mantle dynamics beneath the gibraltar arc western mediterranean from shear wave splitting measurements on a dense seismic array 1 controversial evolutionary models have been proposed for the gibraltar arc system a complex interaction zone between the eurasia and african plates here we derive new mantle anisotropic constraints from sks splitting measurements on a dense network of about 90 broad band stations deployed over south iberia and north morocco the inferred fast polarization directions fpd clearly show a spectacular rotation along the arc following the curvature of the rif betic chain while stations located at the south and south east edges show distinct patterns these results support geodynamical processes invoking a fast retreating slab rather than convective removal and delamination models the fpd variations along the gibraltar arc can be explained by fossil anisotropy acquired during the western mediterranean eocene subduction while changes to the south and south east of the rif betic chain could be the imprint of a flow episode around an alboran high velocity slab during its miocene fragmentation from the algerian slab

perspectives on basaltic magma crystallization and differentiation lava lake blocks erupted at mauna loa volcano summit hawaii explosive eruptions at mauna loa summit ejected coarse grained blocks free of lava coatings from moku aweoweo caldera most are gabbronorites and gabbros that have 0 26 vol olivine and 1 29 vol oikocrystic orthopyroxene some blocks are ferrogabbros and diorites with micrographic matrices and diorite veins 2 cm cross cut some gabbronorites and gabbros one block is an open textured dunite the mgo of the gabbronorites and gabbros ranges 7 21 wt those with mgo 10 wt have some incompatible element abundances zr y ree positive eu anomalies lower than those in mauna loa lavas of comparable mgo gabbros mgo 10 wt generally overlap lava compositions olivines range fo83 58 clinopyroxenes have mg s 83 62 and orthopyroxene mg s are 84 63 all evolved beyond the mineral mg s of mauna loa lavas plagioclase is an75 50 ferrogabbro and diorite blocks have 3 5 wt mgo tio2 3 2 5 4 k2o 0 8 1 3 la 16 27 ppm and a diorite vein is the most evolved sio2 59 k2o 1 5 la 38 ppm they have clinopyroxene mg s 67 46 and plagioclase an57 40 the open textured dunite has olivine fo83 5 seven isotope ratios are 87sr 86sr 0 70394 0 70374 and 143nd 144nd 0 51293 0 51286 and identify the suite as belonging to the mauna loa system gabbronorites and gabbros originated in solidification zones of moku aweoweo lava lakes where they acquired orthocumulate textures and incompatible element depletions these features suggest deeper and slower cooling lakes than the lava lake paradigm kilauea iki which is basalt and picrite clinopyroxene geobarometry suggests crystallization at 1 kbar p highly evolved mineral mg s 75 are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional shifts ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes clinopyroxene geobarometry suggests 300 bar p open textured dunite represents olivine melt mush precursor to vertical olivine rich bodies as in kilauea iki its fo83 5 identifies the most primitive lake magma as 8 3 wt mgo mass balancing and melts show that such a magma could have yielded both ferrogabbro and diorite by 50 fractional crystallization but under different fo2 fmq 250 bar led to diorite and fmq 250 bar yielded ferrogabbro these segregation veins documented as similar to those of kilauea testify to appreciable volumes of rhyolitic liquid forming in oceanic environments namely sio2 rich veins are intrinsic to all shields that reached caldera stage to accommodate various sized cooling differentiating lava lakes

neotectonics and morphotectonics of the southern almeria region betic cordillera spain kinematic implications most previous studies related to the recent tectonic development of southeastern betic cordillera and in particular the south of almeria agree with the kinematic variety that can be observed in the post miocene structures at different scales this variety is difficult to explain if we consider only the regional stress field related to the convergence of the eurasian and african plates that remained relatively steady over the last 9 ma this work proposes a block tectonics with horizontal and vertical relative movements driven by strike slip movements of major faults to explain the variety of kinematics observed in the area from middle miocene to present the combined activity of the alpujarras fault zone dextral strike slip and the carboneras fault zone sinistral strike slip induces within the major block bounded by them the squeezing of minor pre existing fault bounded crustal blocks this process generates traction areas inside the escaping block which are linked with the existence of gradients of westward movement rate this tectonics can explain some of the structures related with the extensional tectonics predominant in the south of almeria province and the development of the minor and major morphostructures driven by faults with different kinematics the tectonics of blocks proposed is useful to give a coherent interpretation of apparently incoherent local structural data into a regional and homogeneous compressional stress field

intercalibration of the hb3gr 40ar 39ar dating standard abstract the 40 ar 39 ar dating technique is based on the knowledge of the age of neutron fluence monitors standards recent investigations have improved the accuracy and precision of the ages of most of the phanerozoic aged standards e g fish canyon tuff sanidine fcs alder creek sanidine ga1550 biotite and lp 6 biotite however no specific study has been undertaken on the older standards i e hb3gr hornblende and nl 25 hornblende generally used to date precambrian high ca k and or meteoritic rocks in this study we show that hb3gr hornblende is relatively homogenous in age composition ca k and atmospheric contamination at the single grain level the mean standard deviation of the 40 ar 39 ar k f value derived from this study is 0 49 comparable to the most homogeneous standards the intercalibration factor which allows direct comparison between standards between hb3gr and fcs is r fcs hb3gr 51 945 0 167 using an age of 28 02 ma for fcs the age of hb3gr derived from the r value is 1073 6 5 3 ma 1 internal error only and 8 8 ma including all sources of error this age is indistinguishable within uncertainty from the k ar age previously reported at 1072 11 ma turner g huneke j c podosek f a wasserburg g j 1971 40 ar 39 ar ages and cosmic ray exposure ages of apollo 14 samples earth planet sci lett 12 19 35 the r value determined in this study can also be used to intercalibrate fcs if we consider the k ar date of 1072 ma as a reference age for hb3gr we derive an age of 27 95 0 19 ma 1 internal error only for fcs which is in agreement with the previous determinations altogether this shows that hb3gr is a suitable standard for 40 ar 39 ar geochronology

fluvial system response to tectonically induced base level change during the late quaternary the rio alias southeast spain abstract the rio alias of southeast spain crosses a major tectonically active structure the carboneras fault zone within the central portion of the drainage basin the fault originated in the mid miocene and has c 40 km of strike slip offset the impact of late quaternary movement along the fault zone on the evolution of the rio alias is addressed geomorphological and sedimentological analysis of the landform assemblage preserved suggests the development of highly sinuous meander loops and laterally incising streams during phases of tectonic activity continued tectonic activity led to the abandonment of the meander loops and a wave of incision that propagated c 2 km upstream the propagation of this incision wave was restricted by the development of a nickpoint itself controlled by an anticlinal structure sub parallel to the fault zone syn sedimentary and post depositional faulting is evident within the late quaternary fluvial sediments and suggests dominantly vertical movement along the fault lines with downthrow to the southeast regional climatic patterns of aggradation and incision resulting in a basinwide terrace sequence were complicated i e an additional terrace was created and meander wavelength was changed through the study reach by tectonic activity this study highlights the importance of tectonically modified sequences when interpreting climatically generated sequences of aggradation and incision

origin of ultramafic xenoliths containing exsolved pyroxenes from hualalai volcano hawaii hualalai volcano hawaii is best known for the abundant and varied xenoliths included in the historic 1800 kaupulehu alkalic basalt flow xenoliths which range in composition from dunite to anorthosite are concentrated at 915 m elevation in the flow rare cumulate ultramafic xenoliths which include websterite olivine websterite wehrlite and clinopyroxenite display complex pyroxene exsolution textures that indicate slow cooling websterite olivine websterite and one wehrlite are spinel bearing orthopyroxene olivine cumulates with intercumulus clinopyroxene plagioclase two wehrlite samples and clinopyroxenite are spinel bearing olivine cumulates with intercumulus clinopyroxene orthopyroxene plagioclase two pyroxene geothermometry calculations based on reconstructed pyroxene compositions indicate that crystallization temperatures range from 1225 to 1350 c migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045 and 1090 c comparisons of the abundance of k2o in plagioclase and the abundances of tio2 and fe2o3in spinel of xenoliths and mid ocean ridge basalt and a single 87sr 86sr determination indicate that these hualalai xenoliths are unrelated to mid ocean ridge basalt similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is hualalai tholeiitic basalt xenoliths probably crystallized between about 4 5 and 9 kb the 155 230 c of cooling which took place over about 120 ka the age of the youngest hualalai tholeiitic basalt yield maximum cooling rates of 1 3 10 3 1 91 10 3 c yr hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel we suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel

timing of tectonic events in the alpujarride complex betic cordillera southern spain ar ar and fission track dating from four low grade sections in the alpujarride complex sheds light on the timing of the early contractional history of these high pressure metamorphic units and their subsequent exhumation during the early miocene extensional collapse of the betic orogen white mica grains from the lowest grade rocks yield partly reset detrital ar ar ages where deformation and recrystallization are more intense however they yield reproducible ages as old as c 48 ma which we interpret as the approximate timing of the main contractional event and associated high pressure metamorphism in these units rocks from slightly higher grade sections give reproducible ar ar ages between 30 and 40 ma but these younger ages probably reflect the interplay of growth and cooling in fine grained micas heated to between 300 c and 400 c during prograde metamorphism in each of the four areas studied the low grade rocks are separated by an extensional detachment or shear zone from medium grade rocks below in two areas these detachments have been folded around major overturned north vergent folds ar ar data from structurally below the detachments and fission track ages from all structural levels indicate the onset of significant cooling in the early miocene which corresponds to the age of the extensional event the folds must therefore be early miocene or younger

the alpujarride nevado fibabride extensional shear zone betic cordillera se spain in the betic cordillera the initial thickening of the alboran domain produced an alpine high pressure low temperature hp lt metamorphism in the alpujarride and nevado filabride complexes after 19 ma this thickening process evolved to an extensional stage with uplift rates of between 1 and 2 mm year 1 the extensional stage generated structures in a heterogeneous shear regime with a top to the west sense of movement the present day alpujarride nevado filabride contact is a detachment fault generated in this shearing regime in the footwall the first structure developed in this extensional stage is a planar linear fabric characterized by constrictive strain that is axial planar to kilometric recumbent tight to isoclinal folds the planar linear fabric is folded by inclined close to open folds with axes parallel to the stretching lineation the planar linear fabric and folds are deformed by a late extensional crenulation cleavage near the detachment surface finally brittle deformation was generated within the same kinematic framework as that of the ductile structures in the hanging wall meanwhile deformation had a brittle character producing faults and joints

andesites from northeastern kanaga island aleutians kanaga island is located in the central aleutian island arc northeastern kanaga is a currently active late tertiary to recent calc alkaline volcanic complex basaltic andesite to andesite lavas record three episodes series of volcanic activity series i and series ii lavas are all andesite while series iii lavas are basaltic andesite to andesite four series ii andesites contain abundant quenched magmatic inclusions ranging in composition from high mgo low alumina basalt to low mgo highalumina basalt the spectrum of lava compositions is due primarily to fractional crystallization of a parental low mgo high alumina basalt but with variable degrees of crustal contamination and magma mixing the earliest series i lavas represent mixing between high alumina basalt and silicic andesite with maximum sio2 contents of 65 67 wt later series i and all series ii lavas are due to mixing of andesite magmas of similar composition the maximum sio2 content of the pre mixed andesites magmas is estimated at 60 63 wt the youngest lavas series iii are all non mixed and have maximum estimated sio2 contents of 59 wt the earliest series i lavas contain a significant crustal component while all later lavas do not it is concluded that the maximum sio2 contents of silicic magmas the contribution of crustal material to silicic magma generation and the role of magma mixing all decrease with time furthermore silicic magmas generated by fractional crystallization at this volcanic center have a maximum sio2 content of 63 wt all of these features have also been documented at the central aleutian cold bay volcanic center brophy 1987 based on data from these two centers a model of aleutian calc alkaline magma chamber development is proposed the main features are 1 a single low pressure magma chamber is continuously supplied by primitive low alumina basalt 2 non primary high alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase 3 sidewall crystallization accompanied by crustal melting produces buoyant silicic 63 wt sio2 liquids that pond at the top of the chamber and 4 continued sidewall crystallization now isolated from the chamber wall produces silicic liquids with 63 wt sio2 that increase the thickness and lowers the overall sio2 content of the upper silicic zone it is suggested that the maximum sio2 content of 63 imposed on fractionation generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal liquid mush along the chamber wall

petrology of lavas from the puu oo eruption of kilauea volcano iii the kupaianaha episode 1986 1992 the puu oo eruption has been remarkable in the historical record of kilauea volcano for its duration over 13 years volume 1 km3 and compositional variation 5 7 10 wt mgo during the summer of 1986 the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser like fountaining at puu oo to virtually continuous relatively quiescent effusion at the kupaianaha vent this paper examines this next chapter in the puu oo eruption episodes 48 and 49 and presents new icp ms trace element and pb sr and nd isotope data for the entire eruption 1983 1994 nearly aphyric to weakly olivine phyric lavas were erupted during episodes 48 and 49 the variation in mgo content of kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of kilauea both of which probably were controlled by variations in kilauea s magma supply rate these lavas contain euhedral olivines which generally are in equilibrium with whole rock compositions although some of the more mafic lavas which erupted during 1990 a period of frequent pauses in the eruption accumulated 2 4 vol olivine the highest forsterite content of olivines 85 in kupaianaha lavas indicates that the parental magmas for these lavas had mgo contents of 10 wt which equals the highest observed value for lavas during this eruption the composition of the puu oo lavas has progressively changed during the eruption since early 1985 episode 30 when mixing between an evolved rift zone magma and a more mafic summit reservoir derived magma ended the normalized to 10 wt mgo abundances of highly incompatible elements and cao have systematically decreased with time whereas ratios of these trace elements and pb sr and nd isotopes and the abundances of y and yb have remained relatively unchanged these results indicate that the hawaiian plume source for puu oo magmas must be relatively homogeneous on a scale of 10 20 km3 assuming 5 10 partial melting and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued the rate of variation of highly incompatible elements in puu oo lavas is much greater than that observed for kilauea historical summit lavas e g ba y 0 09 a 1 vs 0 03 a 1 this rapid change indicates that puu oo magmas did not mix thoroughly with magma in the summit reservoir thus except for variable amounts of olivine fractionation the geochemical variation in these lavas is predominantly controlled by mantle processes

seismic velocity modelling of the carboneras fault zone se spain abstract the carboneras fault zone forms part of a major strike slip fault system in se spain striking ne sw and accommodating up to 40 km displacement it affects basement metamorphic rocks and unconformably overlying upper miocene sediments and volcanic rocks high resolution shallow seismic tomographic sections were made across the fault zone in two localities from the same areas fault rocks and their wallrocks were collected for laboratory seismic velocity measurements the laboratory data were corrected for the substantial effects of near surface crack damage by combining these results with geological cross sections forward velocity models for the fault zone were constructed to compare with field seismic measurements and hence to ground truth the inferences made from them these velocity depth relationships matched moderately well with those extracted from the in situ tomography results aspects of the in situ seismic sections matched features on the forward modelled sections but the comparisons showed that it is important to have some degree of foreknowledge of the geology to be able successfully to interpret seismic tomography sections as an exploration tool

