an alternative interpretation of the cayman trough evolution from a reidentification of magnetic anomalies magnetic data were collected during the wilkes 1973 and seacarib ii 1987 cruises to the cayman trough a new interpretation of magnetic data is carried out an isochron pattern is drawn up from our anomaly identifications an early eocene age 49 ma ypresian for cayman trough opening is proposed instead of the late oligocene or middle eocene ages suggested by previous studies our plate tectonic reconstruction is simpler and fits the on land geology jamaica and cuba and the tectonics our reconstruction shows a southward propagation of the spreading centre between magnetic anomalies 8 and 6 26 and 20 ma the trough width increases by 30 km in this period the southward propagation of the cayman spreading centre from the middle oligocene to the early miocene induced the development of the restraining bend of the swan islands the formation of a 1 km high scarp on the eastern trace of the cayman trough transform fault walton fault and the formation of a pull apart basin hendrix pull apart magnetic anomalies and magnetization maps give information about the deformation and the rocks the proposed evolutionary model of the cayman trough from the inception of seafloor spreading to the present configuration is presented in relation to the tectonic escape of the northern boundary of the caribbean plate from the maastrichtian to the present

from transtension to transpression along the northern caribbean plate boundary off cuba implications for the recent motion of the caribbean plate abstract marine geophysical surveys using seabeam single channel seismic reflection gravimetric and magnetic measurements have been conducted along a segment of the northern caribbean transcurrent plate boundary seacarib ii cruise the data allow a better definition of the geometry and the tectonic regime of this major strike slip area they support the following results 1 1 along the southern cuban margin the oriente fault displays a discontinuous trace mainly composed of dextral offset en echelon segments some pull apart basins are located between fault segments cabo cruz basin chivirico and baitiquiri basins in the windward passage area the plate boundary enters into the tortue channel and is not connected with the subduction front off northern hispaniola 2 2 the eastern part of the oriente deep and the santiago promontory are characterised by active compressional tectonics they form the santiago deformed belt described here for the first time this deformed belt can be divided longitudinally into three main segments each one characterised by a particular tectonic style its development is related to a transpressional mechanism along the left lateral oriente strike slip fault our observations suggest that a tectonic and kinematic reorganisation occurred recently in this area probably in the late pliocene which may be compared with the recent geological events recorded on land in the northern caribbean domain the precise knowledge of both geometry and structures along the oriente strike slip fault south of cuba provides new constraints for the recent kinematic evolution along the northern caribbean transcurrent plate boundary it leads us to infer the existence of a convergence component associated with the slip component along the oriente transform fault

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

total moment spectra of fourteen large earthquakes total moment spectra mt m where m is the moment rate tensor are computed for 14 large earthquakes recorded by the international deployment of accelerometers ida network using the scalar moment retrieval method proposed by silver and jordan 1982 for each event we obtain estimates of mt averaged over the 10 disjunct 1 mhz intervals in the low frequency band 1 11 mhz typical ida record sets from events with mt 0 2 a 1 a 1027 dyne cm yield standard errors on the 1 mhz averages that are generally less than 20 our multiple band estimates of mt are usually consistent with comparable single band values found by other investigators from the total moment spectra we derive the zero frequency static moment mt 0 and the characteristic source duration c 2 var f t where f t is the time function of the moment rate tensor assumed to be the same for all components the parameter c formally depends on the second degree temporal spatial and mixed spatial temporal moments of an extended source but calculations with realistic source geometries indicate that the interpretation of tc strictly in terms of the second temporal moment leads to very little error a plot of c against shows considerable scatter some events lie significantly above the empirical scaling curve of kanamori and given 1981 slow earthquakes and some below fast earthquakes examples of the former include all three deep focus events analyzed here as well as the 1970 colombia and 1963 peru bolivia deep focus earthquakes whose total moment spectra are calculated from the moment tensor solutions of gilbert and dziewonski 1975 an example of the latter is the great sumbawa earthquake of august 9 1977 24 3 a which is inferred to have a relatively shallow spatial centroid 20 km and a nearly flat moment spectrum suggesting that the faulting was confined to the upper more brittle portion of the oceanic lithosphere three other large earthquakes tonga june 22 1977 23 2 a kuril islands december 6 1978 3 6 0 2 a and tumaco december 12 1979 25 2 a exhibit moment spectra which significantly decrease toward higher frequencies indicating larger values of c the aftershocks of both the tonga and kuril islands events are distributed over a considerable range of depths consistent with rupture into the lower more ductile portions of the oceanic lithosphere we speculate that the characteristic source duration may be causally related to the depth of coseismic rupturing within the lithosphere

