a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

balanced section in thrust belts part 1 construction balanced geological cross sections are an important aid to understanding thrust belt structures and to estimating their hydrocarbon bearing potential two approaches to section balancing have been taken in the past construction of retrodeformable sections from raw data and modification of previous interpretations after an evaluation procedure these approaches may be unified by use of a langrangian grid projection of datum locations dips and stratigraphic thicknesses onto the section plane is performed directly by calculation or graphically using plunge lines spline interpolation and parallel fold modeling are the simplest ways of filling gaps in the sectional data but isogon interpolation yields rheologically more realistic results faults are interpolated using cutoff geometries as constraints fault tips are located from distance displacement plots whereas the depth to detachment is obtained by a modified chamberlin construction interpolations made on the section are directed back to the geologic map via plunge lines although these techniques of section construction do not guarantee a balance especially when complexities such as growth structures diapirs or strike slip faults are present they eliminate many potential errors

evaluation of paleomagnetism as a correlation criterion for mogollon datil ignimbrites southwestern new mexico paleomagnetic and rock magnetic data show that 36 24 ma ignimbrite outflow sheets in the mogollon datil volcanic field carry thermoremanent magnetizations trms that provide useful reliable correlation criteria over 90 of 404 ignimbrite sites exhibit well grouped 95 500 m intracaldera ignimbrite sequences agree in polarity with associated outflow sheets but show less consistent trm directions probably reflecting syncooling secular variation and discrepancies between eutaxitic foliations and the syncooling paleohorizontal mean virtual geomagnetic pole positions vgps of normal and reverse polarity ignimbrites are nearly antipodal 175 6 apart showing no evidence of long term differences in the zonal harmonics of the normal and reverse polarity states less antipodal mean vgps 161 2 apart from normal and reverse polarity lavas are interpreted as defects in the data set probably including incompletely removed normal polarity viscous overprints the mean paleomagnetic pole position for mogollon datil ignimbrites n 68 independent unit mean vgps is located at 78 8 n 166 6 e k 19 8 a95 4 0 18 2

spatial and temporal relationships between mid tertiary magmatism and extension in southwestern arizona cenozoic magmatism in southwestern arizona which is within the basin and range tectonic province occurred almost entirely between 15 and 25 ma volcanic rocks typically consist in ascending order of 1 a thin sequence of mafic to intermediate lava flows 2 voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows and 3 basalt and andesite volcanic rock sequences rest disconformably on pre tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism prevolcanic clastic debris is interpreted as a consequence of local early normal faulting in most regions tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism extension generally ended before about 17 ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated transition zone tectonic province which is adjacent to the colorado plateau rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 ma extension was extreme in this belt whereas it was generally moderate to slight in other parts of southwestern arizona large magnitude extension was not associated with areas of greatest igneous activity and rapid cooling and exhumation of core complexes postdated local magmatism these relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust except for young extension in this northwest trending belt there are no apparent regional migration trends for either magmatism or extension within southwestern arizona lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension the long duration and large magnitude of extension adjacent to the transition zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally

the effects of k metasomatism on the mineralogy and geochemistry of silicic ignimbrites near socorro new mexico abstract k metasomatism of the upper lemitar and hells mesa silicic ignimbrites near socorro new mexico is thought to be the result of downward percolation of alkaline saline brines in a hydrologically closed basin chapin c e lindley j i 1986 potassium metasomatism of igneous and sedimentary rocks in detachment terranes and other sedimentary basins economic implications arizona geological society digest xvi 118 126 during the chemical changes associated with metasomatism na rich phases primarily plagioclase are replaced by secondary mineral phases adularia a low temperature k feldspar is the dominant mineral formed during k metasomatic alteration but mixed layer i s discrete smectite and kaolinite can also be present in the assemblage the formation of discrete smectite within the assemblage during k metasomatism may have occurred during periods of low cation h ratios in the alkaline saline brine upon an increase in the cation h ratio such as could occur during evaporation of the alkaline lake the solution may have become sufficiently concentrated to cause illitization of smectite resulting in the formation of mixed layer i s within the assemblage distribution of phases in the alteration assemblage strongly suggests a dissolution precipitation reaction for k metasomatism in the socorro area as indicated by the presence of dissolution embayments in plagioclase crystals the presence of euhedral adularia and the common occurrence of authigenic clay minerals in the assemblage k metasomatism causes significant chemical modification of the silicic ignimbrite particularly increases in k 2 o and rb and depletion of na 2 o and sr with increasing adularia abundance the correlation between rb and k 2 o suggests that rb is enriched during alteration due to substitution for k in adularia the effect of hydrothermal activity either prior to or following metasomatism is also observed in some samples as shown by high concentrations of elements such as ba as sb pb and cs enrichment of middle and hree in the upper lemitar tuff samples and depletion of middle and hree in hells mesa tuff samples suggests attempted re equilibration between the secondary alteration assemblage and the metasomatizing fluid preliminary data indicate clay minerals within the secondary assemblage may have played an important role in the incorporation of ree during redistribution

inner forearc response to subduction of the panama fracture zone southern central america abstract subduction of the right lateral panama fracture zone along the convergent margin of central america creates abrupt lateral variations in convergence rate obliquity and subducting crustal thickness at its intersection with the middle america trench this intersection known as the panama co nz ca triple junction is migrating to the southeast at a rate of 55 mm yr and currently coincides with the lateral termination of the fila costena thrust belt in the inner forearc of the overriding plate mapping in the inner forearc in the area that straddles the subducting panama fracture zone reveals that cocos caribbean convergence west of the triple junction leads to the development of an inner forearc thrust belt inboard of the colliding cocos ridge while little deformation is evident inboard of nazca caribbean convergence east of the triple junction this results in the lateral termination of the fila costena thrust belt in the region of the forearc that projects over the panama fracture zone where four out of five mapped thrust faults tip out and are buried by lahars three new balanced cross sections indicate a steep gradient in shortening from the center of the thrust belt to its southeastern termination the short term history of the inner forearc recorded in the landscape and topography of the fila costena is consistent with the southeastward migration of the thrust belt and the panama triple junction throughout the past 3 ma with evidence for the growth of a new topographic divide and reorganization of stream channel networks

predicting patterns of strain from three dimensional fold geometries neutral surface folds and forced folds the geometries and densities of fractures associated with fold structures can be predicted by assuming that the strains accommodated by fractures mimic the bulk strains induced in the strata during folding this paper examines from a theoretical standpoint the distributions of bedding plane strains expected in folds formed by various folding mechanisms the relationship between the state of bedding plane strain and fold surface geometry is found to vary according to different fold types distinguished on the basis of their curvature properties the first type are developable fold surfaces which have gaussian curvature equal to zero folding mechanisms which are dominated by the mechanical strength of the layering such as buckling produce surfaces of this type folds of this type allow the possibility of estimating the bedding plane strains from the geometrical features of the folded layer neutral surface folds and flexural slip folds are discussed as examples the other main class of folds have non developable surfaces which have non zero gaussian curvature folded surfaces with this form arise predominantly from mechanisms that involve the passive deflection of the layering in response to displacement gradients originating outside of the layer e g drape folding although the geometry of these surfaces implies the presence of bedding plane strains the quantification of these strains cannot be made from the fold geometry but requires additional information on these displacement patterns

implications of the north and south panama thrust belts for the origin of the panama orocline high resolution imaging of the north nptb and south panama thrust belts using the seamarc ii seafloor mapping system and digital seismic reflection profiling provides an excellent data base with which to determine the kinematics of oroclinal deformation of panama remarkable continuity of structural orientations over 300 km along the nptb argues for widespread uniformity in direction of thrusting between panama and the caribbean plate westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence two major cross faults cut the wedge in its eastern part forming two minor blocks with different relative motions with respect to the caribbean frontal thrust orientations change across these faults major faults have been proposed to cut the isthmus of panama but simple cross isthmus faulting alone cannot explain the orientations of the major thrusts of the nptb bending of a flexible beam can explain much of the observed structure both directions and rates though the scale of the orocline precludes considering this as an elastic phenomenon plastic deformation of panama may be a useful mechanical explanation at least for eastern panama but consideration of plastic slip lines must include the northern andes as well changing from more rigid blocklike offsets on faults in western panama to a continuum of strain in eastern panama may be the best explanation for observations at present

middle miocene to present plate tectonic history of the southern central american volcanic arc abstract new mid miocene to present plate tectonic reconstructions of the southern central american volcanic arc cava reveal that the inception of cocos ridge subduction began no earlier than 3 ma and possibly as late as 2 ma the cocos ridge has been displaced from the malpelo ridge to the southeast since 9 ma along the panama fracture zone pfz system ambiguous pfz and coiba fracture zone cfz interaction since 9 ma precludes conclusively establishing the age of initial cocos ridge subduction detailed reconstructions based on magnetic anomalies offshore reveal several other variations in subduction parameters beneath southern central america that preceded subduction of the cocos ridge including southeastward migration of the nazca cocos caribbean triple junction along the middle america trench mat from 12 ma to present and subduction of 2 km high scarps both parallel and perpendicular to the trench from 6 to 1 ma the timing of changes in subduction processes has commonly been determined by and correlated with geologic changes in the upper plate however reliable 40 ar 39 ar dating of these events has become available only recently abstr programs geol soc am 2002 these new dates better constrain the magmatic and structural history of southern costa rica observations from this data set include a gap in the volcanic record from 11 to 6 ma which coincides temporally with emplacement of most plutons in southern costa rica normal arc volcanism ceased after 3 5 ma in southern costa rica and pliocene mostly 1 5 ma adakite volcanism was widely distributed from central panama to southern costa rica though volumetrically insignificant this new data reveals that many geologic phenomena commonly attributed to subduction and underplating of the buoyant cocos ridge in fact precede inception of cocos ridge subduction and seem to correlate more favorably in time with earlier tectonic events adakite volcanic activity corresponds in space and time with the subduction of a large scarp associated with a tectonic boundary off southern panama regional unconformities and an 11 6 ma gap in arc volcanism match temporally with oblique subduction of the nazca plate beneath central and southern costa rica cessation of volcanic activity low temperature cooling of plutons in the cordillera de talamanca ct and rapid increases in sedimentation in the fore arc and back arc basins coincide with passage of the nazca cocos caribbean triple junction and initiation of subduction of rough crust associated with cocos nazca rifting 3 5 ma closely followed by initial subduction of the cocos ridge 2 3 ma none of the aforementioned geologic events occurred at a time that would allow for underplating by the cocos ridge rather they are probably related to complex interactions with subduction of complicated plates offshore all of the aforementioned events indicate that the southern central american subduction system has been in flux since at least 12 ma