the ninole basalt implications for the structural evolution of mauna loa volcano hawaii lava flows of the ninole basalt the oldest rocks exposed on the south side of the island of hawaii provide age and compositional constraints on the evolution of mauna loa volcano and the southeastward age progression of hawaiian volcanism although the tholeiitic ninole basalt differs from historic lavas of mauna loa volcano in most major element contents e g variably lower k na si higher al fe ti ca ree and other relatively immobile minor elements are similar to historic and prehistoric mauna loa lavas and the present major element differences are mainly due to incipient weathering in the tropical environment new k ar whole rock ages from relatively fresh roadcut samples suggest that the age of the ninole basalt is approximately 0 1 0 2 ma although resolution is poor because of low contents of k and radiogenic ar originally considered the remnants of a separate volcano the ninole hills are here interpreted as faulted remnants of the old south flank of mauna loa deep canyons in the ninole hills eroded after massive landslide failure of flanks of the southwest rift zone have been preserved from burial by younger lava due to westward migration of the rift zone landslide induced depressurization of the southwest rift zone may also have induced phreatomagmatic eruptions that could have deposited widespread basaltic ash that overlies the ninole basalt subaerial presence of the ninole basalt documents that the southern part of hawaii island had grown to much of its present size above sea level by 0 1 0 2 ma and places significant limits on subsequent enlargement of the south flank of mauna loa

late orogenic doming in the eastern betic cordilleras final exhumation of the nevado filabride complex and its relation to basin genesis the geometry timing and kinematics of late orogenic extension in the betic cordilleras pose the problem of a decoupling of upper crustal and lower crustal deformation regimes perpendicular directions of extension in metamorphic domes and nearby sedimentary basins remain unexplained this paper puts kinematic constraints on the final exhumation of the nevado filabride complex focusing on the formation of metamorphic domes and their relations with the adjacent basins structural fabrics and kinematic indicators below the main shear zones as well as their relations with both published changing metamorphic p t conditions and geochronological data were studied our approach describes 1 a consistent top to the west shear parallel to dome axes of during d2 i e during decompression with distributed ductile flow and the onset of strain localization along major shear zones 2 further strain localization along the major shear zones under greenschist facies conditions during d3 leading to s c mylonites formation accompanied with a rock strong thickness reduction 3 the divergence of shear direction on either limbs of domes during d3 showing the appearance of the dome geometry and 4 a local evolution toward n s brittle extension d4 in the upper plate and formation of sedimentary basins continuous ductile to brittle top to the west shear is compatible with the slab retreat hypothesis from the miocene the formation of domes which adds gravitational forces responsible for the final stages of exhumation is thus characterized by important kinematics changes necessary to explain coeval n s opened basins later from the upper tortonian a contractional event d5 amplified the earlier domal structures forming the present north vergent folds

partial melting of mantle wedge and evolution of island arc crust evolution of the crust in the northeast honshu arc is discussed on the basis of the melting experiments of a peridotite under hydrous conditions and the crustal compositions estimated from deep crustal xenoliths and seismic velocity data the chemical compositions of melts formed by partial melting of a hydrous peridotite within the pressure temperature conditions for shallow parts of the mantle wedge 1 2 2 0 gpa have been experimentally determined with the sandwich experiment technique the melts range from olivine tholeiitic to magnesian andesitic compositions sio2 48 56 wt mgo 9 14 wt beneath the volcanic front in the northeast honshu arc and are slightly less silicic beneath the coast of the sea of japan the crustal compositions of the volcanic zone in the northeast honshu arc estimated from deep seated crustal inclusions in volcanic rocks and the velocity structures of the crust are andesitic in the central zone and high alumina basaltic near the coast of the sea of japan the compositions of possible primary melts formed in the mantle wedge are significantly more mafic than these crustal compositions consequently if the major part of the crustal materials in the volcanic zone was formed by magmas generated in the mantle wedge thick up to 30 km ultramafic cumulates including olivine pyroxenite must have been formed beneath moho during fractional crystallization and would constitute the upper part of the mantle wedge the volume of the mantle wedge consisting entirely of even fertile peridotite was not sufficient to produce crustal materials and cumulates in the volcanic zone by such processes in the northeast honshu arc a static mantle wedge model is therefore not satisfactory for the growth of the crust in northeast honshu arc a dynamic model is suggested in which the mantle materials have been supplied into the mantle wedge from the back arc side by a convective flow

the role of extension in the miocene denudation of the nevado filabride complex betic cordillera se spain the internal zone of the betic cordillera se spain consists of a nappe stack of three complexes the deepest of which is the nevado filabride complex the zone is separated from the overlying alpujarride complex by a crustal scale shear zone that has variously been interpreted as a thrust or an extensional detachment a suite of 74 new apatite and zircon fission track results have been obtained from the nevado filabride complex and these have been used to define regional cooling patterns for the complex rapid cooling 105 c 200 c ma 1 is spatially related to the tectonic contact with the overlying alpujarride complex cooling to near surface temperatures occurred first in the east sierra de los filabres during the mid serravallian 12 1 ma and was completed by the early tortonian 9 8 ma in the west sierra nevada there is no correlation between fission track age and sample elevation these results are consistent with tectonic unroofing of this complex a finding that favors extension as the mechanism by which the two complexes were brought into contact extension spans the middle and earliest upper miocene 12 8 ma in the study area and therefore lasted much longer than previously documented a hypothesis is advanced which links oblique convergence between the iberian plate and the betic internal zones resulting in crustal contraction at depth with orogen parallel extension in the middle and upper crust

evolution of mauna kea volcano inferences from lava compositions recovered in the hawaii scientific drilling project the lower 776 m of core recovered during the initial phase of the hawaii scientific drilling project hsdp contains lavas erupted from mauna kea volcano tholeiitic and alkalic basalts including an fe ti rich flow are intercalated in the upper 58 m of mauna kea lavas similar basaltic sections are subaerially exposed on the lower east flank of mauna kea the fe ti rich lavas reflect large amounts of clinopyroxene plagioclase and olivine fractionation within the crust and upper mantle but the range from tholeiitic to alkalic compositions reflects variable extents of melting of a garnet bearing source based on abundances of incompatible elements the extent of melting for a basanitoid was a factor of 2 less than that for nearly coeval tholeiitic lavas all flow units in the lower 718 m of the hsdp core are tholeiitic lavas their variability in major element compositions reflect variable accumulation of olivine incompatible element abundance ratios in these lavas reflect a complex temporal variation in extent of melting within the tholeiitic part of the core lavas from 800 m to 950 m formed by the largest extent of melting whereas tholeiitic lavas from the bottom of the core and from just below the tholeiitic to alkalic transition formed by lower degrees of melting inferred melt compositions at 16 mgo show that the 200 to 400 ka mauna kea lavas from the hsdp core and the 250 ka subaerial exposures define an inverse correlation between sio2 and feo contents based on experimental studies this correlation is caused by differing pressures of melt segregation furthermore abundances of nb and sio2 are also inversely correlated in these calculated melts in general the younger lavas are relatively enriched in feo and incompatible elements but are depleted in sio2 these trends are interpreted to reflect an overall trend of increasing pressure of melt segregation and decreasing extent of melting with decreasing eruption age there are however geochemical variations which indicate short term reversals in this long term trend previously the geochemical trends accompanying the transition from tholeiitic to alkalic volcanism at hawaiian volcanoes have been interpreted as reflecting the effects of increasing distance from the plume axis the long term geochemical trends of tholeiitic lavas in the hsdp core also reflect migration of mauna kea away from the hawaiian plume

