late quaternary arroyo formation and climate change in the american southwest arroyos entrenched ephemeral streams that form in desert environments first appeared in the arid and semiarid american southwest after 8000 14 c yr b p for at least 7 k y prior to that time climate vegetation and groundwater conditions were not conducive for arroyo formation along the floors of desert valleys after a hiatus in arroyo formation the frequency of arroyo cutting and filling increased dramatically after 4000 14 c yr b p the early holocene arroyos and increased frequency of arroyo incision after 4000 14 c yr b p are related to the establishment and changes in postglacial vegetation climate and groundwater conditions as a result arroyo sequences preserve a record of large scale climate change and small scale climatic perturbations that occurred during the holocene human modification of valley flood plains is an additional factor that contributed to mid nineteenth and early twentieth century arroyo cutting

alluvial archives of the nochixtlan valley oaxaca mexico age and significance for reconstructions of environmental change abstract a reconnaissance of the nochixtlan valley confirms that thick alluvial deposits of the late quaternary are both well preserved and well exposed along the heavily incised modern streams their geometry and age were ascertained by logging representative cutbank exposures and radiocarbon dating buried palaeosols the last four cut and fill cycles recorded cover the period since 14 000 bp but older alluvial insets are also present contrary to previous reconstructions the streams have had an arroyo type morphology throughout the late quaternary the climatic oscillations at the transition from the pleistocene to the holocene seem to be expressed in the alternating formation of clayey organic rich cumulic a horizons and the precipitation of secondary carbonates rapid though intermittent aggradation set in after 10 300 bp in response to the establishment of a rather dry warm and strongly seasonal climate and of open canopy forest or scrub on slopes the unstable arroyo floodplains offered a favorable niche for the establishment of secondary vegetation and foragers exploiting annual and heliophilous plants after 4000 bp sedentary farmers cleared the valley of its natural vegetation increasing sediment transfers and the frequency of local cut and fill cycles they modified streams intentionally by building long flights of cross channel agricultural terraces known as lama bordos the remains of which are preserved in many cutbanks the oldest specimen dated goes back to ca 3000 bp a remarkably synchronous incision swept through the fluvial system close to 800 bp possibly as a result of a marked climatic shift or valley wide changes in land use the widespread gullying of the latest pre conquest settlements and fields suggests the introduction of grazing or the collapse of hillside terraces as cause but coeval alluvial fills have not yet been positively identified

sources of glacial moisture in mesoamerica paleoclimatic records from mesoamerica document the interplay between atlantic and pacific sources of precipitation during the last glacial stage and holocene today and throughout much of the holocene the entire region receives its principal moisture in the summer from an interaction of easterly trade winds with the equatorial calms glacial records from sites east of 95 w in guatemala florida northern venezuela and colombia record dry conditions before 12 ka however west of 95 w glacial conditions were moister than in the holocene for example pollen and diatom data show that lake patzcuaro in the central mexican highlands was cool deep and fresh during this time and fossil pinyon needles in packrat middens in chihuahua sonora arizona and texas indicate cooler glacial climates with increased winter precipitation cold gulf of mexico sea surface temperatures and reduced strength of the equatorial calms can explain arid full and late glacial environments east of 95 w whereas an intensified pattern of winter westerly air flow dominated hydrologic balances as far south as 20 n overall cooler temperatures may have increased effective moisture levels during dry summer months in both areas

signatures of climate vs sea level change within incised valley fill successions quaternary examples from the texas gulf coast abstract the passive margin texas gulf of mexico coastal plain consists of coalescing late pleistocene to holocene alluvial deltaic plains constructed by a series of medium to large fluvial systems alluvial deltaic plains consist of the pleistocene beaumont formation and post beaumont coastal plain incised valleys a variety of mapping outcrop core and geochronological data from the extrabasinal colorado river and the basin fringe trinity river show that beaumont and post beaumont strata consist of a series of coastal plain incised valley fills that represent 100 kyr climatic and glacio eustatic cycles valley fills contain a complex alluvial architecture falling stage to lowstand systems tracts consist of multiple laterally amalgamated sandy channelbelts that reflect deposition within a valley that was incised below highstand alluvial plains and extended across a subaerially exposed shelf the lower boundary to falling stage and lowstand units comprises a composite valley fill unconformity that is time transgressive in both cross and down valley directions coastal plain incised valleys began to fill with transgression and highstand and landward translation of the shoreline paleosols that define the top of falling stage and lowstand channelbelts were progressively onlapped and buried by heterolithic sandy channelbelt sandy and silty crevasse channel and splay and muddy floodbasin strata transgressive to highstand facies scale architecture reflects changes through time in dominant styles of avulsion and follows a predictable succession through different stages of valley filling complete valley filling promoted avulsion and the large scale relocation of valley axes before the next sea level fall such that successive 100 kyr valley fills show a distributary pattern basic elements within coastal plain valleys can be correlated with the record offshore where cross shelf valleys have been described from seismic data falling stage to lowstand channelbelts within coastal plain valleys were feeder systems for shelf phase and shelf margin deltas respectively and demonstrate that falling stage fluvial deposits are important valley fill components signatures of both upstream climate change vs downstream sea level controls are therefore interpreted to be present within incised valley fills signatures of climate change consist of the downstream continuity of major stratigraphic units and component facies which extends from the mixed bedrock alluvial valley of the eroding continental interior to the distal reaches wherever that may be at the time this continuity suggests the development of stratigraphic units and facies is strongly coupled to upstream controls on sediment supply and climate conditions within hinterland source regions signatures of sea level change are critical as well sea level fall below the elevation of highstand depositional shoreline breaks results in channel incision and extension across the newly emergent shelf which in turn results in partitioning of the 100 kyr coastal plain valleys moreover deposits and key surfaces can be traced from continental interiors to the coastal plain but there are downstream changes in geometric relations that correspond to the transition between the mixed bedrock alluvial valley and the coastal plain incised valley channel incision and extension during sea level fall and lowstand with channel shortening and delta backstepping during transgression controls the architecture of coastal plain and cross shelf incised valley fills

fluvial response to holocene climate change in low order streams of central mexico alluvial sequences constitute a recognised source of information on past environmental change but one that has scarcely been tapped in central mexico this paper reviews what is currently known about the holocene alluvial stratigraphy of the region focusing on the interplay between climate and the pace and style of sedimentation in the incised headwater reaches of stream networks the records obtained in five different drainage basins four in the state of tlaxcala and one in guanajuato are presented and compared to published reconstructions of climate change a near synchronous incision of all stream networks occurred close to 10 200 14c a bp in response to an increase in precipitation and stream discharge a spell of very humid but markedly seasonal conditions ensued resulting in the formation of wet meadows along streams and the accumulation of thick fine textured valley fills dominated by cumulic soil a horizons after 9100 14c a bp a transition to a warmer and more arid climate provoked the thinning of vegetation cover on slopes accelerated runoff and increased sediment delivery to streams the aggradation of coarser textured valley fills poor in organic matter set in it ceased or slowed down significantly after a few millennia as the studied stream reaches achieved a near graded condition adjusted to the relatively stable climate arid mid holocene conditions are also reflected in the abundant precipitation of secondary carbonates in guanajuato at 3100 14c a bp higher precipitation caused more frequent flooding and a resumption of aggradation shortly after that date sedentary farmers colonised tlaxcala agriculture altered runoff and sediment delivery to streams and accelerated cut and fill cycles on a scale that masked the impact of any climatic fluctuations guanajuato was colonised later and its alluvial record suggests the persistence of a humid climate at least until 1000 14c a bp copyright 2009 john wiley sons ltd