deformation of erosive and accretive forearcs during subduction of migrating and non migrating aseismic ridges results from 3 d finite element models and application to the central american peruvian and ryukyu margins subduction of aseismic oceanic ridges causes considerable uplift and deformation of the upper plate and may lead for example to the indentation of the forearc the formation of marine terraces or distinct fault patterns in the upper plate depending on the orientation of the ridge relative to the plate convergence direction the ridge may either be stationary or migrate along the margin here we use three dimensional numerical models to investigate the tectonic evolution of forearcs affected by ridge subduction in different experiments we distinguish between migrating non migrating ridges and accretive erosive margins respectively our results reveal that displacement and strain fields above migrating and non migrating ridges are asymmetric with respect to the ridge axis unless both ridge and plate convergence direction are perpendicular to the trench as the asymmetric deformation pattern shifts along the margin through time uplift caused by the underthrusting ridge is followed by subsidence when the ridge crest passed by and regions initially experiencing shortening may subsequently undergo extension and vice versa if the forearc comprises an accretionary prism the ridge induced reentrant is larger than those in models with erosive forearcs and strain localizes in the frontal part of the wedge additional models with a setup adjusted to the cocos and gagua ridges provide constraints on the onset of their subduction at the central american and ryukyu margins at 2 ma and 1 ma respectively displacement and strain fields from a model for the nazca ridge collision zone show good agreement with geological data from marine terraces and quaternary faulting

neotectonics of the nanga parbat syntaxis pakistan and crustal stacking in the northwest himalayas the northwest termination of the himalayan arc is marked by two large antiformal structures termed syntaxes and a nw se zone of intermediate depth earthquakes which have yielded both strike slip and thrust sense fault plane solutions this contribution presents new structural data from the nanga parbat area the northern syntaxis one of the fastest rising portions of the orogen 7 mm yr 1 our studies show uplift related to syntaxis growth to be accommodated by structures located along its western margin fault kinematics in the southern part of the margin indicate nw directed thrusting along a shear zone and high level cataclastic faults termed the liachar thrust zone this carries amphibolite facies basement rocks out onto quaternary sediments further north along the western margin active faults are dominantly dextral strike slip oriented north south e g the shahbatot fault overprinting relationships suggest that the strike slip fault zone has migrated southwards into the nw directed thrust zone these relationships are consistent with the northwest termination of the arcuate himalayan thrust belt at a lateral tip generating folding and radial thrust directions faulting patterns at nanga parbat suggest that this tip zone has migrated southwards active faulting is now concentrated in the northwest continuation of the hazara southern syntaxis along a seismogenic strike slip and thrust zone these deep level tips lie on the crustal stacking thrusts which cut through the higher level sse directed thrusts of pakistan

thermal history of a bishop tuff section as determined from the width of cryptoperthite lamellae the lamellar spacings in the sanidine cryptoperthites bulk composition or 66 4 ab 32 3 an 1 3 from the bishop tuff near long valley california have been determined using transmission electron microscopy the lamellar spacings show a sharp maximum at about 43 m below the topographically highest sample and a weak minimum about 4 m above this this pattern indicates that two flows compose the section the gradational boundary between the flows is about 37 m below the highest sample heat flow calculations applied to the pattern of lamellar spacings indicate that these flows were emplaced within 1 5 yr of each other the result illustrates the power of this technique in deciphering the geologic history of igneous rocks

closure of the isthmus of panama the near shore marine record of costa rica and western panama the final closure of the isthmus of panama at 3 5 ma divided the american tropical ocean into two separate and different oceanographic regions consequences for the marine biota were profound but hitherto correlation of the pacific and caribbean coastal sections has not been precise enough to track biologic patterns we present here a correlation of 31 sections from the pacific and caribbean coasts of costa rica and western panama using calcareous nannofossils and planktonic foraminifera at both the tops and bottoms of each formation we estimate that the caribbean section ranges from 8 2 ma to 1 7 ma and the pacific sequence from 3 6 ma to 1 7 ma these intervals bracket postulated dates for final closure of the isthmus and provide the first well dated record of middle and late pliocene faunas from the region the caribbean and pacific sections include very different environments of deposition yet there is sufficient overlap and diversity of habitats to permit meaningful biological comparisons on the caribbean side formations tied together by the overlap of the upper pliocene markers sphenolithus abies and pseudoemiliana lacunosa 3 5 ma to 3 6 ma range from very shallow to shallow inner shelf 200 m and upper slope 200 800 m the pacific coast sections were mostly deposited in a trench slope environment which is absent on the caribbean side these sections fortuitously include abundant thick intra formational slumps containing shallow water fauna more appropriate for biological comparison with the caribbean biota similarly the 1 9 ma to 1 5 ma interval well constrained by various taxa includes middle to outer shelf and inner shelf to upper slope deposits on the caribbean side and marginal marine to inner shelf deposits on the pacific coast using our new biostratigraphic framework to correlate previously poorly constrained mollusc collections we show that evolutionary divergence of the pacific and caribbean near shore marine faunas had occurred by 3 5 ma this strongly suggests that the isthmus was effectively closed by this time

does stommel s mixed layer demon work abstract stommel argued that the seasonal cycle leads to a bias in the coupling between the surface mixed layer and the main thermocline of the ocean he suggested that a demon operated that effectively only allowed fluid at the end of winter to pass from the mixed layer into the main thermocline in this study stommel s hypothesis is examined using diagnostics from a time dependent coupled mixed layer primitive equation model of the north atlantic cme the influence of the seasonal cycle on the properties of the main thermocline is investigated using two methods in the first the rate and timing of subduction into the main thermocline is diagnosed using kinematic methods from the 1 resolution cme fields in the second tracer diagnostics of the cme and idealized experiments using a date tracer identifying the timing of subduction are performed over the subtropical gyre both approaches generally support stommel s hypothesis that fluid is only transferred from the mixed layer into the main therm

three dimensional retro deformation of the lechtal nappe northern calcareous alps we determine the amount of shortening depth of detachment and the style of deformation of the lechtal nappe the largest nappe within the northern calcareous alps german austrian border by three dimensional retro deformation analysis of ca 40 km2 area a three dimensional model was constructed by splining lines from eight n s cross sections spaced e w at ca 4 km intervals to define fault geometrics and seven stratigraphic layers within the permo triassic to cretaceous strata the modelled volume has four main thrusts which link to a common detachment at 2 5 km depth three dimensional fault displacements and heaves were determined using allan diagrams the algorithms fault parallel flow and flexual slip unfolding were used to restore northwards movement on the thrusts and folding of beds over thrust planes respectively minimum shortening estimates vary from east to west from 25 to 42 typical error 6 but additional shortening in the west is due only to folding

strain displacement and rotation associated with the formation of curvature in fold belts the example of the jura arc abstract a new simplified genetic classification scheme for arcuate fold thrust belts is proposed based on total strain patterns and displacement vector fields we distinguish three extreme end member models 1 oroclines pure bending of an initially straight belt 2 piedmont glacier with divergent transport directions and 3 primary arcs a simple geometric model set up for the simulation of strain patterns in primary arcs with uniform transport direction demonstrates that divergent strain trajectories and rotations of passive marker lines do not require any divergence in displacement directions these often quoted arguments are insufficient for the identification of oroclinal bending or piedmont glacier type of arc formation only three dimensional restorations of an arc provide the critical information about displacement directions in their absence arc parallel stretches and rotations in comparison with total strains provide the most useful criteria for the distinction of arc formation modes as an example the jura fold thrust belt of the external alps is discussed a large set of strain data includes total shortening estimates based on balanced cross sections local strain axes orientations from the inversion of fault populations homberg c 1996 unpublished phd thesis universite de paris vi france tectonic stylolites and micro strains from twinning in sparry calcite strain trajectories maximum shortening direction computed from these data define a strongly divergent fan with a 90 opening a complete displacement vector field for the entire jura has been determined from balanced cross sections augmented with three dimensional block mosaic restorations philippe y 1995 unpublished phd thesis universite de chambery france displacement vectors diverge by about 40 markedly less than strain trajectories the non parallelism between strain trajectories and transport directions indicates that considerable wrenching deformation did occur in both limbs of the jura arc paleomagnetically determined clockwise rotations of 0 13 from ten sites kempf o et al terra nova 10 6 10 behind the right hand half of the jura arc and two sites with a combined 23 anticlockwise rotation behind the left hand half of the arc are and additional argument in favor of such a wrenching deformation we conclude that the jura arc formed as a primary arc with a minor component of piedmont glacier type divergence in transport directions

establishing a 14 6 0 2 ma age for the nordlinger ries impact germany a prime example for concordant isotopic ages from various dating materials abstract 40ar 39ar dating of recrystallized k feldspar melt particles separated from partially molten biotite granite in impact melt rocks from the approximately 24 km nordlinger ries crater southern germany yielded a plateau age of 14 37 0 30 0 32 ma 2 this new age for the nordlinger ries is the first age obtained from 1 monomineralic melt 2 separated from an impact metamorphosed target rock clast within 3 ries melt rocks and therewith extends the extensive isotopic age data set for this long time studied impact structure the new age goes very well with the 40ar 39ar step heating and laser probe dating results achieved from mixed glass samples suevite glass and tektites and is slightly younger than the previously obtained fission track and k ar and ages of about 15 ma as well as the k ar and 40ar 39ar age data obtained in the early 1990s taking all the 40ar 39ar age data obtained from ries impact melt lithologies into account data from the literature and this study we suggest an age of 14 59 0 20 ma 2 as best value for the ries impact event

integrated paleontologic and paleomagnetic stratigraphy of the upper neogene deposits around limon costa rica a coastal emergence record of the central american isthmus a chronostratigraphic study of mixed carbonate and siliciclastic sediments limon group along the caribbean side of the central american isthmus was conducted to provide refined depositional ages on the uplifted nearshore marine sequence exposed near limon costa rica these upper neogene sediments provide a rich faunal history that spanned the closure of the isthmus and cessation of marine circulation between the pacific ocean and caribbean sea this faunal archive provides a critical link in regional assessment of evolutionary changes that resulted from the development of the central american isthmus results from this study have shown that integrated biostratigraphic and magnetostratigraphic techniques can be successfully combined to provide refined age dating in shallow marine sediments even where some microfossil reworking has occurred we propose the new quebrada chocolate formation to represent latest early early late pliocene mixed reefal and siliciclastic deposits that overlie the early pliocene rio banano formation the superjacent moin formation is expanded to include a second reefal interval deposited near the late pliocene pleistocene boundary as well as fine grained mollusk rich deposits in the forereef and lagoon the style of mixed system reef geometry between the two units is distinct the quebrada chocolate formation reefs consist of a lower interval of alternating reefal units buried by siliciclastic sediment and an upper unit comprised of stacked reefal buildups with relatively minor siliciclastic matrix these stacked reefs formed on a rising sea level in contrast the moin formation reefs are isolated corals and patch reefs within a siliciclastic matrix that were deposited during the peak maximum flooding of the sea level highstand our refined age model for the limon group sediments allows reassessment of pleistocene uplift rates we calculate a rate of about 50 m m y slightly less than previous rate estimates attributed to the shallow subduction in the pacific of the cocos ridge beneath the central american island arc system development of similar high resolution age models from other marine basins should provide the chronostratigraphic control necessary to assess environmental events and evolutionary trends in shallow marine faunas separated and isolated by the central american isthmus