tectonic controls on tholeiitic and calc alkaline magmatism in the aleutian arc the tectonic position of aleutian arc volcanic centers and their magmatic differentiation trends calc alkaline or tholeiitic appear to correlate from 160 w to 175 e the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates the tholeiitic volcanoes are large primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow closed system differentiation the calc alkaline volcanoes are smaller more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers the chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma probably derived from mantle peridotite conditions for the derivation of the two trends through crystal fractionation support the proposed model the tholeiitic magmas show characteristics i e no hydrous phenocrysts fe enrichment trend parallel ree patterns vitrophyric and esites and dacites consistent with low pressure high temperature crystallization in large shallow magma chambers the calc alkaline magmas show characteristics i e some hydrous phenocrysts no fe enrichment trend nonparallel rare earth patterns porphyritic lavas consistent with higher pressure and lower temperature of crystallization than the tholeiitic series tertiary plutons also show both calc alkaline and tholeiitic trends and appear to be chemically similar to the quaternary volcanoes the larger plutons are calc alkaline and probably represent the extensive 1002 km magma chambers of the small calc alkaline volcanoes the small shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series

evaluation of preconsolidation pressure and the overconsolidation ratio from piezocone tests of clay deposits in quebec the preconsolidation pressure and overconsolidation ratio profiles are the most important factors related to the mechanical behaviour of clay deposits they are interpreted on the basis of a limited number of laboratory tests but the near continuous information provided by the piezocone can allow local data to be extrapolated to an entire site numerous methods have been proposed for relating piezocone data to preconsolidation pressure and the overconsolidation ratio but their validity is still uncertain in this paper nine methods are compared on the basis of data collected on 31 sensitive clay sites in quebec whose preconsolidation pressure profiles are well known each method is briefly presented and the most promising are identified the simplest method which directly relates preconsolidation pressure to net tip resistance also appears to be the most effective key words clay preconsolidation pressure overconsolidation ratio piezocone correlations

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

active faulting offshore se spain alboran sea implications for earthquake hazard assessment in the southern iberian margin the southern margin of the iberian peninsula hosts the convergent boundary between the european and african plates the area is characterised by low to moderate magnitude shallow earthquakes although large historical events have also occurred in order to determine the possible sources of these events we recently acquired swath bathymetry tobi sidescan sonar and high resolution seismic data on the almeria margin eastern alboran sea the new dataset reveals the offshore continuation of the ne sw trending carboneras fault a master fault in the eastern betic shear zone and its associated structures n150 and ns faults these structures are active since they cut the late quaternary sedimentary units the submarine carboneras fault zone is 100 km long 5 10 km wide and is divided into two n045 and n060 segments separated by an underlapping restraining stepover geomorphic features typically found in subaerial strike slip faults such as deflected drainage water gaps shutter ridges pressure ridges and ben echelonq folds suggest a strike slip motion combined with a vertical component along the submarine carboneras fault considering the nnw sse regional shortening axis a left lateral movement is deduced for the carboneras fault whereas right lateral and normal components are suggested for the associated n150 and ns faults respectively the offshore portion of this fault is at least twice as long as its onshore portion and together they constitute one of the longest structures in the southeastern iberian margin despite the fact that present day seismicity in the almeria margin seems to be associated with the n150 to ns faults the carboneras fault is a potential source of large magnitude mw 7 2 events hence the carboneras fault zone could pose a

late neogene recent uplift of the cabo de gata volcanic province almeria se spain abstract cabo de gata is a miocene volcanic province in the betic cordillera in southeastern spain the distribution of coastal deposits in the successive marine sedimentary units overlying the last volcanic episode about 7 5 ma old has been used to reconstruct the post volcanic palaeogeographic evolution of the region during the late neogene the current elevation of well dated shoreline marker rocks has been used to estimate uplift amount and rates since the late tortonian a n45 e aligned the strike of the carboneras fault system topographic relief was emergent in the cabo de gata region the extension and height of this island increased throughout the late neogene smaller independent islands were emergent and finally became connected to the main island during the messinian the carboneras and agua amarga pliocene sub basins were the last two marine basins prior to the final emergence of the region since the last volcanic eruption ca 7 5 ma maximum uplift of sedimentary rocks in cabo de gata has taken place on the western margin of the n45 e aligned palaeorelief the altitude of the shoreline marker rocks in the successive sedimentary units decreases eastwards to the present day coast and northwards of the rambla del plomo uplift rates since deposition remain nearly constant for the successive messinian rocks and decrease slightly for the lower pliocene outcrops most of the uplift took place before the pliocene while the main island enlarged uplift amounts and rates since deposition of the upper neogene sedimentary units in the cabo de gata area are similar to the ones estimated for laterally equivalent units in the eastern betic basins approximately 50 m ma despite its volcanic nature and the occurrence of the carboneras fault system the cabo de gata probably became elevated as a consequence of regional uplift in connection with the rest of the betic cordillera

morphology and topology of dolostone lithons in the regional carboneras fault zone southern spain abstract dolostones in a neogene strike slip fault zone are described two main types of structural features are recognised i background deformation in the form of a network of bedding perpendicular and hybrid conjugate fractures barren fractures and bedding parallel stylolites ii fault related features include breccias and cataclasites orthorhombic rock lithons are generated from the intersection of fracture sets with bedding and or joints lithon size and morphology change across the dolostone fault block gradually producing a tetragonal or isometric shape the lithons are 1 2 cm in dimension only 20 outside this range and have an average cross sectional aspect ratio of 1 6 irrespective of size or structural position topology is analysed using nodes and branches ranging from isolated i node and i i branch to connected y and x nodes and i c to c c branches respectively the quantitative description of the geometrical and topological analysis of the dolostone lithons suggest that they become more connected and interact within the dolostone fault blocks assessing the change in topology and lithons connectivity have important implications for subsurface reservoirs and aquifers hosted in dolostone fault zones

patterns and average rates of late neogene recent uplift of the betic cordillera se spain abstract the facies distribution in the sedimentary units infilling a series of neogene basins has been used to reconstruct the relief generation and uplift across the internal zone of the betic cordillera in southern spain uplift amounts and average rates can be estimated using the current elevation of the outcrops of well dated deposits indicative of ancient sea level positions coral reefs and coastal conglomerates record the initial development of emergent betic relief during the langhian continental and marginal marine deposits indicate the existence of a large island centred on the present sierra nevada sierra de los filabres chain by the end of the middle miocene the precursor of the sierra nevada sierra de los filabres chain originally part of this large island remained emerged whilst the surrounding areas were re invaded by the sea during the early tortonian at the end of the tortonian the inland basins granada and guadix basins became continental while the sierras de la contraviesa sierra de gador and sierra alhamilla emerged separating the alboran basin from the alpujarra tabernas and sorbas basins which became narrow passages of the mediterranean sea in contrast the sierra cabrera emerged during the late messinian suggesting a progressive uplift from west to east of the sierras south of the sierra nevada sierra de los filabres chain during the pliocene only the low areas closest to the present day coast remained as marine basins and progressively emerged throughout this stage the highest average uplift rate recorded is 280 m ma for the sierra de gador although the average uplift rates of upper neogene coastal marine rocks since depositon have maximum values of approximately 200 m ma most of the uplift of the betic mountains took place before the early pliocene the recorded uplift of neogene rocks was highest at the margins of western sierra nevada where peaks higher than 3000 m occur the average rates of uplift were lower to the east of this major relief the main sierras and depressions in the present day landscape correspond respectively to the emergent land in which uplift was concentrated and to the marine basins that existed before the final emergence of the region the altitude of the sierras reflects the time at which they became emergent the highest mountains being the first to rise above sea level