global anisotropy in the upper mantle inferred from the regionalization of phase velocities a data set of 2600 paths for rayleigh waves and 2170 paths for love waves was selected in order to resolve azimuthal anisotropy of surface waves in the period range 70 250 s the epicenters were chosen so that the paths provide uniform spatial and azimuthal coverage of earth a synthetic seismogram is computed for each path and compared to the observed seismogram in order to infer the phase velocity along each path to process such a large data set a new tomographic technique is designed a good correlation is found with surface tectonics in the whole period range for love waves and at shorter periods for rayleigh waves the directions of maximum velocities are also well correlated with plate tectonics motions for the 2 azimuthal term of rayleigh waves when the power spectrum of heterogeneities is computed it is found that for love waves the even orders are usually larger than the odd orders for rayleigh waves the degrees 2 and 6 dominate only at long periods suggesting a deep origin for these components

lateral heterogeneity and azimuthal anisotropy of the upper mantle love and rayleigh waves 100 250 s the lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring rayleigh and love wave phase velocities in the period range 100 250 s spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1 m 10 slow regions are evident at the east pacific rise northeast africa tibet tasman sea southwestern north america and triple junctions in the northern atlantic and indian oceans fast regions occur in australia western pacific and the southern atlantic these features are also found by a completely different analysis based on the backus gilbert method the backus gilbert method also shows that the obtained phase velocities are averaged values within an area of about 2000 km radius and the errors are about 1 of the phase velocity in the zeroth order spherically symmetric earth inversion for azimuthal dependence shows that for low angular order the fast phase velocity directions seem to correlate well with the plate motion vectors however resolution analysis by the backus gilbert method shows that the current data do not have enough resolution for everywhere on the globe only a few regions currently have adequate azimuthal coverage thus confirmation of the above correlation requires a more complete data set

is the ventersdorp rift system of southern africa related to a continental collision between the kaapvaal and zimbabwe cratons at 2 64 ga ago abstract rocks of the ventersdorp supergroup were deposited in a system of northeast trending grabens on the kaapvaal craton approximately 2 64 ga ago contemporary with a continental collision between the kaapvaal and zimbabwe cratons we suggest that it was this collision that initiated the ventersdorp rifting individual grabens strike at high angles toward the continental collision zone now exposed in the limpopo province where late orogenic left lateral strike slip faulting and anatectic granites are recognized we relate the ventersdorp rift province to extension in the kaapvaal craton associated with the collision and see some analogy with such rifts as the shansi and baikal systems associated with the current india asia continental collision

evolution of middle america and the in situ caribbean plate model james k h 2009 evolution of middle america and the in situ caribbean plate model origin and evolution of the caribbean plate 328 127 138 james k h lorente m a pindell j l eds conference on geology of the area between north and south america with focus on the origin of the caribbean plate siguenza spain may 29 jun 02 2006

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

tectonic and sedimentary evolution of a cretaceous continental arc backarc system in the korean peninsula new view abstract this paper focuses on the tectonic and sedimentary evolution of the cretaceous volcanic rocks and nonmarine successions in the southeastern part of the korean peninsula which constitute an ancient continental arc system the arc system gyeongsang arc system comprising an arc platform gyeongsang volcanic arc and a backarc basin gyeongsang backarc basin was a southwestward extension of the japanese arc formed by oblique northward subduction of the proto pacific izanagi plate under the asian continent the backarc basin was initiated in the southeastern part of the korean peninsula in the early cretaceous as a narrow ns trending trough the basin was bounded by a major fault in the northern part of the basin from which sediments largely emanated sediments were deposited in streamflow dominated alluvial fans adjacent to the fault bounded basin margin in the north and low gradient fluvial systems of braided channels that extended southward and southeastward for tens of kilometers sediments were also derived from the western highland margin draining precambrian to jurassic basement rocks the initially narrow trough progressively expanded toward the east resulting in eastward migration of depocenters that eventually generated a broad fluvio lacustrine plain fringing the volcanic arc platform the arc platform played an important role for the derivation of volcanogenic materials and accreted sediments into the backarc basin via extensive fluvial network pyroclastic density currents and landslides which originated from the arc platform also entered the basin in addition extrusion of basaltic volcanic rocks was continual within the basin during basin expansion the resultant succession of mixed sedimentary and volcanogenic rocks is generally indicative of a temporal increase in volcanic activity in the arc platform and in subsidence rate of the basin in the late cretaceous andesitic to rhyolitic volcanism became climactic in the arc platform producing a number of calderas or volcanotectonic depressions volcaniclastic aprons fringing the arc platform encroached upon the basin fill intraarc basins produced by sinistral shearing of the arc platform received sediments from active volcanoes and footwalls of contemporaneous faults forming small scale streamflow dominated alluvial fans and floodplains with later development of a deep lake overall the gyeongsang arc system formed under an extensional or transtensional sinistral strike slip stress regime suggesting that the subducting and the overriding plates were not strongly coupled both the volcanic arc and the backarc basin ceased to develop as volcanic activities shifted progressively eastward in the paleogene accompanied with rollback of the subduction of the izanagi plate