magnetic fabrics of the bloodgood canyon and shelley peak tuffs southwestern new mexico implications for emplacement and alteration processes anisotropy of magnetic susceptibility ams of the middle tertiary bloodgood canyon and shelley peak tuffs of the mogollon datil volcanic field has been used to 1 evaluate the ability of ams to constrain flow lineations in low susceptibility ash flow tuffs 2 establish a correlation between magnetic fabric magnetic mineralogy tuff facies and characteristics of the depositional setting and 3 constrain source locations of the tuffs the tuffs are associated with the overlapping bursum caldera and gila cliff dwellings basin the high silica bloodgood canyon tuff fills the gila cliff dwellings basin and occurs as thin outcrops outside of the basin the older shelley peak tuff occurs as thin outcrops both along the boundary between the two structures and outside of the complex ams data were collected from 16 sites of bloodgood canyon tuff basin fill 19 sites of bloodgood canyon tuff outflow and 11 sites of shelley peak tuff sites were classified on the basis of within site clustering of orientations of principal susceptibility axes based on the categories of knight et al 1986 most microscopically visible oxide minerals in the bloodgood canyon tuff outflow and basin fill and in the shelley peak tuff are members of the hematite ilmenite solid solution series however irm acquisition data indicate that bloodgood canyon tuff basin fill and shelley peak tuff have magnetic mineralogy dominated by single or pseudo single domain magnetite and that the magnetic mineralogy of the bloodgood canyon tuff outflow is dominated by hematite hematite in bloodgood canyon tuff outflow is likely to be the result of deuteric and or low temperature alteration of magnetite and iron silicate minerals bulk magnetic susceptibility is higher in magnetite dominated ash flow tuff bloodgood canyon tuff basin fill and shelley peak tuff than it is in hematite dominated ash flow tuff bloodgood canyon tuff outflow bloodgood canyon tuff outflow has the highest total anisotropy h of the three units followed by shelley peak tuff and bloodgood canyon tuff basin fill all three ash flow tuffs are genearlly characterized by oblate susceptibility ellipsoids with those of the bloodgood canyon tuff basin fill nearest to spherical at high values of total anisotropy shelley peak tuff susceptibility ellipsoids attain a prolate shape those of bloodgood canyon tuff outflow attain an increasingly oblate shape three factors may influence differences in total anisotropy and susceptibility ellipsoid shape 1 ash which travelled the greatest distance before deposition may show the best development of magnetic fabric particularly of magnetic lineation 2 deposition of ash in a closed basin may inhibit laminar flow throughout the sheet and the resulting development of flow textures and 3 replacement of magnetite and iron silicates preferentially oriented within the foliation plane by hematite with strong crystalline anisotropy may enhance the magnetic susceptibility within that plane scatter in ams axis orientation within sites may result from 1 greater orientation inaccuracy in block sampled than in fielddrilled samples 2 rheomorphism and 3 low accuracy of ams measurement in low susceptibility ashflow tuffs evaluation of flow lineation based on ams of sites with well clustered k1 axes indicates that 1 bloodgood canyon tuff basin fill flowed along a generally northwest southeast azimuth 2 shelley peak tuff located on the boundary of the bursum caldera and the gila cliff dwellings basin flowed along a nearly east west azimuth and 3 bloodgood canyon tuff outflow sites have k1 susceptibility axes generally radial to the bursum gila cliff dwellings complex but within site scatter of k1 orientations is generally too large to draw conclusions about flow lineation orientation limited petrographic work on pilot thin sections adds flow direction information to ams derived flow lineation information

active thrusting landscape evolution and late pleistocene sector collapse of baru volcano above the cocos nazca slab tear southern central america in this paper stratigraphy geochronology and geologic mapping are used to characterize a sequence of quaternary deposits associated with edifice failure of baru volcano which together with balanced cross sections illustrate the upper plate s response to along strike variations in subduction properties that occur across the tear in the subducting slab located at the panama fracture zone the subducting panama fracture zone is an active transform that separates disparate styles of subduction between the thick rapid and flat subduction of the cocos plate to the west and the thinner more oblique and steeper subduction of the nazca plate to the east we focus on the arc forearc region above the cocos nazca slab tear where both the fila costena inner forearc fold and thrust belt and the exhumed cordillera de talamanca terminate along strike to the southeast the fila costena thrust belt imbricates an eocene late miocene forearc basin sequence with up to 40 km of shortening inboard of cocos plate subduction and crosscuts a sequence of pleistocene and younger volcano sedimentary units on the southwestern flanks of the active baru volcano at its southeastern termination these units constrained in age by radiocarbon dating n 11 soil color and 40 ar 39 ar n 5 data include a sequence of late pleistocene to holocene debris avalanche and lahar deposits landscape morphology the areal distribution of units and new shortening estimates from balanced cross sections collectively suggest that the southeastern termination of the fila costena thrust belt actively propagates to the southeast coeval with migration of the panama triple junction the prevailing map pattern suggests that the forearc s response to southeastward migration of the slab tear varies with distance to the trench in contrast to deformation patterns in the outermost forearc plate boundary tractions associated with shallow subduction and cocos caribbean convergence are more influential in the development of arc forearc deformation than either oblique subduction of the 2 km bathymetric scarp or right lateral shear which both occur astride the subducting panama fracture zone

a rhaetian 40 ar 39 ar age for the rochechouart impact structure france and implications for the latest triassic sedimentary record ar 39 ar dating of potassium feldspar primary spherulitic blocky and secondary idiomorphic k feldspar separated from impact metamorphosed gneiss found near videix in the western central part of the rochechouart impact structure nw massif central france yielded a rhaetian combined age of 201 2 ma 2r indistinguishable within uncertainty from the age of the triassic jurassic boundary ballen quartz intergrown with the primary k feldspar indicates post shock temperatures exceeding approximately 1000 c that affected the precursor gneiss geochemically both feldspar types represent essentially pure potassium end members apart from the approximately 15 km diameter impact deposit area the youngest crystallization age known for basement rocks in this part of the massif central is approximately 300 ma no endogenic magmatic thermal events are known to have occurred later in this region the k feldspar recrystallized from local feldspar melts and superimposed post shock hydrothermal crystallization probably within some thousands of years after the impact it is therefore suggested that the 40 ar 39 ar age for the videix gneiss as a potassic impact metasomatite dates the rochechouart impact in consistence with evidence for k metasomatism in the rochechouart impactites the new age value is distinctly younger than the previously obtained karnian norian age for rochechouart and thus contradicts the late triassic multiple impact theory postulated some years ago in agreement with the paleogeographic conditions in the western tethys domain around the triassic jurassic boundary the near coastal to shallow marine rochechouart impact is compatible with the formation of seismites and tsunami deposits in the latest triassic of the british isles and possible related deposits in other parts of europe

thrust surface geometry implications for thrust belt evolution and section balancing techniques abstract the application of modern concepts of thrust tectonics to orogenic belts has yielded many detailed restorable sections thrusts generally restore with a staircase geometry onto the undeformed stratigraphic template since the work of rich 1934 it has been generally assumed that thrusts have propagated with a staircase trajectory and that hangingwall structures result from the movement of the thrust sheet up footwall ramps however thrusts often cut through previously folded strata with a smooth trajectory smooth trajectory thrusts can be identified from footwall geometry and strain state once a smooth trajectory thrust has been identified it is insufficient to produce a balanced deformed section and its restored counterpart sequential balanced sections must be constructed reversing the deformation history the restoration of thrusts to a staircase trajectory has no mechanical significance as the thrust is merely acting as a marker if restored back beyond the time at which it propagated thrust ramps are not necessarily actual steps in the thrust surface they are often just zones where the thrust cuts through bedding in a smoothly curving trajectory ductile deformation in thrust belts can reduce bed length at depth thus removing the need for structures which are necessary to preserve constant bed lengths ductile deformation in the internal zones of orogenic belts renders the construction of balanced sections in these zones highly problematical

forearc response to subduction of the cocos ridge panama costa rica stratigraphic paleontologic and structural data from two panama costa rica forearc peninsulas located landward of the aseismic cocos ridge along the southernmost middle america trench document rapid pliocene subsidence and basin infilling followed by quaternary deformation and uplift on the burica peninsula an approximately 3 000 m thick pliocene pleistocene clastic sedimentary sequence characterized by fine grained turbidite deposits and volumetrically minor coarser grained channel fill slump and debris flow deposits bears depth diagnostic foraminifera that document rapid pliocene subsidence and deposition on a south dipping paleoslope new paleobathymetric and age estimates from foraminifera in these rocks indicate shallowing of depositional paleodepths from 2 000 to 1 200 m during the late pliocene present day exposure of these rocks indicates an average quaternary uplift rate of 1 mm yr the turbidite section on the burica peninsula is interpreted to represent a trench slope deposit and is correlated to a similar marine sedimentary section on the adjacent osa peninsula of costa rica structures developed within pliocene pleistocene strata of the outer forearc burica and osa peninsulas record minor 5 subhorizontal margin perpendicular shortening and margin perpendicular extension bedding orientations on the osa and burica peninsulas generally record arcward tilting by 10 30 of these outer forearc blocks on the burica peninsula the medial fault zone a north striking high angle right lateral strike slip fault has truncated and reoriented the pliocene pleistocene sedimentary sequence by fault bend forced folding a linear margin parallel fault herein named the ballena celmira fault zone bounds the arcward side of the osa and burica peninsulas and has accommodated vertical displacements between the outer forearc burica osa area and inner forearc terraba area throughout the tertiary structural and seismicity observations from the forearc region suggest that neogene deformation of the leading edge of the arc in this area reflects the effects of isostatic uplift and possibly increased interplate shear stresses as a result of subduction of the cocos ridge beneath this part of the arc since 1 m y ago

geomorphic and exhumational response of the central american volcanic arc to cocos ridge subduction 1 the timing of collision of the cocos ridge at the middle america trench remains one of the outstanding questions in the tectonic evolution of the central american convergent margin new analyses of the tectonic geomorphology of the cordillera de talamanca the extinct volcanic arc inboard of the cocos ridge coupled with low temperature thermochronometry data provide insight into the cooling and erosional history of the arc from late miocene to present we identify a low relief surface at high elevation along the northeastern flanks of the range which represents a relict erosional landscape cut across shallow plutonic rocks of the arc edifice longitudinal profiles of rivers on this surface are isolated from steep downstream sections by prominent knickzones that are interpreted to reflect a migrating wave of transient incision generated during differential rock uplift of the range reconstruction of pre incision profiles suggests that rock uplift during the growth of the cordillera de talamanca is no greater than 2 km this inference is corroborated by results from our apatite u th he and apatite fission track analyses along an elevation transect on mt chirripo the highest mountain in the cordillera de talamanca low temperature cooling ages overlap significantly with published high temperature40ar 39ar ages the combined results imply that rapid cooling in the late miocene was related to secular cooling of a shallow pluton rather than exhumation our results imply that rapid incision along downstream channel segments differential rock uplift and growth of the talamanca as a bivergent orogenic wedge associated with the onset of cocos ridge subduction are relatively young characteristics of the range a review of previously published radiometric ages and revised plate reconstructions for the late miocene further suggest that the cessation of arc volcanism in both the cordillera de talamanca and the cordillera central of western panama was coeval with the initiation of oblique subduction of the nazca plate during the late miocene thus we conclude that the cessation of arc volcanism in the late miocene 5 8 ma and the onset of cocos ridge collision 3 ma are separate events that reflect recent changes in the configuration of the plate boundary system

