jurassic sedimentary melange and associated facies baja california mexico the middle jurassic coloradito formation of cedros island baja california is a fine grained flysch sequence that includes some coarse debris flow filled channels sedimentary melange olistostrome is widespread and is composed of sheared black argillite with megablocks of triassic cherts shallow water origin paleozoic limestone and continental sandstone derived from the north this unit represents the earliest record of arc continent interaction in the baja area the eugenia formation is locally present in erosional contact above the coloradito formation the unit is a northerly derived channel fill deposit in which the lower one third is dominated by metasedimentary detritus and first cycle calc alkaline volcanic detritus dominates above this the long axes of clasts in sheared argillite of the coloradito have orientations parallel to clast imbrication in associated conglomerate channels suggesting shearing due to downslope sliding contrasting deformation styles sheared versus open folded of fine grained coloradito units may be due to abundance of smectite in the latter allowing higher water retention and a more plastic style of deformation jurassic sedimentary sequences on isla cedros record a complex history of explosive island arc volcanism gran canon formation arc continent collision coloradito formation and renewal of magmatic arc volcanism eugenia formation

eclogite blueschist relationships as evidenced by mineral equilibria in the high pressure metabasic rocks of sifnos cycladic islands greece

mineralogy and phase relations in the blueschist facies of the black butte and ball rock areas northern california coast ranges microprobe study of minerals in rocks of the blueschist facies from the black butte and ball rock areas of the south fork mountain schist northern california coast ranges is the basis in this report for analysis of reactions and phase relations among lawsonite pumpellyite epidote and associated minerals observed assemblages are intermediate in a progression from those observed elsewhere in high p low t schists of the franciscan e 9 panoche pass where lawsonite is present in nearly all lithologies to the relatively low phigh t assemblages of the shuksan blueschist washington where epidote is ubiquitous two successive reactions in this sequence in order of increasing temperature are l lawsonite na amphibole epidote quartz albite chlorite hzo 2 na amphibole pumpellyite quartz epidote ca amphibole albite chlorite h2o the pressure of metamorphism for the black butte and ball rock areas is estimated from the jadeite content of pyroxene and from the stability of aragonite to have been approximately 7 kbar temperature estimates are not tightly constrained by mineral equilibria but are suggested to be in the range of 250 to 300 c

rapid and quantitative major element analysis method for wet fine grained sediments using an xrf microscanner several types of x ray fluorescence xrf core scanners have been developed to fulfill the demands for high resolution rapid chemical analysis of wet sediment cores however their application to quantitative analysis has been hampered by the difficulty of evaluating the xrf absorption effect of water in a water film on the sediment surface and interstitial water within the sediments especially for lighter elements in this study we established a non destructive rapid high resolution and quantitative chemical analysis method for wet fine grained sediment samples using an xrf microscanner results of our experiments suggest that the degree of xrf absorption by the water film which develops on the wet sample surface covered with mylar film shows a clear positive correlation with the water content of the sediments under the measurement area a clear positive correlation also exists between the degree of xrf absorption by the interstitial water and the water content of the sediments under the measurement area we evaluated the xrf absorption by the thin water film using the relationship between the water content and thickness of the thin water film and the xrf absorption by the interstitial water using its relation with the water content the water content under the measurement area is estimated from the average transmitted x ray intensity that is measured on the same measurement area simultaneously with the measurement of xrf intensities the dry sample equivalent xrf intensities of major elements al si k ca ti and fe which are the xrf intensities of wet samples corrected for the xrf absorption by the thin water film and the interstitial water show better correlation with the major element concentrations determined by conventional xrf methods using this new method it is possible to measure the average water content and average concentrations of the 6 major elements simultaneously for a square area as small as 0 5 0 5 mm on the wet fine grained sediment within 6 min which is nearly ten times faster than the conventional xrf method

model for late mesozoic early tertiary tectonics of coastal california and western mexico and speculations on the origin of the san andreas fault cretaceous and early tertiary arc and forearc rocks found along the coast in southern california and baja california have been shown by paleomagnetic measurements to have originated many hundreds or even several thousands of kilometers south of their current locations northward transport also is found in cretaceous batholithic rocks near the edge of the continent in washington and british columbia the consistency of this pattern suggests that slices of arc and forearc rock originating on the western edge of north america have been translated along the coast by strike slip faulting strike slip faults are shown to be an expected response to north oblique subduction of the farallon and or kula plates faulting and subduction probably were concurrent activities i e the arc and forearc grew and were displaced simultaneously because displacement was dominantly parallel to subduction related lithic belts arc and forearc rocks may have traveled large distances without producing either significant hiatuses in the rock record or conspicuous disruptions in the distribution of lithic belts subduction related strike slip faulting is shown to be favored by high angles of oblique convergence shallow dip of the subducting slab relatively easy slip on potential faults within the arc or forearc and the existence of a place for the moving sliver to go it is suggested that these conditions are not difficult to satisfy and that subduction related strike slip faulting may thus be a common feature of convergent orogens the location of the san andreas fault inboard from the continental margin may be attributable to localization of the pacific north american transform by a system of precollision subduction related faults

mesozoic paleomagnetism and northward translation of the baja california peninsula paleomagnetic data from cretaceous igneous and sedimentary rocks of the peninsular ranges and continental borderland of the baja california peninsula yield an average pole position of 83 n 344 e a95 3 4 suggesting that the peninsula as a whole has moved northward 11 and rotated 32 clockwise with respect to the north american craton since cretaceous time only a small portion of this movement is due to the neogene opening of the gulf of california results from stratified rocks giving both positive and negative fold tests are included in the average cretaceous pole the rocks that fail fold tests have been deformed and remagnetized by nearby intrusives that are only slightly younger unconnected directions from these sites closely match corrected directions from sites where the fold tests are positive oceanic rocks were also sampled in the continental borderland from upper triassic and upper jurassic tectonostratigraphic terranes which are apparently allochthonous with respect to the western baja peninsula and each other these oceanic rocks show magnetizations indicating low paleolatitudes although the hemisphere of origin is presently unknown the cretaceous rocks of the baja california peninsula may have originally formed adjacent to the now truncated continental margin along the northern middle america trench northward transport of the peninsula probably occurred by transcurrent faulting associated with oblique subduction of the farallon plate beneath western north america between late cretaceous and late miocene time

holocene climate in the atlantic sector of the southern ocean controlled by insolation or oceanic circulation abstractthe holocene climate of the southern ocean is not well un derstood mainly because of the lack of high resolution reconstruc tions of ocean surface properties here we present a 12 500 yr long decadal scale record of holocene sea surface temperatures and sea ice presence from the polar front of the east atlantic southernocean the record shows gradual climate change with no abruptneoglacial cooling and an unprecedented late holocene warming the dominant forcing factor appears to be precessional insolation northern hemisphere summer insolation correlates to at least theearly to middle holocene climate trend spectral analysis revealscentennial scale cyclic climate changes with periods of 1220 1070 400 and 150 yr the record shows good correlation to east ant arctic ice cores and to climate records from south georgia andbunger oasis however the record shows out of phase behaviorwith regard to climate records from the western antarctic penin sula and the peru chile current such behavior hints at a climaticdivide through patagonia the drake passage and between westand east antarctica keywords southern ocean paleoclimate diatoms modern analogtechnique holocene insolation antarctic polar front introductionthe holocene climate variability has received less attention thanthe more dramatic changes during the last glacial period the holoceneis generally assumed to be a stable interglacial period when climatechanges in the north atlantic are transmitted to the rest of the worldthrough a strong thermohaline circulation imbrie et al 1992 thelarge uctuations of the last glacial period appear to continue into thisinterglacial although in a subdued manner bond et al 2001 the southern ocean has received even less attention than the highnorthern latitudes oceanic records are few and coarse they generallydisplay a warm early holocene followed by cooling labracherie etal 1989 pichon et al 1992 hodell et al 2001 similar to northatlantic climate e g koc and jansen 1994 high resolution recordsare restricted to antarctic ice cores which mostly display little climatevariability during the holocene jouzel et al 2001 an exception isthe taylor dome deuterium record in which the holocene appears tobe split by an abrupt cooling event ca 5 5 5 ka steig et al 1998 this cooling appears to t the late holocene global cooling trend por ter 2000 as a nonlinear response from the climate system to insolationchanges demenocal et al 2000 hodell et al 2001 high resolution holocene records available for southern oceanclimate reconstruction are rarely from the antarctic circumpolar cur rent proper e g domack et al 2001 stenni et al 2001 lamy et al 2002 with the exception of a multiproxy approach to holocene cli mate that includes a high resolution description of lithic fragments hodell et al 2001 we present data from site tn057 17 near the polar front in theeast atlantic southern ocean it is based on quantitative estimates ofsummer sea surface temperature ssst and sea ice presence sip utilizing the down core variability of diatom assemblages the recordshows good correlation to antarctic ice cores but a gradual change inphase relationships during the early holocene the record shows anin phase relationship with the ross sea region ice cores during thelate holocene the record shows an in phase relationship with the eastantarctic region ice cores antarctic polar frontthe antarctic polar front apf fig 1 is an important climateboundary in terms of air sea uxes and the heat and salt budgets ofthe oceans at the surface the apf is a steep sea surface temperaturegradient while the subsurface expression is the southern limit for sub duction of cold fresh antarctic surface water below warmer and moresaline subantarctic surface water orsi et al 1995 globally the apfcould be the southern limit of synchrony with northern hemisphereclimate broecker 1996 the apf is subject to large spring and sum mer blooms of diatoms fischer et al 2002 and achieves the largestbiogenic sedimentation rates of the diatom ooze belt of the southernocean burckle and cirilli 1987 it is these high sedimentation ratesof diatomaceous oozes and the sensitivity of diatoms especially to lightand temperature that make sites in the apf region suitable for pro ducing high resolution records of climate change