testing the preservation potential of early diagenetic dolomites as geochemical archives early marine diagenetic dolomite is a rather thermodynamically stable carbonate phase and has potential to act as an archive of marine porewater properties however the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity here the archive potential of early marine dolomites exposed to extreme post depositional processes is tested using various types of analyses including petrography fluid inclusion data stable 13c and 18o isotopes 87sr 86sr ratios and u pb age dating of various dolomite phases in this example a triassic carbonate platform was dissected and overprinted diagenetic temperatures of 50 to 430 c in a strike slip zone in southern spain eight episodes of dolomitization a dolostone cataclasite and late stage meteoric vadose cementation were recognized the following processes were found to be diagenetically relevant i protolith deposition and fabric preservation and marine dolomitization of precursor aragonite and calcite during the middle late triassic ii intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a proto atlantic opening stress regime t ca 250 c during the early middle jurassic iii dolomite cement precipitation during early alpine tectonism rapid burial to ca 15 km and high grade anchizone overprint during alpine tectonic evolution in the early eocene to early miocene iv brecciation of dolostones to cataclasite during the onset of the carboneras fault zone activity during the middle miocene and v late stage regression and subsequent meteoric overprint data shown here document that under favourable conditions early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years evidence for this preservation includes preserved late triassic seawater 13cdic values and primary fluid inclusion data data also indicate that oversimplified statements based on bulk data from other petrographically complex dolomite archives must be considered with caution

modified cam clay modelling of temperature effects in clays the cam clay model for isothermal mechanical behaviour of clays has been extended to take account of the effects of temperature on stress strain behaviour the assumptions used in constructing the new model are based on published results and on new data presented in the paper the model allows prediction of how heating and cooling af fect volume changes pore water pressures and strengths for both normally consolidated and overconsolidated saturated clays it permits modelling of observed reductions in the overconsolidation ratio with increasing temperature the model provides accessible qualitative explanations for temperature effects that were previously difficult to understand it will also allow easy implementation for quantitative modelling in triaxial stress fields results predicted by the model are compared with data collected by the authors at temperatures up to 100 c the model does not account for changes that occur in clay minerals at higher temperatures for example in bentonites at temperatures higher than about 150 c

influence of thermomechanics in the catastrophic collapse of planar landslides frictional heating has long been considered a mechanism responsible for the high velocities and long run out of some large scale landslides in this work a landslide model is presented applicable

modelling unanticipated pore water pressures in soft clays field observations in thin soft clay layers may show pore water pressures that increase for some time after the loading is applied reasons for these observations are not well understood the paper shows how an elastic viscoplastic constitutive model incorporated into the consolidation equation can predict these pore water pressure increases in soils that exhibit significant creep behaviour or secondary compression the phenomenon has been related to relaxation in regions of the profile from which drainage has not yet begun key words clay consolidation creep secondary compression viscous relaxation pore water pressure elastic plastic

the malaguide alpujarride boundary a major extensional contact in the internal zone of the eastern betic cordillera se spain abstract the internal zone of the betic cordillera in southern spain was formed by thrusting and horizontal ductile shortening in the late cretaceous palaeogene but was subjected to a major phase of vertical thinning during the neogene the contact between the malaguide the structurally highest thrust complex and the underlying alpujarride complex is an example of several major tectonic contacts across which the stratigraphy has been repeated by thrusting but the metamorphic grade increases abruptly downwards detailed work in the eastern betic cordillera documents the relationships across this contact in the sierra espuna petrographic and illite crystallinity studies show that the highest imbricate thrusts of malaguide rocks have only been diagenetically altered whereas the deeper parts of the thrust stack exhibit anchizone facies metamorphism in the northern sierra de las estancias apatite fission track studies confirm that there too malaguide rocks have never been above 200 c in both areas the underlying alpujarride rocks exhibit epizone or greenschist facies metamorphism and a major fault zone with both brittle and ductile structures separates the two complexes the observed rapid increase in metamorphic grade over very small structural thicknesses suggests that part of the original metamorphic zonation has been excised across the contact thus in its present form we interpret the malaguide alpujarride boundary as an extensional contact the extensional motion took place in early middle miocene times as constrained by fission track cooling ages the fault zone at the contact is sub horizontal in the sierra espuna but in the sierra de las estancias it is mainly sub vertical kinematic data from a 5 20 m thick calcmylonite layer within the fault zone in the sierra de las estancias corrected for later tilting indicates that the extensional movement was originally directed with the top to the east northeast the interpretation of the malaguide alpujarride boundary as an extensional contact is similar to recent interpretations of the nevado filabride alpujarride contact at deeper structural levels in the central and eastern betics the identification of another major extension fault system confirms that miocene extension was widespread at all structural levels during the early middle miocene the sierra espuna and the northern margin of the sierra de las estancias lay on the boundary between the region undergoing horizontal extension in the internal zones and the alboran sea to the south and the region undergoing horizontal shortening in the external zones to the north as a result the detailed history of the alpujarride malaguide contact is complicated extensional structures cross cut structures related to earlier alpine shortening and were then affected by further shortening as convergence with the external zones continued

proprietes caracteristiques des argiles de l est du canada the compilation of numerous geotechnical data has allowed the definition of the main physical mechanical and hydraulic properties of marine and lacustrine clays of eastern canada as well as the establishment of correlations between some of these properties the possible uses of these correlations are described this study shows that the clays of eastern canada exhibit a behaviour similar to that of other clays but that they are characterized by a higher degree of structuration keywords physical properties mechanical properties hydraulic properties clays canada correlations journal translation

40ar 39ar age of the cabo de gata volcanic series and displacements on the carboneras fault zone se spain the sw ne trending carboneras fault zone of se spain separates a terrain comprising uplifted massifs of the metamorphic basement of the betic cordilleras and intervening neogene sedimentary basins from the cabo de gata volcanic series along the southeastern boundary of the carboneras fault burdigalian marls and tuffs mark the beginning of the volcanic episode rest unconformably on the basement and were tilted to the vertical during the earliest stages of movement on the fault zone 40 ar 39 ar dating of hornblende grains and igneous clasts from the tuffs constrain the onset of the volcanic episode at 21 ma some 4 ma earlier than previously reported volcanic rocks higher in the sequence that overstep unconformably onto the southern edge of the fault zone yielded an age of 11 ma thus constraining the earliest episode of motion on the fault zone between these age limits the main phase of left lateral strike slip movement on the carboneras fault may be later than this time movements on more northerly strands of the fault zone including the uplift of the sierra cabrera basement block to the nw continued at least through pliocene times the onset of volcanism is broadly coeval with the unroofing of the orogen by extensional collapse and supports earlier inferences of very rapid rates of uplift and cooling of the basement rocks

active transfer fault zone linking a segmented extensional system betics southern spain insight into heterogeneous extension driven by edge delamination pliocene and quaternary tectonic structures mainly consisting of segmented northwest southeast normal faults and associated seismicity in the central betics do not agree with the transpressive tectonic nature of the africa eurasia plate boundary in the ibero maghrebian region active extensional deformation here is heterogeneous individual segmented normal faults being linked by relay ramps and transfer faults including oblique slip and both dextral and sinistral strike slip faults normal faults extend the hanging wall of an extensional detachment that is the active segment of a complex system of successive wsw directed extensional detachments which have thinned the betic upper crust since middle miocene two areas which are connected by an active 40 km long dextral strike slip transfer fault zone concentrate present day extension both the seismicity distribution and focal mechanisms agree with the position and regime of the observed faults the activity of the transfer zone during middle miocene to present implies a mode of extension which must have remained substantially the same over the entire period thus the mechanisms driving extension should still be operating both the westward migration of the extensional loci and the high asymmetry of the extensional systems can be related to edge delamination below the south iberian margin coupled with roll back under the alboran sea involving the asymmetric westward inflow of asthenospheric material under the margins