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

shear wave anisotropy of active tectonic regions via automated s wave polarization analysis abstract using an automated aspect ratio method to evaluate shear wave anisotropy from earthquakes recorded on arrays of three component digital seismographs over the bucaramanga nest of colombia the east rift zone of hawaii and the phlegraean fields uplift area of italy a shear wave velocity anisotropy of 4 7 is observed the anisotropy is attributed to tectonic processes acting either to align anisotropic minerals or to align stress related cracks at most fixed seismographs the polarization of the leading s phase is consistent from event to event independent of earthquake location on a regional basis the polarization may vary from station to station but a regional trend is apparent for events from the bucaramanga nest colombia a small seismogenic volume at 160 km depth in a subducted plate shear wave splitting averages 0 5 0 1 s for more than 100 events the polarization direction of the fast shear wave is 355 n 15 parallel to the strike of the benioff zone at the phlegraean fields in italy which represent an active quaternary caldera the average shear wave splitting observed from 51 events is 0 2 s corresponding to a minimum velocity anisotropy of 7 the strike of the inferred maximum compressive stress is consistent with values obtained from fault plane solutions in hawaii the polarization of the leading shear wave tends to parallel the strike of the east rift zone average velocity anisotropy is about 5 if one assumes that the entire travel path contributes to the observed splitting however station variations in fast shear wave polarization direction imply that some of the splitting is due to near station effects

recently formed elastic anisotropy and petrological models for the continental subcrustal lithosphere in southern germany this paper advances new evidence for elastic anisotropy in the continental subcrustal lithosphere in southern germany the range of petrological models compatible with the observed azimuthal variation of seismic p wave velocity is explored the azimuthal distribution of amplitudes of mantle phases and the observed increase of p velocity with depth both indicate a continuation of anisotropy with depth together with an increase of preferred orientation even depletion of the upper mantle in basaltic components as suggested by mantle xenoliths from various parts of germany cannot explain the velocity depth and azimuthal amplitude observations without an increase of anisotropy with depth preferred orientation of olivine is the most likely mechanism for the observed phenomena its fast a axis at the moho level is directed towards n22 5 e the b axis is also required to be horizontal i e the b plane one of the preferred glide planes of olivine is vertical with a strike of n22 5 e therefore this preferred glide plane of olivine practically coincides with the plane of maximum horizontal shear stress deduced from fault plane solutions of earthquakes in western germany this is a strong indication that the preferred orientation of olivine is formed in the recent west european crustal stress field leaking into the upper mantle the distribution of velocities to a depth of at least 50 km requires slight horizontal rotation of the a axis with depth by 10 towards n32 e and a change in the modal composition towards a depletion increasing with depth compatible with the composition of mantle xenoliths from western germany further experiments are needed to substantiate this suggestion which could lead to a better understanding of the interaction of crustal and upper mantle stress strain fields

anisotropy and shear velocity heterogeneities in the upper mantle long period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle the dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure the data are corrected for the effect of surface layers and both love and rayleigh waves are used shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km the shear wave velocity distribution to 200 km depth correlates with surface tectonics except in a few anomalous regions below that depth the correlation vanishes cold subducted material shows up weakly at 350 km as fast s wave anomalies in the transition region a large scale pattern appears with fast mantle in the south atlantic s anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones the pacific shows a nw se fabric

petrochemistry and tectonic significance of cretaceous island arc rocks cordillera oriental dominican republic abstract cretaceous island arc rocks of the caribbean island arc system have been exposed by cenozoic faulting in the cordillera oriental in eastern hispaniola high k 2 o intermediate to felsic volcanic rocks loma la vega volcanics are interbedded with marine epiclastic sedimentary rocks and tuffs las guajabas tuffs and unconformably overlie pre aptian sedimentary rocks low k 2 o volcanic rocks guamira volcanics and a granodioritic to tonalitic intrusion el valle pluton the petrology and geochemistry of these units in conjunction with regional stratigraphic data are used to speculate on the tectonics of the newly developing caribbean island arc system during early and late cretaceous time the loma la vega volcanics are characterized by the presence of large phenocrysts of sanidine and minor amounts of clinopyroxene opaque oxides and rare leucite in a devitrified matrix of chlorite and clay although the volcanic rocks have undergone some low temperature alteration metamorphism which redistributed some major elements and large ion litho phile trace elements the high field strength elements rare earth elements and radiogenic isotopes appear to have been minimally affected based on abundances of the relatively immobile elements trace element enrichment patterns and isotopic compositions the loma la vega volcanics are considered part of the high k calc alkaline ca or shoshonitic island arc volcanic series in contrast pre aptian early cretaceous volcanic and plutonic rocks of the underlying los ranchos formation have chemical characteristics similar to rocks in the island arc tholeiitic or primitive island arc pia series that form coeval and along strike sections of the early cretaceous caribbean island arc in other parts of present day hispaniola cuba puerto rico and the virgin islands an abrupt and regional change in composition from island arc tholeiites to high k calc alkaline rocks is coincident with a hypothesized reversal in subduction polarity in pre aptian time as inferred from previously published tectonic models polarity reversal may have been triggered by attempted subduction of the caribbean oceanic plateau beneath this segment of the caribbean island arc the observed magmatic and tectonic effects of the proposed cretaceous caribbean arc reversal are similar to the better documented neogene subduction reversal event in the solomon islands arc in the southwest pacific