kinematic retro modelling of a cross section through a thrust and fold belt the western irish namurian basin the western irish namurian basin winb developed into a fold and thrust belt at the front of the northward propagating variscan orogenic wedge part of this basin is well exposed along the coast of county clare ireland from a detailed study that used an integrated gps mapping approach we produced a c 50 km long balanced cross section parallel to the tectonic transport vector we sequentially decompacted and retro deformed the namurian strata in 7 stages to evaluate the palinspastic situation of the basin and the amount of shortening by using passive markers in the model and a highly detailed timescale we were able to determine that shortening of the winb from the onset of central clare group sedimentation was 7 44 or 4 07 km of which shortening due to folding accounts for c 2 64 c 1 37 km and therefore c 4 80 c 2 69 km was solely because of thrusting the rate of horizontal shortening ranges from 1 34 mm a 1 this is within typical orogenic values

kinematic 3 d retro modeling of an orogenic bend in the south limon fold and thrust belt eastern costa rica prediction of the incremental internal strain distribution the south limon fold and thrust belt in the back arc area of southern costa rica is characterized by a 90 curvature of the strike of the thrust planes and is therefore a natural laboratory for the analysis of curved orogens the analysis of curved fold and thrust belts is a challenge because of the varying structural orientations within the belt based on seismic reflection lines we created a 3 d subsurface model containing three major thrust faults and three stratigraphic horizons 3 d kinematic retro deformation modeling was carried out to analyze the spatial evolution of the fold and thrust belt the maximum amount of displacement on each of the faults is from hinterland to foreland thrust 1 800 m thrust 2 600 m thrust 3 250 m the model was restored sequentially to its pre deformational state the strain history of the stratigraphic horizons in the model was calculated at every step this shows that the internal strain pattern has an abrupt change at the orogenic bend contractional strain occurs in the forelimbs of the hanging wall anticlines while a zone of dilative strain spreads from the anticline crests to the backlimbs the modeling shows that a nne directed transport direction best explains the structural evolution of the bend this would require a left lateral strike slip zone in the north to compensate for the movement and thereby decoupling the south limon fold and thrust belt from northern costa rica therefore our modeling supports the presence of the trans isthmic fault system at least during the plio pleistocene

the seismic source parameters of the 1991 costa rica aftershock sequence evidence for a transcurrent plate boundary the april 22 1991 valle de la estrella costa rica earthquake ms 7 6 was a back arc thrusting event associated with the underthrusting of the caribbean plate beneath central america a network of three passcal type portable instruments was deployed to monitor the aftershock activity in southern costa rica 2 to 6 weeks after the main shock the waveforms recorded on three component midperiod seismometers were used to recover source information for 15 small aftershocks magnitudes between 3 2 and 4 4 with a linear moment tensor inversion method we conducted several tests to investigate the effects of unknown structure and event mislocation on source parameter recovery the longer period waveforms in general are less sensitive to the effects of the structural details so that the essential source information can be successfully extracted from the waveform data the earlier part of the seismic wave forms has proven to be the most important carrier of the source information a gross crustal model can be used to describe the structure for the source study the small changes in the waveform character resulting from the mislocation of the events or inexact green s functions generated from the oversimplified crustal model do not prohibit us from the recovery of the source orientation at local distances in contrast the determination of the focal depth is subject to uncertainty because of the lack of detailed structural information our focal mechanisms are generally in good agreement with p wave first motion fault plane solutions determined from a local short period network the aftershocks show a clear spatial segmentation based on focal mechanism type most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock in contrast a cluster of aftershocks northwest of the main shock showed dominantly left lateral strike slip motion on a northeasterly striking nodal plane this suggests that a diffuse deformation zone exists in central costa rica and is characterized by left lateral strike slip motion this diffuse transcurrent deformation zone coincides with several geologic and geophysical features and perhaps is a result of the slower subduction rate of the buoyant cocos ridge than its adjacent segments along the middle america trench mat the diffuse transcurrent boundary may intersect with the north panama deformed belt npdb near limon costa rica and is very likely a plate boundary for the proposed panama block

early neogene history of the central american arc from bocas del toro western panama a newly discovered sequence of lower to middle miocene rocks from the eastern bocas del toro archipelago western panama reveals the timing and environment of the earliest stages in the rise of the isthmus of panama in this region two new formations the punta alegre formation lower miocene aquitanian to burdigalian and the valiente formation middle miocene langhian to serravallian are here named and formally described the punta alegre formation contains a diagnostic microfauna of benthic and planktic foraminifera and calcareous nannofossils that indicate deposition in a 2000 m deep pre isthmian neotropical ocean from as old as 21 5 18 3 ma its lithology varies from silty mudstone to muddy foraminiferal ooze with rare thin microturbidite layers near the top the valiente formation which ranges in age from 16 4 to ca 12 0 ma lies with slight angular unconformity on the punta alegre formation and consists of five lithofacies 1 columnar basalt and flow breccia 2 pyroclastic deposits 3 coarse grained volcaniclastic deposits 4 coral reef limestone with diverse large coral colonies and 5 marine debris flow deposits and microturbidites these lithofacies are interpreted to indicate that after ca 16 ma a volcanic arc developed in the region of bocas del toro and that by ca 12 ma an extensively emergent archipelago of volcanic islands had formed 3 9 ar 4 0 ar dating of basalt flows associated with the fossiliferous sedimentary rocks in the upper part of the valiente formation strongly confirms the ages derived from planktic foraminifera and nannofossils paleobathymetric analysis of the two new formations in the valiente peninsula and popa island in the bocas del toro archipelago shows a general shallowing from lower through upper bathyal to upper neritic and emergent laharic and fluviatile deposits from ca 19 to 12 ma the overlying nonconformable bocas del toro group contains a lower transgressive sequence ranging from basal nearshore sandstone to upper bathyal mudstone ca 8 1 5 3 ma and an upper regressive sequence 5 3 3 5 ma a similar paleobathymetric pattern is observed from the gatun to chagres formations 12 6 ma in the panama canal basin area and in the uscari rio banana quebrada chocolate and moin formations 8 1 7 ma in the southern limon basin of costa rica

correlation of ash flow tuffs discrimination and correlation of ash flow sheets is important in structurally complex long lived volcanic fields where such sheets provide the best keys to the regional stratigraphic framework three dimensional complexities resulting from pulsatory eruptions sectorial emplacement mechanical sorting during outflow thermal and compositional zoning of magmas the physical zoning of cooling units and structural and erosional disruption can make such correlation and discrimination difficult when lithologic magnetic petrographic chemical and isotopic criteria for correlating ash flow sheets are critically evaluated many problems and pitfalls can be identified distinctive phenocrysts pumice clasts and lithic fragments are among the more reliable criteria as are high precision k ar ages and thermal remanent magnetization trm directions in unaltered welded tuff chemical correlation methods should rely principally upon welded or nonwelded pumice blocks not upon the ash flow matrix which is subject to fractionation mixing and contamination during emplacement compositional zoning of most large sheets requires that many samples be analyzed before phenocryst glass or whole rock chemical trends can be used confidently as correlation criteria

new model for evolution of fold and thrust belt curvature based on integrated structural and paleomagnetic results from the pennsylvania salient in a number of curved fold and thrust belts worldwide such as the cantabrian arc and the wyoming idaho salient paleomagnetic data indicate vertical axis rotations inconsistent with structural findings this apparent conflict is especially pronounced in the pennsylvania salient where the paleomagnetically defined vertical axis rotations are opposite of the rotation sense indicated by structural studies we resolve this apparent disparity by developing an integrated and kinematically admissible model that has implications for other curved mountain belts the curvature of the pennsylvania salient may be explained by deformation partitioning in an initially laterally tapered basin between the cambrian ordovician carbonates and the overlying siliciclastic rocks earliest deformation occurred in the lowest strata and progressed toward the foreland and up section with time lateral differences in initial layer parallel shortening in the cambrian ordovician carbonates caused differential translation and rotations about a vertical axis in the cover sequence because of initial basin geometry the orogenic wedge developed a lateral taper with further shortening in the wedge and involvement of the cover rocks in the fold and thrust deformation gravitational driving forces became more important and the paleostress trajectories diverged in opposite directions on the two arms of the salient in response to the lateral taper

40ar 39ar laser dating of single grains ages of quaternary tephra from the east eifel volcanic field frg four late pleistocene tephra deposits from the east eifel volcanic field have been dated by 40ar 39ar single grain laser fusion of k feldspar and phlogopite crystals this is a high speed and high precision technique which because it is grain specific can directly detect the presence of older contaminating material our results indicate that the huttenberg tephra from wehr volcano erupted 213 4 ka bp and that the rieden volcanic complex erupted at least over the period 410 to 470 ka bp essential rieden phonolite lapilli contain abundant sanidine with apparent ages older than the eruption age