hawaiian magma reservoir processes as inferred from the petrology of gabbro xenoliths in basalt kahoolawe island gabbro xenoliths in a tholeiitic lava of kahoolawe island hawaii a 1 3 1 4 ma shield volcano are 1 3 cm in size and comprised of plagioclase clinopyroxene and orthopyroxene gabbro textures while intergranular and in part subophitic are open due to 28 48 vol of vesicular basalt occupying xenolith space vesicles in and around the xenoliths are lined or filled with rhyolitic glass segregation vesicles the host is evolved tholeiite mgo 6 1 wt with phenocrysts microphenocrysts and glomerocrysts of olivine clinopyroxene orthopyroxene and plagioclase and megacrysts 1 cm of plagioclase the sr isotope ratio of one xenolith is 0 70489 the host basalt ratio is 0 70460 xenolith isotope composition grain resorption and clinopyroxene fs12 5 15wo38 35 5 orthopyroxene fs19 5 24wo4 1 and plagioclase an68 65or0 8 1 2 compositions suggest that these gabbros crystallized from kahoolawe tholeiitic magma of essentially the same composition as the host basalt but pre dating the magma represented by the host based on the absence of intergranular fe ti oxide phases from the pl cpx opx assemblages and the open vuggy textures we envision crystallization on a reservoir roof at temperatures 1100 c entrainment of gabbro assemblages and plagioclase megacrysts from a roof mush suspension zone occurred during convection associated with replenishment of the magma reservoir these open textured gabbro xenoliths are therefore not fragments of preexisting coarse grained bodies such as sills or segregation veins rhyolitic glass in vesicles represents a gas effervescence filtration process that forced fractionated residual liquids from the groundmass into voids associated with the xenoliths

comportement d un pieu bi fonction fondation et echangeur de chaleur the behaviour of a pile subjected to thermo mechanical loads was studied in situ with the aim of quantifying the thermal influence on the bearing capacity of heat exchanger piles to accomplish this a pile situated in a building under construction was equipped with a pipe system to inject heat into it using a special heat pump load cells deformation gauges and thermometers were installed to evaluate the behaviour of the pile during seven tests with coupled thermo mechanical loads the temperature variations applied to the pile were of the order of 15degreesc and the mechanical load reached 1300 kn the results permitted the quantification of three significant effects brought about by the temperature increase i pile uplift ii mobilization of skin friction due to the relative displacement of the pile with respect to the ground iii additional load generated in the pile by constrained dilation

koolau shield basalt as xenoliths entrained during rejuvenated stage eruptions perspectives on magma mixing rejuvenated stage tuff cones honolulu volcanics on koolau volcano oahu hawaii contain xenoliths of koolau shield basalt because koolau subaerial shield lavas represent a hawaiian geochemical end member and submarine shield lavas have compositions with some affinities to mauna loa and kilauea we analyzed 28 xenolithic basalts from salt lake and koko head cones to determine how these seemingly random samplings of the koolau profile compare to established koolau geochemistry analyses reveal that 24 are shield tholeiitic basalt the focus of this study and 4 are rejuvenated stage basaltic rocks the tholeiitic xenoliths represent largely upper koolau shield lavas as these samples 8 3 to 5 8 wt mgo have with one exception overall major and trace element compositions that overlap those of koolau subaerial shield lavas secondary processes however created some distinctions namely enrichments depletions in k ba sr sio2 and feo and due to zeolitization chabazite with attending okenite and apophyllite elevated cao one xenolithic basalt with 8 2 wt mgo has higher ti zr nb and sc and lower zr nb than subaerial lavas and appears to represent relatively early deeper shield thereby reinforcing that the koolau shield source varied temporally olivine orthopyroxene and plagioclase are the phenocrysts clinopyroxene is rare and their core compositions range widely across the suite fo87 8 72 orthopyroxene mg s 85 72 and an74 60 several xenolithic basalts have both normally and reversely zoned orthopyroxene and plagioclase with a variety of core compositions e g orthopyroxene core mg s 82 77 and 72 all in one sample these compositions and zonations record evidence for wide compositional ranges of replenishment mgo 13 8 wt and reservoir mgo 7 to 5 wt magmas mixing in varying proportions however extreme mgo lavas 13 and 5 wt are not observed as either subaerial or xenolithic basalt but are indicated by phenocryst cores of fo87 8 and orthopyroxene mg 72 the koolau magma mixing history resembles that of kilauea and is unlike the steady state mixing known for mauna loa finally these basalt samples show that any xenolithic occurrence of koolau lava is subject to the zeolitization prevalent in the tuff cone hosts

constraints on the movement history of the carboneras fault zone se spain from stratigraphy and 40ar 39ar dating of neogene volcanic rocks

role of strike slip faults in the betic rifian orogeny abstract a new model for the betic rifian orogeny of the western mediterranean spain and north africa is proposed in which four strike slip faults play an important role the faults are not of the same age two faults the left lateral jebha fault to the south in morocco and principally in the mediterranean sea and the right lateral north betic fault southern spain to the north define the boundaries of the alboran block betic and rifian internal zones final movement along these faults was during the burdigalian time two other faults the left lateral nekor fault north africa to the south of the jebha fault and the right lateral crevillente fault somewhat to the north of the north betic fault define a larger alboran block including part of the betic and rifian external zones that was present during the tortonian the following sequence of events is proposed 1 a during the eocene and oligocene the african and european plates converged in a n s sense causing the breakup and overthrusting of the betic rifian and kabyle internal zones and then the movement towards the wsw of the alboran block by slip along the jebha and north betic faults 2 b by the end of burdigalian time movement along the jebha and north betic faults ceased 3 c with continued n s convergence the nekor and crevillente faults which bound a larger alboran block were formed during the mid and late miocene the arc of gibraltar the zone lying between the four major faults seems to be a result of wsw motion of a crustal block being thrust over external zones the model proposed adds to the earlier idea that tectogenesis proceeds from the interior to the exterior of an erogenic belt in the betic rifian orogeny major strike slip fracture zones shifted to the exterior of the orogenic belt as the orogeny progressed in order to relieve the stress caused by locking of the more internal faults

intracrustal controls on the coexistence of tholeiitic and calc alkaline magma series at aso volcano sw japan aso caldera sw japan was formed incrementally over a period of 300 kyr by the eruption of four basaltic to dacitic pyroclastic flow and fall deposits aso 1 4 these units are collectively referred to as the aso pyroclastic flow deposit apfd in which tholeiitic and calc alkaline magmas have been erupted in close temporal and spatial proximity detailed petrological and geochemical studies indicate that a major shift in magmatic composition occurred between aso 1 and 4 as the caldera forming stage evolved changing from a predominantly tholeiitic to a calc alkaline signature the mineralogical and geochemical characteristics of aso 1 4 suggest that magmatic evolution within each unit is controlled by fractional crystallization associated with magma mixing crustal assimilation by altering the fractional crystallization mineral assemblage as well as the relative amounts of fractional crystallization magma mixing and crustal assimilation the systematic shifts in differentiation trends and magmatic series observed between the four eruptive units can be reproduced this suggests that the coexistence of tholeiitic and calc alkaline magma series can be attributed to shallow level intracrustal processes

geochemistry of the 1984 mauna loa eruption implications for magma storage and supply the 1984 eruption of mauna loa volcano hawaii produced about 220 106 m3 of lava of remarkably homogeneous composition no significant compositional changes occurred during the 22 day eruption and along the 20 km vent system even though crystallinity increased substantially accompanying a decline in the eruption rate such homogeneity in lava composition is typical of many historic mauna loa eruptions the 1984 lava along with most other homogeneous mauna loa lavas is multiply saturated crystallizing olivine clinopyroxene and plagioclase bulk compositions are perched at the intersection of an olivine control trend with a trend for more differentiated lavas there is no evidence to indicate that the 1984 lava is a differentiate of the 1975 magma or that it results from mixing a new batch of magma with differentiated melt remaining in the magmatic plumbing system after the 1975 eruption it is proposed that these lavas are erupted directly from a small continuously replenished magma reservoir located about 3 4 km beneath the summit caldera successive eruptions therefore provide a sampling of magma chamber processes and small but discernible changes in lava composition reflect the competing processes of magma supply differentiation and eruption

