seismicity and stress distribution in the copiapo northern chile subduction zone using combined on and off shore seismic observations abstract historic as well as teleseismically and locally recorded seismicity of the subduction margin of copiapo northern chile is analysed an on and off shore temporary network was deployed during 2 months in the region in order to obtain a detailed definition of the wadati benioff zone an average dip angle of 20 along the subduction thrust interplate contact was determined this value is similar to that observed along the chilean margin suggesting that there are no variations in the dip angle of the nazca plate for depths less than 50 60 km locally recorded data do not provide seismic evidence to determine whether subhorizontal subduction exists north of latitude 28 s teleseismically recorded events exhibit a low rate of occurrence of inland earthquakes with a tendency for epicentres to be located off shore in contrast locally recorded seismicity reveals a homogeneous epicentral distribution without a predominance of off shore events along the subduction thrust contact there is a reverse stress regime with the greatest compression axis 1 horizontal and oriented perpendicular to the strike of the trench events located above the main decollement are in a normal stress regime with the minimum compressive stress axis 3 horizontally oriented parallel to the convergence direction the events located below the coupled interplate contact also lie in a normal faulting regime but the 3 axis plunges almost parallel to the subducting plate the intermediate depth fault plane solutions exhibit a substantial scatter suggesting a complicated faulting regime similar to that observed in previous studies of northern chile they are associated with a normal faulting stress regime in which 3 is oriented in a n14 e direction parallel to the strike of the trench high resolution bathymetric data and seismicity located above the thrust contact suggest that seamounts are being subducting

observations on the role of the bernice coal field sullivan county pennsylvania anthracites in the development of coalification theories in the appalachians abstract the bernice coal field sullivan county pennsylvania contains anthracite rank coal in relatively unfolded strata that setting along with a misinterpretation of the significance of apparent rank anomalies led to examples from the bernice coal field being used in the development of coalification theories in the late 1800 s and early 1900 s the evidence available was used to justify both theories of thrust pressure coalification and rank variation due to original differences in the peat or peat accumulation processes as opposed to thermal metamorphism

notes on the mechanisms of coal metamorphism in the pennsylvania anthracite fields abstract charred devolatilized megasporinite and featureless vitrinite with devolatilization pores filled with pyrolytic carbon suggest that eastern pennsylvania anthracites may have been altered by to localized episodes of metamorphism from superheated steam co2 co hydrocarbons in addition to the pervasive regional metamorphism based on the extant vitrinite textures the lack of mesophase anisotropic textures and on comparisons to laboratory studies of the thermoplasticity of bituminous coals the rank at the time of metamorphism would have been high volatile c or lowermost high volatile b bituminous the temperature of alteration would have been in the 350 400 c range corresponding to the critical point of steam 374 15 c and 22 1 mpa about 1 km depth it is speculated that while increased pressure would have prolonged the duration of fluid properties fluidity is still a transient property and the vitrinite would have returned to a solid state after a few hours the heat would have persisted dissipating over the course of days to months to years depending upon the depth and ambient pre heating geothermal gradient but contributing to the coal metamorphism long after the vitroplast solidified

residence of ree y th and u in granites and crustal protoliths implications for the chemistry of crustal melts a systematic study with laser ablation icp ms scanning electron microscopy and electron microprobe revealed that 70 95 wt of ree except eu y th and u in granite rocks and crustal protoliths reside within reeythu rich accessories whose nature composition and associations change with the rock aluminosity the accessory assemblage of peraluminous granites migmatites and high grade rocks is composed of monazite xenotime in low ca varieties apatite zircon thorthosilicate uraninite and betafite pyrochlore metaluminous granites have allanite sphene apatite zircon monazite and thorthosilicaie peralkaline granites have aeschinite fergusonite samarskite bastnaesite fluocerite allanite sphene zircon monazite xenotime and th orthosilicate granulite grade garnets are enriched in nd and sm by no less than one order of magnitude with respect to amphibolite grade garnets granulitegrade feldspars are also enriched in lree with respect to amphibolite grade feldspars accessories cause non henrian behaviour of ree y th and u during melt solid partitioning because elevated fractions of monazite xenotime and zircon in common migmatites are included within major minerals their behaviour during anatexis is controlled by that of their host settling curves calculated for a convecting magma show that accessories are too small to settle appreciably being separated from the melt as inclusions within larger minerals biotite has the greatest tendency to include accessories thereby indirectly controlling the geochemistry of ree y th and u we conclude that ree y th and u are unsuitable for petrogenetical modelling of granitoids through equilibrium based trace element fractionation equations

eruptive processes and caldera formation in a nested downsagcollapse caldera cerro panizos central andes mountains abstract the cerro panizos ignimbrite center in the central andes mountains produced two ignimbrite sheets and many lava flows the ignimbrite of quebrada cienago was erupted at 7 9 ma and effusive eruptions continued until the two cooling units of the crystal rich dacitic cerro panizos ignimbrite were emplaced at 6 7 ma the lower unit has no laterally continuous flow breaks and was erupted from a single vent or small cluster of vents with limited fluctuation in discharge lithic fragments reach significant concentrations 5 only in the uppermost two meters of this cooling unit where they document vent wall collapse or the opening of a new vent the upper cooling unit contains many flow units with variations in welding thicknesses and lithic fragment concentrations implying an unsteady eruption column the opening of many vents and probable caldera collapse triangulation of anisotropy of magnetic susceptibility flow direction measurements locate a single vent for the lower cooling unit whereas the upper cooling unit had many vents within the present dome cluster a 15 km diameter topographic depression marked by inward dips of 4 8 at the cooling unit contact is centered on the vent area of the lower cooling unit the depression is interpreted as a downsag caldera formed during emplacement of the lower cooling unit collapse began late in the eruption of the lower cooling unit and continued through the emplacement of the upper cooling unit resurgent magmatism occurred as lava flows that mundated the caldera area a ring of dacite domes erupted until at least 6 1 ma in the northern half of the downsag caldera traces the margin of a collapse caldera associated with the upper cooling unit maximum caldera subsidence 353 km 3 is not enough to account for the erupted volume 652 km 3 dre minimum the ash poor crystal rich nature up to 50 crystals in the pumice 75 in the matrix of the cerro panizos pyroclastic flows resulted in poor retention of gases and there is little evidence of fluidization the high crystal content however favored modified grain flow processes in which several particle support mechanisms combine with grain dispersive forces it is postulated that the flows were initially partially fluidized but rapidly lost their gases and grain flow processes grew in relative importance grain flow processes destroyed evidence of fluidization before deposition

the petrogenesis of the neoarchean kukuluma intrusive complex nw tanzania abstract the kukuluma intrusive complex kic is a late archean igneous complex dominated by monzonite and diorite with subordinated granodiorite the monzonite and the diorite suites have low silica content sio2 62 wt moderate mg mg average 49 high sr y sr yaverage 79 and high la yb la ybaverage 56 ratios and strongly fractionated lan ybn 9 69 ree patterns their moderate ni niaverage 50 ppm cr craverage 85 ppm variable cr ni ratio 0 65 3 56 and low tio2 tio2average 0 5 wt indicate little to no interaction with the peridotitic mantle for most major elements al2o3 feot na2o tio2 and p2o5 the monzonite and the diorite suites display subparallel trends for the same sio2 content indicating they represent distinct melts intrusions belonging to the diorite suite have high na2o na2oaverage 4 2 wt dy ybn dy ybn average 1 6 a positive sr anomaly and uncorrelated nb la and zr sm ratios suggesting derivation from partial melting of garnet bearing amphibolite intrusions belonging to the monzonite suite have higher na2o na2oaverage 5 61 wt dy ybn dy ybn average 2 21 a negative sr anomaly and correlated nb la and zr sm ratios consistent with derivation from partial melting of eclogite with residual rutile small variations in the th u ratio and near chondritic morb average values th umonzonite 3 65 th udiorite 2 92 are inconsistent with a subducting slab signature and it is proposed that the monzonite and the diorite suites of the kic formed by partial melting of garnet bearing lower mafic crust of an oceanic plateau the granodiorite suite has lower mg mg average 41 moderately fractionated ree low sr y sr yaverage 20 la yb la ybaverage 15 dy ybn dyybn average 1 24 and small negative eu anomalies suggesting derivation from partial melting of amphibolite and plagioclase fractionation near morb th u th uaverage 2 92 and zr sm zr smaverage 30 21 ratios are consistent with intracrustal melting of amphibolite archean rocks with an adakitic geochemical signature have been used to argue in favour of a plate tectonics regime in the archean the results presented here suggest that tectonic models for the tanzania craton which invoke a subduction related setting for all greenstone belts may need revision