40ar 39ar age spectrum analysis of detrital microclines from the southern san joaquin basin california an approach to determining the thermal evolution of sedimentary basins abstract detrital microcline grains from sedimentary strata preserve a record of thermal evolution in the temperature range 100 to 200 c which can be revealed by40ar 39ar age spectrum analysis microcline separates from deep drill hole intersections with eocene to miocene sediments in the basin and tejon blocks of the southern san joaquin valley california analysed by the age spectrum approach show radiogenic40ar 40ar gradients that record both the slow cooling of the uplifting sediment source 65 ma ago and a recent thermal event this information in conjunction with the observation of fission track annealing in the coexisting apatites allows estimation of the temperature time conditions of this thermal event at about 140 c for 200 ka present and paleotemperature data is in accord with heating related to several kilometers of pleistocene sediment deposition heat flow calculations suggest that this recent subsidence has depressed the thermal gradient from about 30 c km 1 to the present apparent gradient of 24 c km 1 40ar 39ar analysis of detrital microcline crystals yields thermochronological information in the temperature time range of petroleum maturation and provides this technique with potential as both a useful exploration tool and as a means of probing the fundamental geodynamic processes of basin evolution

finite strain and strain variation analysis in the sheeprock thrust sheet an internal thrust sheet in the provo salient of the sevier fold and thrust belt central utah abstract the sheeprock thrust sheet in west central utah is an internal thrust sheet in the provo salient of the sevier fold and thrust belt we have measured finite strain in quartzites the dominant lithology sampled along a square grid within the thrust sheet using the modified normalized fry method mcnaught m a 1994 modifying the normalized fry method for aggregates of non elliptical grains journal of structural geology16 493 503 the x y and x z axial ratios from unsampled locations within the sample area were estimated using the spatial statistics approach the strain ellipsoids exhibit a variable three dimensional orientation pattern resulting from modification of the initial layer parallel shortening lps strain ellipsoid by fault parallel shear in conjunction with vertical flattening and or horizontal stretching indicating that the thrust sheet did not undergo plane strain deformation in the transport plane this suggests that the plane strain assumption used in drawing restorable balanced cross sections breaks down for internal thrust sheets with more than one penetrative strain producing deformation event the x z strain axial ratios decrease away from the thrust towards the middle of the sheet the x y strain axial ratios from interpolated image diagrams indicate transport parallel stretching at the front end of the sheet and strike parallel stretching at the back end of the sheet the footwall and hanging wall finite strain patterns are similar indicating that most of the strain in the sheeprock thrust sheet developed early in the deformation history of the thrust sheet before and perhaps during the growth of a large fault propagation fold pair

predicting the orientation of joints from fold shape results of pseudo three dimensional modeling and curvature analysis we treat layers of sedimentary rock as elastic plates and predict the orientations of joints by assuming that they open parallel to the maximum instantaneous stretch of a layer because the direction of maximum instantaneous stretch is parallel to the maximum curvature of a surface we hypothesize that joints will trend parallel to the minimum curvature of an elastically deformed layer after constructing pseudo three dimensional trishear models of laramide style uplifts that grow self similarly we calculated the direction of minimum curvature axes during the evolution of the fold our analysis of minimum curvature axes in evolving folds suggests several important characteristics for fold related joint sets 1 joints that are neither parallel nor perpendicular to the fold axis may be induced by local fold related strains 2 at any time during folding joint orientations may vary according to the structural position on a fold 3 at any location on a fold joint orientation may depend on when a joint forms during the evolution of the fold and 4 joint patterns in trishear folds may vary with stratigraphic position natural folds that evolve along simple geometric pathways may develop fold related joint sets the orientation dominance abutting relationships spacing and continuity of which will vary systematically throughout the structure this variation in joint system architecture may reflect the history of fold growth

high precision 40ar 39ar dating of oligocene rhyolites from the mogollon datil volcanic field using a continuous laser system replicate 40ar 39ar analyses of sanidine from seven oligocene 28 2 ma rhyolite flows using a continuous laser dating system routinely yield analytical precisions of less than 0 45 1s on samples smaller than 0 5 mg this is as good or better than has been obtained by any other method even though the sample mass is several orders of magnitude smaller the system has the potential of resolving differences in apparent age of as little as 0 2 for high k ca samples as young as a few ma the analyses suggest that the seven volcanic units analyzed were erupted in an interval of about 0 1 m y or less

geometric constraints derived from the law of conservation of volume and applied to evolutionary models for detachment folding this paper presents geometric arguments for modifying the conventional concept for the evolution of concentric fold trains above nonductile detachment zones the common geometric model for concentric folding above a detachment invokes a wave train with fixed fold axes regularly spaced along the fold panel limb lengths from crest to trough remain constant as the limb dips increase to uplift the anticlinal crests while the synclinal troughs slide parallel to the detachment plane the basic depth to detachment calculation which derives from the law of conservation of volume requires that the area of uplift above regional for a concentric anticline be equal to the arithmetic product of the shortening and the depth to detachment this imposes a geometric constraint with which a constant limb length anticline growing by increasing limb dip above a fixed detachment plane cannot comply unless in the early stages mobile material in the detachment zone flows from the synclinal troughs to the anticlinal crests and then in the later stages the flow direction reverses relatively few detachment zones contain a significant amount of mobile salt or shale therefore in most concentric detachment fold trains a basic geometric incompatibility exists between the conventional geometric model and themore law of conservation of volume the conflict can be resolved by an evolutionary fold model wherein the anticlinal fold limbs are short at the inception of folding and grow longer as dips increase and the fold grows in this model the anticlinal axes generally are fixed and the synclinal axes active which has structural and economic implications for the distribution of permeability and syntectonic sediments as well as for the migration and entrapment of hydrocarbons 18 figs less

crossing of the equator by the deep western boundary current in the western atlantic ocean abstract property distributions and geostrophic shear from a hydrographic section near 37 w in the atlantic ocean show the deep western boundary current dwbc in the north atlantic deep water nadw established against the western boundary of the brazil basin immediately south of the equator between 2 and 5 s the dwbc thus has directly crossed the equator to the south atlantic following the east southeast trend of the continental slope isobaths the estimated dwbc transport of nadw is 35 106 m3 s 1 similar to other estimates from the tropics discussed here these large dwbc transports are opposed by flow of deep water to the north atlantic immediately offshore of the dwbc with as much as two thirds of the dwbc transport being returned as these recirculations one recirculation center is the guiana basin north of the equator but extends at least a few hundred kilometers south of the equator another is visible at 11 s in the brazil basin the degree of connection of these two observed recirculatio

depositional history of the western colombian basin caribbean sea revealed by seismic stratigraphy seismic stratigraphic analysis of multichannel seismic reflection profiles indicates that the sedimentary section in the western colombian basin comprises five principal units the deposition of these units was strongly influenced by the basement relief of an oceanic plateau the five units define three depositional episodes the middle three units defining a single episode the depositional history of the western colombian basin supports plate tectonic models of this area based on the insertion of pacific lithosphere the first depositional episode is interpreted to be an extended period of biogenic pelagic sedimentation before and during the insertion of pacific lithosphere it is characterized by accumulation of a thick 0 5 km layer of pelagic deposits on the crest of the plateau above the carbonate compensation depth these deposits probably correlate with similar rocks of late cretaceous to eocene age bounded by reflection horizons a and b in the venezuelan basin the second depositional episode represents a major change in sedimentation with the influx of a large volume of clastic sediment transported mainly by unconfined turbidity flows this episode is inferred to span eocene through miocene time sediments were derived from sources north and west of the basin resuspension of some of these sediments in nepheloid layers probably led to hemipelagic deposition on topographically elevated areas pervasive internal deformation and physical properties suggest that this hemipelagic layer is undercompacted the beginning of this episode apparently corresponds to initiation of predominantly eastward motion of the caribbean plate relative to the americas the final depositional episode began in the late miocene epoch and extends to the present it is characterized by deposition of turbiditefan complexes including a large fan dominated by leveed channel systems offshore of costa rica development of this fan was in response to uplift and increased volcanism in southern central america caused by the collision between the panama arc and south america

a lagrangian numerical investigation of the origins and fates of the salinity maximum water in the atlantic the origins and fates of the atlantic salinity maximum water smw formed through excess evaporation in the tropics and subtropics of both hemispheres are studied using monthly mean outputs of a numerical simulation of the world ocean climatological circulation after defining formation domains from the surface salinity field and the vertical stratification a lagrangian technique is used to estimate the formation rates and main pathways in each hemisphere and the role of this water in the framework of the warm water return flow of the meridional overturning cell formation rates around 9 and 11 sv are found in the southern and northern hemispheres respectively while the export of the southern smw from its formation area is realized by the western boundary currents that of the northern smw mainly results from interior subduction equatorward of the formation regions a fraction of each smw variety is entrained in the subtropical cells that connect the subtropics to the equatorial region poleward of them both varieties are seen to feed the regions of subtropical mode water stmw formation around 35 of latitude in both hemispheres the bulk of the transport associated with each variety eventually turns northward this amounts to 6 sv of southern smw gathered in the north brazil undercurrent nbuc and 10 sv of northern smw found in the gulf stream at 35 n of which 8 sv have gone through the caribbean sea of the 13 4 sv northward transport of the meridional overturning cell estimated by the model at 47 n more than 50 6 9 sv is found to have transited through at least one of the smw regions this gives an indication of the likely important role of smw formation in the observed northward salinity increase of the upper atlantic ocean

timing and development of the heise volcanic field snake river plain idaho western usa the snake river plain srp developed over the last 16 ma as a bimodal volcanic province in response to the southwest movement of the north american plate over a fixed melting anomaly volcanism along the srp is dominatedby eruptions of explosive high silica rhyolites and represents some of the largest eruptions known basaltic eruptions represent the final stages of volcanism forming a thin cap above voluminous rhyolitic deposits volcanism progressed generally from west to east along the plain episodically in successive volcanic fields comprised of nested caldera complexes with major caldera forming eruptions within a particular field separated by ca 0 5 1 ma similar to and in continuation with the present day yellowstone plateau volcanic field passage of the north american plate over the melting anomaly at a particular point in time and space was accompanied by uplift regional tectonism massive explosive eruptions and caldera subsidence and followed by basaltic volcanism and general subsidence the heise volcanic field in the eastern srp idaho represents an adjacent and slightly older field immediately to the southwest of the yellowstone plateau volcanic field five large volume 0 5 km 3 rhyolitic ignimbrites constitute a time stratigraphic framework of late miocene to early pliocene volcanism for the study region field relations and high precision 4 0 ar 3 9 ar age determinations establish that four of these regional ignimbrites were erupted from the heise volcanic field and form the framework of the heise group these are the blacktail creek tuff 6 62 0 03 ma walcott tuff 6 27 0 04 ma conant creek tuff 5 51 0 13 ma and kilgore tuff 4 45 0 05 ma all errors reported at 2a the fifth widespread ignimbrite in the region is the arbon valley tuff member of the star light formation 10 21 0 03 ma which erupted from a caldera source outside of the heise volcanic field these results establish the conant creek tuff as a distinct and widespread ignimbrite in the heise volcanic field eliminating former confusion resulting from previous discordant k ar and fission track dates new 4 0 ar 3 9 ar determinations when combined with geochemical lithologic geophysical and field data define the volcanic and tectonic history of the heise volcanic field and surrounding areas volcanic units erupted from the heise volcanic field also provide temporal control for tectonic events associated with late cenozoic extension in the snake range and with uplift of the teton range wyoming in the snake range movement of large 0 10 km 3 slide blocks of mississippian limestone exposed 50 km to the east of the heise field occurred between 6 3 and 5 5 ma and may have been catastrophically triggered by the caldera eruption of the 5 51 0 13 ma conant creek tuff this slide block movement of 300 vertical meters indicates that the snake range had significant relief by at least 5 5 ma in jackson hole the distribution of outflow facies of the 4 45 0 05 ma kilgore tuff related to eruption from the kilgore caldera in the heise volcanic field on the eastern srp indicates that the northern teton range was not a significant topographic feature at this time

protracted near solidus storage and pre eruptive rejuvenation of large magma reservoirs building super eruptive magma reservoirs in the cold upper parts of earth s crust requires a significant influx of magma over an extended period sufficient to allow the magma to accumulate differentiate and periodically erupt some models favour magma storage in a cold non eruptible state requiring extensive reactivation of the reservoirs before eruption whereas others suggest storage at higher temperature and lower crystallinity implying that magma in such reservoirs is readily eruptible consequently constraining volcanic hazards requires observations directly linking magma residence timescales to the thermal state and crystallinity of storage here we simultaneously determine crystallization temperatures and ages of magmatic crystals of zircon and titanite in the 900 km3 kneeling nun tuff new mexico usa which allows us to place tight constraints on the long term thermal evolution of the magma reservoir we show that zircon and titanite crystals record more than 600 000 years of magma assembly and constrain the dominant storage conditions to low temperatures set between the granitic solidus 680 to 700 c and the temperature of the onset of titanite crystallization about 720 to 730 c we apply the zircon titanite systematics to a suite of other super eruptions and suggest that protracted low temperature storage culminating in late stage reheating is a widespread feature of large crystal rich eruptions super eruptions are fed by large magma reservoirs geochemical analyses of volcanic rocks erupted in new mexico suggest the magma was stored under cool conditions in the crust for 600 000 years before late stage heating triggered an eruption