structure and geological history of the carboneras fault zone se spain part of a stretching transform fault system abstract the carboneras fault zone cfz is a major ne sw trending tectonic lineament in se spain active from upper miocene to recent times it separates the volcanic cabo de gata terrain to the se accumulated over 18 6 ma bp from the tract of uplifted alpine metamorphic basement blocks and post orogenic basins that comprise the betic cordilleras lying to the nw new geological mapping and age determinations have been used to constrain the geometry and geological history of the fault zone the cfz consists of left lateral strike slip faults bearing fault gouge formed in the uppermost 5 km of the crust the faults cut metamorphic basement and folded post orogenic sediments and volcanic rocks and acted as a conduit for calc alkaline volcanic rocks rising to the surface nw of the cfz a series of unconformities and deformation episodes affect successive sedimentary formations of upper miocene age the cfz is interpreted as part of a transform fault system separating ne sw stretched and nw se shortened crust deformed above a south westward retreating subducted slab from a less deformed terrain lying to the south east total offset on the cfz may be up to 40 km but is at least 15 km

on the lateral variations in chemical composition and volume of quaternary volcanic rocks across japanese arcs the fe mg fe ratios xfe of the quaternary basalts sio2 53 wt in the japanese arcs were examined the xxfe of relatively magnesian basalts decreases from the volcanic front toward the japan sea across the arcs based on the partition coefficient of mg fe2 between olivine and liquid it is suggested that all the basalts near the volcanic front which are mostly tholeiitic basalts are significantly fractionated whereas many basalts near the japan sea which are mostly alkali basalts are little fractionated the k2 o content in the primary basalt magmas increases toward the japan sea combining the xfe and k2 o data it is suggested that relatively large amounts of tholeiitic magmas are produced near the volcanic front but they fractionate during their ascent whereas smaller amounts of alkali basalt magmas are formed near the japan sea but they can ascend with less fractionation the density of primary tholeiite magma is significantly larger than that of primary alkali basalt magmas it is most likely that primary tholeiite magmas cannot ascend beyond the upper crust and would fractionate to produce less dense tholeiitic magmas near the volcanic front whereas primary alkali basalt magmas can ascend through the upper crust without fractionation as far as buoyancy is the principal ascending force in the japanese arcs the stress field may be less compressional near the japan sea than near the volcanic front so that magmas can ascend more rapidly in the latter region than in the former these two factors may be responsible for the above mentioned chemical variations of basalt magmas across the arcs the variation in volume of the quaternary volcanic rocks across the arcs can be explained by the presence of a melt rich zone above but nearly parallel to the subducted slab

the general and congruent effects of structure in natural soils and weak rocks the engineering properties of naturally occurring sedimentary and residual deposits which are usually treated in geotechnical engineering as soils are reviewed and it is shown that usually they have characteristics due to bonded structure which are similar to those of porous weak rock while this structure can arise from many causes its effects follow a simple general pattern that involves stiff behaviour followed by yield this yield can be described in a similar way to that occurring due to overconsolidation although it is a separate phenomenon the effects of structure are as important in determining engineering behaviour as are the effects of initial porosity and stress history which are the basic concepts of soil mechanics as it can be described in a general way it is concluded that structure and its effects should be treated as a further basic concept of equal importance l article passe en revue les proprietes des depots sedimentaires et residuels naturels qui sont nor malement traites comm

influence of strike slip fault segmentation on drainage evolution and topography a case study the palomares fault zone southeastern betics spain abstract the palomares fault zone pfz is one of the main sinistral strike slip faults in the betics se spain with approximately 16 km of north south displacement the pfz initiated during the tortonian messinian as a transfer structure linking areas subject to nw se shortening during the plio quaternary the fault zone lengthened cutting previous fold structures and widened towards the east displacing the active mountain front to the western border of sierra almagrera sierra almenara these sierras which show moderate uplift rates of 0 05 0 15 m ka formed in response to the oblique slip regime of the pfz the drainage system in the vicinity of the pfz is asymmetric with respect to a main axial valley that runs parallel to the pfz on the downthrown fault block in this block the drainage density is lower and the streams are longer than in the uplifted block furthermore west of the main axial valley the streams describe eastward directed deflections indicating the progressive eastward migration of the main axial valley during the pleistocene the drainage system on the uplifted ranges shows a consequent pattern indicating recent uplift and folding under nnw sse convergence recent activity along segments of the pfz has increased topographical gradients favouring dissection of previous streams by headward erosion of streams transverse to the active fault segments

the messinian of the nijar basin se spain sedimentation depositional environments and paleogeographic evolution abstract the reconstruction of the depositional events related to the messinian salinity crisis msc of the mediterranean is generally hampered by an incomplete stratal record in the circum mediterranean basins the sediments of the northern part of the nijar basin however provide an excellent and continuous record of late messinian sediments because features of severe erosion are lacking especially the successions of the deeper part of the basin had sufficient accommodation space to warrant ongoing deposition and may thus serve as a testing ground for existing hypotheses regarding the msc conformable contacts with the overlying pliocene and good correlation possibilities with the adjacent astronomically dated messinian of the sorbas basin provide the necessary age constraints the main body of evaporites in the nijar basin yesares formation has been affected by local dissolution and erosion prior to deposition of the latest messinian lago mare facies pelitic float breccias show textures indicating flowage and or mass transport and include slumped and slided stratal packets due to foundering of the mixed evaporitic clastic margin increased runoff of meteoric waters probably played an important role as these packet slides are perfectly sealed by the hyposaline lago mare strata field observations show that marginal sediments commonly classified as the terminal carbonate complex tcc are a lateral equivalent of the basinal yesares evaporites the latest messinian deposits feos formation are characterized by a sedimentary cyclicity related to fluctuating base levels consisting of chalky marly laminitic strata alternating with continental coarser clastic intervals despite considerable w e facies changes and indications for discrete tectonic events a persistent sequential pattern of eight lago mare cycles is present which are interpreted as precession controlled variations in regional climate instead of one major desiccation event in the latest messinian the repeatedly fluctuating water levels of the lago mare episode may have been the cause of the widespread vigorous erosion and canyon cutting in the lower evaporites abrupt non erosional contacts with the normal marine pliocene take place above the continental interval of the last lago mare cycle indicating that flooding took place during a period of lowered water levels the paleogeographic configuration of the nijar sorbas and vera basins has changed considerably during the messinian separation of the formerly interconnected basins is thought to have started in the late yesares times by tectonic uplift of the basement complexes in the latest messinian of the nijar basin two different coarse clastic supply areas can be distinguished which point to the partial emergence of the sierra cabrera and the cabo de gata block and activity of the sierra alhamilla and carboneras faults concerning the overall regional tectonic activity tectonics were probably also instrumental for the restoration of the atlantic gateway in the basal pliocene

quantification of fabrics in clay gouge from the carboneras fault spain and implications for fault behavior abstract clays in fault rocks have the potential to control fault behavior the formation of frictionally weak clays in fault rocks can lead to fault zone weakening and the development of fabrics through authigenesis and mechanical rotation in clay rich fault rocks can influence fault zone permeability structures the left lateral carboneras fault of southwestern spain is a ca 1 km wide fault zone with anastomosing layers of clay rich gouge up to 50 m thick the gouge derived largely from mica schist is composed of neoformed illite and chlorite anisotropy of magnetic susceptibility 201 measurements and x ray pole figures 144 measurements show comparable oblate clay fabrics with the axis of maximum anisotropy sub parallel to strike this dimensional fabric contrasts with previously published maximum permeability results that are sub vertical this observation coupled with the generally poor clay fabric measured in the samples indicates that the orientation of clays is not likely to be the primary control of the permeability anisotropy of clay rich fault gouge becoming important only when gouge zones are 10s of meters thick which is very unusual even in a fault zone like that of the carboneras fault with thick very clay rich gouge zones with a visible foliations and slip surfaces the degree of clay preferred orientation is low above the millimeter scale showing that any strong clay fabric associated with the foliation are extremely localized phenomena this suggests that processes operating below the millimeter scale may significantly influence fault behavior a result compatible with earlier characterizations of faults in a variety of geologic settings highlighting the importance of understanding how the formation and fabric of fault related authigenic clays influence fault behavior