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

moho topography in the central andes and its geodynamic implications p to s converted waves at the continental moho together with waves multiply reflected between the earth s surface and the moho have been used to estimate the moho depth and average crustal vp vs variations in the central andes our analysis confirms and significantly complements the moho depth estimates previously obtained from wide angle seismic studies and receiver functions the resulting crustal thickness varies from about 35 km in the forearc region to more than 70 km beneath the plateau and thins 30 km further to the east in the chaco plains beneath the andean plateau the moho is deeper in the north altiplano and shallower in the south puna where the plateau attains its maximum elevation a non linear relation exists between crustal thickness and elevation and bouguer gravity suggesting that the crust shallower than 50 55 km is predominately felsic in contrast to a predominately mafic crust below such a relation also implies a 100 km thick thermal lithosphere beneath the altiplano and with a lithospheric thinning of a few tens of kilometers beneath the puna absence of expected increase in lithospheric thickness in regions of almost doubled crust strongly suggests partial removal of the mantle lithosphere beneath the entire plateau in the subandean ranges at 19 20 s the relation between altitude and crustal thickness indicates a thick lithosphere up to 130 150 km and lithospheric flexure beneath a relative topographic low at the salar de atacama a thick crust 67 km suggests that the lithosphere in this region is abnormally cold and dynamically subsided possibly due to coupling with the subducting plate this may be related to the strongest ms 8 0 known intra slab earthquake in the central andes that happened very close to this region in 1950 the average crustal vp vs ratio is about 1 77 for the altiplano puna and it reaches the highest values 1 80 1 85 beneath the volcanic arc indicating high ambient crustal temperatures and wide spread intra crustal melting 2002 elsevier science b v all rights reserved

geologic controls on thermal maturity patterns in pennsylvanian coal bearing rocks in the appalachian basin thermal maturation patterns of pennsylvanian strata in the appalachian basin were determined by compiling and contouring published and unpublished vitrinite reflectance vr measurements vr isograd values range from 0 6 in eastern ohio and eastern kentucky western side of the east kentucky coal field to greater than 5 5 in eastern pennsylvania southern anthracite field schuylkill county corresponding to astm coal rank classes of high volatile c bituminous to meta anthracite vr isograds show that thermal maturity of pennsylvanian coals generally increases from west to east across the basin the isograds patterns which are indicative of maximum temperatures during burial can be explained by variations in paleodepth of burial paleogeothermal gradient or a combination of both however there are at least four areas of unusually high rank coal in the appalachian basin that depart from the regional trends and are difficult to explain by depth of burial alone 1 a west northwestward salient centered in southwestern pennsylvania 2 an elliptically shaped northeast trending area centered in southern west virginia and western virginia 3 the eastern part of black warrior coal field alabama and 4 the pennsylvania anthracite region in eastern pennsylvania high rank excursions in southwest pennsylvania the black warrior coal field and the pennsylvania anthracite region are interpreted here to represent areas of higher paleo heat flow related to syntectonic movement of hot fluids towards the foreland associated with alleghanian deformation in addition to higher heat flow from fluids the pennsylvania anthracite region also experienced greater depth of burial the high rank excursion in southwest virginia was probably primarily controlled by overburden thickness but may also have been influenced by higher geothermal gradients

role of decollement material with different rheological properties in the structure of the aljibe thrust imbricate flysch trough gibraltar arc an analogue modelling approach abstract the miocene aljibe thrust imbricate flysch trough in the external zone of the gibraltar arc shows large changes in structural style towards the foreland rheological variation of the decollement rocks triassic evaporitic rocks and mesozoic limestones appears to be the main factor controlling the contrasts in structural style that characterise adjacent wedge domains in order to test this hypothesis we carried out analogue experiments involving three dimensional variations of decollement material a plate of viscous material was placed in the central part of the models at the base of the sand multilayers different geometries of the viscous plate were used experimental results show a strong link between the rheological properties of the decollement material and the structure of the overlying thrust wedge these results compare well with the internal geometry and vergence distribution in the aljibe thrust imbricate indeed the tectonic style of the wedge can be explained as a consequence of variations of rock type along the basal decollement

geochemistry and isotope systematics of small to medium volume neogene quaternary ignimbrites in the southern central andes evidence for derivation from andesitic magma sources the geochemical and isotopic compositions of small to medium volume 10 km3 50 km3 neogene quaternary ignimbrite sheets of the southern central andes 25 00 26 45 s 67 30 69 30 w have been analysed in order to assess the petrogenesis of this type of calc alkaline magma the ignimbrites are derived from the active volcanic zone around salar de la isla sis in chile and from behind the active arc in the area of the salar de antofalla saf argentina the ignimbrites in both areas are characterized by 87sr 86sr t ratios 0 7066 0 7090 that are lower and end t values 2 4 to 5 1 that are higher than in the large volume caldera sourced ignimbrites of cerro galan 100 km further east and those of ignimbrites from other megacalderas of the central andes this difference reflects smaller amounts of crustal material in the sis and saf ignimbrites rather than a change in basement composition as indicated by pb provinciality the isotope signature of the sis and saf ignimbrites is similar to that of late to post miocene composite calc alkalic andesite dacite stratovolcanoes of the frontal arc in this region this finding combined with major and trace element modelling lends support to a petrogenetic model for the sis and saf ignimbrites involving fractional crystallization from a lil depleted andesitic parental magma which was beforehand modified by crustal contamination

correlation between optical chemical and micro structural parameters of high rank coals and graphite abstract in order to identify the parameters that best characterize the chemical and structural evolution of organic matter during coalification the relationships between optical chemical and micro structural parameters in high rank coals and natural graphite were studied the samples include anthracites from penarroya belmez espiel basin spain douro basin portugal and alto chicama basin peru and natural graphite from canada mozambique and austria correlations between the following optical parameters were assessed vitrinite random reflectance rr reflectance indicating surfaces ris axis rmax rint and rmin and ris parameters ram rev and rst as well as bw and ai anisotropy parameters furthermore the chemical parameters used were chosen according to their significant variation in coals namely volatile matter carbon and hydrogen contents calculated in dry ash free basis vmdaf cdaf hdaf as well as the h c atomic ratio structural organization was characterized by micro raman spectroscopy and xrd raman parameters used were the full width at half maximum fwhm and position of g and d1 bands on the first order raman spectrum and the id1 ig intensity area ratio the selected xrd parameters were interlayer spacing d002 and crystallite sizes la and lc results show that i rmax ris axis seems to correlate best with chemical and micro structural parameters ii for the majority of studied samples hdaf and h c atomic ratio are the only chemical parameters with significant correlations with rmax iii the fwhm of the g band of raman spectrum shows good linear correlation with the xrd parameter d002 and iv structural organization of carbon materials as measured by trends in their optical and crystalline parameters is influenced by their hydrogen content daf basis and therefore by the h c atomic ratio

geochronology and stratigraphy of the ignimbrites from the 21 30 s to 23 30 s portion of the central andes of northern chile the central volcanic zone cvz of the andes which extends from 14 to 28 s is one of the largest provinces of late tertiary to recent ignimbrite volcanism in the world in the 21 30 s to 23 30 s portion of the cvz in northern chile ignimbrite volcanism was initiated at the beginning of the late miocene 10 4 ma ago and continued until the recent the stratigraphy is dominated by the products of five major dacitic ignimbrite eruptions 100 km3 the artola ignimbrite member 9 4 ma the sifon ignimbrite member 8 3 ma the pelon ignimbrite member 5 5 ma the puripicar ignimbrite member 4 2 ma and the atana ignimbrite member 4 1 ma at least three of these represent only portions of larger ignimbrite sheets erupted from centres of sw bolivia two of the ignimbrites the sifon and atana ignimbrites must represent erupted volumes in excess of 1000 km3 each the major units which represent 90 of the total volume of ignimbrite in this area are intercalated with smaller 50 km3 less extensive volcanic units some of which were erupted from source structures within the study area the upper tertiary to recent volcanic stratigraphy has been revised using the large ignimbrites as the major stratigraphic horizons the ignimbrite formations can be divided into two groups on a geographic basis those in the northern and central portions of the study area are known as the san bartolo group while those in the southern portion belong to the silapeti group these two groups appear to be contemporaneous although the silapeti group may represent a period of ignimbrite volcanism which was initiated later about 6 ma ago the ignimbrite formation of sw bolivia and the punense arancaniano formation of nw argentina are equivalent to the san bartolo and silapeti groups based on correlation of the stratigraphy into adjacent areas in sw bolivia and nw argentina the beginning of the late miocene 10 4 ma marks the onset of the most intense phase ignimbrite volcanism in the cvz this phase of volcanism is suggested to be the result of crustal melting in response to crustal thickening during the quechua phase of deformation and subduction approximately 10 000 km3 of ignimbrite erupted over a period of 9 ma in the 21 30 23 30 s portion and led to the development of a major ignimbrite province