structural history of high pressure metamorphic rocks in the southern vanoise massif french alps and their relation to alpine tectonic events abstract a nappe of amphibolite facies metamorphic rocks of pre permian age in the southern vanoise massif the arpont schist has been affected by an alpine hp lt metamorphism the first mesoscopically recognizable deformation d 1 post dated the high pressure peak jadeitic pyroxene quartz glaucophane lawsonite and was associated with glaucophane epidote d 1 produced a flat lying schistosity and a nw trending glaucophane lineation and was probably associated with nappe displacement involving nw directed subhorizontal shear d 2 formed small scale folds and a foliation associated with chlorite albite the changing parageneses during the period pre d 1 to d 1 to d 2 suggest decreasing pressure so that the deformation appears to have been related to the uplift history rather than to the process of tectonic burial d 2 was followed by a static metamorphism green biotite chlorite albite possibly of lepontine age se directed backthrusting and folding d 3 and later differential uplift along steep faults took place under low grade conditions

metamorphic p t paths from pelitic schists and greenstones from the south west tauern window eastern alps petrological data from intercalated pelitic schists and greenstones are used to construct a pressure temperature path followed by the upper schieferhulle ush series during progressive metamorphism and uplift in the south west tauern window italy pseudomorphs of al epidote fe epidote albite oligoclase chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite albite chlorite subfacies of the blueschist facies at p 7 8 kbar geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475 25 c 5 6 kbar calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of 7 5 kbar garnet zoning patterns are complex and reversals in zoning can be correlated between samples thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity fluid inclusion data from coexisting immiscible h2o co2 nacl fluids constrain the uplift path to have passed through temperatures of 380 30 c at 1 3 0 2 kbar there is no evidence for metamorphism of ush at pressures greater than 7 5 kbar in this area of the tauern window this is in contrast to pressures of 10 kbar recorded in the lower schieferhulle only 2 3 km across strike a history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the p t data obtained from these adjacent tectonic units

blueschist facies metamorphism shear heating and p t t paths in subduction shear zones the low temperatures recorded by blueschist facies metamorphic rocks place upper bounds on the magnitude of shear stresses in subduction zones at depths of 15 50 km in this paper steady state and transient pressure temperature time p t t paths followed by subducted material are calculated using analytical expressions presented by molnar and england 1990 supplemented by two dimensional numerical calculations in the absence of shear heating calculated steady state p t t paths are very cold intersecting 1 gpa 10 kbar at t 200 c for a wide range of subduction parameters for the case of constant shear stress in the subduction shear zone and typical thermal parameters steady state p t t paths intersect the blueschist facies for 10 60 mpa and 0 100 mpa for convergence rates of 10 and 3 cm yr 1 respectively blueschist facies metamorphism will not occur in rapid subduction zones if shear stresses exceed 60 mpa for the case of shear stress increasing linearly with depth steady state p t t paths intersect the blueschist facies when 1 9 p and 0 14 p for convergence rates of 10 and 3 cm yr 1 respectively subduction zone shear stresses may be considerably lower and possibly zero if blueschist terrains represent material metamorphosed during the early stages of subduction first 5 10 m y while the thermal structure is still evolving or if maximum temperatures in blueschist terrains are achieved during exhumation subduction zone shear stresses of several tens of mpa at 35 km depth or a few percent of lithostatic pressure are consistent with a steady state model of franciscan metamorphism a progressive underplating model for the formation of the pelona schist inverted metamorphic gradient and surface heat flow measurements in the ne japan subduction zone

contrasted metamorphic evolutions in thrusted cover units of the brianconnais zone french alps a model for the conservation of hp lt metamorphic mineral assemblages the evolution of organic matter silicate and fluid phases in cover units of the three structural zones of the vanoise area allows one to distinguish differentp t cooling paths these paths depend on whether the cover has been transported western and northern vanoise or not southern vanoise all units first underwent a common high pressure low temperature hp lt metamorphic stage 300 c 6 kbar of blueschist type fe mg carpholite chloritoid facies the cover units transported on the extermal colder zone coal measure series metamorphosed in albite chlorite facies preserved their hp mineralogy fe mg carpholite lawsonite and organic matter content oils wet gases and kerogen while the unit which remained in contact with its more thermally conductive basement polymetamorphic now shows extensive greenschist facies overprinting breakdown of fe mg carpholite and lawsonite appearance of chlorite pyrophyllite chloritoid and clinozoisite absence of oils and wet gases these two tectonic evolutions might serve as a more general model for the preservation of hp lt mineral assemblages

lawsonite zone blueschists and a sodic amphibole producing reaction in the tavsanli region northwest turkey the petrology and mineralogy of lawsonite zone metabasites have been studied northeast of town of tavsanli nw turkey in the field the metabasites are characteristically green and lack foliation the essential mineral assemblage being sodic pyroxene lawsonite chlorite quartz sodic amphibole sodic pyroxene of aegirine jadeite composition occurs as pseudomorphs after magmatic augite lawsonite and chlorite are the other two dominant minerals sodic amphibole forms progressively from a reaction between sodic pyroxene chlorite and quartz and an isograd representing the first abundant occurrence of sodic amphibole in basic rocks has been mapped the widespread occurrence of sodic pyroxene pseudomorphs in other blueschist terrains indicates that the inferred sodic amphibole producing reaction is of general significance for blueschist metabasites

blueschist metamorphism in an active subduction zone the high pressure low temperature metamorphic rocks known as blueschists have long been considered to form in subduction zones where the descent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures until now however the link between blueschist facies rocks and subduction zones has been indirect relying on a spatial association of blueschists with old subduction complexes and estimates of the geothermal gradients likely to exist in subduction zones here we strengthen this link by reporting the discovery of blueschist facies minerals lawsonite aragonite sodic pyroxene and blue amphibole in clasts from a serpentinite seamount in the forearc of the active mariana subduction zone the metamorphic conditions estimated from the mineral compositions are 150 250 c and 5 6 kbar 16 20 km depth the rocks must have been entrained in rising serpentine mud diapirs and extruded from mud volcanoes onto the sea floor further study of these rocks may provide new insight into the tectonics of trench forearc systems and in particular the processes by which blueschist facies clasts come to be associated with forearc sediments in ancient subduction complexes

optical determination of topotactic aragonite calcite growth kinetics metamorphic implications textures produced by the partial recrystallization of aragonite to calcite in the blueschist facies show that heterogeneous topotactic nucleation is rapid enough to permit growth kinetics to dominate the overall geologic rate of transformation both grain boundary velocities measured microscopically between 375 c and 455 c and analysis of topotactic relations predict a growth mechanism involving only small atomic displacements near a highly coherent boundary between the two phases the kinetic equation of turnbull for interface controlled growth based on that mechanism accurately describes the experimental results when calculated rates are integrated over geologically plausible pressure temperature time paths for convergent plate margins even the most extreme values lead to the conclusion that aragonite bearing rocks enroute to the surface must enter the calcite stability field between 125 c and 175 c the widespread occurrence of aragonite along the pacific margin of north america and by contrast