the curinga girifalco line in the framework of the tectonic evolution of the remnant alpine chain in calabria southern italy in the peri mediterranean metamorphic belts the tectonic evolution of the calabria peloritani terrane during the dominant compressive tectonics of the eocene represents one of the most problematic points in palinspastic restorations a matter of particular debate is its shortening which could have occurred during the alpine or the apennine subduction in this regard a crucial joint is provided by the kinematics of one of the most relevant shear zones such as the curinga girifalco line cropping out in central calabria this shear zone juxtaposed a nearly complete hercynian crustal section i e the sila and serre unit onto the remnants of the castagna unit the data in the available literature on ductile kinematics from the south eastern branch of the curinga girifalco line indicate a downward movement of the hanging wall in the present paper we show new ductile kinematic data and petrographic evidence from outcrops in the north western and south eastern branches of the curinga girifalco line our results highlight the coherent kinematics of the eocene shortening during the alpine subduction system followed by late eocene oligocene to early miocene dominantly ductile extensional reworking relating to the apennines subduction system

alpine orogenic p t t deformation history of the catena costiera area and surrounding regions calabrian arc southern italy the nappe edifice of north calabria revised with insights on the tyrrhenian apennine system formation 1 the nappe structured belt of calabria constitutes the eastward termination of the southern branch of the alpine mediterranean belt that delimits the northern edge of the africa plate contrasting hypotheses for the origin and tectonic significance of the north calabrian nappe edifice have been proposed and kinematic data from north calabria have been used to support different interpretations of the alps apennines linkage and the polarity of the tethyan subduction in the apennine region we reconstruct the architecture of the north calabria nappe edifice through a multidisciplinary approach which integrates structural investigations with metamorphic thermobarometry and 40ar 39ar geochronology results from this study indicate that north calabria consists of a tertiary nappe stack resulting from superimposed top to the west extensional shearing late oligocene to middle miocene in age onto a previously structured top to the east compressional belt eocene to oligocene in age this study also documents that the top to the west extensional tectonics was achieved by means of regionally sized extensional detachment fault systems stretching apart and translating as allochthonous fragments the previously accreted units thinning operated by top to the west extensional detachment tectonics also resulted in the direct juxtaposition of non alpine or slightly alpine metamorphosed units upper plate complex onto the previously exhumed deep seated portions of the orogenic wedge metamorphosed under blueschist facies metamorphic conditions lower plate complex these findings support a new tectonic scenario for the orogenic history of north calabria which may be adequately framed within the tertiary apennine tyrrhenian system evolution

alpine tectonics in the calabrian peloritan belt southern italy new 40ar 39ar data in the aspromonte massif area this study provides new ar 40 ar 39 geochronological constraints on the age of the alpine tectonics in the aspromonte massif southern part of the calabrian peloritan belt this massif exposes the upper units of the calabride complex which originated from the european continental margin the calabride complex was incorporated in the alpine orogenic wedge and then integrated into the apennines and maghrebides fold and thrust belts throughout the calabride complex there is evidence for a two stage tectonic history which remains however rather poorly dated alpine nappe stacking is followed by extensional reworking along the former thrust contacts or along new detachment surfaces our new ages suggest that exhumation of the uppermost units which accompanied nappe stacking probably started at 45 ma and that the deepest units were almost completely exhumed at 33 ma this kinematics probably corresponds to syn orogenic extension while the end of exhumation is clearly related to the extensional tectonics dated at 28 6 ma along detachment structures our geochronological data reveal a very short lag time between accretional and extensional processes in this part of the mediterranean alpine orogenic belt the direction of extension when the units are restored to their initial position i e before the opening of the western mediterranean basins and the bending of the arc is nne ssw such a direction does not fit with the eastward slab retreat model generally put forward to explain extension in the western mediterranean in contrast we provide evidence for roughly n s middle oligocene extension in the accretionary prism not previously described in this part of the mediterranean domain

stable isotope stratigraphy of holocene speleothems examples from a cave system in rana northern norway high precision tims u series dates and continuous stable oxygen and carbon isotope profiles of a 4000 year stalagmite record from rana northern norway are presented and compared with data from two other speleothems from the same cave the dating results yield ages from 3875 34 to 296 3 years before ad2000 with 2s errors from 0 5 to 1 the overall growth rate is 35 mm ka corresponding to a temporal resolution of 29 years mm the stalagmite is tested for isotopic equilibrium conditions where all hendy tests except one indicate isotopic equilibrium or quasi equilibrium deposition both the stable oxygen and carbon isotope records reveal a strong and abrupt enrichment in the near top measurements this corresponds in time to the opening of a second cave entrance in the late 1960s which caused changes in the cave air circulation the stable oxygen isotope signal is enriched compared to the modern value over the last 300 years indicating a negative response to temperature changes likewise the stable carbon isotope record is enriched in this period however both of the stable isotope records are shown to be significantly enriched compared to the isotope ranges displayed by other stalagmites in the same cave and this questions the reliability of the proxy records derived from the presented stalagmite still a general good correspondence of large scale fluctuations is found between the three stable oxygen isotope records from this cave the stable carbon isotope records show large variations within the cave and are believed to be governed by soil zone conditions percolation pathways and possibly driprates q 2001 elsevier science b v all rights reserved