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

the geological and structural evolution of the long lived miocene pleistocene la hoyada volcanic complex in the geodynamic framework of the central andes argentina abstract the long lived la hoyada volcanic complex lhvc is located in the east of cordillera de san buenaventura in argentina this region is important for understanding of the geodynamic evolution of the southern puna since it is located in its southern edge where the transition between normal and low angle subduction occurs the aim of this work is to contribute to the knowledge of the volcanism of the southern puna through the analysis of the stratigraphy and geological evolution of lhvc fieldwork and image analysis have led to a new lithostratigraphic scheme that identifies lithosomes and unconformity bounded stratigraphic units defining the stratigraphic architecture of the lhvc new 40ar 39ar ages further constrain the geological evolution of this complex the resulting stratigraphic scheme combines the lithostratigraphic units into lithosomes and synthemic units with the identification of at least six superimposed eruptive centers and their products during the lhvc evolution a new stratigraphic framework for the eastern cordillera de san buenaventura composed by four supersynthems separated by unconformities that reflect periods of quiescence of the volcanic activity catastrophic destructive events and changes in the magmatic plumbing system is proposed 1 san buenaventura supersynthem ca 9 7 ma including mainly explosive rhyolitic volcanism 2 lower la hoyada supersynthem ca 7 d pyroclastic and epiclastic deposits and 4 purulla supersynthem

lead isotope provinces of the central andes inferred from ores and crustal rocks the central andean cordillera of peru bolivia northwestern argentina and northern chile may be divided into three fundamental geologic provinces based on the age and dominant geology of the exposed rocks we define these provinces as follows province i the coastal volcanic arc comprising the jurassic early tertiary coastal batholith complex and related volcanic rocks of the western cordillera and coastal belt based on differences in the isotopic composition of ore lead province i is further subdivided into subprovince ia comprising the coastal volcanic belt of northern and central chile subprovince ib in central peru north of 13 degrees s and subprovince ic in southern peru and northernmost chile between 19 degrees s and 13 degrees s province ii the jurassic and cretaceous miogeosynclinal sedimentary belt that dominates the geology of the high andes of peru province iii the eastern cordillera of southeastern peru central bolivia and northwestern argentina made up mainly of early paleozoic clastic sediments we have also divided this into two subprovinces due to different ore lead isotope characteristics subprovince iiia in bolivia and northern argentina and subprovince iiib in southeastern peru these provinces are bordered by noncordilleran units of the brazil and guyana shields the arequipa massif and the metamorphic terranes of southern chile and central western argentina we present 45 new analyses of ore minerals from 31 mining districts and two of host rocks from province iii and summarize our results together with other data available in the literature isotopic compositions of ore lead from the above provinces conform to neighboring but distinct fields on pb isotope covariation diagrams lead from province i ores has 206 pb 204 pb 18 21 to 18 82 207 pb 204 pb 15 55 to 15 69 and 208 pb 204 pb 38 11 to 38 95 lead from province ii ores is less variable with consistently higher 206 pb 204 pb and 208 pb 204 pb than the province i ores and slightly higher average 207 pb 204 pb we consider the province i and ii ore leads to reflect different degrees of mixing between magmatic upper mantle derived lead and crustal sources most assimilation of crustal lead is probably magmatic and results from incorporation of lead from subducted sediments at the magma source and or from the crust during ascent of the parent magma from the upper mantle province iii ore leads are much more variable than those of provinces i and ii having 206 pb 204 pb 17 97 to 25 18 207 pb 204 pb 15 51 to 16 00 and 208 pb 204 pb 37 71 to 40 07 they have consistently higher 207 pb 204 pb and 208 pb 204 pb for given values of 206 pb 204 pb than ore leads from the other provinces province iii leads indicate more radiogenic heterogenous source regions and suggest that fusion of continental crust has played an important role in the genesis of magmas and associated ore deposits this is substantiated by the metaluminous to strongly peraluminous character of much of the magmatism in province iii detailed comparisons of lead isotope data available in the literature for andean volcanic rocks with compositions of ores are problematic because analyses are rarely reported from related ores and igneous bodies where such data exist ore pb isotope compositions correlate closely with those in the associated intrusions

on the fundamental difference between coal rank and coal type abstract this article addresses the fundamental difference between coal rank and coal type while theoretically settled long ago as being different aspects of coal systems science the two concepts are still often confounded in recent years this has resulted in the publication of several works stating that coal type changes with coal rank coal type refers solely to coals depositional origin and the maceral mineral admixture resulting from that origin coal types typically fall in to two categories humic coals developed from peat and sapropelic coals developed from organic mud either type may be allocthonous or autochthonous and within types further refinement of depositional environment can be made coal rank refers to the changes in geochemistry and resultant changes in reflectance caused by increasing thermal maturity of the coal thus it provides an overprint of maturity on existing coal types with proper techniques such as use of crossed polars and etching maceral forms can be differentiated even at high ranks and the original coal type determined

late paleozoic to triassic evolution of the gondwana margin evidence from chilean frontal cordilleran batholiths 28 s to 31 s upper paleozoic to triassic chilean granitoids in the andean frontal cordillera between 28 s and 31 s record crustal and mantle conditions at the gondwana margin during the final assembly and initial breakup of the pangea supercontinent this period overlaps the end of paleozoic terrane accretion and precedes andean subduction integration of new trace element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the gondwana margin in the model the carboniferous to early permian is a period of oblique convergence associated elqui complex granitoids are diverse those in the guanta and montosa units are predominantly related to subduction processes whereas those in the cochiguas and el volcan units are dominated by melting of the subduction complex and older crust progressive oblique collision of the last pre pangea terrane equis along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the elqui complex subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt which intruded and melted the crust producing the post collisional ingaguas complex the los carricitos granitoids formed in thickened crust whereas the chollay el colorado and el leon units formed in thinner crust the ingaguas complex is part of the choiyoi granite rhyolite province whose formation similar to that of other gondwana silicic provinces was probably accentuated by anomalously hot upper mantle associated with the pangea supercontinent

geochemical isotopic and single crystal 40ar 39ar age constraints on the evolution of the cerro galan ignimbrites the giant ignimbrites that erupted from the cerro galan caldera complex in the southern puna of the high andean plateau are considered to be linked to crustal and mantle melting as a consequence of delamination of gravitationally unstable thickened crust and mantle lithosphere over a steepening subduction zone major and trace element analyses of cerro galan ignimbrites 68 71 sio2 that include 75 new analyses can be interpreted as reflecting evolution at three crustal levels afc modeling and new fractionation corrected 18o values from quartz 7 63 8 85 are consistent with the ignimbrite magmas being near 50 50 mixtures of enriched mantle 87sr 86sr 0 7055 and crustal melts 87sr 86sr near 0 715 0 735 processes at lower crustal levels are predicated on steep heavy ree patterns sm yb 4 7 high sr contents 250 ppm and very low nb ta 9 5 ratios which are attributed to amphibolite partial melts mixing with fractionating mantle basalts to produce hybrid melts that rise leaving a gravitationally unstable garnet bearing residue processes at mid crustal levels create large negative eu anomalies eu eu 0 45 0 70 and variable trace element enrichment in a crystallizing mush zone with a temperature near 800 850 c the mush zone is repeatedly recharged from depth and partially evacuated into upper crustal magma chambers at times of regional contraction crystallinity differences in the ignimbrites are attributed to biotite zoned plagioclase and other antecrysts entering higher level chambers where variable amounts of near eutectic crystallization occurs at temperatures as low as 680 c just preceding eruption 40ar 39ar single crystal sanidine weighted mean plateau and isochron ages combined with trace element patterns show that the galan ignimbrite erupted in more than one batch including a 2 13 ma intracaldera flow and outflows to the west and north at near 2 09 and 2 06 ma episodic delamination of gravitationally unstable lower crust and mantle lithosphere and injection of basaltic magmas whose changing chemistry reflects their evolution over a steepening subduction zone could trigger the eruptions of the cerro galan ignimbrites