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

deformation style and basin fill architecture of the offshore limon back arc basin costa rica abstract the limon back arc basin located at the eastern coast of costa rica is part of the southern central american arc trench system basin evolution started in late cretaceous times as response to the onset of the subduction of the farallon plate below the caribbean plate the limon basin can be subdivided into a northern and a southern sub basin separated by the e w trending trans isthmic fault system a regional grid of offshore seismic lines allows the comparative analysis of the basin fill architecture and the deformation style in both sub basins the northern sub basin shows exclusively normal faults the basin fill architecture resembles a passive continental margin setting showing a distinct wedge shaped geometry and seaward propagating depositional units the area of the rio san juan delta is characterized by gravity driven deltaic deformation exhibiting a series of listric growth faults both on the shelf and in slope areas in contrast to the northern basin the southern sub basin is characterized by the development of thin skinned fold and thrust belt tectonics best recorded in concentric or asymmetric hangingwall anticlines separated by listric or planar southwestward dipping thrust faults due to the pronounced ne propagating of folding the shelf is broader and the slopes are steeper in the southern sub basin compared with the northern sub basin thus both sub basins show a very different tectonic style in the north limon basin an extensional regime established whereas in the south limon basin compression dominated however in both sub basins the basal detachment is probably controlled by a lithological change from limestone to shale

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

fracturing of the panamanian isthmus during initial collision with south america tectonic collision between south america and panama began at 23 25 ma the collision is significant because it ultimately led to development of the panamanian isthmus which in turn had wide ranging oceanic climatic biologic and tectonic implications within the panama canal zone volcanic activity transitioned from hydrous mantle wedge derived arc magmatism to localized extensional arc magmatism at 24 ma and overall marks a permanent change in arc evolution we interpret the arc geochemical change to result from fracturing of the panama block during initial collision with south america fracturing of the panama block led to localized crustal extension normal faulting sedimentary basin formation and extensional magmatism in the canal basin and bocas del toro synchronous with this change both panama and inboard south america experienced a broad episode of exhumation indicated by u th he and fission track thermochronology coupled with changing geographic patterns of sedimentary deposition in the colombian eastern cordillera and llanos basin such observations allow for construction of a new tectonic model of the south america panama collision northern andes uplift and panama orocline formation finally synchroneity of panama arc chemical changes and linked uplift indicates that onset of collision and isthmus formation began earlier than commonly assumed

constraints on inner forearc deformation from balanced cross sections fila costena thrust belt costa rica 1 the fila costena thrust belt in the forearc of costa rica is accommodating a significant portion of the convergence of the cocos plate and panama microplate geologic mapping of the thrust belt depicts a duplex with three horses that incorporate eocene limestones and oligocene to early miocene clastics inboard of the subducting cocos ridge axis by constructing a cross section at this location along a ne sw trending transect perpendicular to the thrust belt we constrain a shortening rate of approximately 40 mm a and propose that as much as 50 of the total plate convergence rate is taken up in the inner forearc the eocene limestones at the base of the thrust sheets pinch out in both directions away from the onland projection of the cocos ridge axis owing to decrease in slip on faults and a lateral ramp in the basal decollement the thrust belt terminates near the panama border at the onland projection of the subducting panama fracture zone these observations suggest that shortening is propagating to the east with the migration of the panama triple junction and the onset of shallow subduction of the thickened edge of the cocos plate the absence of similar features in the nicaraguan forearc where the subducting crust is older subducts more steeply and lacks incoming ridges and seamounts indicates that deformation of the forearc basin in costa rica reflects greater coupling between the converging plates inboard of the cocos ridge

a plate tectonic model for the mesozoic and early cenozoic history of the caribbean plate abstract we present a model in which the caribbean plate is an intra american feature formed along the caribbean spreading center as opposed to the current model that considers the caribbean plate as a far travelled crustal segment that formed in the pacific region paleomagnetic data which cover an age range from jurassic through paleocene indicate the ophiolite complexes in costa rica and panama formed in an equatorial position the ophiolites did not fundamentally change their position relative to south america since their origin basaltic rocks of the lower part of the ophiolites are of mid ocean ridge type suggesting formation at a spreading center they are interpreted to have formed as part of the proto caribbean crust at a spreading axis in an intra american position during jurassic and early cretaceous times the upper part of the ophiolites is mainly built up by island arc and intraplate basalts the island arc basalts evolved at the central american landbridge which started in the middle cretaceous the intraplate basalts are related to the caribbean plateau basalt which thickened and stiffened the caribbean crust in middle cretaceous to probably campanian times changes in relative plate motions along the middle american convergent plate margin are reflected by orientations and time sequences of paleostress tensors calculated from fault slip data in southern mexico and costa rica kinematic indicators from mylonite zones display sinistral movement along secondary shear zones within the southern mexican crustal complexes and along the northern boundary of the chortis block in the motagua polochic fault system this reflects the eastward drift of the chortis block since the beginning of the cenozoic at this time the chortis block became part of the caribbean plate which moved more than 1000 km towards the east relative to the north and south american plates during the cenozoic

kinematic model for the curvature of the northern subalpine chain france abstract analyses of cross sectional shortening mapped faults and calcite twin strain are used in conjunction with geometric kinematic modeling to interpret the mechanism of curvature formation in the northern subalpine chain of france there is a 45 change in strike around the chain that is convex towards the foreland northwest five models for fold thrust belt curvature are applied to the northern subalpine chain they can reproduce the geometry and the kinematic history of a curved fold thrust belt and can be used to predict the magnitude and orientations of strains and the three dimensional geometry of the curvature the models are pure bending radial thrusting curve parallel simple shear uniform displacement uniform shortening and transport parallel simple shear two principal criteria for distinguishing among the models geometrically are the horizontal tangential strain extension or contraction parallel to strike and cross sectional shortening the transport parallel simple shear model predicts no tangential extension in the central domain and greater radial shortening and significant tangential extension in the northeastern and southern domains this is the pattern observed in the northern subalpine chain therefore the curvature of the northern subalpine chain is attributed to a transport parallel simple shear mechanism

geometric constraints derived from the law of conservation of volume and applied to evolutionary models for detachment folding geologic note 1 this paper presents geometric arguments for modifying the conventional concept for the evolution of concentric fold trains above nonductile detachment zones the common geometric model for concentric folding above a detachment invokes a wave train with fixed fold axes regularly spaced along the fold panel limb lengths from crest to trough remain constant as the limb dips increase to uplift the anticlinal crests while the synclinal troughs slide parallel to the detachment plane the basic depth to detachment calculation which derives from the law of conservation of volume requires that the area of uplift above regional for a concentric anticline be equal to the arithmetic product of the shortening and the depth to detachment this imposes a geometric constraint with whi h a constant limb length anticline growing by increasing limb dip above a fixed detachment plane cannot comply unless in the early stages mobile material in the detachment zone flows from the synclinal troughs to the anticlinal crests and then in the later stages the flow direction reverses relatively few detachment zones contain a significant amount of mobile salt or shale therefore in most concentric detachment fold trains a basic geometric incompatibility exists between the conventional geometric model and the law of conservation of volume the conflict can be resolved by an evolutionary fold model wherein the anticlinal fold limbs are short at the inception of folding and grow longer as dips increase and the fold grows in this model the anticlinal axes generally are fixed and the synclinal axes active which has structural and economic implications for the distribution of permeability and syntectonic sediments as well as for the migration and entrapment of hydrocarbons

controls on the geometry of fold thrust belt salients salients convex to the foreland curves in the map traces of orogens typically involve fold thrust belts formed along the foreland margin of orogens an examination of the map view shape degree of asymmetry dimensions and structural trend line patterns i e the variation in fold trend and fault strike along the length of the belt of 20 fold thrust belt salients worldwide illustrates relationships between salient geometry and the geologic setting in which the salient formed in order to understand these relationships we used simple sandbox models to simulate the formation of three of the several types of salients specifically we examined indenter controlled salients the geometry of which reflects interaction of a fold thrust belt with a hinterland indenter basin controlled salients the geometry of which reflects lateral variations in the thickness of predeformational basin fill and detachment controlled salients the geometry of which reflects lateral variations in the strength of a detachment surface our models demonstrate that thrusts in these three types of salients initiate with a curved shape and that rocks within thrust sheets on the limbs of salients can undergo a vertical axis rotation in response to simple shear without requiring major rotation of the fault trace furthermore trend line patterns distinguish basin controlled from indenter controlled salients specifically structural trend lines in basin controlled salients converge at the end points of the curve while in indenter controlled salients they diverge at the end points of the curve the asymmetry of a salient correlates with the asymmetry of the predeformational basin and or of the indenter and with the convergence direction thus our results explain geometric characteristics of individual salients and address controversies concerning paleomagnetic data in salients they also may help prioritize petroleum exploration targets in salients foreland basin oil fields concentrate at the apex of basin controlled salients because the geologic factors that generate the salient also contribute to localizing oil fields

a hierarchical tectonic model of the gulf of mexico and caribbean region abstract a quantitative computer graphic methodology hierarchical tectonic analysis has been used to model the tectonic evolution of the gulf of mexico and caribbean region this method employs a highly structured hierarchical description of the relative motions of pairs of tectonic elements in order to quantify the timing and style of plate interactions the main features described in this model are 1 1 a revised fit of pangea 2 2 the opening of the gulf of mexico in the middle late jurassic 3 3 the formation of a proto caribbean ocean due to the separation of north and south america during the early cretaceous 4 4 the formation of the greater antilles arc along the western margin of the proto caribbean ocean 5 5 the insertion of the farallon plate between north and south america during the middle cretaceous resulting in the northeastward advance of the greater antilles arc and the subduction of proto caribbean oceanic crust 6 6 the complete subduction of the proto caribbean ocean and the collision of the greater antilles arc with the bahamas platform in the late cretaceous 7 7 the truncation of the caribbean section of the farallon plate from its pacific counterpart and the formation of the panama arc latest cretaceous and 8 8 the westward movement of north and south america with respect to the caribbean plate resulting in the opening of the yucatan basin paleocene and subsequently in the evolution of the cayman trough and e w zone of strike slip