sta cruz atizapan a 22 ka lake level record and climatic implications for the late holocene human occupation in the upper lerma basin central mexico abstract the upper lerma is a high altitude basin with three water bodies linked by the lerma river this basin has a long archaeological history characterised by the establishment of settlements within the lacustrine ecosystem itself man made islands during the late classic to epiclassic ad 550 900 which were abandoned by the end of the epiclassic the upper lerma is an ideal site to study climatic and environmental conditions during the period of human occupation as well as during the last full glacial interglacial cycle two sediment cores stcrz 9 54 m and almoloya del rio 5 12 m were recovered from the highest lake in the system chignahuapan ten radiocarbon dates provide chronologies for these sequences in which the tres cruces tephra c 8500 yr bp and the upper toluca pumice c 11 600 yr bp serve as stratigraphic markers magnetic properties loss on ignition and diatom analyses were used to infer lake level fluctuations during the last c 22 000 yr bp the late pleistocene environment was characterised by a freshwater lake high sediment input and variable lake levels are recorded during the last glacial maximum c 19 000 16 000 yr bp while slightly higher water levels and reduced sediment input are recorded during the late glacial c 16 000 11 000 yr bp a short episode of shallow conditions is inferred by c 12 400 yr bp holocene lake levels were generally shallower and three episodes of very shallow slightly alkaline waters are identified the first dates to the early holocene c 11 000 7000 yr bp the second is centred at c 4600 4500 yr bp the third occurred between c 2000 and 800 yr bp c 200 bc ad 1100 calibrated ages with very shallow water after c 1400 yr bp ad 550 calibrated age lake level increased after c 800 yr bp these three shallow water events are also recorded at other sites in central mexico indicating regional climatic trends rather than local events a deeper water phase occurred between 7000 and 6400 6200 yr bp the last shallow water phase correlates with the classic and epiclassic periods ad 200 900 and shallowest conditions occurred in the late classic to epiclassic c ad 550 900 when the construction of man made islands reached a peak an increase in lake level after c 800 yr bp ad 1100 calibrated age may have led to the abandonment of this life strategy

a high resolution record of holocene climate change in speleothem calcite from cold water cave northeast iowa high precision uranium thorium mass spectrometric chronology and 18o 13c isotopic analysis of speleothem calcite from cold water cave in northeast iowa have been used to chart mid holocene climate change significant shifts in 18o and 13c isotopic values coincide with well documented holocene vegetation changes temperature estimates based on 18o 16o ratios suggest that the climate warmed rapidly by about 3 c at 5900 years before present and then cooled by 4 c at 3600 years before present initiation of a gradual increase in 13c at 5900 years before present suggests that turnover of the forest soil biomass was slow and that equilibrium with prairie vegetation was not attained by 3600 years before present

deep rooted overthrusting and blueschistic metamorphism in compressive continental margins an example from calabria southern italy at the base of austroalpine nappes in calabria alpine greenschist dynamometamorphism is overprinted by blueschist metamorphism tectonic overpressures and deep rooted movements of upthrusting type in compressive regime are the key to generation of blueschistic conditions and to greenschist bluecschist evolution of metamorphism the tectonic overpressures are generated because the rocks at higher confining pressures and lower pore pressures have relatively higher resistance to deformation the evolution from greenschist to blueschist conditions results from cooling during upward motion with overpressures remaining concentrated along the thrust zones

structural and petrological analyses of the frido unit southern italy new insights into the early tectonic evolution of the southern apennines calabrian arc system abstract this study provides new data on the deformation and metamorphic evolution of the jurassic to upper oligocene frido unit an ocean continent transition oct unit belonging to the ligurian accretionary complex lac by means of the integration of structural analysis petrological investigations and a revision of the stratigraphical setting the frido unit representing the main metamorphic component of the lower middle miocene lac in southern italy is characterized by a multistage tectonic evolution including i two progressive deformation phases involving the development under relatively high pressure metamorphic conditions of an early cleavage associated with isoclinal folds within the framework of a dominant se ward tectonic transport ii a third deformation stage involving the growth of lower pressure na amphibole along extensional shear surfaces therefore probably marking the onset of tectonic exhumation and iii two very low temperature deformation phases characterized by the development of kink folds and associated thrusts probably related to the late tectonic emplacement of the accretionary wedge onto the outer sectors of the apennine domain with a prevailing tectonic transport first toward the ne and then toward the nw in order to clarify the metamorphic evolution of the frido unit petrologic analyses were focused on its metasedimentary pelitic succession here mineral parageneses including carpholite well documented for the first time in this study and potassic white mica yielded peak pressure of 1 2 1 4 gpa and temperature around 350 c thus indicating a high pressure very low temperature metamorphism and a p t t path characterized by a rapid exhumation without any greenschist facies overprint in comparison with similar units cropping out in northern calabria tuscany and corsica the frido unit experienced one of the coldest burial exhumation histories

microstructures and petro fabrics of lawsonite blueschist in the north qilian suture zone nw china implications for seismic anisotropy of subducting oceanic crust abstract we conducted a detailed study on the microstructures and petro fabrics of massive and foliated lawsonite blueschist lbs in north qilian suture zone nw china the lattice preferred orientation lpo of glaucophane and lawsonite in foliated lawsonite blueschist lbs is considered to be dominantly formed by the deformation mechanism of dislocation creep and rigid body rotation respectively the lpo of glaucophane is mainly characterized by the 001 axis aligning parallel to lineation and the 100 axis and 110 pole plunging perpendicular to foliation in contrast the lpo of lawsonite features the maximum 010 axis concentrated close to lineation and the 001 axis strongly clustered normal to foliation the preferred orientation of 010 axis of lawsonite parallel to lineation is supported by a two dimensional numerical modeling using the finite volume method fvm the mineral lpos are much stronger in foliated lbs than in massive lbs in addition a kinematic vorticity analysis suggests that both pure shear dominant wm 0 18 0 26 and simple shear dominant wm 0 86 0 93 deformation regimes are present in foliated lbs the 001 axis and 010 pole of glaucophane and the 100 and 010 axes of lawsonite tend to distribute in a foliation parallel girdle in the pure shear dominant samples but simple shear dominant samples display more lineation parallel concentrations of a 001 axis of glaucophane and a 010 axis of lawsonite because the whole rock seismic anisotropies in foliated lbs are significantly higher than those in massive lbs and a counteracting effect on seismic anisotropies occurs between glaucophane and lawsonite the delay time of fast s wave polarization anisotropy induced by an actual subducting oceanic crust with a high subducting angle 45 60 is expected to range from 0 03 to 0 09 s lower bound for massive lbs and from 0 1 to 0 3 s upper bound for foliated epidote blueschist

glacial meltwater cooling of the gulf of mexico gcm implications for holocene and present day climates an atmospheric general circulation model the ncar ccm has been used to investigate the possible effects that reduced gulf of mexico sea surface temperatures sst could have on regional and hemispheric climates 18o records and terrestrial evidence indicate at least two major glacial meltwater discharges into the gulf of mexico subsequent to the last glacial maximum it is probable that these discharges reduced gulf of mexico sst we have conducted three numerical experiments with imposed gulf wide sst coolings of 3 c 6 c and 12 c and find in all three experiments significant reductions in the north atlantic storm track intensity along with a strong decrease in transient eddy water vapor transport out of the gulf of mexico surface pressures are higher over the north atlantic indicating a reduction of the climatological icelandic low the region is generally cooler and drier with a reduction in precipitation that agrees well with evidence from greenland ice cores other statistically significant changes occur across the northern hemisphere but vary between the three experiments in particular warmer wetter conditions are found over europe for both the 6 c and 12 c sst reductions but cooler conditions are found for the 3 c reduction this indicates a dependence in both the sign and magnitude of the model response on the magnitude of the imposed sst anomaly the results suggest that the present day north atlantic storm track is dependent on warm gulf of mexico sst for much of its intensity they also suggest that meltwater induced coolings may help account in part for some of the climatic oscillations that occurred during the last glacial interglacial transition