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

crustal thickness in the central andes from teleseismically recorded depth phase precursors summary although crustal thickness is a critical constraint for geodynamic models of andean orogenesis relatively few measurements exist from local seismic studies in this study we use reflections from the underside of the moho as illuminated by intermediate depth earthquakes to provide new estimates of crustal thickness for the central andes 16 34 s these reflected signals pmp are identified as precursors to the depth phase pp when recorded at teleseismic distances 35 85 although relatively small in amplitude the pmp phase is often clear even on single seismograms less obvious pmp phases were enhanced by stacking traces from arrays this method was most effective for events of m w 6 and depth 100 km crustal thickness determined in this study ranges from 59 to 70 km in southern peru from 49 to 80 km across the puna altiplano and from 50 to 60 km above the pampean flat slab the lack of such phases for certain events may be evidence of heterogeneity at the moho and some precursors appear to correspond to intracrustal magma and lithospheric mantle discontinuities the results obtained using the pmp technique combined with those from other studies show the crustal thicknesses under the central andes are more variable than expected from purely isostatic considerations we infer that one or a combination of geodynamic processes involving lower crustal flow lower crustal delamination and mantle crustal lithospheric coupling can explain the highly variable topography on the moho beneath the central andes

afc3d a 3d graphical tool to model assimilation and fractional crystallization with and without recharge in the r environment abstract afc3d is an original graphical free software developed in the framework of the r scientific environment and dedicated to the modelling of assimilation and fractional crystallization without afc and with afc r recharge facilitating the search for the solutions of the equations originally proposed by depaolo 1981 1985 and first solved in a graphical way by aitcheson and forrest 1994 the software presented here allows a graphical 3d representation of mass of assimilated crust mass of original magma as a function of r rate of crustal assimilation rate of fractional crystallization and recharge rate of magma replenishment rate of assimilation for each element isotope finding a coherent set of r parameter triples in a mostly automated way mathematically optimized solutions are derived which can and should then be discussed and evaluated from a geological and petrological point of view by the end user the presented contribution presents the software and a series of models published in the literature which are discussed as case studies of application and whose solutions are sometimes enhanced based on the results provided by the software

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

modelling the geometry of coulomb thrust wedges internal geometry and detachment of coulomb thrust wedges is examined in a combined gravity and pressure force field under various boundary conditions and coulomb parameters frontally buttressed sand wedges accreted with narrow tapers above soles with negligible friction in these models serial imbricates stepped up in conjugate sets which operated synchronously or serially with increases in sole friction model coulomb wedges accreted in piggyback style with enhanced taper but narrower spacing of imbricates in this mode flat topped box fold anticlines nucleated at the frontal tip of a sole thrust prior to the fore limb failing as a ramp the initial ramp thrusts subsequently rotated and steepened to acquire concave upwards listric geometry when cohesion was a significant component of strength and the sand grains interlocked thoroughly ramp climb during piggyback thrusting occurred through the generation of a stack of backthrusts the taper of cohesionless sand wedges which lack any length scale was consistent with the critical cohesionless coulomb wedge taper in contrast the taper of experimental cohesive sand wedges which are scale dependent was consistent with the theoretical cohesive coulomb narrow taper only when the cohesive length scale was lr sr rar 10 6 this was constrained by comparing geometry of cohesive sand wedges shortened at different body force per unit mass

shallowing and steepening subduction zones continental lithospheric loss magmatism and crustal flow under the central andean altiplano puna plateau

estimating basal friction in accretionary wedges from the geometry and spacing of frontal faults elastic theory applied to the deformation in accretionary wedges is used to calculate the condition for slip along an active frontal fault and the basal decollement the equations for calculating the stresses can be solved for the coefficient of basal friction in the situation of the formation of a new frontal thrust fault this allows us to calculate the efficient coefficient of basal friction which includes the weakening effect of pore fluid pressure from geometric parameters and material properties only the geometric parameters like fault dip and layer thickness can be derived from high resolution seismic cross sections application of our analysis to the makran and the nankai accretionary wedge allows us to estimate the upper limit of the effective coefficient of basal friction wbw0 16 and wbw0 2 in these two areas respectively fl 2001 elsevier science b v all rights reserved

phase relationships of gabbro tonalite granite water at 15 kbar with applications to differentiation and anatexis phase relationships have been determined at 15 kbar with variable h 2o contents for three rocks a gabbro a tonalite and a muscovite granite representing the compositional trend of the calc alkaline rock series experiments were conducted in 0 5 in piston cylinder apparatus using ag pd or pt capsules the results obtained are acceptable representations of the phase relationships for elucidation of petrological processes selected runs with 5 h 2o were completed using au capsules to overcome the problem of iron loss up to the temperature limit of au for reliable electron microprobe analyses of glass garnet clinopyroxene amphibole and plagioclase no glasses could be analyzed in the gabbro the data provide k d values as a function of temperature for pairs of coexisting minerals and liquid the chemical variations are used to evaluate 1 the fractionation trends in hydrous magmas at 55 km in thickened continental crust or uppermost mantle and 2 the products of anatexis of the thickened crust at this depth fractionation of hydrous basalt or anatexis of gabbro and tonalite or their metamorphosed equivalents appears to produce liquids diverging from the calc alkaline trend anatexis of granite produces a liquid less siliceous than itself with some chemical characteristics of syenite

pb isotopes define basement domains of the altiplano central andes detailed pb isotopic maps of the central andes based on 345 163 previously published 182 new analyses of ores volcanic rocks and their host rocks elucidate the gross structure of the basement and reveal that several isotopically distinct basement domains are juxtaposed in this region the data clearly show that most of the pb in central andean igneous and ore samples is derived from the local basement including pb in ore deposits of the bolivian tin belt some of the isotopic domain boundaries correspond to geologic structures and the residual gravity pattern as well as to metallogenic boundaries such as the western edge of the bolivian tin belt

crustal recycling of metamorphic basement late palaeozoic granitoids of northern chile 22 s implications for the composition of the andean crust upper palaeozoic silicic magmatism is widespread in the central introduction andes but its origin is poorly constrained we investigated wholethe magmatic arc in the puna altiplano plateau of the rock chemical and isotopic composition of upper palaeozoic grancentral andes rests on a continental crust of a maximum itoids and their early palaeozoic high grade country rocks in thickness of 70 km crustal thickening is mainly atthe chilean coastal cordillera and precordillera at 22 s in tributed to shortening of the crust and not to juvenile comparison with an upper cretaceous granitoid the age of the magmatic additions from the subduction zone see review late palaeozoic granitoids from a rb sr isochron of 300 ma by allmendinger et al 1997 this implies a major is consistent with k ar cooling ages of hornblende and biotite contribution of pre mesozoic material to the present similar major and trace element patterns as well as nd and pb thick crust because mesozoic juvenile additions to the isotopic composition of upper palaeozoic granitoids and gneisses crust are focused in the mesozoic magmatic arcs mainly point to a source of the granitoids that is similar to the gneisses at in the chilean coastal cordillera fig 1 e g scheuber outcrop sr isotope ratios of the upper palaeozoic granitoids are et al 1994 lucassen et al 1996a which is not involved less radiogenic than those in many of the gneisses we propose a in the crustal thickening the last major reorganization of stratification of the early palaeozoic crust with a rb deficient the crust before the cenozoic was in the early palaeozoic granulitic mid lower crust resulting in less radiogenic sr compared orogeny between 500 ma the age of peak metawith the upper crust based on the interpretation of the p t t morphism and 400 ma the age of the final exhumation history and isotopic composition of the lower palaeozoic meta damm et al 1990 1994 lucassen et al 1996b unmorphic basement and of the isotopic composition of the late published data 1998 becchio et al 1997 the widepalaeozoic granitoids and younger magmatic rocks nd isotopic spread upper palaeozoic granitoid magmatism starting composition is identical in lower and upper crust and in crustal at 300 ma was assigned to arc magmatism e g brown melts from the late palaeozoic to recent the cretaceous granitoid 1991 bahlburg herve 1997 or to melting of the evolved from partial melts of a mantle derived source with concrust at a passive margin breitkreuz zeil 1984 and siderable contamination by the old crustal component the crust references therein the metamorphic history of the prethat formed in the early palaeozoic is the major source of material mesozoic crust was studied in detail before lucassen et for the cenozoic tectonic thickening of the andean crust al 1999a unpublished data 1998 summary of previous