a north to south transect of holocene southeast atlantic continental margin sediments relationship between aerosol transport and compound specific δ13c land plant biomarker and pollen records 1 we examined near surface late holocene deep sea sediments at nine sites on a north south transect from the congo fan 4 s to the cape basin 30 s along the southwest african continental margin contents distribution patterns and molecular stable carbon isotope signatures of long chain n alkanes c27 c33 and n alkanols c22 c32 are indicators of land plant vegetation of different biosynthetic types which can be correlated with concentrations and distributions of pollen taxa in the same sediments calculated clusters of wind trajectories and satellite aerosol index imagery afford information on the source areas for the lipids and pollen on land and their transport pathways to the ocean sites this multidisciplinary approach on an almost continental scale provides clear evidence of latitudinal differences in lipid and pollen composition paralleling the major phytogeographic zonations on the adjacent continent dust and smoke aerosols are mainly derived from the western and central south african hinterland dominated by deserts semi deserts and savannah regions rich in c4 and cam plants the northern sites congo fan area and northern angola basin which get most of their terrestrial material from the congo basin and the angolan highlands may also receive some material from the chad region very little aerosol from the african continent is transported to the most southerly sites in the cape basin as can be expected from the present position of the phytogeographic zones the carbon isotopic signatures of the n alkanes and n alkanols both become isotopically more enriched in 13c from north to south the results of the study suggest that this combination of pollen data and compound specific isotope geochemical proxies can be effectively applied in the reconstruction of past continental phytogeographic developments

deformation and metamorphism in the bard area of the sesia lanzo zone western alps during subduction and uplift the structure microstructure and petrology of a small area close to the village of bard in val d aosta italy has been studied in detail the area lies across the contact between the gneiss minuti gm and the eclogitic micaschist ems complexes of the lower element of the sesia portion of the sesia lanzo zone western alps both complexes have undergone high pressure metamorphism but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the gm to the ems therefore we interpret the contact as a thrust dividing the lower element of the sesia into two sub elements this interpretation is supported by structural evidence the early alpine 90 70 ma metamorphic history is best preserved in the ems and is one of increasing pressure associated with thrusting the maximum p t recorded in the ems is 1500 mpa 15kbar and 550 c and in the gm is 1500 1300 mpa 15 13 kbar and 500 550 c we suggest that the rocks were probably in an active benioff zone during this time from then on the histories of the gm and ems are the same deformation continued and the thrust and thrust slices were folded during decreasing pressure we interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the benioff zone had become inactive and a new benioff zone had developed further to the north west a still later deformation and the lepontine metamorphism 38 ma are related to continued uplift much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface

experimental investigations of blueschist greenschist transition equilibria pressure dependence ofal2o3 contents in sodic amphiboles a new geobarometer

metamorphic petrology of a high pressure low temperature subduction complex in west central baja california mexico the western baja terrane wbt of west central baja california is an uplifted subduction complex that is divided into smaller subterranes on the basis of bounding faults and petrological differences each subterrane contains coherent early jurassic to early cretaceous sedimentary and mafic volcanic rocks not melange that have been metamorphosed under blueschist facies conditions key phases in metabasites and metaturbidites include jadeitic to acmitic clinopyroxene sodic amphibole lawsonite aragonite chlorite titanite and white mica pressure indicators include the jadeite content of clinopyroxene and the presence of aragonite temperature indicators include the presence of lawsonite the absence of greenschist facies minerals and results from vitrinite reflectance studies conditions at the peak of metamorphism were 8 kbar 225 325 c for subterrane 1 7 8 kbar 170 220 c for subterrane 2 and 5 6 kbar 175 200 c for subterrane 3 these correspond to cold geothermal gradients 6 9 km vein assemblages that include aegerine jadeite and aegerine albite aragonite lawsonite and sodic amphibole indicate uplift during continued cold conditions probably during steady state subduction

incipient blueschist metamorphism and metasomatism in the tavsanli region northwest turkey volcano sedimentary rocks in an imbricate tectonic zone around a peridotite massif have been studied northeast of the town of tavsanli in northwest turkey basic volcanic rocks which are the dominant rock type in this zone show incipient blueschist metamorphism and associated metasomatism while the igneous textures of the volcanic rocks are retained augites are partially to completely replaced by sodic pyroxene and plagioclase is albitised resulting in rocks with 6 8 wt na2o the volcanic rocks are cross cut by numerous veins of calcite aragonite quartz pumpellyite albite lawsonite and sodic pyroxene pelagic limestones which are interbedded with the basic volcanic rocks consist of coarse aragonite grains showing partial replacement by calcite the occurrence of aragonite lawsonite and albite indicates conditions of metamorphism for the whole zone in the range of 5 8 kb and 150 200 c metasomatism probably related to high pressure serpentinization has occurred contemporaneously with the incipient high pressure metamorphism

geochronology of blueschists from west central baja california and the timing of uplift in subduction complexes a geochronologic study of blueschists from west central baja california has provided constraints on the timing of high pressure low temperature metamorphism and the subsequent uplift of this subduction complex mineral separates from coherent blueschists and blueschist blocks enclosed in serpentinite melange were analyzed by 40 ar 39 ar fission track and rb sr methods 40 ar 39 ar age spectra for white mica separated from four blueschist blocks and an overprinted eclogite block in serpentinite matrix melange show slow cooling gradients between 115 95 ma rb sr apparent ages for one block are concordant with these results 40 ar 39 ar ages of blue amphibole are primarily the result of the degassing of white mica intergrowths as confirmed by microprobe analyses apatite fission track ages indicate a blueschist block and an eclogite block cooled to a temperature below 100 c at 22 ma and 32 ma respectively subduction related metamorphism of the coherent blueschists occurred in la

sea ice as the glacial cycles climate switch role of seasonal and orbital forcing a box model of the coupled ocean atmosphere sea ice and land ice climate system is used to study glacial interglacial oscillations under seasonally and orbitally varying solar forcing the dominant 100 kyr oscillation in land ice volume has the familiar sawtooth shape of climate proxy records and to zeroth order it does not depend on the seasonal and milankovitch forcing the sea ice controls via its albedo and insulating effects the atmospheric moisture fluxes and precipitation that enable the land ice sheet growth this control and the rapid growth and melting of the sea ice allow the sea ice to rapidly switch the climate system from a growing ice sheet phase to a retreating ice sheet phase and to shape the oscillation s sawtooth structure a specific physical mechanism is proposed by which the insolation changes act as a pacemaker setting the phase of the oscillation by directly controlling summer melting of ice sheets this mechanism is shown to induce deglaciations during periods of lower summer insolation superimposed on the 100 kyr are the linear milankovitch forced frequencies of 19 23 and 41 kyr the transition from 41 kyr glacial cycles to 100 kyr cycles one million years ago may be explained as being due to the activation of the sea ice switch at that time this would be the case if sea ice extent was more limited during the warmer climate of the early pleistocene

ultra long distance littoral transport of orange sand and provenance of the skeleton coast erg namibia abstract quantitative provenance analysis based on high resolution bulk petrography and heavy mineral data on beach and dune sands integrated with detrital zircon geochronology and chemical analyses of pyroxene garnet and staurolite demonstrates that sand derived from the orange river is carried by powerful and persistent longshore currents as far as northern namibia and southern angola 1750 km north of its mouth this is the longest cell of littoral sand transport documented so far compositional forward modeling indicates that 80 of dune sand in the skeleton coast is orange derived the remaining 20 being supplied by slow erosion of the damara orogen chiefly via the swakop river a decrease in basaltic rock fragments and pyroxene with relative enrichment in garnet staurolite tourmaline and other metamorphic minerals north of walvis bay indicates that only one third of beach sand in the 350 km dune free gap between the namib and skeleton coast ergs is orange derived the remaining two thirds being supplied largely by the swakop river draining the damara orogen although volcanic gravel becomes dominant in beaches of the skeleton coast north of the uniab mouth detritus from cretaceous etendeka lavas accounts for only 4 of beach sand reflecting limited sand generation in the arid catchment contributions from the kunene river to either dune or beach sands is ruled out indicating that gem diamonds found in placer deposits along the coast of namibia are all derived from the orange river