pumpellyite actinolite facies of the sanbagawa metamorphism the pumpellyite actinolite facies proposed by hashimoto is defined by the common occurrence of the pumpellyite actinolite assemblage in basic schists it can help characterize the paragenesis of basic and intermediate bulk compositions which are common constituents of various low grade metamorphic areas the dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite actinolite and prehnite pumpellyite facies in p t space in the sanbagawa belt in japan the mineral parageneses of hematite bearing and free basic schists as well as pelitic schists have been well documented the higher temperature limit of this facies is defined by the disappearance of the pumpellyite epidote actinolite chlorite assemblage in hematite free basic schists with xfe3 of epidote around 0 20 0 25 and the appearance of epidote actinolite chlorite assemblage with xepfe3 0 20 in hematite bearing basic schists there is a continuous change of paragenesis to higher grade epidote glaucophane or epidote blueschist facies in pelitic schists the albite lawsonite chlorite assemblage does occur but only rarely and its assemblage cannot be used to determine the regional thermal structure the lower temperature equivalence of the pumpellyite actinolite assemblage is not observed in the field the mikabu greenstone complex and the northern margin of the chichibu complex which are located to the south of the sanbagawa belt are characterized by clinopyroxene chlorite or lawsonite actinolite assemblages which are lower temperature assemblages than the pumpellyite actinolite assemblage these three metamorphic complexes belong to the same subduction metamorphic complex the pumpellyite actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes

palaeozoic sequences and evolution of the calabrian peloritan arc southern italy three palaeozoic sequences belonging to three different basins crop out in the calabrian peloritan arc their age covers the time span from middle cambrian to early carboniferous the sequences comprise terrigenous volcanic and carbonate rocks and show low grade metamorphism the basement is of pre middle cambrian age crops out in the calabrian peloritan arc and was metamorphosed prior to the opening to the palaeozoic basins the palaeozoic basins existed no later than the middle carboniferous with inferred maximum crustal extension in the cambro ordovician by devonian time the tectonic regime was compressional overall with middle devonian island arc type volcanic activity that continued until the closure of the basins approximately 330 myr the palaeozoic sequences experienced low pressure greenschist facies metamorphism and continuing subduction controlled the variscan tectonogenesis with 280 myr island arc type intrusive magmas subduction ceased and late stage 280 270 myr granitic magmas were emplaced during continental collision the crustal sector carrying the palaeozoic basins is interpreted as comprising fragments of an active continental margin one of the several microplates located at the southern margin of the euro asia continent which faced a large at least 1500 km in width ocean basin

pressure temperature evolution of lawsonite eclogite in sivrihisar tavsanli zone turkey abstract the cretaceous blueschist belt tavsanli zone representing the subducted and exhumed northern continental margin of the anatolide tauride platform is exposed in western anatolia the sivrihisar area east of tavsanli is made up of tectonic units consisting of i metaclastics and conformably overlying massive marbles coherent blueschist unit ii blueschist eclogite unit iii marble calcschist intercalation and iv metaperidotite slab the metaclastics are composed of jadeite lawsonite glaucophane and jadeite glaucophane chloritoid schists phengite phyllites and calcschists with glaucophane lawsonite metabasite layers the blueschist eclogite unit representing strongly sheared deeply buried and imbricated tectonic slices of accreted uppermost levels of the oceanic crust with minor metamorphosed serpentinite bodies consists of lawsonite bearing eclogitic metabasites approximately 90 of the field lawsonite eclogites metagabbros serpentinites pelagic marbles omphacite glaucophane lawsonite metapelites and metacherts the mineral assemblage of the lawsonite eclogite garnet omphacite 70 is omphacite garnet lawsonite glaucophane phengite and rutile lawsonite eclogite lenses are enclosed by garnet lawsonite blueschist envelopes textural evidence from lawsonite eclogites and country rocks reveals that they did not leave the stability field of lawsonite during subduction and exhumation the widespread preservation of lawsonite in eclogitic metabasites and eclogites can be attributed to rapid subduction and subsequent exhumation in a low geothermal gradient of the oceanic crust material without experiencing a thermal relaxation peak p t conditions of lawsonite eclogites are estimated at 24 1 kbar and 460 25 c these p t conditions indicate a remarkably low geotherm of 6 2 c km corresponding to a burial depth of 74 km

very low temperature record of the subduction process a review of worldwide lawsonite eclogites abstract lawsonite eclogites preserve a record of very low temperature conditions in subduction zones all occur at active margin settings typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite matrix melange peak lawsonite eclogite facies mineral assemblages garnet omphacite lawsonite rutile typically occur in prograde zoned garnet porphyroblasts their matrix is commonly overprinted by higher temperature epidote bearing assemblages greenschist or amphibolite facies conditions erase former lawsonite eclogite relics various pseudomorphs after lawsonite occur particularly in some blueschist eclogite transitional facies rocks coesite bearing lawsonite eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low temperature ultrahigh pressure eclogites are known using inclusion assemblages in garnet lawsonite eclogites can be classified into two types l type such as those from guatemala and british columbia contain garnet porphyroblasts that grew only within the lawsonite stability field and e type such as from the dominican republic record maximum temperature in the epidote stability field formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation the earliest occurrences of lawsonite eclogite facies mineral assemblages are early paleozoic in spitsbergen and the new england fold belt of australia this suggests that since the phanerozoic secular cooling of earth and subduction zone thermal structures evolved the necessary high pressure temperature conditions buoyancy of serpentinite and oblique convergence with a major strike slip component may facilitate the exhumation of lawsonite eclogites from mantle depths

the carbonate tectonic units of northern calabria italy a record of apulian palaeomargin evolution and miocene convergence continental crust subduction and exhumation of hp lt rocks in northern calabria italy the metasedimentary succession of the lungro verbicaro tectonic unit preserves mineral assemblages suggesting underthrusting to depths in excess of 40 km internal deformation of these rocks occurred continuously during the following decompression index mineral composition associated with progressively younger tectonic fabrics indicates that a substantial part of the structural evolution took place within the blueschist facies p t field despite their tectonic and metamorphic history the rocks of the lungro verbicaro unit preserve significant sedimentary and palaeontological features allowing correlations with successions included in adjacent thrust sheets and the reconstruction of the mesozoic continental margin architecture the subduction exhumation cycle recorded by the lungro verbicaro unit is entirely of miocene age this portion of the apulia continental palaeomargin was involved in convergence related deformation not earlier than the aquitanian the integration of our results with available constraints on the tectonic evolution of the apennine calabrian arc system suggests that subduction and most of the subsequent exhumation of the lungro verbicaro unit occurred up to langhian time at maximum vertical rates in excess of 15 mm a 1 the exhumation process was then completed at much slower rates 1 in late miocene time as indicated by both apatite fission track data and stratigraphic information