petrogenesis of early neogene magmatism in the northern puna implications for magma genesis and crustal processes in the central andean plateau of uolm 2 and mm group rocks assuming the same endnew compositional data and petrogenetic models are presented for member compositions the modelling suggests genesis of the mm pre upper miocene volcanism in the northern puna of argentina magmas at higher pressure than the uolm 2 centres 10 kbar 22 s 24 s two phases of volcanism produced small dome vs 7 kbar which may reflect the influence of crustal thickening in complexes of mainly silicic andesite to low sio2 rhyolite the the plateau region by the mid miocene the felsic dome complexes upper oligocene lower miocene phase uolm 20 17 ma of this study are compositionally similar to the large volume calderaproduced two distinct groups of rocks the uolm 1 group is sourced felsic ignimbrites that dominated volcanism in the region metaluminous and mainly andesitic with isotopic compositions like from 10 to 2 ma and our results suggest that there is no fundamental those of the recent arc sr srt 0 706 ndt 3 the difference in magma genesis between them the differences in the uolm 2 group is more silicic and peraluminous and has isotopic volumes and the mode of eruption reflect changes in the stress and compositions indicating a substantial crustal contribution sr thermal regime with time srt 0 713 ndt 8 the mid miocene phase mm 15 12 ma produced rocks similar in composition to those of the uolm 2 group sr srt 0 710 ndt 7 but with higher incompatible element contents ratios of ba nb and zr

andean cenozoic volcanic centers reflect basement isotopic domains isotopic data for quaternary volcanic centers from lat 17 5 to 22 s in northern chile provide evidence for crustal contributions to arc magma genesis in the central andes and delineate basement domains crustal thickness distance from trench height of volcanic edifices above the seismically active subduction zone and sediment supply to the trench are all constant along this segment of the arc the only significant variable is crustal age paleozoic in the south proterozoic in the north pb isotopic compositions of quaternary lavas in the northern sector are lower 206 pb 204 pb 17 89 18 28 and sr and o isotope ratios are less variable 87 sr 86 sr 0 7058 0 7077 18 o 6 5 8 4 than in the southern sector 206 pb 204 pb 18 59 18 79 87 sr 86 sr 0 7053 0 7083 18 o 6 4 9 3 for rocks similar in bulk composition from 53 to 74 wt sio 2 together these features are interpreted to reflect interaction of magmas with crustal contaminants of different isotopic character rather than changes in the mantle source of central andean are magmas resulting from either subduction erosion or involvement of old enriched lithosphere

magma evolution in the purico ignimbrite complex northern chile evidence for zoning of a dacitic magma by injection of rhyolitic melts following mafic recharge the 1 3 ma purico complex is part of an extensive neogene pleistocene ignimbrite province in the central andes like most other silicic complexes in the province purico is dominated by monotonous intermediate ash flow sheets and has volumetrically minor lava domes the purico ignimbrites total volume 80 100 km3 are divided into a lower purico ignimbrite lpi with two extensive flow units lpi i and lpi ii and a smaller upper purico ignimbrite upi unit crystal rich dacite is the dominant lithology in all the purico ignimbrites and in the lava domes it is essentially the only lithology present in the first lpi flow unit lpi i and in the upper purico ignimbrite but the lpi ii flow unit is unusual for its compositional diversity it constitutes a stratigraphic sequence with a basal fall out deposit containing rhyolitic pumice 68 74 wt sio2 overlain by ignimbrite with dominant crystal rich dacitic pumice 64 66 wt sio2 rare andesitic and banded pumice 60 61 wt sio2 are also present in the uppermost part of the flow unit the different compositional groups of pumice in lpi ii flow unit rhyolite andesite dacite have initial nd and sr isotopic compositions that are indistinguishable from each other and from the dominant dacitic pumice end 6 7 to 7 2 and 87sr 86sr 0 7085 0 7090 however two lines of evidence show that the andesite dacite and rhyolite pumices do not represent a simple fractionation series first melt inclusions trapped in sequential growth zones of zoned plagioclase grains in the rhyolite record fractionation trends in the melt that diverge from those shown by dacite samples second mineral equilibrium geothermometry reveals that dacites from all ignimbrite flow units and from the domes had relatively uniform and moderate pre eruptive temperatures 780 800 c whereas the rhyolites and andesites yield consistently higher temperatures 850 950 c hornblende geobarometry and pressure constraints from h2o and co2 contents in melt inclusions indicate upper crustal 4 8 km magma storage conditions the petrologic evidence from the lpi ii system thus indicates an anomalously zoned magma chamber with a rhyolitic cap that was hotter than and chemically unrelated to the underlying dacite we suggest that the hotter rhyolite and andesite magmas are both related to an episode of replenishment in the dacitic purico magma chamber rapid and effective crystal fractionation of the fresh andesite produced a hot rhyolitic melt whose low density and viscosity permitted ascent through the chamber without significant thermal and chemical equilibration with the resident dacite isotopic and compositional variations in the purico system are typical of those seen throughout the neogene ignimbrite complexes of the central andes these characteristics were generated at moderate crustal depths 30 km by crustal melting mixing and homogenization involving mantle derived basalts for the purico system assimilation of at least 30 mantle derived material is required

controls on magmatism in an island arc environment study of lavas and sub arc xenoliths from the tabar lihir tanga feni island chain papua new guinea the tabar lihir tanga feni tltf islands of papua new guinea mainly comprise high k calc alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction related magmatism numerous sedimentary mafic and ultramafic xenoliths recovered from tubaf seamount located on the flank of lihir island provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region the sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with pacific affinity a majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the tltf arc leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50 or more of elements such as cu zn rb u pb and light ree rare earth elements most likely introduced in the xenoliths via hydrous fluids released from a subducted slab the leaching experiments demonstrated that the light ree enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit ree patterns that range from flat to light ree depleted sr nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87sr 86sr with decreasing 143nd 144nd ratios the labile phases in the ultramafic xenoliths represented by the leachates show decoupling between sr and nd with distinctly more radiogenic 87sr 86sr than the residues both leachates and residues exhibit very wide range in their pb isotopic compositions indicating the involvement of three components in the mantle wedge under the tltf islands two of the components can be identified as pacific oceanic mantle and pacific sediments some of the ultramafic samples and clinopyroxene separates however exhibit relatively low 206pb 204pb at elevated 207pb 204pb suggesting that the third component is either indian ocean type mantle or australian subcontinental lithospheric mantle geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized it did not significantly contribute to the isotopic and incompatible trace element compositions of tltf lavas compared to the mantle samples the tltf lavas have very restricted pb isotopic compositions that lie within the pacific morb range indicating that magma compositions were dominated by melts released from a stalled subducted slab with pacific morb affinity interaction of slab melts with depleted peridotitic component in the mantle wedge followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area

recognition of vitrinite with non uniaxial negative reflectance characteristics non uniaxial negative reflectance characteristics are being reported for coals from around the world with increasing frequency these data have traditionally been collected from oriented sections cut from a block of coal to determine the magnitudes of the principal reflectance axes to make data collection less time consuming a method of determining the principal reflectance axes from a standard crushed coal sample has been developed the method requires measurement of the maximum and minimum apparent reflectances from a series of randomly oriented vitrinite particles the reflectance crossplot displays these two reflectance values against their difference or bireflectance the paper describes the method of interpreting the resultant pattern to determine the values of the three principal reflectance axes analysis of coals from the british columbia rocky mountains and foothills revealed biaxial and uniaxial positive as well as uniaxial negative reflectance characteristics a new measure of maturation rev is proposed which is based purely on the lengths of the three principal reflectance axes rm approximates this value but is influence by any eccentricity of the reflectance indicating surface ris vitrinite with a non uniaxial reflectance characteristics cannot be adequately described by a single measure such as rmaxorrm but requires reference to both the size and shape of the ris a reflectance determination such as rev or rm provides rank astm information whereas the new measures rst and ram describe a reflectance indicating surface shape which is the result of maturation and tectonic histories vitrinites with non uniaxial negative reflectance characteristics are abundant in the coalfields of the rocky mountains and foothills of british columbia rmax does not fully describe the reflectance character of these coals and does not describe the potentially useful ris shape

