geology geochronology and tectonic setting of the jorullo volcano region michoacan mexico abstract the jorullo monogenetic volcano erupted 250 years ago at the southern border of the trans mexican volcanic belt tmvb an area that records a long history of magmatic and tectonic activity the oldest rocks that crop out in the area are early oligocene 32 7 0 2 ma dioritic to granitic 60 72 wt sio2 plutons intruded by younger to contemporaneous 30 3 0 1 and 33 3 0 7 ma aplitic 76 78 wt sio2 dikes and mineralized veins volcanic rocks related to these intrusives form a thick sequence exposed in cliffs from the base of which an altered andesite 58 wt sio2 was collected the nearby epithermal cu mineralization of the inguaran mining area to the southeast is also related to this tertiary volcanic episode the oldest expression of the tmvb in this area is represented by pliocene 3 2 2 4 ma basaltic andesite to andesite 53 63 wt sio2 lavas with pyroxene olivine forming extensive mesas eroded mounds and thick elongate flows that were unconformably deposited on top of the oligocene rocks their emplacement was followed by erosion approximating a rate of 100 m ma over the past 3 ma this led to the formation of an inverted topography reflecting the higher resistance to erosion of these lavas that were originally deposited in valleys and today constitute prominent mesas during the last 1 ma at least twenty six monogenetic scoria cones formed in the study area erupting about 10 2 km3 of lava and pyroclastics dominantly composed of olivine and pyroxene bearing andesites 61 vol dre lesser basaltic andesites 30 vol dre and a minority of olivine rich basalts 10 vol dre these eruptive products include the k rich basaltic trachyandesites of cerro la pilita a scoria and lava cone located close to jorullo the 27 ka san hilario and esmeralda lavas which are located 13 km southeast of jorullo are the southernmost eruptive products of the tmvb and also the youngest volcanoes after jorullo in this area mapping of main faults and scoria cone alignments on regional and local scales reveals that the ascent of dikes feeding quaternary eruptions in the jorullo area was controlled by west northwest and west southwest basement structures northeast oriented subduction related horizontal compressional stress probably associated with crustal fractures and a major northwest trending seismically active fault zone that passes near paricutin and jorullo

petrogenesis of the early cretaceous adakite like porphyries and associated basaltic andesites in the eastern jiangnan orogen southern china abstract early cretaceous quartz porphyries are associated with mantle derived mafic rocks basaltic andesites in the eastern segment of the neoproterozoic jiangnan orogen southern china zircons from two quartz porphyries yield identical u pb ages at 142 1 ma by the la icp ms method hornblende and augite are the dominant phenocrysts in the basaltic andesites and the groundmass plagioclase has a high anorthite content 80 both of which indicate high h 2 o 6 wt contents in the magma the basaltic andesites have low tio 2 0 68 0 78 wt and nb la 0 23 0 28 nb and ta depletions high initial 87 sr 86 sr ratios 0 710 and unradiogenic nd isotopes e nd t 3 4 to 3 7 implying that they were generated from the partial melting of an isotopically enriched lithospheric mantle source that had been metasomatized by subduction related fluids the quartz porphyries sio 2 65 6 70 5 wt show adakite like geochemical features with high sr y 18 1 53 4 and la yb 18 6 39 4 ratios but low y 9 44 15 2 ppm and yb 0 84 1 43 ppm contents they have low mgo 0 72 1 46 wt ni 1 04 14 7 ppm and cr 2 34 36 7 ppm contents and lower nd isotopic ratios e nd t 6 02 to 7 64 than the basaltic andesites suggesting that they may have been generated from the partial melting of the lower part of a thickened continental crust without interaction with the underlying lithospheric mantle the mafic magmas ascended from the metasomatized and hydrated lithospheric mantle and heated the lower part of thickened continental crust triggering partial melting to form the adakite like porphyries the moderate levels of sr 204 546 ppm and negative eu anomalies eu eu 0 69 0 86 in the porphyries require the presence of minor plagioclase as residual phase in the magma source indicating the continental crust may be not as thick 50 km as previously suggested the new recognition of adakite like porphyries and associated mafic rocks in the eastern jiangnan orogen reveals the significant role of neoproterozoic orogen related crust and oceanic sediments in the formation of mesozoic adakite like rocks of central eastern china

evaluation of recent tectonomagmatic discrimination diagrams and their application to the origin of basic magmas in southern mexico and central america discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years although old diagrams have been extensively used the recent ones based on discriminant functions of ratio variables with or without log transformation proposed during 2004 2010 for the discrimination of four tectonic settings of island arc continental rift ocean island and mid ocean ridge were newly evaluated to show their high success rates of 57 3 100 and 58 5 100 for major element and immobile element based diagrams respectively for the continental arc of the andes evaluated for its similarity to island arc these four sets of diagrams showed success rates of 62 1 83 8 these four sets of five diagrams per set were therefore used to infer tectonic setting of the mexican volcanic belt mvb los tuxtlas volcanic field ltvf and central american volcanic arc cava using this approach the mvb especially its western central and eastern parts and the ltvf of southern mexico show a dominantly continental rift setting and the cava shows an arc setting the west central part of the mvb is consistent with dual tectonics of arc and rift these results confirm the application of an unusual mantle upwelling rift model for the mexican on land volcanism whereas the conventional plate tectonic subduction model seems to be applicable for the cava from guatemala to north western costa rica