contrasting evolution of sea surface temperature in the benguela upwelling system under natural and anthropogenic climate forcings we present alkenone derived sea surface temperature sst records from three marine cores collected within the southern benguela upwelling system bus spanning the last 3 ka the sst evolution over the last 3 millennia is marked by aperiodic millennial scale oscillations that broadly correspond to climatic anomalies identified over the north atlantic region the bus sst data further suggest cooling and warming trends opposite to the temperature evolution in the moroccan upwelling region and in antarctica in contrast the last decades are marked by a cooling of unprecedented magnitude in both the benguela and moroccan upwelling systems which is not observed in the antarctic record these contrasted responses in atlantic upwelling systems triggered by natural and anthropogenic forcings shed light on how different climatic mechanisms are mediated by ocean atmosphere interactions and transmitted to the geological records of past and present climate changes citation leduc g c t herbert t blanz p martinez and r schneider 2010 contrasting evolution of sea surface temperature in the benguela upwelling system under natural and anthropogenic climate forcings geophys res lett 37 l20705 doi 10 1029 2010gl044353

geometry and evolution of low angle normal faults lanf within a cenozoic high angle rift system thailand implications for sedimentology and the mechanisms of lanf development 1 at least eight examples of large 5 35 km heave low angle normal faults lanfs 20 30 dip occur in the cenozoic rift basins of thailand and laterally pass into high angle extensional fault systems three large displacement lanfs are found in late oligocene miocene onshore rift basins suphan buri phitsanulok and chiang mai basins they have 1 developed contemporaneous with or after the onset of high angle extension 2 acted as paths for magma and associated fluids and 3 impacted sedimentation patterns displacement on low angle faults appears to be episodic marked by onset of lacustrine conditions followed by axial progradation of deltaic systems that infilled the lakes during periods of low or no displacement the chiang mai lanf is a low angle 15 25 high displacement 15 35 km heave ese dipping lanf immediately east of the late early miocene doi inthanon and doi suthep metamorphic core complexes early cenozoic transpressional crustal thickening followed by the northward motion of india coupled with burma relative to east burma and thailand 40 30 ma caused migmatization and gneiss dome uplift in the late oligocene of the core complex region followed by lanf activity lanf displacement lasted 4 6 ma during the early miocene and possibly transported a late oligocene early miocene high angle rift system 35 km east other lanfs in thailand have lower displacements and no associated metamorphic core complexes the three lanfs were initiated as low angle faults not by isostatic rotation of high angle faults the low angle dips appear to follow preexisting low angle fabrics thrusts shear zones and other low angle ductile foliations predominantly developed during late paleozoic and early paleogene episodes of thrusting and folding

late holocene dynamics of sea surface temperature and terrestrial hydrology in southwestern africa southwest africa is an important region for paleo climatic studies being influenced by both tropical and temperate climate systems and thus reflecting the interplay of variable controls the aim of this study was to unravel the interaction of sea surface temperature sst changes in the southernmost benguela upwelling system with precipitation changes in south africa s winter rainfall zone wrz during the late holocene therefore a marine sediment core from the southernmost benguela upwelling system was investigated to reconstruct climate changes in this region for the past 2000 years grain size and geochemical analyses were conducted to reconstruct changes in fluvial sediment discharge and weathering intensity while sst changes were estimated using alkenone paleo thermometry results show that the southernmost benguela behaves distinctly in comparison with the rest of the benguela system reflecting amplified sst changes decreasing ssts accompanied increasing river discharge during times of increas

surface wave potential for triggering tectonic nonvolcanic tremor corrected abstract source processes commonly posed to explain instances of remote dynamic triggering of tectonic nonvolcanic tremor by surface waves include frictional failure and various modes of fluid activation the relative potential for love and rayleigh wave dynamic stresses to trigger tectonic tremor through failure on critically stressed thrust and vertical strike slip faults under the coulomb griffith failure criteria as a function of incidence angle are anticorrelated over the 15 to 30 km depth range that hosts tectonic tremor love wave potential is high for strike parallel incidence on low angle reverse faults and null for strike normal incidence the opposite holds for rayleigh waves love wave potential is high for both strike parallel and strike normal incidence on vertical strike slip faults and minimal for 45 incidence angles the opposite holds for rayleigh waves this pattern is consistent with documented instances of tremor triggered by love waves incident on the cascadia megathrust and the san andreas fault saf in central california resulting from shear failure on weak faults apparent friction is 0 2 documented instances of tremor triggered by surface waves with strike parallel incidence along the nankai megathrust beneath shikoku japan however are associated primarily with rayleigh waves this is consistent with the tremor bursts resulting from mixed mode failure crack opening and shear failure facilitated by near lithostatic ambient pore pressure low differential stress with a moderate friction coefficient 0 6 on the nankai subduction interface rayleigh wave dilatational stress is relatively weak at tectonic tremor source depths and seems unlikely to contribute significantly to the triggering process except perhaps for an indirect role on the saf in sustaining tremor into the rayleigh wave coda that was initially triggered by love waves

paleomagnetism of mesozoic red chert from cedros island and the san benito islands baja california mexico revisited previous paleomagnetic study of mesozoic subduction complex rocks on cedros island and the san benito islands mexico shows that these rocks have been remagnetized a single polarity magnetization was found in chert sections and underlying pillow basalt deposited during a mixed polarity interval of the geomagnetic field reanalysis of the cedros and san benito chert samples 101 total shows that 13 samples also retain a high blocking temperature component of magnetization 600 to 680 c indicating a polarity stratigraphy and deposition of the bedded chert near 2 3 paleolatitude these data are similar to those for red chert in california and japan implying that many of the mesozoic ophiolitic chert sequences now exposed around the pacific rim were initially deposited within the equatorial zone of high biologic productivity these new data also support accretion of the cedros and san benito chert to the american margin at tropical paleolatitudes requiring significant northward translation and clockwise rotation to bring them to their present position with respect to the continental interior

evolution and structure of the upper rhine graben insights from three dimensional thermomechanical modelling the evolution and geometry of the tertiary upper rhine graben were controlled by a continually changing stress field and the reactivation of pre existing crustal discontinuities a period of wnw ese extension in the late eocene and oligocene was followed by lateral translation from the early miocene onwards this study utilizes 3d finite element techniques to simulate extension and lateral translation on a lithospheric scale brittle and creep behaviour of lithospheric rocks are represented by elastoplasticity and thermally activated power law viscoplasticity respectively contact elements allocated with cohesion and frictional coefficients are used to describe pre existing zones of weakness in the elastic brittle field our results suggest that 1 extension is accommodated along listric border faults to midcrustal depth of 15 16 km beneath pure shear stretching occurs without a need for localized shear zones in lower crust and upper mantle 2 ductile flow at midcrustal depth across the graben accounts for the pronounced halfgraben morphology thereby the shape of the border faults their frictional coefficients and sedimentary loads have profound effects on the rate of ductile flow across the graben 3 horizontal extension of 8 8 5 km and sinistral displacement across the rift of 3 4 km are needed to accommodate the observed sediment thickness

calc silicate mineralization in active geothermal systems detailed study of the calc silicate mineralogy in the cerro prieto geothermal system was used to demonstrate examples for thermodynamic evaluation of phase relations among minerals of variable composition and to calculate the chemical characteristics of hydrothermal solutions compatible with the observed calc silicate assemblages in general there is a close correlation between calculated and observed fluid compositions calculated fugacities of o 2 at about 320 degrees c in the cerro prieto geothermal system are about five orders of magnitude less than those at the nearby salton sea geothermal system this observation is consistent with the occurrence of fe super 3 rich epidotes and andradite garnets in the latter system and the presence of prehnite at cerro prieto modified journal abstract