tectono stratigraphic and kinematic evolution of the southern apennines calabria peloritani terrane system italy abstract temporal controls such as sedimentation ages in foredeep and wedge top basins combined with information about stratigraphic and metamorphic evolution ages and characterization of magmatic rocks deep structures burial and exhumation histories allowed us to obtain kinematic estimations of the southern apennines calabria peloritani terrane system evolution from the late oligocene to recent calculated thrust front velocities suggest to subdivide the orogenic evolution in main five kinematic stages characterized by different velocity trends nine kinematic complexes from a to i i e sets of tectonic units deformed in the same time range are determined according to foredeep ages these complexes bounded by main regional thrust faults encompass one or more tectonic sub units and successions of different paleoenvironment domains paleogeographic evolution model indicates according to available paleomagnetic data a counterclockwise rotation with a mean angle of 60 for the apennine platform carbonates whereas a mean angle of 20 for the eastern sector of the apennine platform and western side of apulian platform shortening estimations for the apennine successions show values ranging between 55 and 88 and a linear best fit indicating an increase from 60 nw sector to ca 90 se sector such high values are consistent with a deformation dominated by a thin skinned tectonics on the contrary the low value of 28 calculated for mt alpi suggests an exhumation ruled by high angle deep seated structures characterized by limited displacements tectonic vergences referred to middle late miocene late miocene and pliocene middle pleistocene when restored indicate an average eastward tectonic transport finally tectonic evolution is summarized in eleven schematic cross sections and corresponding paleogeographic maps

agricultural slope management and soil erosion at la laguna tlaxcala mexico abstract several superimposed systems of agricultural terracing are preserved at the archaeological site of la laguna in central mexico excavations uncovered complex stratigraphic sequences of terrace fills and colluvial deposits their chronology was established by radiocarbon optically stimulated luminescence osl and the presence of diagnostic features and artifacts the formative occupations of the site ca 600 400 bc and 100 bc ad 100 resulted in severe erosion and exposure of indurated subsoil horizons in the middle to late postclassic ad 1150 1520 stone walled terraces were built to recover the degraded slopes for cultivation renewed reclamation works have been undertaken since the colonial period eventually taking the form of sloping field terraces with berms planted in maguey the reconstructed sequence of local land use underscores the risks of dating agricultural features by association with sherd scatters as well as the complexity of the relationships between demographic variables agricultural intensity and terracing

late pleistocene holocene stratigraphy and radiocarbon dating of la malinche volcano central mexico abstract previous studies of la malinche identified and radiocarbon dated several volcanic layers the youngest of which yielded an age of ca 7 5 ka an additional ash fallout layer that crops out at high altitudes was considered the most recent deposit with an estimated age of 6 ka in the present work 38 new radiocarbon ages are presented from these several date the young ash fallout layer and lie around 3 1 ka with the aid of these dates a new and comprehensive stratigraphy documenting the late pleistocene holocene eruptive history of la malinche is presented the stratigraphy indicates two main stages of volcanic activity pre malinche and malinche the first undoubtedly comprises the major part of the eruptive history but its deposits are largely covered by the products of the latter stage on which this study is focused the malinche stage was subdivided into three eruptive periods period 1 started with the emplacement of the huamantla pumice more than 45 ka ago this deposit consists of a thick pumice fallout overlain by pyroclastic flow deposits subsequently several episodes of construction and collapse of summit domes occurred the oldest dome was dated at ca 45 ka period 2 started 21 5 ka ago with the malinche pumice i a widespread pumice fallout covering the entire slopes of the volcano pyroclastic flows and lahars related to this eruption were channeled along deep barrancas and reached considerable distances deposits produced by partial sector collapse and dated at ca 20 9 ka and a pumice and ash flow deposit dated at 15 9 ka were also generated during this period the last period started with the eruption of the malinche pumice ii a distinctive fallout deposit overlain by ash flow deposits on the ne slope of the volcano the age of this pumice layer is estimated between 12 and 9 ka formation of block and ash flows lahars and pumice and ash flows followed during this period and peaked in a most intensive episode that was dated at 7 5 ka after this a long interval of quiescence was interrupted 3 1 ka ago with the eruption of an ash fallout accompanied by small pyroclastic flows whose deposits are the youngest volcanic products recognized at la malinche the new stratigraphic findings identify la malinche as a potentially active and dangerous volcano that could severely affect densely populated areas at its lower slopes in addition the results of this work can shed some light on archaeological research that is being carried out in the puebla tlaxcala region

paleosols bones phytoliths and δ13c signatures of humus and teeth in the alluvial sequence of axamilpa puebla inferences for landscape evolution and megafauna paleoecology during mis 3 2 in southern mexico abstract alluvial paleosol sedimentary sequences containing animal fossils and paleobotanical and paleopedological archives have a potential for reconstructing quaternary paleoenvironments in southern mexico and for understanding the paleoecology of pleistocene megafauna in the section of axamilpa river four pedostratigraphic units were recognized the lower gleyic and vertic pedological units are at the end of marine isotopic stage mis 3 while the upper humic and most probably calcic units correspond to mis 2 according to the radiocarbon dates of humus the micromorphological analysis indicates an incipient development and syn sedimentary character of paleopedogenesis in the periodically water saturated environment indicated by redoximorphic features hydrogenic carbonates and low values of magnetic susceptibility in most strata the 13 c values of the paleosol humus range from 22 6 to 24 7 indicating a c3 vegetation whereas in the modern soil it is higher 15 2 pointing to an abundance of the c4 and cam plants the 13 c signature of the vegetation inferred from the teeth enamel of the fossil fauna appeared to be less negative than that of humus from corresponding paleosols the alimentary preferences of the paleofauna which fed on grasses with high proportion of c4 plants explain this difference the phytolith analysis shows dominance in conifer forms followed by c4 and c3 grasses the content of conifer phytoliths is progressively reduced towards the top of the sequence we interpret the late pleistocene landscape of this area as a heterogeneous forest grassland landscape with the forest shrinking through time and the grass areas increasing swampy floodplain meadows inferred from paleopedological results could provide a habitat for grassy vegetation both isotope and phytolith records demonstrate a contrasting difference between pleistocene and recent vegetation dominated by xerophytic shrubs and succulents this vegetation change could play important role in the extinction of pleistocene megafauna