the amazon laurentian connection as viewed from the middle proterozoic rocks in the central andes western bolivia and northern chile middle proterozoic rocks underlying the andes in western bolivia western argentina and northern chile and early proterozoic rocks of the arequipa massif in southern peru form the arequipa antofalla craton these rocks are discontinuously exposed beneath mesozoic and cenozoic rocks but abundant crystalline clasts in tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton near berenguela western bolivia the oligocene and miocene mauri formation contains boulders of granodiorite augen gneiss 1171 20 ma and 1158 12 ma u pb zircon quartzose gneiss and granofels that are inferred to have arkosic protoliths 1100 ma source region u pb zircon quartzofeldspathic and mafic orthogneisses that have amphibolite and granulite facies metamorphic mineral assemblages 1080 ma metamorphism u pb zircon and undeformed granitic rocks of phanerozoic age the middle proterozoic crystalline rocks from berenguela and elsewhere in western bolivia and from the middle proterozoic belen schist in northern chile generally have present day low 206pb 204pb 15 57 and elevated 208pb 204pb 37 2 to 50 7 indicative of high time averaged th u values the middle proterozoic rocks in general have higher present day 206pb 204pb values than those of the early proterozoic rocks of the arequipa massif 206pb 204pb between 16 1 and 17 1 but lower than rocks of the southern arequipa antofalla craton 206pb 204pb 18 5 a difference inferred to reflect grenvillian granulite metamorphism the pb isotopic compositions for the various proterozoic rocks lie on common pb isotopic growth curves implying that pb incorporated in rocks composing the arequipa antofalla craton was extracted from a similar evolving pb isotopic reservoir evidently the craton has been a coherent terrane since the middle proterozoic moreover the pb isotopic compositions for the arequipa antofalla craton overlap those of the amazon craton thereby supporting a link between these cratons and seemingly precluding part of the arequipa antofalla craton from being a detached fragment of another craton such as eastern laurentia which has been characterized by a different u pb history pb isotopic compositions for the arequipa antofalla craton are furthermore distinct from those of the proterozoic basement in the precordillera terrane western argentina indicating a pb isotopic and presumably a tectonic boundary between them the pb isotopic compositions for the precordillera basement are similar to those of eastern laurentia and support other data indicating that these rocks are a detached fragment of north america finally the distinct pb isotopic evolution history of the arequipa antofalla craton and eastern laurentia require minor modification to tectonic models linking eastern north america scotland to the oroclinal bend in western south america

the incapillo caldera and dome complex 28 s central andes a stranded magma chamber over a dying arc abstract the plio pleistocene incapillo caldera and dome complex 5570 m is situated at the southernmost termination of the andean central volcanic zone 28 s where the more steeply dipping segment of the subducting nazca plate transitions into the chilean pampean flat slab to the south the 250 m high walls of the elliptical caldera 6 5 km are composed of a homogeneous pumice rich rhyodacitic ignimbrite that radiates 15 km from the caldera rim new biotite 40 ar 39 ar ages along with previous k ar ages reveal that the final eruption of the incapillo caldera occurred at 0 51 0 04 ma flanking the caldera are 40 steep sided rhyodacitic domes that predate the main caldera forming ignimbrite the morphology of these domes reflects the non explosive effusive eruption of a crystal rich gas poor rhyodacitic magma a volumetrically minor debris flow composed of abundant pumice dacites and lithic clasts is topographically confined to a glacial valley south of the caldera and unconformably overlies the ignimbrite mixed pumices at the distal end of the ignimbrite reflect the convective physical condition within the magma chamber immediately prior to the pyroclastic eruption including the domes and ignimbrite the total estimated volume of erupted incapillo magma is 37 km 3 based on its age small size and effusive eruptive style the incapillo caldera and dome complex provides an endmember model for an andean caldera erupting within a waning magmatic arc over a shallowing subduction zone

geological mapping of the schuppen belt of north east india using geospatial technology abstract a revised geologic map of the schuppen belt of northeast india has been prepared based on interpretation of digitally enhanced satellite images the satellite image interpretation is supported by limited field work and existing geologic maps available geological maps of this fold thrust belt are discontinuous and multi scaled the authors are of multiple opinions regarding the trajectory of formation boundaries and fault contacts digital image processing of satellite images and limited field surveys have been used to reinterpret and modify the existing geological maps of this fold thrust belt optical data of landsat thematic mapper enhanced thematic mapper and elevation data of aster have been used to prepare this revised geological map the study area extends from hajadisa in south to digboi oilfield in north bounded by naga thrust in the west and disang thrust in the east pca image fusion linear contrast stretch histogram equalization and painted relief algorithms have been used for the delineation of major geological lineaments like lithological boundary thrust and strike slip faults digital elevation maps have enabled in the discrimination between thrust contacts and lithological boundaries with the former being located mostly in the valleys textural enhancements of pca colour composites and painted relief algorithm have been used to discriminate between different rock types few geological concepts about the terrain have been revisited and modified it is assumed that this revised map should be of practical use as this terrain promises unexploited hydrocarbon reserves

la pacana caldera and the atana ignimbrite a major ash flow and resurgent caldera complex in the andes of northern chile the recently discovered la pacana caldera 60 35 km is the largest caldera yet described in south america this resurgent caldera of pliocene age developed in a continental platemargin environment in a major province of ignimbrite volcanism in the central andes of northern chile at about 23 s latitude collapse of la pacana caldera was initiated by the eruption of about 900 km3 of the rhyodacitic atana ignimbrite the atana ignimbrite was erupted from a composite ring fracture system and formed at least four major ash flow tuff units that are separated locally by thin air fall and surge deposits all four sheets were emplaced in rapid succession about 4 1 0 4 ma ago caldera collapse was followed closely by resurgent doming of the caldera floor accompanied by early postcaldera eruptions of dacitic to rhyolitic lava domes along the ring fractures the resurgent dome is an elongated asymmetrical uplift 48 5 12 km which is broken by a complex system of normal faults locally forming a narrow discontinuous apical graben later postcaldera eruptions produced large andesitic and dacitic stratocones along the caldera margins and dacitic domes on the resurgent dome beginning about 3 5 ma ago and persisting into the quaternary hydrothermally altered rocks occur in the eroded cores of precaldera and postcaldera stratovolcanoes and along fractures in the resurgent dome but no ore deposits are known a few warm springs located in salars within the caldera moat appear to be vestiges of the caldera geothermal system

isotopic composition of late mesozoic basic and ultrabasic rocks from the andes 23 32 s implications for the andean mantle mesozoic mantle derived magmatic rocks from the jurassic early cretaceous active continental margin of n chile near 24 s and from the cretaceous rift system of nw argentina near 26 s central rift and 32 s southern rift are used to constrain the isotopic composition of mantle reservoirs before the onset of the cenozoic andean orogeny they are largely uncontaminated by assimilation of the local palaeozoic basement as seen in the major and trace element patterns and sr and nd isotopic composition of most samples but small amounts of crustal contamination are detected in the pb isotope composition two different types of mantle sources have been identified 1 for rocks from the jurassic arc now at the continental margin at 24 s and for rocks from the central rift at 26 s a depleted mantle source with 143nd 144nd t 0 5127 0 5128 87sr 86sr t ca 0 7033 may be inferred pb isotopic ratios of most samples from the magmatic arc are overall less radiogenic than those from the rift 206pb 204pb t ca 18 0 18 4 vs 18 1 19 1 207pb 204pb t 15 55 15 59 vs 15 58 15 64 and 208pb 204pb t 37 8 38 6 vs 38 4 38 9 2 in the southern rift at ca 30 32 s sierras los condores region lc and sierras chajan region ch an enriched mantle source has been identified which is similar to the composition of the mantle lithosphere beneath the brazilian shield samples from lc and ch show some isotopic differences because of varying degrees of crustal contamination in the lc samples the lc samples have a higher th u ratio than the ch samples and different pb isotopic ratios 206pb 204pb t 18 0 18 6 lc vs 17 7 and 17 8 ch 207pb 204pb t 15 50 15 66 lc vs 15 50 ch and 208pb 204pb t 38 6 39 2 lc vs ca 37 5 ch the nd isotopic ratios of lc and ch samples are similar 143nd 144nd t 0 5123 0 5122 lc vs 0 5123 ch whereas their sr isotopic ratios differ 87sr 86sr t 0 7054 0 7079 lc vs ca 0 704 ch electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00410 001 0344 3