uplift related retrogression history of aragonite marbles in western crete greece in the low grade high pressure 400 c 10 kbar metamorphic phyllite quartzite unit of western crete widespread occurrences of aragonite marbles have been discovered recently a sedimentary precursor is proved by relic structures bedding fossils partial or complete transformation of aragonite into calcite is ubiquitous compositional and microstructural features reflect the metamorphic history 1 the high pressure stage is documented by aragonite that is chemically characterized by incorporation of variable amounts of sr and the lack of mg the most sr rich aragonite has about 9 wt sro xsrarag 0 09 a compositional zoning observed in some aragonite crystals may be due to the prograde divariant calcite aragonite transformation in the system caco3 srco3 because the parent rocks probably were sr poor calcite limestones one can speculate that strontium has been supplied from an external source under high pressure conditions 2 during uplift calcite replacing aragonite did not equilibrate with unreplaced aragonite disequilibrium is indicated by highly variable compositions of calcite crystals that show topotactic relations to the host aragonite the calcite compositions range from that of the host aragonite sr rich and mg free to mg bearing and sr poor 3 calcite that recrystallized during retrogression is generally sr poor mean value ofxsr 0 005 mg bearing xmg 0 010 and chemically homogeneous because practically no sr remains in the calcite an interaction with a fluid phase is indicated in fine grained calcite marbles rich in solid organic matter microstructural features indicative of former aragonite may be present 4 the last stage of retrogression is documented by the appearance of radiating aragonite in veins and nodules this aragonite which shows neither deformation nor retrogression was probably formed metastably in a near surface environment

high and low latitude climate control on the position of the southern peru chile current during the holocene 1 we reconstructed changes of temperature salinity and productivity within the southern peru chile current during the last 8000 years from a high resolution sediment core recovered at 41 s using alkenones isotope ratios of planktic foraminifera biogenic opal and organic carbon paleotemperatures and paleosalinities reached maximum values at 5500 years ago and thereafter declined to modern values whereas paleoproductivity continuously increased throughout the last 8000 years we ascribe these long term holocene trends primarily to latitudinal shifts of the antarctic circumpolar current acc the concurrence with shifts in the position of the southern westerlies points to a common response of atmospheric and oceanographic circulation patterns off southern chile millennial to centennial scale fluctuations of paleotemperatures and paleosalinities on the other hand lag displacements in the position of the southern westerlies but reveal a significant correlation to short term temperature changes in antarctica indicating a high latitude control of the acc at these timescales

magma faults and gravitational loading at mount etna the 2002 2003 eruptive period 1 mount etna is characterized by a complex structural setting that influences its evolution in order to understand how the interaction between dike forming intrusions and faulting influences the kinematics of the volcanic edifice we developed a numerical model it takes account of the topography the medium heterogeneities the gravitational loading and the most active crustal discontinuities a parameterization of the apparent coefficient of friction as a function of the depth has been considered in the range from 0 01 to 0 5 the density values used young s modulus and poisson s ratio range from about 1700 to 3200 kg m3 8 5 to 140 gpa and 0 15 to 0 35 respectively the model was applied to the 2002 2003 etna eruption the resulting deformation pattern was in agreement with the data provided by the continuous gps stations and the seismological knowledge obtaining displacements up to 1 m some discrepancies between the recorded ground deformation field and the modeled displacements allowed us to make some hypotheses on the volcano tectonic e g for the pernicana fault or regional tectonic origin acireale s alfio fault system of the crustal discontinuities moreover our results suggested that the pernicana fault is characterized by a very low apparent coefficient of friction less than 0 1 furthermore we evaluated the contribution of the gravitational loading and found that it implies a variation of about 10 of the overall deformation pattern our results clearly showed that the weight of the volcanic edifice acts in opposition to the magmatic intrusions conversely we found that the presence of medium heterogeneities may favor the eastern flank movements toward the se and the spreading of the summit area playing a fundamental role in ascending magma finally we investigated the shallow sliding model and found that dike forming intrusions produce negligible displacements less than 1 mm along the subhorizontal detachment surface therefore the gravitational loading during dike forming intrusions is not able to trigger sliding processes along this plane

comparison of the high pressure schist belts of new caledonia and sanbagawa japan the high pressure schist terranes of new caledonia and sanbagawa were developed along the oceanic sides of sialic forelands by tectonic burial metamorphism the parent rocks were chemically similar as volcanic sedimentary trough or trench sequences and metamorphic temperatures in both belts were 250 to 600 c from phase equilibria curves total pressures were higher for new caledonia 6 15 kb than for sanbagawa 5 11 kb and the estimated thermal gradients were 7 10 c km and 15 c km respectively pt paths identify the higher pressure in new caledonia p differences 2 kb at 300 c and 4 kb at 550 c with consequent contrast in progressive regional metamorphic zonation for pelites in the two areas lawsonite epidote omphacite new caledonia and chlorite garnet biotite sanbagawa in new caledonia the na amphibole is dominantly glaucophane and na pyroxenes associated with quartz are jadeite jd95 100 and omphacite in sanbagawa the amphibole is crossite or riebeckite and the pyroxene is omphacite jd50 for both areas garnet rims show increase in pyrope content with advancing grade but sanbagawa garnets are richer in almandine progressive assemblages within the two belts can be equated by such reactions as new caledonia sanbagawa glaucophane paragonite h2o albite chlorite quartz glaucophane epidote h2o albite chlorite actinolite and the lower pressure japanese associations appear as retrogressive phases in the new caledonia epidote and omphacite zones

relationship between antarctic sea ice and southwest african climate during the late quaternary here we compare late quaternary southwest african climate records from the west coast of southern africa published winter rainfall and trade wind intensity records from a core off the coast of namibia to records of antarctic sea ice extent this comparison reveals coherent changes between antarctic sea ice extent and the southwest african winter rain region since 45 k y b p with enhanced winter rainfall and trade wind vigor during periods of increased sea ice presence we propose an oceanic and atmospheric coupling between antarctic sea ice and the winter rainfall zone of southwest africa that may lead to increased desertification in the region if global warming persists

changes in seismic activity following the 2011 tohoku oki earthquake effects of pore fluid pressure abstract we examined the effects of pore fluid pressure on seismicity changes following the 2011 off the pacific coast of tohoku tohoku oki earthquake through the analysis of aftershocks based on the coulomb failure criterion background tectonic stress fields in northeast japan are generally characterized by e w compression after the tohoku oki earthquake as expected from decrease in the coulomb failure function shear stress 0 4 normal compressive stress seismicity in the upper crust of northeast japan decreased except some restricted regions where we observed many aftershocks with unfavourable focal mechanisms to the background stress fields by mapping the focal mechanisms of aftershocks on the 3 d mohr diagram region by region we confirmed that the aftershocks occurred on optimally oriented faults in some regions but on misoriented faults in other regions the aftershocks on optimally oriented faults indicate the increase in regional ambient fluid pressure caused by the flow of over pressurized fluid from a deep reservoir on the other hand the aftershocks on misoriented faults which cannot be attributed to coseismic stress rotation indicate the increase in fault confined fluid pressure relative to the ambient fluid pressure

denudation rates thermal evolution and preservation of blueschist terrains the erosion of low temperature high pressure blueschist facies metamorphic rocks presents a number of problems if their rate of erosion is not sufficiently rapid then the rise in temperature caused by thermal relaxation and radioactive self heating is great enough to obliterate the original low temperature mineral assemblages the problem has been investigated using a one dimensional two layer model to mimic an accretionary prism overlying oceanic lithosphere the results suggest that if no erosion takes place the maximum time that blueschist conditions may exist is about 26 m y if levels of radioactive self heating are high and approximately 100 m y if levels are very low if erosion takes place the maximum temperature reached by an element of rock is at a higher level in the earth s crust than its initial depth of burial rocks may re equilibrate at this temperature or be overprinted by incomplete reactions depending on the temperature rise and rocks involved this could result in the geotherm

paleomagnetic and geologic data indicating 2500 km of northward displacement for the salinian and related terranes california results of a pair of paleomagnetic studies of sedimentary rocks in the salinian terrane of westernmost california indicate a northward displacement of about 2500 km since cretaceous time stratigraphic relations suggest that the salinian terrane became amalgamated with several others by the late cretaceous and all underwent the same northward translation this composite terrane allochthon became sutured to cratonal north america by early tertiary time one paleomagnetic study involved upper cretaceous turbidites of the pigeon point formation fine grained thin bedded turbidites yielded the most reliable results whereas medium to coarse grained sandstones are not always stably magnetized contorted layers resulting from soft sediment deformation usually gave directional results parallel to those of undeformed sedimentary layers thereby implying a postdepositional remanence process there is no indication of inclination error or other processes affecting the fidelity of the remanent magnetization paleomagnetic results from four separate locations in the pigeon point formation gave positive fold tests and mean inclination values near 36 a mean geomagnetic latitude calculated from these results is 21 2 5 3 much more southerly than the value of 46 5 2 3 calculated for pigeon point using paleomagnetic pole data from late cretaceous rocks from cratonic north america declination values are not antipodal for normal and reversed polarity sites analysis of depositional current directions from sandstone beds of the same outcrops shows that the difference is the result of neogene rotations of small crustal blocks adjacent to the san gregorio fault in a sequence of paleocene turbidites that crop out at point san pedro four sample localities yielded positive fold tests with mean inclination values near 40 declination values however were spread over a range between 230 and 320 comparison with depositional current directions from the same outcrops indicates clockwise rotations related to neogene right slip on the pilarcitos fault the mean geomagnetic latitude calculated for the point san pedro sediments is 24 5 4 0 and is more southerly than the 43 3 2 5 latitude expected for that location for the salinian terrane and the west coast of cratonic north america absolute rates of latitudinal motion were 8 5 cm yr northward and 3 cm yr southward respectively