buried palaeosols of the nevado de toluca an alternative record of late quaternary environmental change in central mexico buried palaeosols of central mexico not previously analysed from a palaeopedological standpoint have helped to develop a reliable regional model of late quaternary climatic change this paper focuses upon morphological and micromorphological properties particle size distribution and extractable fe al and si of seven palaeosols named pt1 pt7 in the vicinity of the nevado de toluca volcano the characteristics of andic cambisol pt1 similar to those of modern soils in semi arid environments indicate a drier climate in the first half of the holocene humic andosols pt2 pt4 are analogous to modern volcanic soils of humid forest environments they show evidence that a moist palaeoclimate prevailed during marine oxygen isotope stages ois 2 and 3 luvisols pt5 and pt6 which are assumed to have formed at the end of marine ois 5 to marine ois 4 also indicate humid conditions we attribute the differences between andosols pt2 pt4 and luvisols pt5 pt7 to variations in the duration in the pedogenesis rather than to changes in palaeoclimate micromorphological features of andosol to luvisol transition confirm that these soils form an evolutionary sequence being for the most part consistent with lacustrine records the palaeosol properties do not reflect the episodes of drier climate during the last glacial maximum however as shown by diatom and palynological data from lake sediments mesoclimate variations and or incompleteness of soil memory reflecting mostly periods of humid pedogenesis probably provide the reason for this disagreement copyright 2001 john wiley sons ltd

past hydrological events reflected in the holocene fluvial record of europe abstract a comprehensive database of radiocarbon dated fluvial units in great britain poland and spain has been compiled to investigate the relationship between environmental change flooding and holocene river dynamics following the methodology recently developed by macklin and lewin macklin m g lewin j 2003 river sediments great floods and centennial scale holocene climate change journal of quaternary science 18 101 105 radiocarbon dates in fluvial sequences that coincide with a modification in sedimentation rate or style have been highlighted allowing geomorphologically significant changes in holocene river activity to be identified data analysis has been undertaken at both national and sub national scales and on catchments of different size type and land use history multiple phases of higher flood frequency characterized by accelerated erosion and sediment deposition on floodplains are recognized and compared with a range of climate proxies the relative and varying roles of climate and land use on river dynamics are considered and the value of the database for reconstructing past hydrological events as well as for predicting river response to future environmental change is assessed

ductile brittle transition in pre alpine amphibolite facies mylonites during evolution from water present to water deficient conditions mont mary nappe italian western alps in the austroalpine mont mary nappe italian western alps discrete zones of mylonites ultramylonites developed from coarse grained upper amphibolite facies metapelites of pre alpine age the syn mylonitic mineral assemblage is quartz biotite muscovite plagioclase garnet sillimanite ilmenite graphite and formed via the model hydration reaction grt 1 kfs h 2 o bt 2 ilm 2 qtz ms sil grain size reduction of about three orders of magnitude was accompanied by extensive recrystallization of all minerals except sillimanite and by compositional changes of garnet and biotite deformation took place at temperatures of 510 580 c under low pressure conditions 0 25 0 45 gpa and corresponds to the latest stages of pre alpine metamorphic evolution the pre alpine mylonitization conditions were close to the brittle ductile transition as indicated by syn mylonitic generation of pseudotachylytes and high diverential stress inferred from quartz grain size piezometry the brittle ductile behaviour at a relatively high temperature and the absence of annealing textures in quartz aggregates are suggestive of water deficient conditions during mylonitization these were accomplished through progressive consump tion of water by syn kinematic hydration reaction and by adsorption onto the greatly increased grain boundary area resulting from dynamic recrystallization

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

palaeoclimatic interpretation of stable isotope data from holocene speleothems of the waitomo district north island new zealand one straw stalactite and three stalagmites from the waitomo district of north island new zealand were examined for stable isotopes of oxygen and carbon with a view to interpreting their palaeoclimate signal dating was by uranium series and ams 14c for the stalagmites and by gamma ray spectrometry for the straw records were thus established for about 100 years for the straw and 3 9 10 1 and 10 2 ka for the stalagmites the range of variability in d18oc and d13cc this century is about two thirds of that experienced over the entire holocene and is most simply explained in terms of the oceanic source area of rain stable isotope variations in three stalagmites show some general similarities but have significant differences in detail which underlines the necessity to base palaeoclimatic interpretations on more than one speleothem record the d18oc of each stalagmite varies positively with temperature indicating the dominance of the ocean source of evaporation in determining the isotopic composition of

alpine structural and metamorphic signature of the sila piccola massif nappe stack calabria italy insights for the tectonic evolution of the calabrian arc combined structural and petrographical investigations coupled with 40ar 39ar geochronology were carried out in the sila piccola massif of the calabrian arc in order to define the structural geometry and map out the major structural and metamorphic breaks within the exposed nappe sequence on the basis of the contrasting alpine pressure temperature p t and structural signatures the nappe stack can be divided in two major tectonic complexes bounded by a flat lying ductile to brittle extensional shear zone the upper complex consists of a nappe like structure where a major top to the east compressional shear is recorded the lower tectonic complex consists of an ophiolite bearing sequence showing typical high p low t parageneses mg carpholite and na amphibole the 40ar 39ar geochronology on phengites in equilibrium with blueschist minerals provided a minimum age estimate for the blueschist event in the lower complex rocks at the oligocene eocene boundary around 35 ma ductile to brittle top to the west extensional shear accompanied the nearly isothermal retrogression and exhumation of the lower complex rocks reworking the previous nappe contacts with shear localization along the upper lower tectonic complex discontinuity the 40ar 39ar dating indicates that this postnappe stacking tectonic evolution took place from 30 ma onward it is proposed that exhumation of the deep seated rocks occurred below a top to the west extensional detachment active during convergence and orogenic complex formation synorogenic extension the age of this detachment is bracketed between 30 ma and the post orogenic neogene basin sedimentation middle upper miocene the revised structural and metamorphic scenario is here integrated into a new tectonic evolutionary reconstruction which involves an early high p low t top to the east crustal thickening episode during the construction of the apennine orogenic wedge eocene oligocene followed and overprinted by a top to the west extensional shear probably active from the late oligocene