cenozoic tectonic history of the north america caribbean plate boundary zone in western cuba structural studies of well dated jurassic to lower miocene rocks in western cuba constrain the sequence of structural events affecting this oblique collisional zone between the late cretaceous island arc and the jurassic cretaceous north america passive margin in the southeastern gulf of mexico and straits of florida results of detailed mapping and collection of fault slip data at 34 sites define a regionally consistent five phase tectonic model for the period from the late paleocene to the post early miocene during the late paleocene to the early eocene the cuban island arc collided with the north american passive margin bahamas platform northwest ward overthrusting during the collision defines tectonic phase i a nne ssw compression concurrent with early eocene left lateral strike slip faulting along the pinar fault zone defines phase ii this result is consistent with structural mapping showing sinistral shear within the 065 striking pinar fault zone an ene wsw to e w compression defining phase iii overprinted phase ii faults in the lower eocene and older rocks post early miocene normal faulting characterizes phase iv inversion of fault slip data indicates two contemporaneous directions of tension of 120 and 170 strike slip faults that overprint phase iv normal faults yield a 120 compression phase v the direction of compression associated with the arc continent collision rotates clockwise from nw se in the late paleocene early eocene phase i to nne ssw phase ii and to ene wsw by the middle eocene phase iii the rotation in the compression direction occurred because the arc turned toward an oceanic area in the present day area of central and eastern cuba progressive collision led to complete subduction of the remnant oceanic crust by middle to late eocene time

observations of laterally inhomogeneous anisotropy in the continental lithosphere elastic anisotropy may be an important factor when seismic data are used to determine the structure composition and dynamics of the earth s interior1 2 azimuthal anisotropy has been found in the oceanic and although less definitely in the continental lithosphere from field experiments where the pn wave velocity was observed to vary with direction3 5 these studies suggest a 180 periodicity in travel time variations as the only criterion of the presence of azimuthal anisotropy this criterion however may fail to distinguish between anisotropy and lateral inhomogeneity of the isotropic medium a new method for measuring azimuthal anisotropy has been described recently6 based on the phenomenon of shear wave splitting in anisotropic media which suggests three criteria instead of one to identify azimuthal anisotropy we have now applied this method to the records of permanent seismograph stations in southern germany our results provide strong evidence for the presence of azimuthal anisotropy in the lithosphere of the region and reveal pronounced lateral variations in the parameters of anisotropy on a scale of about 200 km

cretaceous paleogene thrust emplacement of serpentinite in southwestern puerto rico serpentinite emplacement mechanisms along convergent plate boundaries are complex and not well understood this investigation combined structural and stratigraphic data from serpentinite in puerto rico to suggest an emplacement mechanism along the north american caribbean plate boundary northward dipping thrust faults and foliations along the southern margins of the monte del estado and rio guanajibo serpentinite bodies in southwestern puerto rico indicate southwestward thrust movement of serpentinite structural analysis of faults folds and foliation recorded within the serpentinite bodies and analysis of contact relationships with volcano sedimentary rocks lead to the interpretation that thrust movements record contractional deformations occurring during the maastrichtian paleocene and late eocene early oligocene intervals the conclusion that serpentinite has been most recently emplaced above thrust faults during transpression between the caribbean north america plates is in contrast to the earlier hypothesis that serpentinite was emplaced in response to diapirism late eocene transpression reactivated older faults and also displaced the serpentinite although the timing of the serpentinite emplacement in southwestern puerto rico is similar to emplacement events in cuba and hispaniola there are differences in the origin mechanism and direction of emplacement of the serpentinite masses the heterogeneity in the serpentinite emplacement mechanism and transport direction evokes reconsideration of the deformation history along the caribbean north american plate boundary

a theory for plastic deformation and textural evolution of olivine polycrystals seismic anisotropy in the upper mantle is due primarily to preferred orientation of olivine crystals induced by progressive deformation in order to understand better the origin of seismic anisotropy we present a simple theory for plastic deformation and textural evolution of olivine polycrystals each crystal in the aggregate is assumed to deform by intracrystalline slip on three major slip systems whose hardnesses and stress exponents are known from experiments the evolving grain orientation distribution in the aggregate is calculated by minimizing the difference between the local crystal deformation and the global aggregate deformation subject to the constraint of global strain compatibility the axial compression texture predicted by our model agrees well to first order with that determined experimentally by nicolas et al 1973 although there are significant discrepancies in the details our results suggest that the orientation texture developed during progressive plane strain deformation is a nearly unique function of the finite strain such that the crystallographic axes 100 010 and 001 are concentrated around the finite strain axes a c and b respectively a b c this result may allow the state of finite strain at depth to be estimated from observations of seismic anisotropy