creeping subduction zones are weaker than locked subduction zones faults that are fully or partially locked pose the greatest seismic hazard because they accumulate stress that can then be released in large earthquakes in contrast other faults continuously creep the creeping versus locked behaviour is probably related to the frictional properties of the fault and the effective normal stress on the fault but it is unclear whether locked faults are weaker or stronger than creeping faults here we use stress orientations in subduction zones from inversion of earthquake moment tensors and find that geodetically determined creeping versus locked behaviour is correlated with the orientation of the subduction zone plate boundary fault relative to the principal stress axes globally locked subduction zones appear well oriented for failure assuming a typical laboratory friction coefficient creeping subduction zones are more poorly oriented implying a lower apparent friction coefficient due to either low intrinsic friction or reduced effective normal stress the spatial variations of stress orientation on the japan trench are similarly correlated with spatial variations in coupling with creeping regions having a lower apparent friction coefficient than locked regions the absolute strength of faults is influenced by the ambient fluid pressure which is often elevated in subduction zones this suggests low overall strength for locked subduction zone faults and additional strength reduction in creeping zones that may be due to transient elevated fluid pressures the faults in creeping segments of subduction zones are weaker than those in locked segments according to analyses of stress orientations and gps data from subduction zones globally

distribution of major elements in atlantic surface sediments 36 n 49 s imprint of terrigenous input and continental weathering 1 numerous studies use major element concentrations measured on continental margin sediments to reconstruct terrestrial climate variations the choice and interpretation of climate proxies however differ from site to site here we map the concentrations of major elements ca fe al si ti k in atlantic surface sediments 36 n 49 s to assess the factors influencing the geochemistry of atlantic hemipelagic sediments and the potential of elemental ratios to reconstruct different terrestrial climate regimes high concentrations of terrigenous elements and low ca concentrations along the african and south american margins reflect the dominance of terrigenous input in these regions single element concentrations and elemental ratios including ca e g fe ca are too sensitive to dilution effects enhanced biological productivity carbonate dissolution to allow reliable reconstructions of terrestrial climate other elemental ratios reflect the composition of terrigenous material and mirror the climatic conditions within the continental catchment areas the atlantic distribution of ti al supports its use as a proxy for eolian versus fluvial input in regions of dust deposition that are not affected by the input of mafic rock material the spatial distributions of al si and fe k reflect the relative input of intensively weathered material from humid regions versus slightly weathered particles from drier areas high biogenic opal input however influences the al si ratio fe k is sensitive to the input of mafic material and the topography of andean river drainage basins both ratios are suitable to reconstruct african and south american climatic zones characterized by different intensities of chemical weathering in well understood environmental settings

imbricate low angle faulting in uppermost franciscan rocks south yolla bolly area northern california the south yolla bolly area of northern california contains the intersection of the franciscan complex the klamath mountains and the great valley sequence four distinct lithic units among them the south fork mountain schist as well as several subunits have been discerned in the franciscan rocks of this area these units are composed mostly of coherent or semi coherent layered sedimentary and mafic igneous rocks and their metamorphic equivalents rather than melange and are arranged in largely upright parallel imbricate fault slices four major low angle faults have been clearly recognized paleontologic data indicate that at least one fault places older material over younger lithic units exhibit various degrees of differences in metamorphic mineral assemblages textural grade and amount of internal deformation across the faults with more intensely deformed and metamorphosed material occupying successively higher structural positions these contrasts indicate that some of these faults served as surfaces by which deformed rocks were emplaced differentially upward during and or after metamorphism original bedding and compositional layering in all of the coherent units generally parallel the thrust faults two fault bounded units are each composed of a coherent sequence of basalt at the base overlain by bedded chert and further overlain by mudstone and graywacke or their metamorphic equivalents the faults which underlie these two sequences seem to have originated as decollements localized at or near a sedimentary rock mafic crust interface perhaps as discrete underthrust slices the distinct contrasts in original rock composition among these units plus the aforementioned characteristics seem to indicate that these faults had an early history in the accretionary prism apart from their later use as convenient surfaces for differential upward emplacement of metamorphosed material

high pressure low temperature metamorphism in the gran paradiso basement western alps the gran paradiso basement complex of the french and italian alps is composed of metasediments termed the gneiss minuti and metabasic rocks both of which are intruded by a late hercynian granite the bonneval gneiss which crops out at the western edge of the complex is composed of highly deformed metasediments volcanics and volcaniclastic rocks eclogites now highly altered occur in the metabasic rocks kyanite and blue green amphibole are locally present in the gneiss minuti and aegirine plus riebeckite occur in the bonneval gneiss a moderately high pressure low temperature metamorphic event of probable alpine age occurred in the basement complex this metamorphic event differs from that in the overlying sesia unit and ophiolites of the schistes lustres nappe in being at lower pressures below the ab jd100 qz transition and post dating the major d2a deformation the origin of the metamorphism is discussed and interpreted as a probable consequence of the overlying nappe pile which was emplaced during the d2a event subsequent greenschist facies metamorphism in the basement complex is a consequence of thermal relaxation during uplift

regional progressive high pressure metamorphism seward peninsula alaska blueschist facies rocks on the seward peninsula constitute a structurally coherent terrane measuring at least 100 150 km radiometric age data indicate that high pressure metamorphism probably occurred in jurassic rather than in palaeozoic or precambrian time as previously suggested protolith sediments nome group are of intracontinental basin or continental margin type and of lower palaeozoic and possibly late precambrian age thus predating the high pressure metamorphism by more than 200 m y blueschist facies mineral assemblages were developed in almost all lithologies of the nome group and are best preserved in feti rich metabasites glaucophane almandine epidote and pelites glaucophane chloritoid phengite a lawsonite crossite subfacies was developed in possible nome group rocks on the east flank of the darby mountains albite epidote amphibolite facies assemblages characterize nome group rocks in the southwestern part of the peninsula metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote almandine glaucophane schist subfacies with the local development east of the nome river of eclogitic assemblages the high pressure metamorphic minerals were synkinematic with the development of mesoscopic scale intrafolial isoclinal folds and a flattening foliation of consistent orientation initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles and partial post kinematic greenschist facies replacements record later stages of decompression ophiolites and melange are not associated with the seward peninsula blueschists the high pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin the pt conditions of high pressure metamorphism were approximately 9 11 kbar 400 450 c thus falling between the pt paths of the shuksan and franciscan terranes

retrograde evolution of eclogite facies rocks from ne oman evidence from fluid inclusions and petrological data eclogite facies rocks from eastern saih hatat oman metamorphosed under peak conditions of p 12 kbar and t of 550 580 c have several types of fluid inclusions in quartz clinopyroxene and garnet most of these inclusions are two phase at room t and contain an aqueous fluid belonging to the system nacl mgcl2 h2o final melting temperatures for most inclusions range from 4 to 0 c but values as low as 13 c have been recorded indicating salinities of 490 c inclusions with three phases at room t also contain a predominantly low salinity aqueous fluid with xco2 12 kbar to 4 kbar or less at t 500 550 c this conclusion is supported by the scatter in homogenization temperatures for these inclusions and their large variety of shapes and sizes which range from irregular with hair like fractures surrounded by satellite inclusions to negative crystals the replacement of ferroan dolomite ankerite and quartz by calcite talc and of rutile calcite by titanite or rutile glaucophane by ilmenite albite in these eclogites are also consistent with a stage of isothermal decompression following near isothermal decompression to depths of 10 15 km brittle deformation prevailed leading to the formation of layer parallel and crosscutting fractures that were filled by quartz calcite this stage was followed by near isobaric cooling recorded by the isochores for fluid inclusions in vein quartz and by the occurrence of dendritic networks of decrepitated and imploded fluid inclusions in these veins