late holocene stratigraphy of the tetimpa archaeological sites northeast flank of popocatepetl volcano central mexico late holocene 2500 yr b p tephras bury a sequence of pre hispanic archaeological sites in the tetimpa area on the northeast flank of popocatepetl volcano from measured stratigraphic sections 14c dates and isopach maps this paper reconstructs the eruptive chronology and the regional extent of deposits associated with the tetimpa archaeological sites a regionally extensive paleosol defines the base of the late holocene sequence in the tetimpa area deposits from two periods of explosive volcanism unconformably overlie this paleosol eruptive sequence i was deposited at ca 2100 yr b p and blanketed late tetimpa archaeological sites with 1 1 5 m of yellow andesitic 62 sio2 pumice and locally with 20 40 m of olivine bearing andesitic lavas isopach and isopleth maps of the pumice deposit suggest a plinian style eruption event that covered 240 km2 on the east side of the volcano with 25 cm of tephra lavas from eruptive sequence i dammed drainage in the lowland area near the town of san nicolas and caused local upstream deposition of as much as 30 m of lacustrine silts clays and sands these lacustrine deposits record an eruptive hiatus for the tetimpa area of about 750 14c yr between ca 2100 and ca 1350 yr b p no major tephras were deposited in the tetimpa area in upland areas this time period is represented by an unconformity and by entisols formed in the top of pumice deposits and lavas from eruptive sequence i artifacts agricultural furrows and dwellings record human reoccupation of this surface at the end of this hiatus several lahars were deposited above the lacustrine sequence and locally above the entisol in upland positions adjacent to streams between ca 1350 and ca 1200 yr b p tephras from eruptive sequence ii buried these paleosols occupation sites lacustrine sediments and lahars andesitic 62 sio2 pumice lapilli deposits in the tetimpa area record three pumice fall eruptions directed northeast and east of the crater the first and smallest of these maximum tetimpa area thickness 12 cm 52 km2 covered by 25 cm took place at ca 1350 yr b p and was accompanied by pyroclastic surge events preserved in the tetimpa area by charcoal sand waves and cross stratified sand sized tephra at ca 1200 yr b p the products of two plinian style events and additional pyroclastic surges reached the tetimpa area the largest of these tephra fall events covered the tetimpa area with 0 5 1 m of tephra and blanketed an area of 230 km2 with a thickness of 25 cm the tetimpa record confirms two of the four periods of explosive volcanism recognized by studies conducted around popocatepetl in the past 30 yr eruptive sequence i corresponds to the explosive period between 2100 and 2500 yr b p and eruptive sequence ii corresponds to the period between 900 and 1400 yr b p the archaeology and lacustrine stratigraphy of the tetimpa area help constrain the timing of the plinian phase of eruptive sequence i to ca 2100 yr b p and suggest that the pumice fall eruptions of eruptive sequence ii took place in at least two intervals between ca 1350 and ca 1200 yr b p

structural signature of tectonic processes in the calabrian arc southern italy evidence from the oceanic derived diamante terranova unit the diamante terranova unit is an oceanic derived element within the thrust and nappe structure of the northern sector of the calabrian arc its metamorphic and structural signature reveals a long history of se dipping subduction and collision related deformation of an oceanic lithospheric section underlying the narrow diamante terranova basin the deformation history of the diamante terranova unit may be related to three main tectonic phases the first one is characterized by a structural association which includes features that developed during a progressive deformation related to the evolution of a deep seated shear zone kinematic indicators characterizing subduction related structures clearly show a top to the northwest sense of overthrust shear the second tectonic phase that can be recognized in the area is characterized by structures that may be related to the onset of shear deformation induced by a postcollisional northeastward motion of the overriding plate the third tectonic phase is recorded by brittle features related to the latest tectonic events affecting the calabrian arc during the opening of the tyrrhenian sea shear sense indicators and available radiometric and stratigraphic information together with plate kinematic data allowed us to infer the time evolution of this sector of the calabrian arc and to assess that the diamante terranova unit cannot be considered as an eoalpine element since its deformation related to subduction collision processes occurred prior to the end of the cretaceous

mineral and organic components of the buried paleosols of the nevado de toluca central mexico as indicators of paleoenvironments and soil evolution abstract results of earlier studies of quaternary tephra paleosol sequences in central mexico revealed contradictions between paleopedological and lacustrine records to settle the contradictions selected quantitative characteristics of mineral and organic components of the paleosols pt1 pt7 from the nevado de toluca sequence were studied as independent paleoclimate proxies mineralogical composition of sand and clay fractions allows assessment of weathering and secondary mineral formation both being climate dependent processes stable carbon isotope ratios of humus and phytolith morphology indicate paleovegetation in all paleosols the sand fractions consist mainly of unstable volcanogenic minerals although the grains have weathering features short durations of pedogenesis are responsible for limited weathering status the clay fraction is dominated by halloysite and dehydrated halloysite in units pt1 pt4 which suggests drier periods during paleosol formation whereas major amounts of kaolinite in the units pt5 pt7 indicate extended humid pedogenesis 13c values of pt2 pt4 humus are intermediate between those characteristic of c3 and c4 plants phytolith assemblages which include both forms of c3 dominant and c4 minor quantities grasses agree with carbon isotope data these data indicate plant cover changes driven by wet dry paleoclimate oscillations the new results showing drier stages during the formation of late pleistocene paleosols pt2 and pt3 correct the primary interpretation of these units as indicating only humid pedogenesis and settles the contradiction between paleopedological and lacustrine records

an exposed hercynian deep crustal section in the sila massif of northern calabria mineral chemistry petrology and a p t path of granulite facies metapelitic migmatites and metabasites in the sila massif of northern calabria a continous section is exposed through a segment of a hercynian deep continental crust which has been interpreted by previous workers as a stack of basement nappes monte gariglione complex the section consists essentially of metapelitic migmatites and subordinate metabasites and marbles which were metamorphosed at medium pressure high temperature granulite facies conditions a continuous metamorphic gradient through the exposed segment can be deduced from the systematic change in the compositions of ferromagnesian minerals in divariant metapelitic assemblages this gradient is partly supported by conventional geothermobarometry and by applying the tweequ method however peak metamorphic conditions are better defined by dehydration melting reactions which reveal similar to4 kbar and 740 degreesc at the top and similar to6 kbar and 770 degreesc at the base of the section therefore the exposed rocks represent a former crustal depth range of similar to 14 21 km at the thermal peak of metamorphism the metamorphic evolution of the former deep crustal rocks has been reconstructed from reaction textures the prograde p t path took place in the stability field of sillimanite the retrograde path is characterized by a stage of isothermal uplift after peak metamorphism to mid crustal levels 10 15 km followed by near isobaric cooling to greenschist facies conditions the deep crustal section in northern calabria resembles that of the serre massif in southern calabria which represents the lowermost part of an exposed tilted cross section through the hercynian continental crust postkinematic mineral growth reaction textures and isotopic age constraints indicate that the thermal conditions in the calabrian crust during the hercynian orogeny were mainly controlled by advective heat input through magmatic intrusions as in southern calabria large granitic bodies were emplaced between the granulite facies lower crust monte gariglione complex and the amphibolite to greenschist facies upper crust during regional metamorphism

migmatite formation at subsolidus conditions an alternative to anatexis this contribution discusses the formation of stromatic high grade migmatites volume considerations require that most of the leucosome material is not added from outside the system a segregation mechanism is necessary except in those cases where the protolith of the migmatite already had a banded structure although partial melting is most often advocated to provide the segregation mechanism several arguments can be raised against high degrees of melting mineral compositions and even zoning patterns are similar in both mesosomes and leucosomes sufficient degrees of melting at reasonable temperatures require more than the available amounts of water the leucosomes do not always approximate to a minimum melt composition high degrees of melting cannot occur without an appreciable volume increase etc diffusion works as a segregation mechanism at low temperatures as diffusion rates increase exponentially with temperature diffusion must become still more important as a segregation mechanism at high temperatures a model is suggested based on the diffusion of components in response to the gradient x where 1 3 3i 1 i is the mean pressure in homogeneously strained rocks 3 is larger in rock parts rich in incompetent phases than in rock parts depleted in incompetent phases accordingly mechanically competent but chemically incompetent high volume phases like quartz and feldspars stressed in micadominated parts of a rock high migrate to parts of the rock that are depleted in mica low it is suggested that hornblende occurring in many leucosomes may be premigmatitic or early syn migmatitic and due to its mechanical competency it initiates the segregation diffusion occurs along grain boundaries and is enhanced by small amounts of intergranular fluid at the best semiquantitative estimates of diffusion rates and distances indicate that the process should work over geological times