mechanisms of coal metamorphism case studies from paleozoic coalfields controls on coal metamorphism can be complex in this paper we examine four paleozoic coalfields the western kentucky portion of the illinois basin the pennsylvania anthracite fields the south wales coalfield and the bowen basin an increase in temperature with depth of burial is certainly a factor in coal metamorphism in many coalfields however including the coalfields reviewed here it has become apparent that such a simple mechanism does not explain the coal rank patterns observed the flow of hydrothermal fluids through the coals has been proposed as a cause of coal metamorphism evidence includes inverted rank gradients elevated cfl as an indicator of brine fluids isotopic evidence for hydrothermal fluids and vein and cleat mineral assemblages in any case multiple hypotheses must often be evaluated in the examination of any coalfield since the simple paradigm of coal rank increases with a simple increase in temperature with increasing depth does not fit the evidence observed in many cases

seismic imaging of a convergent continental margin and plateau in the central andes andean continental research project 1996 ancorp 96 1 a 400 km long seismic reflection profile andean continental research project 1996 ancorp 96 and integrated geophysical experiments wide angle seismology passive seismology gravity and magnetotelluric depth sounding across the central andes 21 s observed subduction of the nazca plate under the south american continent an east dipping reflector nazca reflector is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km we interpret parts of the nazca reflector as a fluid trap located at the front of recent hydration and shearing of the mantle the fluids being supplied by dehydration of the oceanic plate patches of bright quebrada blanca bright spot to more diffuse reflectivity underlie the plateau domain at 15 30 km depth this reflectivity is associated with a low velocity zone p to s wave conversions the upper limits of high conductivity and high vp vs ratios and to the occurrence of neogene volcanic rocks at surface we interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during neogene shortening and plateau growth the imaging properties of the continental moho beneath the andes indicate a broad transitional character of the crust mantle boundary owing to active processes like hydration of mantle rocks in the cooler parts of the plate margin system magmatic underplating and intraplating under and into the lowermost crust mechanical instability at moho etc hence all first order features appear to be related to fluid assisted processes in a subduction setting

magmatic evidence for neogene lithospheric evolution of the central andean flat slab between 30 s and 32 s abstract geochemical data from andean miocene magmas erupted through the now volcanically inactive flat slab between 30 s to 32 s coupled with geological and geophysical data illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone pb sr and nd isotopic and trace element data show that main cordilleran precordilleran and pampean magmas were contaminated in both the mantle and crust and that the nature of the contaminants varied in time and space reflecting tectonic events contaminants included both enriched high lil element high sr and pb and low nd isotopic ratios and depleted low lile element low sr and pb and high nd isotopic ratios types in the western region main cordilleran earlier miocene lavas had contaminants with less enriched signatures than later miocene lavas progressive enrichment is attributed to a increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge and b contamination in a thickening main cordilleran lower crust whose composition was progressively enriched this enrichment occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging heating and deformation of crust from beneath the shortening precordillera thrust belt to the east further east magmas erupted through back arc crust have more depleted signatures those erupted in the central part through the evolving precordilleran thrust belt were contaminated by an older thinner grenville 1100 ma basement whose depleted signature is unique among central andean terranes late miocene pocho lavas erupted further east in conjunction with uplift of the sierras pampeanas show enrichment through time arguably these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west and progressively incorporated into the convecting asthenosphere first order mass balance calculations comparing the lithospheric geometry proposed for 20 ma with that today suggests that 60 of the lithosphere within 600 km of the trench at 20 ma has been lost this lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab and thickening and shortening of the crust continental lithosphere recycled into the asthenosphere could contribute to the sources of the enriched em1 and em2 mantle components in oceanic island basalts

the geochemistry of a dying continental arc the incapillo caldera and dome complex of the southernmost central andean volcanic zone 28 s the pleistocene incapillo caldera and dome complex 5 570 m marks the southernmost siliceous center of the andean central volcanic zone 28 s where the steeply dipping 30 segment of the subducting nazca plate transitions into the chilean flatslab to the south the eruption of the incapillo caldera and dome complex began with a 3 1 ma effusive phase characterized by 40 rhyodacitic dome eruptions this effusive phase was terminated by an explosive caldera forming event at 0 51 ma that produced the 14 km3 incapillo ignimbrite distinctive and virtually identical chemical signatures of the domes and ignimbrites sio2 67 72 wt la yb 37 56 ba la 16 28 la ta 30 50 87sr 86sr 0 70638 0 70669 e nd 4 2 to 4 6 indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between units were minor the strong hree depletion sm yb 6 8 that distinguishes incapillo magmas from most of the large ignimbrites of the altiplano puna plateau can be explained by the extent and degree of partial melting at lower crustal depths 40 km in the presence of garnet at upper crustal depths this high pressure residual geochemical signature also common to adjacent late miocene pliocene pircas negras andesites was partially overprinted by shallow level assimilation and fractional crystallization processes energy constrained afc modeling suggests that incorporation of anatectic upper crustal melts into a fractionated adakite like dacitic host best explains the petrogenesis of incapillo magmas the diminution of the sub arc asthenospheric wedge during nazca plate shallowing left the incapillo magma chamber unreplenished by both mafic mantle derived and lower crustal melts and thus stranded at shallow depths within the andean crust based on its small size and distinctive high pressure chemical signature the incapillo caldera and dome complex provides an endmember model for an andean caldera erupting within a waning magmatic arc over a shallowing subduction zone

reflectance anisotropy and syn deformational coalification of the jewel seam in the cadomin area alberta canada abstract the jewel seam of the cadomin area is complexly folded and cut by several thrust faults vitrinite reflectance anisotropy was determined from twenty oriented coal blocks collected from various structural positions five of the blocks have uniaxial anisotropies five have biaxial positive anisotropies the remaining ten are biaxial negative the biaxial coals display maximum reflectance axes parallel to nearby fold axes which suggests a relationship between vitrinite anisotropy and deformation the biaxial vitrinite reflectance ellipsoids result from superposition of tectonic strains on a primary sedimentary burial related uniaxial anisotropy a tectonic stress field was probably present during the later stages of sedimentary burial and subsequent deformation coalification resulted largely from pre deformational sedimentary burial but the final coalification stage was syn deformational as illustrated by the intersection of isorank surfaces and the jewel seam and the biaxial vitrinite reflectance anisotropy

on the optically biaxial character and heterogeneity of anthracites abstract the results of the study of optical properties of 13 anthracites from different parts of the world are presented in this paper measurements of reflectance values were made on non oriented vitrinite grains for a minimum of 300 points per sample the reconstruction of reflectance indicating surfaces ris were made by kilby s method kilby w e 1988 recognition of vitrinite with non uniaxial negative reflectance characteristics int j coal geol 9 267 285 kilby w e 1991 vitrinite reflectance measurement some technique enhancements and relationships int j coal geol 19 201 218 it was found that the use of kilby s method for strongly anisotropic materials like anthracites did not give unambiguous results some improvement in kilby s method consisting of the division of the cumulative cross plot into several elemental components is suggested each elemental cross plot corresponds to a textural class of anthracite which is characterized by the values of ris main axes rmax k rint k and rmin k k 1 2 n n number of classes the global texture of anthracite is characterized as a ris with main axes calculated as the weighted means of r max r int and r min for each class of this anthracite the division of cumulative kilby s cross plot on elemental components makes possible the calculation of new coefficients ht and h10 characterizing the heterogeneity of the structure and texture of anthracites the results of our study show that all anthracites have biaxial negative textures but their heterogeneity varies in a wide range of ht and h10 coefficients depending upon the individual coal basin