deformation structures and processes within the continental crust an introduction the collection of research papers in this volume provides an overview of current trends in the study of deformation structures and deformation processes in rocks from the continental crust the volume has been divided into three sections to reflect the current interest in structures and processes shear zones and folds the interactions between magmatism and structure and the relationship between microstructure and rheology the range of topics includes the geometry of natural structures from field observations the detailed analysis of the microstructure of deformed rocks the timing of deformation by various geochronological techniques the modelling of deformation processes to constrain the development of structures in rocks and the use of labora tory experimentation to reproduce deformation fabrics in rocks the mixture of techniques and methods treated illustrate the broad and multidisciplinary approach to the study of rock defor mation in the continental crust

the betic segment of the lithospheric trans alboran shear zone during the late miocene abstract the betic sinistral transcurrent shear zone betic cordilleras crosses the southeastern part of spain the study of the geodynamics of this area is approached from tectonic sedimentary magmatic and geophysical data large strike slip movements tectono sedimentary processes and magmatic events are seen at the boundary between two different lithospheric slabs they are located in a thinned compartment with a high geothermal gradient the large scale movements vertical and horizontal and the intensity of magmatic activity are the expression of the mechanical behaviour of this hot and thin crust all the geological events from the late miocene to recent are integrated in a crustal scale cross section which shows that the betic sinistral transcurrent shear zone widely affects the lithosphere it appears that subduction zones are not the only geodynamic context in which calc alkaline magmatism occurs

modeling the viscoplastic behaviour of clays during consolidation application to berthierville clay in both laboratory and field conditions to analyze the effects of strain rate and viscoplastic strain on consolidation of natural clay this paper presents a nonlinear viscoplastic model in which viscoplastic behaviour is modeled by a un

apophyllite 40ar 39ar and rb sr geochronology potential utility and application to the timing of secondary mineralization of the kirkpatrick basalt antarctica apophyllite kca 4 si 8 o 20 f oh 8h 2 o which formed as a low temperature secondary mineral in the kirkpatrick basalt has been examined to evaluate its potential for geochronology using the k ar by the 40 ar 39 ar technique and rb sr methods the 40 ar 39 ar total gas ages for apophyllites from the kirkpatrick basalt are 114 133 ma for the central transantarctic mountains 95 114 ma for south victoria land and 76 100 ma for north victoria land within individual hand samples apophyllite 40 ar 39 ar apparent ages show variations of up to 24 m y and within single large up to 2 cm crystals differences of up to 14 m y are observed the 40 ar 39 ar incremental heating spectra are generally flat but have low and high temperature discordances attributed to 39 ar recoil rb sr model ages for the apophyllites range from 94 to 144 ma and vary from concordant with the 40 ar 39 ar dates to as much as 14 m y older the dates are consistent with other temporal indicators of low temperature alteration in the basalts and are interpreted to broadly reflect the time of apophyllite precipitation they document a geographically widespread early to middle cretaceous secondary mineralization which is inferred to be caused by a major period of groundwater movement associated with the initiation of uplift and denudation of the transantarctic mountains the results suggest that apophyllite can produce geologically meaningful ages by both rb sr and k ar methods although further work is required to fully understand the variations in age that may be attributed to ar loss alkali mobility or an extended history of mineral precipitation

melting experiments on a high magnesian andesite abstract melting experiments were conducted on a high magnesian bronzite olivine andesite teraga ike andesite which is considered to be a primary andesite the high magnesian andesite magma is in equilibrium with both olivine and orthopyroxene at about 15 5 kbar and 1080 c under h2o saturated conditions and at lower pressure and higher temperature under h2o undersaturated conditions this suggests that high magnesian andesites could be generated by the partial melting of upper mantle peridotite containing a small amount of h2o

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

tenth canadian geotechnical colloquium recent developments in consolidation of natural clays a global analysis of the consolidation of natural clays is realized considering the consolidation process to be a combination of the effects of compressibility and of permeability the compressibil

petrology of gabbroic xenoliths in 1960 kilauea basalt crystalline remnants of prior 1955 magmatism the 1960 kapoho lavas of kilauea s east rift zone contain 1 10 cm xenoliths of olivine gabbro olivine gabbro norite and gabbro norite textures are poikilitic ol sp cpx in pl and intergranular cpx pl ol opx poikilitic xenoliths which we interpret as cumulates have the most primitive mineral compositions fo82 5 cpx mg 86 5 and an80 5 many granular xenoliths ol and noritic gabbro contain abundant vesicular glass that gives them intersertal hyaloophitic and overall open textures to suggest that they represent mush and crust of a magma crystallization environment their phase compositions are more evolved fo80 70 cpx mg 82 75 and an73 63 than those of the poikilitic xenoliths associated glass is basaltic but evolved mgo 5 wt tio2 3 7 5 8 wt the gabbroic xenolith mineral compositions fit existing fractional crystallization models that relate the origins of various kilauea lavas to one another feo mgo crystal liquid partitioning is consistent with the poikilitic ol gabbro assemblage forming as a crystallization product from kilauea summit magma with 8 wt mgo that was parental to evolved lavas on the east rift zone for example least squares calculations link summit magmas to early 1955 rift zone lavas 5 wt mgo through 28 34 crystallization of the ol sp cpx pl that comprise the poikilitic ol gabbros the other ol gabbro assemblages and the olivine gabbro norite assemblages crystallized from evolved liquids such as represented by the early 1955 and late 1955 lavas 6 5 wt mgo of the east rift zone the eruption of 1960 kapoho magmas then scoured the rift zone reservoir system to entrain portions of cumulate and solidification zones that had coated reservoir margins during crystallization of prior east rift zone magmas

sedimentary facies during early stages of flooding in an extensional basin the breche rouge de carboneras late miocene almeria se spain the intramontane basins of the betic cordil leras se spain formed subsequent to the main phase of orogenic deformation during the middle miocene in a close genetic relation to the trans alboran shear zone left lateral movements along a local branch carboneras fault zone cf strike ne sw of this zone played a major role in controlling the formation and dynamics of the nijar carboneras basin to the south of the fault a major phase of strike slip faulting is recorded during the late tortonian the expression of this event is the brche rouge de carboneras brc which seals a deep denudational surface on top of dislocated fault blocks formed by volcanics of the cabo de gata complex and early tortonian shallow marine calcarenite the sedi mentary facies of this widely distributed unit in the carboneras subbasin mirror the submarine topography and the distribution of the fault zones along strike slip fault zones autoclastic breccias and neptunian dikes preferentially oriented nw se and ne sw occur which are interpreted to represent the near surface expression of the faults red limestone forms the groundmass of the autoclastic breccia and infills of neptunian dikes which exhibit multiple phases of opening of fissures gravita tional sedimentary infill lithification and renewed cre ation of cracks steep relief probably along fault scarps was mantled by epiclastic volcanic conglomerate with a red carbonate matrix well lithified coarse skeletal lime stone rich in planktonic foraminifera formed pavements along sediment starved rocky surfaces in deep water laterally within topographic depressions the pavement limestone grades into thick accumulations of skeletal rudstone composed of fragmented azooxanthellate corals and stylasterid hydrozoans which were concentrated by powerful bottom currents and gravitiy flows within the shallow water zone of dip slope ramps cross bedded calcarenite and calcirudite formed based on textures fabrics and biota rocks of the brc were grouped into nine genetic lithofacies which document cryptic deep aphotic and shallow photic environments typical of a sediment starved extensional basin

viscous behaviour of st roch de l achigan clay quebec this paper describes the viscous behaviour of st roch de l achigan clay a champlain sea clay from quebec canada the general geotechnical profile of the soil deposit was first established with st