evidence from shear wave splitting for the restriction of seismic anisotropy to the upper crust the existence of anistropy in the propagation speed of seismic waves through the earth s crust has been attributed to the non random alignment of microcracks in the crust1 for sparsely distributed parallel vertical cracks perturbation theory predicts the splitting of a shear wave into two orthogonally polarized waves2 4 here we present extremely clear evidence for shear wave splitting in three component seismograms from crustal and sub crustal microearthquakes recorded in the shikoku district of japan quantitative analysis of travel time differences between split shear waves leads to the conclusion that the observed anisotropy is limited to the upper crust shallower than 10 15 km this model is consistent with recent results from reflection seismology5 and implies that microcrack density or orientation changes from the upper to the lower crust

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

variation of palaeostress patterns along the oriente transform wrench corridor cuba significance for neogene quaternary tectonics of the caribbean realm abstract in this study we address the late miocene to recent tectonic evolution of the north caribbean oriente transform wrench corridor in the southern sierra maestra mountain range se cuba the region has been affected by historical earthquakes and shows many features of brittle deformation in late miocene to pleistocene reef and other shallow water deposits as well as in pre neogene late cretaceous to eocene basement rocks these late miocene to quaternary rocks are faulted fractured and contain calcite and karst filled extension gashes type and orientation of the principal normal palaeostress vary along strike in accordance with observations of large scale submarine structures at the south eastern cuban margin initial n s extension is correlated with a transtensional regime associated with the fault later reactivated by sinistral and or dextral shear mainly along e w oriented strike slip faults sinistral shear predominated and recorded similar kinematics as historical earthquakes in the santiago region we correlate palaeostress changes with the kinematic evolution along the boundary between the north american and caribbean plates three different tectonic regimes were distinguished for the oriente transform wrench corridor otwc compression from late eocene oligocene transtension from late oligocene to miocene d1 and transpression from pliocene to present d2 d4 when this fault became a transform system furthermore present day structures vary along strike of the oriente transform wrench corridor otwc on the south eastern cuban coast with dominantly transpressional compressional and strike slip structures in the east and transtension in the west the focal mechanisms of historical earthquakes are in agreement with the dominant ene wsw transpressional structures found on land

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

mountain building strike parallel motion and mantle anisotropy abstract geophysical and geological evidence indicates that tectonic movements parallel to the strike of orogenic belts and suture zones are the most important mode of displacement during continental collision and oblique subduction several hundred to a few thousand kilometers of displacement may have occurred this way resulting in a pervasive ductile deformation of the lithosphere consequently both crustal and mantle fabrics may reflect these movements and their study using both geological and geophysical methods could supply important clues to lithosphere behavior during mountain building on the basis of kinematic analysis of ductilely deformed crustal rocks in zones of movement a variety of tectonic models have been proposed which differ in 1 1 the timing of strike parallel motions early before or during continental accretion or late after accretion was completed 2 2 the geometric pattern of strike parallel displacement zones low angle or high angle and 3 3 the location of orogen parallel movement zones in outboard terranes on the subducting plate along the suture zone or within the continental hangingwall large scale displacements parallel to mountain belts also suggest that they occurred along lithospheric faults rooted in the upper mantle although few studies have been carried out from this view point thermal structural and petrological data support the conclusion that lithospheric faults involving the upper mantle may be discriminated from crustal faults rooted in some crustal decoupling zone where available data on mantle flow beneath orogenic belts are in agreement with mountain parallel displacements inferred from crustal studies this has an important bearing on the seismic anisotropy pattern which can be predicted in the lithospheric mantle below mountain belts the fast velocity direction which is parallel to the flow direction should therefore be recorded parallel to the strike of the belt

age of the los ranchos formation dominican republic timing and tectonic setting of primitive island arc volcanism in the caribbean region this study reports u pb ages for zircons from the los ranchos formation which is part of the primitive island arc sequence the oldest volcanic rocks in the greater antilles zircons were analyzed from three samples quartz porphyry quartz keratophyre from the lower part of the formation quita sueno member fragmental quartz porphyry from the upper part of the formation pueblo viejo member and the cotui quartz diorite stock that intrudes the lower part of the formation the lower part of the los ranchos formation represented by the quita sueno sample formed at either 113 9 0 8 or 118 6 0 5 ma depending on interpretation of the data whereas the upper part represented by the pueblo viejo sample formed ca 110 9 0 8 ma the cotui stock rd 73 601 was emplaced ca 111 8 0 6 ma or 112 9 0 9 ma depending on whether eight grains reflect subtle inheritance these results show that the los ranchos formation was emplaced during the aptian albian transition that the cotui stock could have supplied magma to the volcanic sequence and fluids to the pueblo viejo gold silver deposit in the upper part of the formation and that carbonaceous sediments in the upper part of the formation formed at the same time as ocean anoxic event 1b paquier caribbean plate tectonic models involving invasion of anomalous pacific crust into the caribbean region appear to agree best with this age because they provide a mechanism for the change from primitive island arc pia to calcalkaline magmatism and account for the restricted ocean circulation necessary to generate regional anoxic conditions