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

structural evolution in response to fold and thrust belt tectonics in northern assam a key to hydrocarbon exploration in the jaipur anticline area abstract the upper assam basin is an intermountain basin surrounded on two sides by eastern himalayan fold belt and the third side by assam arakan fold belt the basin has the contradictory characteristics of being a historic oil producing province in one of the most tectonically active areas of the world assam is a seismically active area and the location of major earthquakes yet contains one of the world s largest oil and gas fields an integrated study based on surface and subsurface data and relying on the sequence stratigraphic approach was undertaken to provide a re appraisal of the upper miocene to recent succession and its relationship to the region s structural history detailed data in the jaipur anticline region of the basin is viewed in regional context while the seismic database area includes only the northeastern part of the basin we suggest that the interpretations are applicable to the entire upper assam basin for interpreting the complex relationship between structural evolution and the generation and remigration of hydrocarbons in tectonically active regions a regional geo history analysis is a required component of the petroleum system description necessary for a successful exploration model

coal more than a resource critical data for understanding a variety of earth science concepts abstract coal is one of the world s primary energy sources and it is critical for making coke used in steel making and is used for a multitude of chemical products aside from its significance as a fuel or product however coal and data derived from the exploration or mining of coal have also provided the foundation and testing ground for diverse geologic concepts because of coal s economic importance and common variability in thickness distribution and quality it has been critical to collect and correlate a wide variety of surface and subsurface data sets at relatively high spatial frequency which varies from the mine to basinal scale also because many coal beds occur in most coal basins the collection of coal data from multiple beds creates relatively high temporal frequency data sets at scales from laminae to bed to larger unit scale these data have been important for the development and expansion of many earth science concepts including aspects of basin analyses paleogeography paleoclimatology paleontology stratigraphy sedimentology structural geology and tectonics

deformation metamorphism of bituminous and anthracite coals from china abstract tectonic displacement of coal seams in china has resulted in faulting parallel to coal bedding displacement along these faults caused significant comminution of the coal on the footwall contributing to various mining problems the worst of which is catastrophic failure or outbursting of the working face during mining the granular texture and mostly unconsolidated nature of the coal suggests that faulting occurred relatively late in the coalification sequence at a time of maximum tectonic stress coal samples taken on either side of the fault plane normal and deformed coal layers were obtained in an effort to establish what influence these tectonic stresses might have had on coal properties as well as what they might reveal about the influence of tectonic pressure on organic maturity sample sets were collected within coal beds from undisturbed and adjacent deformed layers including 21 bituminous samples from the pingdingshan coal field and nine anthracite samples from the jiaozuo coal field the tieshenggou coal mine of the yuxi coal field in henan province the beijing xishan coal field the baijiao mine of the furong coal field in sichuang province and the baisha coal field in hunan province china results from vitrinite reflectance proximate and ultimate analyses show some differences in reflectance hydrogen content and nitrogen content of anthracite coal no significant difference was found between volatile matter yields of normal and deformed coal specimens gc measurements of the saturated hydrocarbon fraction of chloroform extracts from bituminous coals showed that lower molecular weight carbon fragments were concentrated in the deformed samples therefore although changes in the gross chemical properties of the deformed coal were insignificant some modification of the chemical structure is seen to have occurred as a result of exposure to tectonic pressure

an unstable kinematic state of the himalayan tectonic wedge evidence from experimental thrust spacing patterns abstract the frontal tectonic wedge in the nw himalayas shows a sequence of imbricate thrusts with increasing spacing in the foreland direction we used analogue model experiments to analyze the thrust pattern in relation to the kinematics of wedge evolution experimental findings reveal two kinematic states of the foreland ward propagation of a wedge unstable state and stable state the unstable state is characterized by continuous vertical growth of the wedge with progressive horizontal contraction on the other hand in the stable state the wedge ceases to grow vertically but propagates laterally in the foreland direction in the experimental runs thrust wedges remained in the unstable state even after large horizontal shortening 50 when the basal friction was high 0 46 our analysis suggests that successive thrusting occurs always with increasing spacing in unstable wedges and the rate of increase is larger for larger basal friction it can maintain uniform spacing only when the wedges turn into the stable state using finite element models we determined the stress distribution in the deforming wedge to find the potential locations of new thrusts in front of a wedge which also show larger spatial distances with increasing vertical thickness of the wedge both physical and numerical models suggest that the tectonic wedge in the nw himalayan frontal belt has evolved in an unstable state

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

probing the evolving andean lithosphere mid late tertiary magmatism in chile 29 30 30 s over the modern zone of subhorizontal subduction in the mid late tertiary 27 6 m y ago volcanism diminished then ceased in the main andean cordillera in chile between 29 and 30 30 s over the same time span the subduction angle shallowed under this region which today lies in the modern shallowly dipping nonvolcanic segment of the south central andes 28 33 s the trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crustal thickening that accompany this major tectonic transition volcanic changes include in addition to a decreased rate of volcanism a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks and an increase in crustal assimilation these volcanic changes also correlate in time with deformational events that help to account for crustal thickening and the space time pattern of deformation and volcanism in the foreland region although the model stands alone support for the tectonic setting and magmatic source region of these tertiary volcanic rocks can be found in comparisons with recent volcanic rocks erupting over more steeply dipping modern andean segments the older dona ana formation 27 18 m y is similar to volcanic rocks in the southern volcanic zone while the younger cerro de las tortolas 14 10 m y and vallecito 6 m y formations are similar to volcanic rocks over regions of thicker crust in the central volcanic zone and the northern part of the southern volcanic zone

crustal thickness beneath the andes and sierras pampeanas at 30 s inferred from pn apparent phase velocities 1 we obtained a 2d crustal model beneath central chile and western argentina using apparent pn phase velocities recorded along an ew trending transect at 30 s this model is characterized by a thick 65 km crustal root beneath the high cordillera a 60 km thick crust beneath the precordillera and a gradual shallowing of moho depths beneath the sierras pampeanas from 55 km in the west to 40 km in the east beneath the sierras pampeanas where very little crustal shortening is observed the thick crust must be explained by means other than simple tectonic shortening isostatically balanced crustal thickness is consistently shallower than our model shows suggesting the existence of dynamic forces possibly linked to the flat slab

the nature of orogenic crust in the central andes 1 the central andes 16 22 s are part of an active continental margin mountain belt and the result of shortening of the weak western edge of south america between the strong lithospheres of the subducting nazca plate and the underthrusting brazilian shield we have combined receiver function and surface wave dispersion results from the banjo seda project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure the major results are as follows 1 the crust supporting the high elevations is thick and has a felsic to intermediate bulk composition 2 the relatively strong brazilian lithosphere is underthrusting as far west 65 5 w as the high elevations of the western part of the eastern cordillera ec but does not underthrust the entire altiplano 3 the subcrustal lithosphere is delaminating piecemeal under the altiplano ec boundary but is not completely removed beneath the central altiplano the altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation leading to lower crustal flow and flat topography in contrast in the high relief inland sloping regions of the ec and sub andean zone the upper crust is still strongly coupled across the basal thrust of the fold thrust belt to the underthrusting brazilian shield lithosphere subcrustal shortening between the altiplano and brazilian lithosphere appears to be accommodated by delamination near the altiplano ec boundary our study suggests that orogenic reworking may be an important part of the felsification of continental crust

estimating basal friction and fluid pressure from the geometry and spacing of frontal faults in naga fold thrust belt india abstract naga fold thrust belt is an emerging destination for petroleum exploration in eastern india it is a thin skinned ftb where new initiatives are being taken after a long gap of initial discoveries in absence of sufficient drilling or quality seismic data in the southern part of the fold thrust belt fluid pressure regimes remain largely speculative using the concept and the formulation for efficient coefficient of basal friction we have revisited the method and concept of pressure dependent coulomb wedge theory for thin skinned thrust belts for deriving the fluid pressure ratio in naga fold thrust belt the efficient coefficient of basal friction on the decollement and the fluid ratio of naga fold thrust belt are estimated to be 0 22 and 0 85 respectively this indicates an overpressure situation in the wedge this method of estimating basal friction and fluid pressure is more case specific and can be obtained from the data of thrust initiation angle and thrust spacing

magma sources and tectonic setting of central andean andesites 25 5 28 s related to crustal thickening forearc subduction erosion and delamination

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry