastern old woman mountains yield fission track ages of 68 ma with mean track lengths of 13 8 m these samples record cooling after batholith emplacement at 73 ma in the northwestern old woman mountains apatite fission track ages decrease to 20 ma and have mean track lengths 15 m indicating very rapid cooling we interpret this age pattern to be the result of minor eastward tilting of the old woman mountains block after 18 19 ma or flexural unroofing of the northwestern old woman mountains due to movement on the ship mountains fault extension in the old woman mountains area occurred between 19 and 16 ma which is coincident with extension in the colorado river extensional corridor and later than major extension in the mojave extensional belt

relationship between mesozoic and cenozoic tectonic features in west central arizona and adjacent southeastern california the maria fold and thrust belt mftb is a narrow belt of mesozoic crustal shortening that trends east west across west central arizona and adjacent southeastern california it is characterized by generally south vergent folds and thrusts that commonly displace proterozoic crystalline rocks over deformed and metamorphosed paleozoic and mesozoic strata the mftb is cut by a south to southeast trending belt of mid tertiary extensional deformation extension was characterized by large displacements on low angle normal faults known as detachment faults and by isostatic uplift of mylonitic midcrustal rocks now exposed in metamorphic core complexes the geometry and style of extensional deformation change along the extensional belt and reveal the influence of the mftb several extensional features are spatially coincident with the root zone of mftb thrusts 1 an areally extensive west northwest trending belt of denuded tertiary mylonitic fabrics in the whipple and harcuvar metamorphic core complexes 2 a style of extension characterized by minimum extensional dismemberment of the upper plate and maximum denudation and uplift of deep seated lower plate rocks 3 an abrupt bend in the belt of arched uplifted rocks below detachment faults and 4 an abrupt bend in the trend of the breakaway zone of the detachment faults and an associated lateral ramp in the detachment fault system the first two features may be the result of isostatic uplift of a crustal root downward protruding moho bulge inherited from crustal thickening within the mftb and the other two features may reflect at the time of detachment fault initiation the influence of stresses in the crust associated with flexural support of the buoyant mesozoic crustal root

neogene rotations and quasicontinuous deformation of the pacific northwest continental margin paleomagnetically determined rotations about vertical axes of 15 to 12 ma flows of the miocene columbia river basalt group of oregon and washington decrease smoothly with distance from the plate margin consistent with a simple physical model for continental deformation that assumes the lithosphere behaves as a thin layer of fluid the average rate of northward translation of the continental margin since 15 ma calculated from the rotations using this model is about 15 mm year which suggests that much of the tangential motion between the juan de fuca and north american plates since middle miocene time has been taken up by deformation of north america the fluid like character of the large scale deformation implies that the brittle upper crust follows the motions of the deeper parts of the lithosphere

paleomagnetic results from the central sierra nevada constraints on reconstructions of the western united states paleomagnetic results from three plutons of the central sierra nevada limit the motion of the sierra nevada relative to the north american craton to 5 8 of poleward motion and 7 11 of clockwise rotation since about 90 ma the indicated rotation and translation are not significant at the 95 confidence level a new tectonic model is presented that is consistent with all available paleomagnetic data from the sierra nevada great valley klamaths oregon coast range cascades and western nevada the paleogeographic reconstructions of this model associate the displacement of major crustal blocks with basin and range extension starting at about 30 ma this model accounts for about 15 of clockwise rotation of the klamaths and southern cascades about 2 of clockwise rotation of the sierra nevada and great valley and 26 of clockwise rotation of the oregon coast range since 30 ma the model is consistent with 40 of clockwise rotation of the oregon coast range before 30 ma as proposed by magill and cox 1980 the amount of basin and range extension in this model at latitude 41 n is 225 km directed e w and at latitude 35 n is 245 km directed wsw ene

a reinterpretation of the timing position and significance of part of the sacramento mountains detachment fault southeastern california a contact previously considered to be part of the sacramento mountains detachment fault sdf exposed in the sacramento mountains metamorphic core complex is reinterpreted as an unconformity between tertiary rhyolite of eagle peak and cataclastically deformed crystalline lower plate rocks this reinterpretation is based on outcrop scale topographic relief and the absence of deformation along the base of the rhyolite even where underlying rocks are severely deformed and altered the rhyolite is dated at about 14 3 ma lower plate rocks of the sdf comprise gneiss intruded by granodiorite tonalite and leucogranite mylonitic fabrics are variably developed in lower plate rocks and are cut by brittle shear zones that contain ultracataclasite and by high and low angle faults upper plate rocks include gneiss nonmylonitic granodiorite tonalite and granite and middle miocene sedimentary and volcanic rocks on the south side of eagle peak a thick ultracataclasite zone dips about 30 west contains subhorizontal hematite stained slickenlines and is interpreted to be part of the sdf system this ultracataclasite zone is unconformably overlain by more gently dipping undeformed 14 3 ma rhyolite of eagle peak the unconformity records unroofing of a domed sdf after a period of 8 to 9 m y of progressive unloading of the lower plate in an evolving extensional shear zone extension on part of the sdf was accommodated by a system of high angle oblique slip faults

implications of low temperature cooling history on a transect across the colorado plateau basin and range boundary west central arizona fission track ages of apatite and zircon from metamorphic plutonic and sedimentary rocks along a 80 km transect across the colorado plateau basin and range boundary in west central arizona show differences in the low temperature cooling histories between the provinces the transect extends from cypress mountain in the colorado plateau transition zone to the eastern buckskin mountains in the basin and range along the northeast margin of the basin and range province metamorphic rocks exposed in the footwall of a major detachment fault system yield zircon and apatite fission track ages of 16 10 ma these ages are similar to k ar fusion ages of biotite and age minima of k feldspar on 40ar 39ar age spectra and collectively indicate rapid cooling one k feldspar age spectrum has an age maximum of about 22 8 ma an age minimum of 11 8 ma and a spectrum whose shape is suggestive of reheating possibly in middle miocene time the heating event was probably related to hydrothermal activity during emplacement of cu and mn deposits in and above the detachment fault zone effects of this heating are only locally detected in rocks above the detachment fault in the poachie range fission track ages of apatite and zircon are 60 50 and 80 70 ma respectively the disparity between the apatite and zircon ages indicates that the rocks cooled slowly in late cretaceous and early tertiary time probably due to gradual uplift and erosion total uplift and denudation in the area of the poachie range since cretaceous time is 6 km or more to the northeast of the range fission track ages of apatite and zircon increase and diverge indicating that apparent uplift decreased in that direction the apatite ages from the poachie range are concordant with early tertiary hornblende ages determined in other studies of lower plate rocks near the southwest end of the transect the ages represent cooling of crystalline rocks after cretaceous regional metamorphism and magmatism near bagdad 20 km northeast of the poachie range 2 km or less of erosion has occurred since intrusion of high level plutons and dikes and caldera formation in late cretaceous time remnants of an erosion surface that developed in middle tertiary time are preserved in the transition zone volcanic and sedimentary rocks at least as old as early miocene were deposited on the erosion surface and filled valleys cut into it dissection of these deposits began about 8 ma we interpret these data combined with those from other studies to indicate that in cretaceous time southward thrusting and later extensive magmatism in the middle crust led to thickening and heating of the crust the cretaceous igneous rocks at bagdad are high level manifestations of this magmatism uplift and slow cooling occurred in late cretaceous and early tertiary time in late oligocene and early miocene time during northeast southwest extension middle crustal rocks moved southwest put from beneath the southwest margin of the transition zone tectonic denudation rapidly exposed the crust that had been brought up from a depth of 10 km or more and rapidly cooled in the eastern buckskin and harcuvar mountains middle miocene reheating occurred locally in the lower plate along the detachment and in nearby parts of the upper plate

intrusion of horizontal dikes tectonic significance of middle proterozoic diabase sheets widespread in the upper crust of the southwestern united states initially horizontal sheet intrusions of middle proterozoic diabase are abundant in a region 650 by 300 km across in arizona and california the diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences massive granite is the most common basement host for the sheets probably because it fractured more easily than foliated hosts during sheet emplacement steep feeder dikes are rare compared to the sheets the diabase in many places is exposed in fault blocks that were tilted during tertiary tectonic extension structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust to depths of at least 13 km sheet intrusion implies a vertical orientation of the least compressive stress so i conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1 1 ga magma overpressures water encountered by rising magma vertical changes in the crustal stress regime and flotation of low density granite all may be important factors for sheet intrusion the stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event the diabase province contrasts structurally with similar age provinces of basaltic magmatism elsewhere in north america that show evidence of tectonic extension such as the midcontinent rift the consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks sheet intrusions in the geologic record may be seriously underreported their recognition is important for the interpretation of seismic reflection profiles of continental crust sheets in arizona may be responsible for the bagdad reflection sequence which extends to depths of at least 15 km

volcanoes in the pre columbian life legend and archaeology of costa rica central america costa rica is located geographically in the southern part of the central american volcanic front a zone where interaction between the mesoamerican and south american cultures occurred in pre columbian times several volcanoes violently erupted during the holocene when the first nomadic human hunters and later settlers were present volcanic rocks were the most important geo resource in making artifacts and as construction materials for pre columbian inhabitants some pottery products are believed to resemble smoking volcanoes and the settlements around volcanoes would seem to indicate their influence on daily life undoubtedly volcanic eruptions disrupted the life of early settlers particularly in the vicinity of arenal and irazu volcanoes where archaeological remains show transient effects and displacement caused by periodical eruptions but later resilient occupations around the volcanoes most native languages are extinct with the exception of those presently spoken in areas far away from active volcanoes where no words are related to volcanic phenomena or structures the preserved legends are ambiguous suggesting that they were either produced during the early spanish conquest or were altered following the pre columbian period

late stage evolution of the chemehuevi and sacramento detachment faults from apatite u th he thermochronometry evidence for mid miocene accelerated slip the unique low temperature sensitivity of the apatite u th he ahe technique provides valuable new information relating to the late stage thermal evolution of metamorphic core complexes in the southern basin and range province in all the ranges studied the data show greater complexity than previously seen from other thermochronometers in the chemehuevi and northern sacramento mountains a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall this is followed by essentially age invariant segments of 15 1 ma reflecting an increase in the rate of detachment fault movement the timing of the change suggested by the data agrees with recently published ahe results from the nearby harcuvar mountains located 125 km to the southeast suggesting that an underlying regional mechanism triggered this change plate reconstructions indicate that the pioneer mendocino fracture zone migrated northward through the area at this time leaving a slab window in its wake we suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this combined with the extensional strain regime present resulted in an increased rate of slip within the actively extending metamorphic core complexes this agrees well with timing constraints for extension onset in the central basin and range province of southern nevada and suggests that weakening of the southern basin and range province at 15 1 ma was partially responsible for the onset of extension in the central basin and range these findings demonstrate that improved constraints on changes in extension rate within the basin and range province should be possible through the application of the ahe technique to other metamorphic core complex footwalls

eruptive history of arenal volcano costa rica 7 ka to present abstract new tephra stratigraphic studies of arenal volcano have been used to update its volcanic history deposits of major eruptions are renamed ar 1 to ar 22 from older to younger in stratigraphic order extending from 7 ka b p to 1968 a d arenal tephras overlie regional tuffs that are 20 000 years old isopachs and characteristics of the most relevant and recognized fall deposits are presented among arenal eruptions plinian events like ar 20 ar 15 ar 12 and ar 9 were the most relevant with tephra volumes up to 0 44 km3 these plinian eruptions are separated by periods of 750 1080 years also 8 subplinian 7 violent strombolian and 2 vulcanian eruptions have been recognized and correlated in arenal s tephra sequence tens of other minor explosive eruptions resembling in size to ar 22 which occurred in july 1968 have been recognized lava cycles have generally followed the plinian eruptions typical erupting volumes for these cycles have been in the order of 0 7 km3 the estimated total volume of tephra fall is 4 5 km3 the rate of total volcanics erupted is 2 7 km3 ka 1 0 086 m3 s 1 the tephra sequence previous to ar 9 3200 b p 7000 b p has no dark soils whereas the overlying sequence does it is presumed to be a consequence of regional climatic changes from a dry environment to a rainy tropical one

late cenozoic tephrostratigraphy offshore the southern central american volcanic arc 1 tephra ages and provenance we studied the tephra inventory of 18 deep sea drill sites from six dsdp odp legs legs 84 138 170 202 205 206 and two iodp legs legs 334 and 344 offshore the southern central american volcanic arc cava eight drill sites are located on the incoming cocos plate and ten drill sites on the continental slope of the caribbean plate in total we examined 840 ash bearing horizons and identified 650 of these as primary ash beds of which 430 originated from the cava correlations of ash beds were established between marine cores and with terrestrial tephra deposits using major and trace element glass compositions with respect to relative stratigraphic order as a prerequisite for marine terrestrial correlations we present a new geochemical data set for significant neogene and quaternary costa rican tephras moreover new ar ar ages for marine tephras have been determined and marine ash beds are also dated using the pelagic sedimentation rates the resulting correlations and provenance analyses build a tephrochronostratigraphic framework for costa rica and nicaragua that covers the last 8 myr we define 39 correlations of marine ash beds to specific tephra formations in costa rica and nicaragua from the 4 15 ma lower sandillal ignimbrite to the 3 5 ka rincon de la vieja tephra from costa rica as well as another 32 widely distributed tephra layers for which their specific region of origin along costa rica and nicaragua can be constrained this article is protected by copyright all rights reserved

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

effects of plate convergence obliquity on timing and mechanisms of exhumation of a mid crustal terrain the central anatolian crystalline complex abstract apatite fission track ft ages from the central anatolian crystalline complex cacc a microcontinent within the turkish segment of the alpine himalayan orogen vary dramatically from north to south this variation correlates with differences in the obliquity of convergence of the continental fragment relative to the pontide belt for the northern cacc and the tauride belt for the southern cacc the northern cacc was deformed and metamorphosed during late cretaceous orogen normal collision and was exhumed from the mid crust to shallow crustal levels 50 ma to the present in contrast the southern cacc nigde massif was initially deformed and metamorphosed during late cretaceous contraction but subsequently developed as a metamorphic core complex in a wrench dominated regime and was exhumed to

correlation of metamorphosed paleozoic strata of the southeastern mojave desert region california and arizona isolated outcrops of deformed regionally metamorphosed paleozoic strata are scattered within the southeastern mojave desert region of california and western arizona these strata unconformably overlie a basement of proterozoic crystalline rocks and are overlain in turn by metamorphosed mesozoic sedimentary rocks the strata can be correlated lithostratigraphically with the classic cratonal paleozoic section of the western grand canyon arizona and with nonmetamorphosed paleozoic sections transitional between cratonal and miogeoclinal in the ship marble and providence mountains california the strata evidently were once continuous with paleozoic epicontinental strata exposed throughout the southern great basin and colorado plateau outcrops of paleozoic strata and of the underlying proterozoic basement in the southeastern mojave desert region define a terrane that has been disrupted by mesozoic thrust faults and by tertiary detachment faults but that nevertheless retain a gross paleogeographic coherence this coherent terrane extends at least as far west and southwest as the big maria palen and calumet mountains and possibly beyond to include paleozoic exposures in the san bernardino mountains and near victorville poorly understood tectonic boundaries separate the area of paleogeographic coherence from known or suspected allochthonous terranes in the western mojave desert and the eastern transverse ranges

plate deformation at depth under northern california slab gap or stretched slab plate kinematic interpretations for northern california predict a gap in the underlying subducted slab caused by the northward migration of the pacific north america juan de fuca triple junction however large scale decompression melting and asthenospheric upwelling to the base of the overlying plate within the postulated gap are not supported by geophysical and geochemical observations we suggest a model for the interaction between the three plates which is compatible with the observations in this slab stretch model the juan de fuca plate under coastal northern california deforms by stretching and thinning to fill the geometrical gap formed in the wake of the northward migrating mendocino triple junction the stretching is in response to boundary forces acting on the plate the thinning results in an elevated geothermal gradient which may be roughly equivalent to a 4 ma oceanic lithosphere still much cooler than that inferred by the slab gap model we show that reequilibration of this geothermal gradient under 20 30 km thick overlying plate can explain the minor neogene volcanic activity its chemical composition and the heat flow in contrast to northern california geochemical and geophysical consequences of a true slab gap can be observed in the california inner continental borderland offshore los angeles where local asthenospheric upwelling probably took place during the miocene as a result of horizontal extension and rotation of the overlying plate the elevated heat flow in central california can be explained by thermal reequilibration of the stalled monterey microplate under the coast ranges rather than by a slab gap or viscous shear heating in the mantle

anatomy of a metamorphic core complex seismic refraction wide angle reflection profiling in southeastern california and western arizona the metamorphic core complex belt in southeastern california and western arizona is a nw se trending zone of unusually large tertiary extension and uplift midcrustal rocks exposed in this belt raise questions about the crustal thickness crustal structure and the tectonic evolution of the region three seismic refraction wide angle reflection profiles acquired and analyzed as a part of the u s geological survey s pacific to arizona crustal experiment were collected to address these issues the results presented here which focus on the whipple and buckskin rawhide mountains yield a consistent three dimensional image of this part of the metamorphic core complex belt the seismic refraction wide angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles these phases include refractions from the near surface and crystalline basement reflections from boundaries in the middle and lower crust and reflections and refractions from the upper mantle the final model consists of a thin veneer 2 km of upper plate and fractured lower plate rocks 1 5 5 5 kms 1 overlying a fairly homogeneous basement 6 0 km s 1 and a localized high velocity 6 4 km s 1 body situated beneath the western whipple mountains a prominent midcrustal reflection is identified beneath the whipple and buckskin rawhide mountains between 10 and 20 km depth this reflector has an arch like shape and is centered beneath or just west of the metamorphic core complex belt this event is underlain by a weaker approximately subhorizontal reflection at 24 km depth together these two discontinuities define a lens shaped midcrustal layer with a velocity of 6 35 6 5 km s 1 the apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift 10 km defined by surface geologic mapping and petrologic barometry studies the layer thins to the northeast and is absent in the transition zone the 6 35 6 5 km s 1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks this midcrustal layer is underlain by a higher velocity lower crustal layer that is modeled as only 3 6 km thick beneath the metamorphic core complex belt and regions to the southwest to the northeast however this layer thickens to 8 10 km as the midcrustal layer pinches out above it the velocity of the lower crust is constrained by traveltime modeling and is 6 6 0 15 kms 1 beneath the western transition zone and the metamorphic core complex belt higher velocities may be present farther to the southwest where the layer is thin the velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust instead we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle derived intrusions the crust mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26 28 km or in the case of the whipple mountains actually deepens a 3 km crustal root is modeled this lack of upward doming of the moho together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension related structures in the middle and lower crust suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension as one would expect if extension occurred along crust penetrating shear zones wernicke 1981 wernicke et al 1985 instead domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust

how the harcuvar mountains metamorphic core complex became cool evidence from apatite u th he thermochronometry apatite u th he ahe data from 60 km of exposed footwall in the harcuvar mountains metamorphic core complex arizona reveal a higher precision late stage cooling history than previously reported for this complex and others in the region progressively younger ahe ages recorded with distance along the slip direction show two distinct trends separated by a sharp inflection at structurally shallower levels ages decrease from ca 23 to 15 ma toward the northeast defining an estimated slip rate of 2 6 0 1 0 2 km m y samples from the structurally deepest 35 km of footwall yield concordant ages of ca 15 ma 2 error limits the effects of mineralizing fluids advection of isotherms toward the surface and flexure induced erosion are considered but are unlikely to have influenced the ages therefore it is suggested that the fault slip rate must have increased ca 15 ma to a rate of 30 km m y evidence for an increased slip rate ca 15 ma has also been observed in other metamorphic core complexes nearby and this increase is attributed to the thermal effects of a slab free window on the extending lithosphere

quantifying recent pyroclastic and lava flows at arenal volcano costa rica using medium footprint lidar 1 arenal volcano is a small active stratovolcano in costa rica in 1998 and 2005 nasa s laser vegetation imaging sensor lvis was used to collect wide swath 3 dimensional topographic images of the volcano the lvis is a full waveform scanning medium sized footprint airborne laser altimeter system by digitally recording the shape of the returning laser pulse waveform the lvis provides a precise and accurate view of both the sub canopy and canopy top topographies as well as the vertical and horizontal structure of vegetation at 15 25 m horizontal resolution by comparing georeferenced waveform data collected in 1998 and 2005 we mapped lava and pyroclastic flows deposited during this period the active crater grew by 3 82 m yr 1 a flow volume estimate of 2 19 107 m3 dense rock equivalent of 1 89 107 m3 or 0 085 m3s 1 was obtained for the period 1998 to 2005 precise elevation and elevation change data such as those provided by the lvis are essential to calculate eruption volume and to study magma supply dynamics as well as assess the danger posed by the volcano to the local population from hazards such as pyroclastic flows

the cultural nature of tephra problematic ecofacts and artifacts and the baru volcano panama abstract this paper presents data from archaeological stratigraphy and lacustrine core samples containing tephra from the baru volcano in western panama the discussion seeks to refine the understanding of medial tephra deposits near baru in relation to the eruption history indicated by geological palaeoecological and archaeological data a primary goal of this fieldwork is the identification and correlation of tephra from major baru eruptions encountered in archaeological contexts by their lithology stratigraphic position and mineralogy without resorting to geochemical fingerprinting the intention is to develop a cost efficient and reproducible means of utilizing tephra as a chronostratigraphic marker in archaeological contexts and examine the intersection of human life with volcanic events and dynamic environments tephra in archaeological contexts can have ambiguous placement between ecofact and artifact as is the case with a small number of crude tephra sculptures in my excavated material and that of a seminal prior study while the research project utilizes visible tephra layers not cryptotephra by all means the tephra data prove to be cryptic engagement with the imperfection entailed in meshing multidisciplinary data sets and different scientific communities with interests in tephra particularly in lesser studied regions is posited as a total view of tephra

high pressure metamorphism and uplift of the olympic subduction complex the discovery of the critical assemblage lawsonite quartz calcite indicates that a significant part of the cenozoic olympic subduction complex of northwestern washington state formed by underplating at a depth of about 11 km the deep structural level exposed in this area is attributed to the presence of a 10 km high arch in the underlying juan de fuca plate we postulate that this arch was formed when the southern cordilleran coastline swung westward as a result of middle miocene to recent extension in the basin and range province

cretaceous eocene laramide landscape development and oligocene pliocene drainage reorganization of transition zone and colorado plateau arizona landscape development of central and northern arizona can no longer be ascribed mainly to events of miocene and pliocene age new information on the age and distribution of older cenozoic deposits has led to the recognition of a regional cretaceous paleocene surface of erosion that conforms to major elements of the present topography and to the recognition that a formerly thick deposit of gravel accumulated on this regional surface of erosion these relations cast new light on the history of evolution of the landscape and indicate a much greater age for the main landscape elements and a more complicated and prolonged history of erosion and deposition than has been previously supposed the timing of events postulated for development of drainage on the colorado plateau can now be compared and partly reconciled with events recognized in the adjacent closely related mountain region transition zone of central arizona as a consequence of late cretaceous paleocene laramide compression central and northern arizona underwent at least 1200 m of uplift documented by paleochannels cut into erosionally truncated paleozoic strata on the hualapai plateau of the southwestern colorado plateau during this time a highly irregular erosion surface was developed on proterozoic rocks across the transition zone south of the mogollon rim the scarp of the mogollon rim was eroded to its present height 600 900 m and an extensive stripped surface was developed on resistant upper paleozoic strata north of the rim deposition of several hundred meters of paleocene eocene rim gravels derived from highlands south and west of the region followed covering much of the cretaceous paleocene erosion surface nearly complete burial of the rim is suggested by the distribution of remnants of the rim gravels across the erosional scarps and on high plateau areas north of the rim a second increment of uplift apparently occurring in late eocene time and apparently recorded by a series of fission track cooling ages from the marble and grand canyons is inferred to have been responsible for ending deposition of the rim gravels for initiating differential uplift of contemporaneous deposits canaan peak and claron formations to their positions in the high plateaus of central utah and for causing the drainage reorganization required to explain the extensive removal of rim gravels from much of the region a southerly flowing ancestral verde river related to the drainage reorganization removed much of the older gravel cover from the transition zone of central arizona resulting in a younger regional erosion surface having 600 900 m of relief a surface closely approximating the cretaceous paleocene erosion surface late oligocene and early miocene rocks locally rest unconformably on remnants of rim gravels in the transition zone indicating that the second episode of regional erosion had been completed by late oligocene time north of the mogollon rim a west flowing ancestral colorado river is inferred to have become established on the rim gravels draining the interior parts of the colorado plateau and transporting detritus off the plateau exhumation of the mogollon rim and development of 600 900 m of topographic relief in the transition zone by an ancestral verde river system suggests the potential for a comparable coeval entrenchment of an ancestral colorado river in paleozoic strata north of the mogollon rim regional extension and volcanic activity ensued in late oligocene to pliocene time the oligocene erosion surface in the extensional basins of central arizona became largely concealed by miocene and pliocene deposits as the neogene climate became drier in late miocene and pliocene time perennial streams appear to have been lacking transport of detritus appears to have been principally by flash flooding little or no detritus appears to have been removed from the region and much of the precipitation presumably moved by groundwater flow through the deposits of aggradation a coeval episode of aggradation in the grand canyon is suggested by deposits that appear to have once choked much of the canyon if this event parallels the episode of late miocene and pliocene aggradation recorded east south and west of the grand canyon the colorado river could have been incised to its present level by late miocene time a return to wetter conditions in late pliocene time presumably was responsible for renewed erosion and reexcavation of older drainages and basins an understanding of this tertiary structural erosional and depositional history can be important for the geological analysis of geophysical transects across the region

plate dynamics caribbean map corrections and hotspot push summary in 1986 the author s previous paper describing a simple mathematical model for the forces driving the plates had an rms error of 1 62 per cent in its best case the model included slab pull and its global and local reactions proportional to speed times age 3 2 viscous drag due to shear under moving plates and in the flow towards mantle sinks at midocean ridges and sources at subduction zones ridge push proportional to plate age and friction at continental collision zones strengths of the various forces were found by least squares fitting three different improvements now reduce the error to 1 14 per cent they are 1 revision of the n s american plate boundary east of the caribbean sea to run along the 15 20 fracture zone ii removal of a transform segment from the caribbean subduction zone and iii allowing for hotspot push hotspots cause upward flow in the mantle which spreads outwards under the lithosphere above and pushes two plates apart if their boundary runs nearby in a way which is calculated the results and their statistical significance are estimated by a method given by backus park garbasz 1981 modified slightly because three of the equations in the model are linearly dependent on the remainder all the physical mechanisms envisaged in this work as driving plates turn out to be significant except local reaction to slab pull the effect of which is very small anyway

thermal and unroofing history of a thick tilted basin and range crustal section in the tortilla mountains arizona we estimate here a geothermal gradient of only 17 5 c km 1 for the tilted grayback fault block in southeastern arizona when extension began 25 ma this gradient is lower than preextension gradients estimated elsewhere in the basin and range is only about 50 of typical gradients in the basin and range today and needs to be accounted for in models of continental extension the grayback block exposes a 12 km thick crustal section of proterozoic and cretaceous granitoids which was tilted 90 during extension between 25 and 15 ma zircon fission track ages decrease structurally downward westward across the block and were all within a zone of partial track annealing prior to tilting and quenching the zircon age gradient suggests that the 220 240 c isotherm migrated downward 5 6 km during paleogene erosion and regional cooling apatite fission track ages decrease westward from 83 ma in the structurally highest crystalline rocks to 24 ma at 6 km paleodepth and then to 15 ma another 6 km farther west track length analysis confirms that apatites above the break in slope in age at 5 7 km paleodepth resided in a zone of partial annealing prior to tilting and deeper apatites record rapid cooling upon tilting and unroofing beginning 25 ma at that time the 110 10 c isotherm determined by the depth at which tracks in apatite were fully erased was at a basement paleodepth of 5 7 km and the 220 30 c isotherm as estimated from zircon data resided at a pretilting basement depth of 12 15 km from consistent values of paleogeothermal gradient for two depth intervals we estimate the pretilt gradient was 17 5 c km 1 from 25 to 15 ma the rotating grayback block cooled rapidly as higher westward moving blocks unroofed it tectonically at a rate of 1 km m y 1

chapter 22 paleomagnetism of continental north america implications for displacement of crustal blocks within the western cordillera baja california to british columbia

miocene pliocene exhumation along the west salton detachment fault southern california from u th he thermochronometry of apatite and zircon 1 the salton trough is the northernmost segment of the active gulf of california oblique rift system the main rift related structure in the western salton trough is the low angle west salton detachment fault wsdf footwall and hanging wall apatite and zircon u th he ages record a significant contrast in thermal history across the wsdf confirming pliocene normal slip along the fault apatite u th he ages record rapid exhumational cooling from 5 to 2 ma consistent with the timing of accelerated tectonic subsidence recorded in wsdf hanging wall sedimentary strata and consistent with the initiation age of dextral plate boundary slip on the southern san andreas fault in the salton trough our results indicate that the wsdf caused at least 2 3 4 km of exhumation and 8 10 km of approximately e directed horizontal extension since 5 ma slip rate of the wsdf at yaqui ridge was likely in the range of 2 3 5 km ma from 7 to 2 ma the wsdf may have been active well before 5 ma middle miocene apatite u th he ages from samples at high elevations have steep age elevation gradients and suggest that exhumation may have initiated by 12 ma about when subduction ceased along a large segment of the baja california margin and consistent with the age of onset of extension in northeastern baja california

tectonics of mid tertiary extension along a transect through west central arizona large magnitude miocene extension in west central arizona occurred primarily along three imbricate northeast dipping normal faults the structurally highest of these faults the gently dipping buckskin rawhide detachment fault accommodated approximately 66 km of crustal extension whereas the two structurally lower faults accommodated a total of about 20 km extension due to this large magnitude extension an area at the earth s surface that was 10 to 20 km wide is now over a 100 km wide and crystalline rocks with mid tertiary mylonitic fabrics uncovered by detachment faulting are exposed over roughly 2000 km in the harcuvar metamorphic core complex most of the upper plate of the buckskin rawhide detachment fault was largely undeformed by internal extension only the thin tapered end of the upper plate was highly extended during extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded forming extension parallel folds in the lower plate low flexural strength characterized the lower plate during denudation and a highly mobile low viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere

thermochronologic evaluation of postcollision extension in the anatolide orogen western turkey 1 to better understand the driving mechanisms behind the transition from collision to extension in a convergent orogen data from multiple low temperature thermochronometers were obtained from the simav detachment fault sdf the earliest developed major extensional structure recognized in the western anatolide orogen of western turkey twenty two zircon fission track ft 26 apatite ft 12 apatite u th he ages and 26 apatite ft track length analyses are presented the data establish that the sdf was a major extensional fault active between 25 and 19 ma the coincidence in timing of magmatism with cessation of sdf activity implies the detachment became locked owing to doming induced by magma emplacement zircon ft ages away from the influence of miocene magmatism record rapid footwall cooling between 25 and 21 ma at a slip rate of up to 15 km myr and demonstrate that active ductile deformation along the sdf migrated northward with time apatite ft ages from the footwall of the sdf are spatially invariant over 100 km and consistently 2 3 myr younger than the zircon fission track ages from the same samples these data are consistent with a regional but relatively rapid erosion linked cooling phase that removed 2 3 km of overburden following cessation of sdf activity but before deposition of sedimentary rocks on the detachment surface at 16 4 ma postcollision extension of the anatolide orogen with development of the sdf can be best explained as the result of postcollision magmatism and thermal weakening of the crust inducing the extensional reactivation of an earlier major out of sequence thrust

the magma budget of volcan arenal costa rica from 1980 to 2004 using topographic data collected by radar interferometry stereo photogrammetry and field survey we have measured the changing surface of volcan arenal in costa rica over the period from 1980 to 2004 during this time this young volcano has mainly effused basaltic andesite lava continuing the activity that began in 1968 explosive products form only a few percent of the volumetric output we have calculated digital elevation models for the years 1961 1988 and 1997 and modified existing models for 2000 and 2004 from these we have estimated the volume of lava effused and coupled this with the data presented by an earlier study for 1968 1980 we find that a dense rock equivalent volume of 551m m 3 was effused from 1968 to 2004 the dense rock equivalent effusion rate fell from about 2m 3 s 1 to about 0 1 0 2m 3 s 1 over the same period with an average rate of about 0 5m 3 s 1 between 1980 and 2004 the average effusion rate was 0 36m 3 s 1 a similar rate to that measured between 1974 and 1980 there have been two significant deviations from this long term rate the effusion rate increased from 1984 to 1991 at the same time as explosivity increased after a period of moderate effusion rates in the 1990s the rate fell to lower levels around 1999 2006 elsevier b v all rights reserved

late miocene ductile extension and detachment faulting mykonos greece the geometry and style of structural relations at map to microscopic scale on mykonos cyclades aegean sea greece suggests ductile extension and detachment faulting during the late miocene this island is dominated by granitic rocks that have a well developed shallowly dipping mylonitic foliation and a northeast southwest trending mineral elongation lineation kinematic indicators in these rocks exhibit a top to the northeast down dip sense of shear textural relations suggest that conditions during mylonitic deformation were greenschist to amphibolite facies and published k ar geochronology suggests a late miocene age for this deformation event mylonitic rocks lie in the footwall of a low angle fault and fault striae parallel to the elongation lineation suggest kinematic coordination between mylonitic deformation and movement along the fault translation along the fault juxtaposes younger tilted sedimentary rocks of low metamorphic grade upon the once deeper mylonitic rocks the style and geometry of deformational fabrics and map relations on this island are strikingly similar to those observed in metamorphic core complexes of the western united states

thermochronological constraints on the eocene exhumation of the grand forks complex british columbia based on 40ar 39ar and apatite fission track geochronology the grand forks complex gfc is a metamorphic core complex within the composite shuswap complex in the southern omineca belt of the canadian cordillera it is juxtaposed against the surrounding lo

evidence for large scale transport on the bullard detachment fault west central arizona the bullard detachment fault is a gently to steeply dipping normal fault that flanks the harcuvar and harquahala mountains of the basin and range province in west central arizona the stratigraphy of upper plate miocene conglomerates and the regional distribution of upper and lower plate pre tertiary units indicate that upper plate rocks were displaced about 50 km to the northeast with respect to the lower plate during middle to late tertiary time normal slip of this magnitude on the regionally northeast dipping bullard fault indicates that deep seated tertiary cretaceous mylonitic gneisses and mesozoic thrust faults of the lower plate were drawn out from beneath precambrian rocks along the margin of the transition zone of central arizona during middle to late tertiary crustal extension the transition zone was therefore affected by deep seated tectonism in both mesozoic and tertiary time