petrogenesis of plagiogranite and associated diorites and mafic rocks in the habana matanzas ophiolites northwestern half of central cuba structural field relationships petrography and geochemical features of plagiogranites and associated intermediate diorite to mafic rocks basalt and gabbro of the habana matanzas ophiolites in northern cuba suggest that these felsic and mafic rock units represent a boundary case between forearc and volcanic arc ophiolite types subduction related basalts can be subdivided into back arc back arc to forearc forearc and volcanic arc types the majority of the basalts plot in the oceanic arc as well as joint oceanic arc continental arc field of the th yb nb yb diagram the samples that classify as volcanic arc basalts plot in the continental arc field of the th yb nb yb diagram and within the island arc tholeiite and boninite fields in the v ti diagram the habana matanzas plagiogranites and associated basalts diabases and gabbros display significantly negative ta nb anomalies and are depleted in heavy rare earth elements 40ar 39ar whole rock plateau ages of these plagiogranites reveal minimum crystallization ages of 98 11 0 50 ma providing the first important time constraint for suprasubduction zone oceanic crust formation on the island of cuba based on our petrogenetic modelling and geochemical characterization we conclude that magmas of the plagiogranites and associated intermediate rocks of the habana matanzas ophiolites were derived from partial melting of highly depleted pre existing crustal rocks basalt and or gabbro thematic collection this article is part of the tethyan ophiolites and tethyan seaways collection available at https www lyellcollection org cc tethyan ophiolites and tethyan seaways

shear wave polarization anisotropy in the upper mantle beneath honshu japan shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth s surface is investigated using three component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in honshu japan eighty nine high quality seismograms were selected from a period of 3 years the data used in this study are restricted such that incidence angles are smaller than the critical angle of 30 to the earth s surface in order to avoid phase shifts in the shear wave train to find directions of the maximum and minimum velocities in split shear waves where shear waves are resolved into two phases with the maximum time separation each set of the two horizontal component seismograms is rotated in the horizontal plane the split shear waves thus obtained are again recombined after the correction of anisotropy and the anisotropy corrected particle motion is compared with the focal mechanism for a cross check of the observed anisotropy directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station the stereograms and the cross sections of seismic ray paths show that 1 the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations and 2 the anisotropic region is separated into two parts one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0 to 30 from the north north anisotropy and the other in the south with it trending 90 to 120 south anisotropy the maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions the horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km perhaps prevalent in west honshu however the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles if we assume the vertical extent of the anisotropic material to be 100 km the maximum velocity contrast would be 4 if we adopt a crack alignment model for the observed anisotropy cracks are inferred to be distributed vertically trending 0 to 30 in the north and 90 to 120 in the south if we assume an olivine alignment model olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0 to 30 and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90 to 120 and the slip plane being horizontal

archean rocks of the black hills south dakota reworked basement from the southern extension of the trans hudson orogen the little elk and the bear mountain terranes on the margin of the precambrian core of the black hills provide the only known exposures of archean rocks between the archean wyoming and superior provinces the little elk terrane consists of a supracrustal sequence dominated by biotite feldspar gneiss meta arkose or metatuff that is intruded by the slightly metaluminous to peraluminous calc alkaline little elk granite which has a u pb intercept age of 2 549 11 m y a biotite plagioclase schist metagraywacke dominates the bear mountain terrane and is intruded subparallel to foliation by the peraluminous granite at bear mountain an isochron calculated from six severely discordant u pb zircon analyses yields an age of 2 393 230 m y the archean terranes participated in the trans hudson orogenic event suggested by a published rb sr age for the little elk granite 1 850 m y 1 87 1 97 gaigneous activity in the black hills and the similarity in orientation of the proterozoic deformational fabric with geophysical anomalies interpreted to be associated with the trans hudson orogen the metamorphic conditions estimated for the archean rocks may be the result of a thermal event that produced the proterozoic harney peak granite and thermal high northeast of lead south dakota the unexposed precambrian basement is probably dominated by granites similar to the little elk granite and supracrustal rocks consisting of continental margin deposits based on the ree contents of the archean schists and on the heavy mineral assemblages in the uraniferous nature of and the metasedimentary clasts in the lower proterozoic box elder creek formation the ages and the relatively long lived crustal sources for these archean granites and the similarity of the inferred basement characteristics to those of rocks in the wyoming province suggest that the black hills archean rocks may be the easternmost exposures of this province the probable trans hudsonian reworking of the black hills proterozoic rocks and their similarity to those in northern saskatchewan imply that the black hills occupies a tectonic position comparable to that of rocks described from the cree lake zone of saskatchewan