seamount ridge and transform subduction in southern central america understanding the factors that control subduction zone processes is a first order goal in the study of convergent margins in southern central america a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion fluid flow deformation and seismogenesis in this paper i evaluate the role that seamount ridge and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a 500 km long region of central america where each of these three scenarios are present along strike the data show that the subduction of short wavelength bathymetry e g seamounts and faults on the seafloor produces short wavelength deformation that persists for relatively short timescales 104 105 years whereas the subduction of longer wavelength bathymetry e g the aseismic cocos ridge results in longer wavelength deformation that endures over a longer time scale 106 years the timing and distribution of upper plate deformation are consistent with subhorizontal cocos ridge subduction driving upper plate deformation and the increased crustal thickness 20 km of the subducting cocos ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening the data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in middle america

high precision 40ar 39ar sanidine geochronology of ignimbrites in the mogollon datil volcanic field southwestern new mexico 40ar 39ar age spectra have been obtained from 85 sanidine separates from 36 ignimbrites and one rhyolitic lava in the latest eocene oligocene mogollon datil volcanic field of southwestern new mexico of the 97 measured age spectra 94 yield weighted mean plateau ages each giving single spectrum 1 precision of 0 25 0 4 0 07 0 14 ma replicate plateau age determinations for eight different samples show within sample 1 precisions averaging 0 25 plateau ages from multiple n 3 8 samples of individual ignimbrites show 1 within unit precision of 0 1 0 4 0 04 0 13 ma this within unit precision represents a several fold improvement over published k ar data for the same ignimbrites and is similar to the range of precisions reported from single crystal laser fusion studies a further indication of the high precision of unit mean 40ar 30ar ages is their close agreement with independently established stratigraphic order two samples failed to meet plateau criteria apparently due to geologic contamination by older feldspars effects of minor contamination are shown by six other samples which yielded slightly anomalous plateau ages 40ar 39ar plateau ages permit resolution of units differing in age by 0 5 0 15 ma or less this high resolution combined with paleomagnetic studies has helped to correlate ignimbrites among isolated ranges and has allowed development of an integrated timestratigraphic framework for the volcanic field mogollon datil ignimbrites range in age from 36 2 to 24 3 ma ignimbrite activity was strongly episodic being confined to four brief 2 6 m y eruptive episodes separated by 1 3 m y gaps ignimbrite activity generally tended to migrate from the southeast toward the north and west

the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake and its tectonic implications a broadband seismic study the rapture process of a large back arc thrusting earthquake the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake is investigated using broadband body waves and long period surface waves we find the source process to be relatively simple with the source models separately obtained from body and surface waves being very consistent the event occurred on a shallow southwest dipping rapture plane on which most energy is released updip of the hypocentral location 10 20 km deep high frequency radiation appears to have been released over a relatively small source area our preferred model has a focal mechanism with strike 102 10 dip 17 14 and rake 63 17 a seismic moment of 3 8 1 5 1020 n m and a total rupture duration of 40 6 s the earthquake appears to be associated with the north panama deformed belt npdb a thrust and fold complex that has accommodated the oroclinal deformation of panama this event along with previous large events north of panama in 1882 and 1916 indicates that there is substantial convergence along the npdb marking the npdb as a probable emerging plate boundary it remains difficult to gauge the earthquake hazard in the region because of the tectonic complexity

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

map view retrodeformation of an arcuate fold and thrust belt the jura case 1 the mechanism of arcuate mountain range formation is a matter of debate here we perform a map view restoration of a detailed three dimensional model of the jura arc a typical arcuate mountain range in the foothills of the swiss and french alps this retrodeformation is performed using the unfold surface balanced program based on a block mosaic method it results in a displacement field divergent toward the deformation front this displacement field suggests counterclockwise rigid rotations in the horizontal plane up to 30 in the southern internal jura and a corresponding vertical axis rotation or shear strain of the savoie molasse basin hinterlandward we also predict a 10 clockwise vertical axis rotation of the molasse basin in switzerland all these rotations agree with those documented by paleomagnetic data the former are taken to result from a substantial decrease in shortening at the southern jura end while the divergence of displacements is interpreted to result from variations in the detachment level distribution

kinematic evolution and structural styles of fold and thrust belts fold and thrust fat belts occur worldwide and have long been the focus of research of structural geologists who have devised a variety of techniques to image characterize and model their main structural features this introductory chapter reviews the principal geological features of fat belts formed in different settings emphasizing aspects related to their kinematic evolution and structural styles despite great advances challenges remain particularly in the understanding of the spatial and temporal evolution 4d of fat belts and their controlling factors these research efforts are being assisted by the growing availability to researchers of relatively new tools to collect field data high quality 3d seismic data and computer and laboratory modelling tools this volume includes technical papers presented in the conference international meeting of young researchers in structural geology and tectonics yorsget 08 held in oviedo spain together with other papers on the same theme these papers deal with fat belts in different parts of the world and cover a broad range of different aspects from detailed structural analysis of single structures to regional issues and from studies based on classical field structural geology to modelling

new 40 ar 39 ar isotopic dates from miocene volcanic rocks in the lake mead area and southern las vegas range nevada new 40 ar 39 ar dates on volcanic rocks interlayered with synextensional miocene sedimentary rocks in the western lake mead area and southern end of the las vegas range provide tight constraints on magmatism basin formation and extensional deformation in the basin and range province of southern nevada vertical axis rotations associated with movement along the las vegas valley shear zone occurred after 15 67 0 10 ma 2 based on a 40 ar 39 ar date from a tuff in the gass peak formation in the southern las vegas range basaltic magmatism in the western lake mead area began as early as 13 28 0 09 ma based on a date from a basalt flow in the lovell wash member of the horse spring formation isotopic dating of a basalt from the volcanic rocks of callville mesa indicates that these rocks are as old as 11 41 0 14 ma suggesting that volcanic activity began shortly after formation of the boulder basin the extensional basin in which the informally named red sandstone unit was deposited the red sandstone unit is at least as old as 11 70 0 08 ma and contains megabreccia deposits younger than 12 93 0 10 ma this result shows that formation of the boulder basin was associated with development of topographic relief that was probably generated by movement along the saddle island low angle normal fault stratal tilting associated with extension occurred both prior to and after 11 5 ma

costa rica petroleum geology of the caribbean margin the existing database for the caribbean margin of costa rica figure 1 in cludes 1 information from wells drilled in the past by sinclair union gulf elf and pemex and more recently at the end of the 1980s with the aid of petrocanada international assistance pciac table 1 2 regional to semidetailed marine and land seismic data acquired in the 1970s and 1980s by gsi elf the university of texas sea gap gulf comesa and petrocanada figure 2 3 complete coverage of aeromagnetic and gravimetric high resolution data 4 geochemical analysis from outcrop samples oil and gas seeps as well as from well cuttings of the majority of the wells drilled in the caribbean and 5 a comprehensive sedimentological analysis of the cretaceous tertiaty sedimentary cover of the basin

calderas and ash flow tuffs of the mogollon mountains southwestern new mexico the mogollon mountains are a major volcanic source area in the southwestern mogollon datil volcanic field where about 2000 m of middle tertiary rocks are exposed the volcanic sequence includes eight major 100 km3 ash flow units believed to represent four caldera related compositionally zoned ash flow sequences each caldera related sequence shows normal compositional zonation from high silica rhyolite to rhyolite and dacite the mogollon caldera 34 ma is preserved only as a fragment in the wall of the bursum caldera the emory caldera 34 ma is about 50 km east of the mogollon mountains and only the high silica facies of its presumed outflow sheet fall canyon tuff reaches the mogollons the gila cliff dwellings caldera 30 ma earlier interpreted as the source of the bloodgood canyon tuff probably is a large remnant of a caldera or pair of calderas that were the source of the davis canyon and shelley peak tuffs the bloodgood canyon tuff was most likely erupted from the bursum caldera 29 28 ma but ponded in the gila cliff dwellings caldera the resurgent dome of the approximately 40 km diameter bursum caldera is the predominant topographic and structural feature of the mogollon mountains the caldera probably collapsed initially in response to eruption of the bloodgood canyon tuff then was largely filled with apache spring tuff prior to resurgent doming the cyclical eruption beginning about 34 ma of similar compositionally zoned ash flows from separate shallow magma chambers suggests derivation from a common parent magma of batholithic proportions a regional bouguer gravity low of about 40 mgals has been interpreted to reflect the presence of such a batholith major ash flow eruptions in the mogollon datil field were followed 26 25 ma by fundamentally basaltic volcanism characterized mainly by andesite about 22 21 ma high silica rhyolite tholeiitic basalt and alkali basalt were erupted as a bimodal suite roughly coincident with the beginning of basin range extensional faulting alkali olivine basalt as young as 5 5 ma is interlayered with basin fill gila conglomerate adjacent to the mogollon mountains

the subpolar mode water of the north atlantic ocean abstract the warm waters of the subtropical and subpolar basins of the north atlantic have tight regional temperature salinity relationships and are conventionally called the regional central waters a volumetric census of the temperature salinity characteristics of the north atlantic by wright and worthington 1970 shows that waters characterized by certain segments of the t s relationships have large volumes compared to those of other segments volumetric mode waters such mode waters appear as layers with increased vertical separation between isopycnals pycnostads the present study reports on the existence of pycnostads in the central and eastern north atlantic these subpolar mode waters are formed by deep winter convection in the subpolar north atlantic and participate in the upper water circulation of the northern north atlantic the seasonal outcropping of the pycnostads occurs within and adjacent to the north atlantic current the irminger current the east and west greenland currents and

the water masses along the western boundary of the south and equatorial atlantic a quasi meridional hydrographic section located offshore from south america from 50 s to 10 n and three shorter transverse lines to the continental slope are used for a descriptive study of the water masses along the western boundary of the south and equatorial atlantic at the upper and intermediate levels the tracer analysis provides geographical limits of the wind driven circulation regimes and a comparison of the tracer values at the continental slope and along the meridional section shows where the boundary currents originate at depths shallower than about 200 m the subdivision of the subtropical gyre into two cells separated by the subtropical countercurrent near 28 s that was pointed out in a previous study is corroborated south of this front a warm variety 18 c of subtropical mode water in the inner recirculation of the brazil current appears despite its limited extent as a southern counterpart of the north atlantic 18 c water at the deep levels the upper circumpolar water and upper north atlantic deep water enter the south atlantic in a significantly overlapping density range the ensuing lateral encounter of both water masses occurs at 26 s near the western boundary where most of the boundary flow of the latter water is stopped and deflected seaward by the base of the subtropical gyre other tracer anomalies signal significant eastward escapes of north atlantic deep water within two jets at about two degrees of latitude on either side of the equator in another narrow current at 10 s and at 34 s the latter latitude marks the confluence and eastward deflection of the opposite boundary currents of lower north atlantic deep water and lower circumpolar water near the bottom of the argentine basin the weddell sea deep water that flows westward north of the zapiola ridge is more recently ventilated than the water carried by the boundary current near the falkland escarpment while a part of it flows anticyclonically around the ridge another part turns equatorward and enhances the southern property signatures of the water farther north