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

subduction and accretion of the permanente terrane near san francisco california previous studies of the calera limestone permanente terrane franciscan complex of california have shown that deposition occurred at approximately 25 n paleolatitude atop an oceanic plateau or rise and that it was subsequently transported northeast and accreted to the north american continental margin in the early tertiary the permanente terrane in the study area along the pacific coast south of san francisco contains a suite of rock types present in north dipping imbricate thrust slices possibly related to the subduction and accretion of an oceanic crustal sequence geochemical studies of greenstones thought to represent fragments of the dismembered oceanic crust indicate affinities with mid ocean ridge basalts and within plate basalt these greenstones may represent fragments of the rise upon which the calera limestone was deposited metamorphic minerals in the greenstones indicate both sea floor metamorphism and locally high pressure low temperature metamorphism it is concluded tentatively that the entire permanente terrane in the study area underwent high pressure low temperature metamorphism but that temporal or geologic conditions prevented widespread recrystallization or development of blueschist facies minerals structures in the deformed rocks can be related to underthrusting beneath the continental margin and attachment to the overriding plate studies of veins indicate several stages of dewatering it is likely that dewatering was a major process in the attachment event as shown by veins associated with this phase of deformation structurally interleaved melanges and broken formation contain sedimentary rock clearly derived from a continental margin these rocks probably represent trench and trench slope facies that were underthrust with the accreting oceanic crustal sequence

mesozoic blueschists and melanges of cedros island baja california mexico a consequence of nappe emplacement or subduction cedros island and the adjacent vizcaino peninsula are characterized by the major overthrust of a volcanic plutonic terrane of middle to late jurassic age san andres cedros volcanic arc and associated late triassic ophiolites on top of blueschist units metamorphosed mainly in early cretaceous time and melange terrancs p t metamorphic conditions for the blueschist units are 5 6 kbar and 200 235 c recwstallizations are located both in foliated rocks dynamic metamorphism and in undeformed protoliths in situ static metamorphisnl a relatively later high pressure low temperature fissural static metamorphism was observed cutting through previously metamorphosed rocks the blueschists are tectonically intermixed with or present as blocks in the unmetamorphosed melanges testifying to superimposed thrust events development of the complex relationship between these metamorphosed and unmetamorphosed franciscan type rocks is not interpreted here as resulting of a simple subduction episode but as the result of an advancing nappe induced by interarc collision in mid cretaceous time along the continental margin of baia california the supposed original thickness of the allochthon present upon these blueschist rocks 18 20 km thick is apparently sufficient to consider these metamorphic rocks as formed in the shear zone at the base of the nappc at the firststage of its development however the alternative formation of these blueschists in a subduction zone just before collision cannot be disregarded

active metasomatism in the cerro prieto geothermal system baja california mexico a telescoped low pressure low temperature metamorphic facies series in the cerro prieto geothermal field carbonate cemented quartzofeldspathic sediments of the colorado river delta are being actively metasomatized into calc silicate metamorphic rocks by reaction with alkali chloride brines between 200 sup 0 and 370 sup 0 c low fluid and lithostatic pressures and low oxygen fugacities petrologic investigations of drill cores and cutting from over 50 wells in this field identified a prograde series of calc silicate mineral zones which include as index minerals wairakite epidote prehnite and clinopyroxene associated divariant mineral assemblages are indicative of a very low pressure temperature metamorphic facies series which encompasses the clay carbonate zeolite greenschist and amphibolite facies this hydrothermal metamorphic facies series which is becoming increasingly recognized in other active geothermal systems is characterized by temperature telescoped dehydration and decarbonation mineral equilibria its equivalent should now be sought in fossil hydrothermal systems

late quaternary palaeoenvironments of the winter rainfall zone of southern africa palynological and sedimentological evidence from the agulhas plain abstract south africa s winter rainfall zone is a climatically sensitive region lying at the interface of the earth s temperate and sub tropical climatic systems its seasonally arid climate has generally prevented the preservation of long and organic rich sedimentary deposits producing a spatially limited and temporally biased quaternary record this paper seeks to address this issue further and develop a fuller understanding of the wider climatic changes in this region during the late quaternary modern climatic data does not support the definitions of the winter rainfall zone presented in previous syntheses and this factor amongst others may explain difficulties in resolving the palaeoenvironmental record in this region we present palaeoenvironmental evidence from the agulhas plain an area marginal to the modern winter rainfall zone cored sedimentological and palynological records from two pans are integrated with previously reported records of aeolian activity providing evidence for phases of enhanced aridity and humidity during the late quaternary the record is fragmentary which may reflect the cycling of sediment between the pans and their accompanying lunette dunes lacustrine sedimentary sequences dating to c 33 ka 47 ka provide evidence for enhanced humidity consistent with evidence in the winter and year round rainfall zones increased humidity is also recorded from c 2 ka following drier conditions than the present c 2 7 2 5 ka palynological evidence supports the sedimentological interpretations of pan water status and offers rare insights into the nature of the region s unique fynbos vegetation during the late pleistocene

rate and state of background stress estimated from the aftershocks of the 1989 loma prieta california earthquake estimates of the tectonic stress state including loading rate and magnitude of background stress are derived from the spatial and temporal distribution of loma prieta aftershocks this technique was previously applied to the landers aftershock sequence gross and kisslinger 1997 and is based upon the seismicity model of dieterich 1994 dieterich s theory suggests that background seismicity should be proportional to stress rate and the number of aftershocks in an area should be proportional to the stress step experienced in that area we used two independently derived source models to compute the stress step from the mainshock and to determine how effective that stress step was in triggering aftershocks a background stress state is then chosen which makes the stress steps at the aftershock locations most distinct from the stress steps at hypocenters of background seismicity the best fitting background stress state has its greatest compressive stress plunging 17 to n13 e and an intermediate stress very close in magnitude to the least principal stress the small shear stresses at depth and low coefficient of friction suggest that high pressure pore fluids may be present inside active faults the estimated stress rate of 70 pa d 0 25 bar yr is comparable to the stress rate found for the southern san andreas fault system we found a best fitting effective coefficient of friction 0 2 for the loma prieta area significantly less than 0 6 estimated for landers in previous work variations in aftershock decay rate within the loma prieta aftershock zone are correlated with static stresses caused by postseismic slip as modeled by burgmann et al 1997 some of the postseismic slip occurred on structures that did not slip during the mainshock so the postseismic and coseismic stress step fields have different spatial distributions the effectiveness of slowly accumulated postseismic static stresses in triggering aftershocks is especially interesting because dynamic stresses are insignificant in this case

hydrographic variability in the st helena bay region of the southern benguela ecosystem cross shelf distributions of temperature salinity water masses and dissolved oxygen in st helena bay revealed substantial vertical and seasonal variations in the surface layers nearshore and offshore temperature and salinity patterns differed with bay scale variability linked to upwelling dynamics and coastal processes while the offshore region was influenced by solar insolation spectral analysis revealed that an annual signal prevailed at most stations and corroborated contrasting patterns between the offshore and nearshore regions with phase differences suggesting shoreward propagation of the offshore temperature signal the shelf was dominated by modified upwelled water muw and subantarctic mode water samw which comprised the primary source of upwelled water clear zonation of muw was evident across the shelf resulting from seasonal variations in locations of the oceanic and bifurcated shelf break fronts dynamics within st helena bay consistently differed from those further offshore due to the influences of the shelf break front cape columbine upwelling plume and cyclonic recirculation which appeared to be associated with an intraannual signal with a periodicity of 3 4 months persistent hypoxia in the bottom waters suggested the occurrence of a permanent reservoir of low oxygen water low seasonal shoreward and offshore expansion of low occurred throughout the upwelling season with maximum extent reached during summer and autumn due to the coupled effects of advection and local phytoplankton decay while wind mixing ventilated the water column at nearshore stations in winter and the onset of upwelling during spring introduced oxygen richer water from further offshore hypoxia persisted in the center of the bay