anisotropy beneath 9 stations of the geoscope broadband network as deduced from shear wave splitting polarization of sks scs and s waves has been analyzed on digital 3 components seismograms recorded at 9 stations of the recently installed geoscope broadband network splitting was observed for about 15 records and is interpreted as being due to seismic anisotropy beneath the stations from every record it is possible to retrieve the fast direction at the station and the time difference between the fast and slow polarization directions some 30 observations of quasi linear s pulses also help constrain the direction of fast velocity synthetic seismograms were built to test the validity of the method used to retrieve the parameters of anisotropy typical time differences are about 1 sec corresponding for example to a 80 km thick zone of perfectly oriented pyrolite the fast directions obtained at 3 continental stations in america westford santa cruz heredia could be related to plate tectonic processes on the other hand the fast directions found in 3 oceanic islands kipapa papete la reunion differ strongly from the oceanic plate velocity direction there

evidence from core reflected shear waves for anisotropy in the earth s mantle seismologists have begun to recognize that the upper mantle at depth is anisotropic and that seismological data are not fully explicable if anisotropy is ignored1 5 on entering an anisotropic zone a plane shear wave splits into two orthogonally polarized phases6 an effect observed by several researchers7 notably by ando et al 5 8 9 the near vertical scs wave an s wave reflected once at the core mantle boundary is one of the most suitable phases to study the phenomenon of shear wave birefringence because it is isolated from other phases and because its particle motion is nearly horizontal and thus free from s to p conversion the split scs shear waves travel with different velocities in the anisotropic zone and consequently there is a time delay between their arrival at stations my aim was to determine this time delay and the azimuth of polarization using the scs records of two horizontal components from the japanese islands for a deep shock beneath the sea of okhotsk from the total of 41 stations 34 show a polarization of the maximum velocity phase in the nnw sse direction with an average time advance of 0 8s from the minimum velocity phase the relevant anisotropic zone seems to be deep seated

archean oceanic flake tectonics assuming that the archean mantle was hotter and the oceanic crust thicker than at present we estimate that the archean oceanic lithosphere had a sandwich rheology with the lower crust behaving as a soft ductile layer accordingly the archean oceanic lithosphere had a rheological structure similar to present day continental lithosphere but unlike present day oceanic lithosphere the thickness of the ductile layer is a decreasing function of age while its strength is an increasing function of age the buoyancy problem in archean subduction could be overcome by delamination of oceanic upper crust but mass balance calculation indicates that most archean oceanic crust must have been recycled where the right dynamic conditions existed for delamination oceanic flake tectonics may have been an important process leading to the preservation of oceanic upper crust in greenstone belts that are characteristic of archean terrains

an automated analytical method to determine shear wave splitting abstract we introduce and discuss an automated analytical method for determination of shear wave splitting to determine the polarization direction of the first arriving shear wave fast wave we evaluate the linearity of the particle motion of shear waves by maximizing as a function of azimuth the aspect ratio i e the ratio of the projections of the particle displacement on orthogonal axes in the horizontal plane in a time window that contains only the first shear wave arrival the azimuth at which the maximum aspect ratio occurs is taken as the polarization direction of the fast shear wave then the time separation between the split shear waves is determined by time shifting the component with the slow shear wave arrival toward the fast shear wave arrival until maximum linearity in particle motion is observed the polarization direction of the fast shear wave can be accurately predicted for seismograms having good signal to noise ratios as are typical of local earthquake recordings similar procedures are used to evaluate particle motion in a vertical plane in the time interval including the first shear wave arrival to examine the apparent incident angle in order to reveal the likelihood of interfering s p conversions

azimuthal anisotropy in the earth from observations of sks at geoscope and nars broadband stations abstract we have analyzed observations of low frequency sks waves recorded on horizontal components at several broadband stations of the geoscope and nars arrays splitting of sks is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis in the upper mantle beneath the receivers we have designed an inversion method which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi shear waves in general the directions of fast velocity obtained both for stations around the atlantic and around the pacific appear to be related to directions of past and present plate motions