stratigraphic evidence for anthropogenically induced coastal environmental change from oaxaca mexico abstract previous interdisciplinary paleoenvironmental and archaeological research along the rio verde valley of oaxaca mexico showed that holocene erosion in the highland valleys of the upper drainage basin triggered geomorphic changes in the river s coastal floodplain this article uses stratigraphic data from sediment cores extracted from laguna pastoria an estuary in the lower rio verde valley to examine changes in coastal geomorphology potentially triggered by highland erosion coastal lagoon sediments contain a stratigraphically and chronologically distinct record of major hurricane strikes during late holocene times three distinct storm facies are identified from sediment cores obtained from laguna pastoria which indicate that profound coastal environmental changes occurred within the region and are correlated with increased sediment supplied from highland erosion the chione laevicardium facies was deposited in an open bay while the mytella barnacle facies and sand facies were deposited in an enclosed lagoon following bay barrier formation we argue that highland erosion triggered major geomorphic changes in the lowlands including bay barrier formation by 2500 cal yr b p these environmental changes may have had significant effects on human populations in the region the lagoon stratigraphy further indicates an increase in mid late holocene hurricane activity possibly caused by increased el nino frequencies

deformation microstructures of glaucophane and lawsonite in experimentally deformed blueschists implications for intermediate depth intraplate earthquakes a series of simple shear experiments on blueschist was performed at 400 500 c and 1 2 5 gpa to understand the deformation of seismically active subducting oceanic crust the experiments show that brittle microstructures are mainly found at pressures of 1 2 gpa whereas ductile microstructures form at 2 5 gpa j indices a measure of fabric intensity of glaucophane crystal preferred orientations change systematically with changing shear strain and confining pressure and the angle between the slip plane and the shear direction of samples deformed at 2 gpa is similar to that of a strain ellipsoid these results together with the variable orientations of fine grains in a selected area electron diffraction image at 2 gpa indicate that the brittle ductile transition for glaucophane occurs at 2 gpa in contrast to this lawsonite in the experiments show abundant fracturing in most specimens and a poor correlation between the j index shear strain and confining pressure this demonstrates that lawsonite deformed by brittle failure under all experimental conditions in the case of a starting material that has a strong fabric and deformed under dry experimental conditions the brittle ductile transition zone of glaucophane will be much shallower than 2 gpa therefore our initial experimental results on the deformation behavior of blueschist indirectly support the dehydration embrittlement of subducting oceanic crust glaucophane as an important factor in the origin of intraplate earthquakes

trace element partition coefficients in the calcite water system and their paleoclimatic significance in cave studies abstract speleothems stalactites stalagmites formed in limestone caves have been found to contain much information on the timing and intensity of past climates from analysis of their u th 13c and 18o contents because the incorporation of certain trace elements e g mg mn and zn in calcite is known to be temperature dependent it may be possible to use variations in trace metal content of fossil speleothems as an alternative paleotem perature indicator using specially developed ion exchange sampling techniques analysis of trace metal content of seepage water and associated fresh calcite deposits in caves in vancouver island and jamaica shows that mg is distributed between phases in a consistent manner within the temperature regimes of the caves 7 and 23 c respectively average values of the distribution coefficient for mg are respectively 0 017 and 0 045 at these temperatures these results indicate that the mg content of calcite varies directly with temperature and in a sufficiently pronounced manner that a 1 c rise in depositional temperature of a speleothem containing 500 ppm mg at 10 c would be seen as an increase of 35ppm mg a readily determinable shift other factors affecting mg content of a speleothem are considered

23 000 yr of vegetation history of the upper lerma a tropical high altitude basin in central mexico abstract pollen analysis on a 9 54 m sediment core from lake chignahuapan in the upper lerma basin the highest intermontane basin in central mexico 2570 m asl documents vegetation and limnological changes over the past 23 000 14c yr the core was drilled near the archaeological site of santa cruz atizapan a site with a long history of human occupation abandoned at the end of the epiclassic period ca 900 ad six radiocarbon ams dates and two well dated volcanic events the upper toluca pumice with an age of 11 600 14c yr b p and the tres cruces tephra of 8500 14c yr b p provide the chronological framework for the lacustrine sequence from ca 23 000 14c yr b p to ca 11 600 14c yr b p the plant communities were woodlands and grasslands based on the pollen data the glacial advances mii 1 and mii 2 correlate with abundant non arboreal pollen mainly grasses from ca 21 000 to 16 000 14c yr b p and at ca 12 600 14c yr b p during the late pleistocene lake chignahuapan was a shallow freshwater lake with a phase of lower level between 19 000 and 16 000 14c yr b p after 10 000 14c yr b p tree cover in the area increased and a more variable lake level is documented late holocene ca 3100 14c yr b p deforestation was concurrent with human population expansion at the beginning of the formative period 1500 b c agriculture and manipulation of the lacustrine environment by human lakeshore populations appear at 1200 14c yr b p 550 a d with the appearance of zea mays pollen and abundant charcoal particles

a high elevation holocene pollen record from iztaccihuatl volcano central mexico pollen data from 3860 m in the agua el marrano valley on the northwest side of iztaccihuatl volcano southeastern part of the basin of mexico 198n combined with data on the glacial chronology and tephrochronology document holocene climatic and vegetation changes near the present timberline glacial advances occurred between 12 000 and 10 500 cal yr bp milpulco 1 and between 8300 and 7300 cal yr bp milpulco 2 a 450 cm core was drilled in sediments accumulated in a milpulco 1 moraine depression the sequence covers the last 11 000 cal yr bp according to three 14 c dates and the presence of a distinctive pumice deposit dated elsewhere at 5700 cal yr bp the pollen record of agua el marrano shows barren conditions after glacier retreat followed by a gradual colonization by elements of the alpine grasslands around 10 900 cal yr bp this plant community remained until c 7200 cal yr bp between c 7200 cal yr bp and 6500 cal yr bp coniferous forest grew close to or at the site but alpine grasslands were re established between 6500 cal yr bp and c 5000 cal yr bp apparently in connection with mid holocene dry conditions recorded in central mexico the upper timberline attained its modern altitude 4000 m in the last 3000 years during this period no significant changes are observed in the pollen spectra comparison with other palaeoecological records from central mexico suggests that the timberline fluctuated in response to both temperature and precipitation changes during the holocene