basin inversion and contractional reactivation of inherited normal faults a review based on previous and new experimental models abstract compressional inversion of former extensional sedimentary basins is a particularly common phenomenon given that passive margins share the same destiny of being subsequently incorporated into fold and thrust belts during basin inversion pre existing faults may concentrate stress and localise future thrust ramps or may be reactivated recording reversal of movement from extension to reverse this process has attracted attention largely because of its economic implications related to petroleum and ore deposit prospects and seismic hazard assessment since more than two decades analogue modelling has been used to indentify and or test a number of parameters relevant for the inversion process this paper aims therefore to offer a reasoned review of the analogue modelling work done on the subject coupling such previous results with those of a new experimental series inspired to the northern apennines stratigraphy and evolution past models have explored the role of main factors governing fault reactivation susceptibility such as fluid pressure fault weakness fault steepness angle of shortening and sediment loading the new models have addressed the role played by geometry and strength of a basal ductile layer during inversion two sets of sand silicone models were first extended orthogonally at different velocities to produce dissimilar pre inversion internal geometries and then shortened coaxially at different velocities to vary the brittle ductile coupling the experimental results confirm the selectiveness of fault inversion and reveal strong similarities with structural styles of tectonic inversion reported from the northern apennines and various areas worldwide the preferred geological model inferred from this modelling involves contribution of 1 syn inversion rotation of pre existing faults to shallower dip and 2 rotation of principal stress axes which could explain the invariable reactivation of only one of the two oppositely dipping graben bounding faults these factors would thus interact with the other well established parameters controlling the fault reactivation process

holocene vegetation history and palaeoenvironments at klaarfontein springs western cape south africa several sedimenit cores ai ounid the coastal lake of verlorenvlei westem cape have thus far revealed important details of the development of this semi arid winter rainfall environment during the late quatemary complex interplay between climate change sea level fluctuation and human activities have characterized the verlorenvlei record and suggests that these environments have been subject to high degrees of disturbance over time however due to the discontiniuous nature of the sedimentary record the holocene record has remained fragmentary investigation of a 6 in vibracore sequence at klaarfontein 32 25 26 s 18 29 40 e an artesian spring site some 18 km inland of eland s bay facilitates a more complete holocene palaeoenivironmental reconstruction pollen concentrations in 37 spectra distributed through the 6 m core vary with organic content but at all depths are stufficient to facilitate interpretation of vegetation history and reveal a high degree of dynamism in the local and regional enviro

organic matter in the franciscan and cedros subduction complexes the problems of instantaneous maturation and missing petroleum in accretionary prisms abstract most accretionary complexes are submarine mountain belts consisting predominantly of sedimentary rock past studies of accretionary complexes have concluded that they have minimal petroleum potential although they do contain potential source rocks owing to their great bulk of sedimentary rock large amounts of petroleum must be generated in accretionary prisms and these hydrocarbons must be trapped in the complexes or vented into the ocean or on land to understand the nature and abundance of organic material in accretionary complexes organic matter in the shales limestones and melange matrix of the franciscan and cedros subduction complexes was studied mostly using rock eval pyrolysis the largest amounts of total organic carbon toc were found in the calera limestone from the san francisco area 1 6 toc terrigenous sedimentary rocks of probable trench or trench slope origin contain from

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

holocene sea level fluctuations inferred from the evolution of depositional environments of the southern langebaan lagoon salt marsh south africa a holocene sea level curve is constructed from the facies distribution and radiocarbon ages of sediment recovered from the distal southern salt marsh of langebaan lagoon on the southwest coast of south africa calibrated radiocarbon analyses of an oyster rich bioclastic gravel indicate that the flandrian transgression flooded the lagoon to 0 3 m above present day levels by 6750 years ago 6 8 ka organic matter and shell material dated in distal lagoonal sediments indicate that sea level returned to present day levels by 4 9 ka and have since remained within 61 m of present day levels bleached shell and a hiatus in sedimentation suggest an approximate 1 m sea level lowstand between 2 5 and 1 8 ka changes in the macrobenthos assemblage since 1 5 ka that include the loss of the oysterostrea atherstonei razor clamsolen capensis brown musselarcuatula capensis and periwinkleoxystele variegata reflect loss of hard substrate decreased tidal flow velocities as reworked sands prograded into the southern la

rapid exhumation and mountain building in the western alps petrology and 40ar 39ar geochronology of detritus from tertiary basins of southeastern france 1 tertiary convergence high pressure low temperature hp lt metamorphism and subsequent exhumation in the internal western alps were concurrent with detrital deposition in foreland basins at the external periphery of the orogen in the present work we have probed the geochronology and chemistry of white mica grains and metamorphic pebbles from the foreland basins of se france in order to gain additional perspective on exhumation rates and mountain building processes in the internal domain of the tertiary orogen our data indicate that throughout the tertiary these basins were mainly fed from a relatively low pressure source dominated by si poor micas yielding pre alpine 40ar 39ar ages small amounts of detritus composed of high pressure minerals and pebbles are first detected in early oligocene 30 32 ma strata of the barreme basin the 40ar 39ar geochronology of phengites from garnet blueschist pebbles from these strata yielded 34 3 ma ages indicating rapid ascent and exposure of hp lt rocks in the internal part of the orogen shortly after metamorphism to explain the preponderance of pre alpine micas alongside the rare presence of alpine hp lt detritus we suggest that the general architecture of the western alps whereby inner hp lt metamorphic units are cut off from the se france basins by a low grade lid of brianconnais back thrusts was rapidly built in the early oligocene and did not change significantly since then

surface wave potential for triggering tectonic nonvolcanic tremor source processes commonly posed to explain instances of remote dynamic triggering of tectonic nonvolcanic tremor by surface waves include frictional failure and various modes of fluid activation the relative potential for love and rayleigh wave dynamic stresses to trigger tectonic tremor through failure on critically stressed thrust and vertical strike slip faults under the coulomb griffith failure criteria as a function of incidence angle is anticorrelated over the 15 to 30 km depth range that hosts tectonic tremor love wave potential is high for strike parallel incidence on low angle reverse faults and null for strike normal incidence the opposite holds for rayleigh waves love wave potential is high for both strike parallel and strike normal incidence on vertical strike slip faults and minimal for 45 incidence angles the opposite holds for rayleigh waves this pattern is consistent with documented instances of tremor triggered by love waves incident on the cascadia megathrust and the san andreas fault saf in central california resulting from shear failure on weak faults apparent friction 0 2 however documented instances of tremor triggered by surface waves with strike parallel incidence along the nankai megathrust beneath shikoku japan is associated primarily with rayleigh waves this is consistent with the tremor bursts resulting from mixed mode failure crack opening and shear failure facilitated by near lithostatic ambient pore pressure low differential stress with a moderate friction coefficient 0 6 on the nankai subduction interface rayleigh wave dilatational stress is relatively weak at tectonic tremor source depths and seems unlikely to contribute significantly to the triggering process except perhaps for an indirect role on the saf in sustaining tremor into the rayleigh wave coda that was initially triggered by love waves

holocene climatic optimum in southwest africa evidence from the marine clay mineral record abstract sediment composition grain size and clay mineral record of a high resolution sediment core from the continental slope off namibia was investigated to gain information on the deposition of terrigenous matter in this part of the southwest african continental margin during the last 18 k y the depositional processes involved are fluvial input by the kunene river and eolian input from the namib and kalahari deserts each supplying characteristic mineral suites during low sea level erosion of the exposed shelf yields additional material the amount of eolian or fluvial matter depends on the strength of the transport process which is a function of aridity or humidity of the source area thus allowing paleoclimatic interpretations arid conditions prevailed during a low sea level from 18 to 15 ka and unconsolidated shelf sediment was mobilized and supplied to the slope by short distance transport by southerly winds a dramatic increase in the accumulation of terrigenous sediment is recorded from 15 to 10 ka without major changes in sediment composition which is attributed to increased runoff of the kunene river and fluvial erosion of shelf sediments this period coincides with a strengthening of the monsoonal system during a precessional minimum which is observed in numerous sites in central africa and indicates an intensified influence of the monsoon at the kunene headwaters a distinct shift in clay mineralogy towards river derived material marks a second period of increased river runoff during high sea level from 9 to 5 ka indicating maximum humidity in the source area from 6 to 5 ka this corresponds to the holocene climatic optimum observed in the arid belts of northern africa the present balance between fluvial and eolian input was reached approximately at 4 ka