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

the variscan belt in central europe main structures geodynamic implications open questions abstract the variscan belt of europe originated from the confrontation of northern europe and gondwana with intervening pre variscan blocks cadomian and older armorica tepla barrandean moravo silesian though a strike slip component cannot be excluded geological evidence suggests a subduction collision process newly discovered features include basaltic rocks with morb affinities high pressure metamorphic rocks trench deposits and large scale allochthonism which also affects the pre variscan basement and has often led to an inversion of the metamorphic isogrades large scale thrusting is evident at all crustal levels and is also observable by geophysical methods the structure of the variscan crust is best described as horizontal tectonic layering late ordovician through carboniferous convergence closed one rhenohercynian and one saxo thuringian basin to the north and at least one mediterranean basin to the south of the pre variscan blocks subduction occurred from the north as well as from the south down under the dorsal core region the exact timing and the driving mechanism of this bilateral activity remains a problem likewise uncertain are the exact limination of the pre variscan blocks and the existence and width of oceanic areas in the saxothuringian zone and in the south facing part of the moldanubian zone there is good evidence of early palaeozoic rifting and of late ordovician through carboniferous plate convergence palaeomagnetic and palaeoclimatic constraints however suggest only a limited extent of oceanic basins in these areas while they appear to allow a wider ocean at the rhenohercynian saxothuringian boundary which is relatively unobtrusive from the geological point of view research during the past decade has produced a wealth of information on the variscan fold belt in europe the resulting geodynamic models however are extremely diverging evidently it is not the time yet for definitive statements the present paper intends to review the state of knowledge to deduce such basic items of a geodynamic model as appear reasonably well documented and to point out the most urgent open questions stress will be laid on central europe which offers a complete cross section through the northern flank of the variscan belt and where we have carried out most of our studies during the last years

seasat derived gravity constraints on stress and deformation in the northeastern indian ocean we use seasat derived gravity data to investigate crustal deformation in the ne indian ocean gravity highs reflecting crustal undulations vary in orientation from e w in the central indian basin to ne sw in the wharton basin the undulations vary in trend similarly to the variation of the maximum compression directions predicted by a plate driving force model cloetingh and wortel 1985 and are essentially restricted to the area of predicted compression for both principal horizontal stresses this agreement implies that the stress model describes the basic features of the deformation observed in the gravity as well as the seismicity the gravity data also provide insight into two enigmatic tectonic features the transition in the morphology of the 90 e ridge at 10 s from a continuous high to a complex blocky structure appears related to the deformation since undulations in the central indian basin can be projected eastward to blocks on the ridge the morphology formerly interpreted as a fossil feature may reflect the recent deformation the discrepancy between the trends of the southernmost 85 e ridge and the 90 e ridge previously thought to exclude similar hot spot track origins appears to result from treating a crustal undulation as part of the 85 e ridge earthquakes do not appear to be preferentially located with respect to the peaks and troughs of the undulations

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

oscillatory zoning in eclogitic garnet and amphibole northern serpentinite melange cuba a record of tectonic instability during subduction exotic blocks of eclogite from distant localities along the northern serpentinite melange of cuba have comparable p t histories that include high pressure prograde sections 450 600 c 15 kbar associ ated with subduction of oceanic lithosphere and retrograde sections within the albite epidote amphibolite facies 500 c 10 kbar related to melange uplift 40 ar 39 ar and rb sr cooling ages 118 103 ma of one of the blocks indicate pre aptian subduction and aptian albian uplift detailed x ray imaging and profiling further reveals that minerals in these eclogite blocks notably garnet and amphibole display subtle but well defined oscillatory zoning that developed along the prograde trajectory of the rocks previous to attainment of peak eclogitic conditions the chemistry e g coupled changes of mg and mn in garnet and of si ti al and na in amphibole and geometry euhedral to anhedral shapes of the oscillations can be interpreted in terms of subtle fluctuations in p t during the general prograde subduction related metamorphic path a near equilibrium model is presented for the formation of oscillations at near peak conditions by means of recurrent dissolution growth reaction processes this model for near peak conditions and the chemical signatures of earlier oscillations notably in amphibole suggest that episodes of retrogression upward movement affected parts of the subducting slab it is proposed that these retrograde episodes record the tectonic rupture of the subducting slab and probably of the upper plate mantle either due to the intrinsic dynamic behaviour of subduction systems or to the effects of the plate tectonic rearrangement of the caribbean region during the early cretaceous

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

azimuthal anisotropy of velocity in the mantle lid beneath the basin and range province the basin and range province in western north america is one of the largest and best studied regions of cenozoic continental extension1 2 yet there is still considerable debate surrounding the geological history of the province and the cause of the observed widespread rifting although recent geological and crustal geophysical observations2 constrain crustal scale processes within the basin and range more information is needed on the structure and rheology of the mantle part of the lithosphere the mantle lid to distinguish between competing geodynamic models1 2 here we present seismic data showing an azimuthal variation of 3 2 in the p wave velocity in the shallowest mantle beneath the basin and range the direction of maximum seismic velocity coincides with the nw se direction of neogene and present day tectonic extension suggesting that penetrative deformation associated with this extension has induced a preferred orientation of the minerals in the mantle lid our results thus imply that deformation associated with present day surface rifting extends at least 30 km into the underlying mantle