40ar 39ar geochronology of jarosite the effectiveness of hf in removing silicate contaminants abstract supergene and hypogene jarosite have been dated successfully but accurate dating of weathering derived jarosite and its application to landscape evolution studies has been limited because of difficulties in obtaining high purity mineral separates hydrofluoric acid hf can remove potassium bearing silicates from supergene samples but its effects on jarosite crystal chemistry and age dating are unquantified three experiments were conducted to determine whether hf treatment removes silicates without altering the potassium or argon composition of the jarosite prior to dating using the 40 ar 39 ar method in the first two experiments pure hypogene jarosite from pena blanca mexico pb 9 42 0 22 ma and a mixture of 85 pb 15 fish canyon sanidine fc 2 28 02 ma were crushed and treated with 25 hf for 0 30 240 and 480 min jarosite partially dissolves during hf treatment with grains becoming increasingly pitted and rounded with time in acid but pb s potassium concentration and apparent age remained constant regardless of treatment time fc 2 was absent from all treated samples suggesting that 30 min in hf is sufficient to remove mechanically mixed sanidine crystals from jarosite methods developed during experiments conducted on pb jarosite were applied to fine grained supergene jarosite from the red river valley rrv in northern new mexico rrv jarosite is mixed with oligocene 24 86 ma potassium bearing silicates that make up 50 or more of each sample four rrv jarosite samples that yielded age spectra with clear signs of contamination by older phases were treated with hf for 30 min and re dated despite treatment back scattered electron images show that silicates including sanidine and illite continued to comprise approximately 30 of hf treated rrv samples and the age spectra produced during laser step heating continued to show signs of contamination in higher wattage steps however the integrated ages of these samples were consistently less than 1 ma at least 7 ma younger than the expected integrated age if the 30 70 mixture of rrv silicates and jarosite degassed completely the young ages suggest that young jarosite controls the apparent age of these samples and that these ages can be interpreted as the maximum timing of supergene jarosite formation

k ar dating incomplete extraction of radiogenic argon from alkali feldspar abstract complete recovery of radiogenic argon from high temperature alkali feldspars is more difficult than is commonly believed at a temperature sufficiently high for quantitative release of ar from biotites and hornblendes as much as 15 of the radiogenic ar can remain trapped in the melt of an alkali feldspar sample feldspar melts at a higher temperature than biotite or hornblende and lacks structural volatiles which act to flush ar from a viscous melt thus even for extractions at 1670 c for 40 min a small but measurable amount usually less than 2 of total remains behind although this quantity of residual gas may be marginally within analytical error its presence can be detected readily by remelting an extracted sample together with recent basalt for routine analyses a recommended procedure is to melt feldspar and basalt together at 1670 c or higher it is clear that inadequate experimental conditions rather than diffusive loss of radiogenic ar can be responsible for some erroneously young feldspar ages

fault related folding a review of kinematic models and their application abstract folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts accretionary wedges fold and thrust belts and intra plate settings in either strike slip compressional or extensional regimes due to their widespread occurrence knowledge about the development of these structures is important to a broad spectrum of geoscience sub disciplines such as structural geology seismology geomorphology petroleum geology and quaternary geology fault related folding has been analyzed intensively over the last 140 years for the sake of this review we divide the folds according to the way the faults and the folds form that is into detachment fault bend and fault propagation folds all fault related folds are caused by changes in fault parameters the simplest method to produce folds is to transport material along faults that have stepped flat ramp flat geometries fault bend fold alternatively the slip can decrease along the length of the fault and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy either a detachment fold or a fault propagation fold is formed respectively detachment folding was first investigated in the early 20th century whereas the full significance of fault propagation folds was recognized quite late in the 1980s seminal work on fault related folding was carried out in the 1930s but quantitative kinematic models have only been available in the last 30 years these models are extremely valuable because they allow a comprehensive understanding of the evolution of fault related folds and lead to more accurate predictions of the sub surface structure from the mid 1990s onwards numerical simulations have been used to identify how fault parameters such as dip and fault bend angle propagation to slip ratio and shape of the trishear zone influence the geometry of the related folding this is directly applicable to the analysis of the shape of anticlines produced however this does not mean that fold geometry is uniquely related to fault geometry on the contrary different kinematic approaches can lead to a similar fold shape

the crustal structure of the cocos ridge off costa rica 1 the submarine cocos ridge in the northwestern panama basin a bathymetric feature more than 1000 km long and 250 500 km broad is about 2 km shallower than the adjacent basin it is generally interpreted as the trace of the galapagos hot spot two 127 and 260 km long seismic wide angle sections were recorded along and across this ridge offshore the osa peninsula costa rica crustal thickening is seen everywhere along the sections on the northwestern outer ridge flank increased thickness is exclusively attributed to the upper crust and expressed by 2 km thick flow basalts the quepos plateau caps the upper crust in this area toward the center of the cocos ridge the moho deepens from 11 12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80 of the crust and is three times the thickness of normal oceanic lower crust it is homogeneously structured and the velocities which range from 6 5 km s at the top to 7 35 km s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition similarities to the crustal velocity structure of iceland central kerguelen plateau and broken ridge are consistent with a formation of this 13 15 ma old cocos ridge segment by excessive magmatism in a near plate boundary setting

arc continent collision and orocline formation closing of the central american seaway closure of the central american seaway was a local tectonic event with potentially global biotic and environmental repercussions we report geochronological six u pb la icp ms zircon ages and geochemical 19 xrf and icp ms analyses data from the isthmus of panama that allow definition of a distinctive succession of plateau sequences to subduction related protoarc to arc volcaniclastic rocks intruded by late cretaceous to middle eocene intermediate plutonic rocks 67 6 1 4 ma to 41 1 0 7 ma paleomagnetic analyses 24 sites 192 cores in this same belt reveal large counterclockwise vertical axis rotations 70 9 6 7 and moderate clockwise rotations between 40 4 1 and 56 2 11 1 on either side of an east west trending fault at the apex of the isthmus rio gatun fault consistent with isthmus curvature an oligocene miocene arc crosscuts the older deformed and segmented arc sequences and shows no significant vertical axis rotation or deformation there are three main stages of deformation 1 left lateral strike slip offset of the arc 100 km and counterclockwise vertical axis rotation of western arc segments between 38 and 28 ma 2 clockwise rotation of central arc segments between 28 and 25 ma and 3 orocline tightening after 25 ma when this reconstruction is placed in a global plate tectonic framework and published exhumation data is added the central american seaway disappears at 15 ma suggesting that by the time of northern hemisphere glaciation deep water circulation had long been severed in central america

paleointensity estimates from ignimbrites an evaluation of the bishop tuff 1 ash flow tuffs or ignimbrites typically contain fine grained magnetite spanning the superparamagnetic to single domain size range that should be suitable for estimating geomagnetic field intensity however ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history we examined three stratigraphic sections through the 0 76 ma bishop tuff where independent information on postemplacement cooling and alteration is available as a test of the suitability of ignimbrites for paleointensity studies thermomagnetic curves suggest that low ti titanomagnetite tc 560 c 580 c is the dominant phase with a minor contribution from a higher tc phase s significant remanence unblocking above 580 c suggests that maghemite and or titano maghemite is an important contributor to the remanence in most samples we obtained successful paleofield estimates from remanence unblocked between 440 c and 580 c for 46 of 89 specimens 15 sites at two of three total localities these specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43 0 t 3 2 equivalent to a vadm of 7 8 1022 am2 the most densely welded sections of the tuff have emplacement temperatures inferred to be as high as 660 c suggesting that the remanence may be primarily thermal in origin though a contribution from thermochemical remanence cannot be excluded these results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations

central costa rica deformed belt kinematics of diffuse faulting across the western panama block fault kinematics seismicity and geodetic data across central costa rica reveal a diffuse fault zone here named the central costa rica deformed belt ccrdb the ccrdb defines the western margin of the panama block and links the north panama deformed belt npdb along the caribbean coast with the middle america trench mat along the pacific coast the junction of the ccrdb and the mat coincides with an abrupt transition from smooth to rough crust on the subducting cocos plate rough smooth boundary shallow subduction of rough thickened oceanic crust associated with the cocos ridge shifts active shortening into the volcanic arc along faults of the ccrdb variable fault kinematics along this zone may reflect three combined deformation mechanisms horizontal shortening and shear from oceanic ridge indentation basal traction from shallow subduction and localized block uplift from subducting seamount roughness within the forearc domain 1 mesoscale faults express transtension where steep ne striking regional scale faults intersect the pacific coast across the volcanic arc domain 2 mesoscale faults exhibit mostly sinistral and dextral slip on ne and nw striking conjugate faults respectively approaching the npdb in the back arc domain 3 transcurrent faulting is modified by transpression and crustal thickening fault kinematics are consistent with earthquake focal mechanisms and global positioning system gps measurements radiometric age constraints confirm that faulting postdates the late neogene onset of shallow subduction the ensuing deformation front has propagated northward into the volcanic arc to its present position along the seismically active ccrdb within the forearc the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts

mechanics of back arc deformation in costa rica evidence from an aftershock study of the april 22 1991 valle de la estrella costa rica earthquake mw 7 7 the april 22 1991 mw 7 7 valle de la estrella costa rica earthquake represents back arc thrusting of the caribbean plate beneath the panama block along the north panama thrust belt large back arc thrusting events are quite rare occurring in only two other locations along the sunda arc and japan sea to better understand the mechanics of back arc thrusting we constrain the faulting geometry associated with the 1991 costa rica earthquake using aftershock locations and focal mechanisms obtained from a three component portable digital network deployed in and around the aftershock area following the mainshock the spatial distribution of aftershocks reveals a complicated faulting geometry in the rupture area focal mechanisms determined from inversion of p wave and tangentially and radially polarized s wave sh and sv respectively amplitudes recorded by this temporary network confirm fault complexity and indicate active thrust normal and strike slip faults in the back arc of costa rica most of the thrust events are confined to the southern portion of the aftershock zone in the vicinity of the mainshock their distribution suggests the existence of a near horizontal basal fault plane at a depth of about 15 km with many imbricate faults having steeper dips extending from the basal plane toward the surface events with strike slip mechanisms locate northwest of the thrust events and define a sw ne trending left lateral strike slip fault zone that represents the nw termination of the mainshock rupture and possibly the maximum nw extension of the panama block the superposition of the aftershock locations on a geologic map of the region shows that aftershocks are restricted to occur in the older more competent rock units volcanic and volcaniclastic rocks interbedded with carbonates of the back arc sedimentary basin shallow events depth 5 km occur only where these oldest units are exposed at the surface this suggests that 1 exposure of the lower units results from repeated earthquake slip on the shallow crustal faults imaged by the aftershocks and 2 as much as 7 km of basin fill material overlying the lower units northeast of the mainshock does not deform seismically but instead folds and possibly faults aseismically