age and petrology of alkalic postshield and rejuvenated stage lava from kauai hawaii at the top of the waimea canyon basalt on the island of kauai rare flows of alkalic postshield stage hawaiite and mugearite overlie tholeiitic flows of the shield stage these postshield stage flows are 3 92 ma and provide a younger limit for the age of the tholeiitic shield stage the younger koloa volcanics consist of widespread alkalic rejuvenated stage flows and vents of alkalic basalt basanite nephelinite and nepheline melilitite that erupted between 3 65 and 0 52 ma all the flows older than 1 7 ma occur in the west northwestern half of the island and all the flows younger than 1 5 ma occur in the east southeastern half the lithologies have no spatial or chronological pattern the flows of the koloa volcanics are near primary magmas generated by variable small degrees of partial melting of a compositionally heterogeneous garnet bearing source that has about two thirds the concentration of p2o5 rare earth elements and sr of the source of the honolulu volcanics on the island of oahu the same lithology in the koloa and honolulu volcanics is generated by similar degrees of partial melting of distinct source compositions the lavas of the koloa volcanics can be generated by as little as 3 percent to as much as 17 percent partial melting for nepheline melilitite through alkalic basalt respectively phases that remain in the residue of the honolulu volcanics such as rutile and phlogopite are exhausted during formation of the koloa volcanics at all but the smallest degrees of partial melting the mantle source for kauai lava becomes systematically more depleted in 87sr 86sr as the volcano evolves from the tholeiitic shield stage to the alkalic postshield stage to the alkalic rejuvenated stage at the same time the lavas become systematically more enriched in incompatible trace elements on a shorter timescale the lavas of the koloa volcanics display the same compositional trends but at a lower rate of change the source characteristics of the koloa volcanics considered along with those of the honolulu volcanics support a mixing model in which the source of rejuvenated stage lava represents large percent melts of a plume source mixed with small amounts of small percent melts of a heterogeneous mid ocean ridge source

simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel topographic interactions generate multidirectional and unsteady air ow that limits the application of velocity pro le approaches for estimating sediment transport over dunes results are presented from a series of wind tunnel simulations using irwin type surface mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp crested model dunes findings complement existing theories on secondary air ow effects on stoss transport dynamics and provide new information on the in uence of lee side air ow patterns on dune morphodynamics for all speeds investigated turbulent unsteadiness at the dune toe indicates a greater more variable surface shear despite a signi cant drop in time averaged measurements of streamwise shear stress at this location this effect is believed suf cient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region on the stoss slope streamline compression and ow acceleration cause an increase in ow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60 70 per cent greater than at ow reattachment on the lower stoss of closely spaced dunes streamwise ow accelerations rather than turbulence have greater in uence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed reversed ow within the separation cell generates signi cant surface shear 30 40 per cent of maximum values for both spacings this supports eld studies that suggest reversed ow is competent enough to return sediment to the dune directly or in a de ected direction high variability in shear at reattachment indicates impact of a turbulent shear layer that despite low values of time averaged streamwise stress in this region would inhibit sediment accumulation downwind of reattachment shear stress and ow steadiness increase within 6 h h dune height of reattachment and approach upwind values by 25 h a distance of at least 30 h is suggested for full boundary layer recovery which is comparable to uvial estimates the irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust simple and cost effective design shows promise for validating these ndings in natural dune settings copyright 2003 john wiley sons ltd

geology and hydrogeochemistry of the jungapeo co2 rich thermal springs state of michoacan mexico abstract we present the first geothermal assessment of the jungapeo co2 rich mineral springs which are located in the eastern part of michoacan state central mexico at the southern limit of the trans mexican volcanic belt all but one of the 10 springs occur at the lower contact of the distal olivine bearing basaltic andesite lavas of the tuxpan shield a 0 49 to 0 60 ma old cluster of monogenetic scoria cones and lava flows the tuxpan shield has a maximum radius of 6 km and was constructed on top of a folded and faulted cretaceous basement consisting largely of marine limestones marls and shales the mineral waters are characterized by moderate temperatures 28 to 32 c mild acidity ph from 5 5 to 6 5 relatively high discharge rates effervescence of co2 gas clarity at emergence and abundant subsequent precipitation of hydrous iron silica oxides and carbonates around pool margins and issuing streamlets chemical and isotopic deuterium oxygen and tritium analyses of water and gas samples obtained during the period 1991 1997 indicate that the springs are largely composed of meteoric water from a local source with relatively short residence times water ages of 7 to 25 years spring waters are chemically characterized by moderate sio2 ca mg nearly equal to na k high hco3 moderate to low cl low f and so4 high b moderate li while br and as are low in contrast fe mn is exceptionally high thus the jungapeo waters cannot be regarded as high temperature geothermal fluids instead they resemble soda spring waters similar to other low to medium temperature soda waters in the world gas samples are extremely rich in co2 with no detectable geothermal h2s or h2 and very low contents of ch4 and nh3 indicating the gases are not derived from a high temperature resource carbon 13 analyses of co2 show a narrow range 6 7 and 7 2 that falls within the range for morb co2 thus most co2 seems to originate from the mantle but some co2 could originate from thermal degradation of organic remains in underlying cretaceous rocks 3 4he ratios range from about 2 to 3 rc ra indicating that a small mantle magmatic he component is present in the gases in conclusion the mineral waters are the surface expression of a low temperature geothermal system of limited size that originates from the combined effects of a high regional heat flow and possibly the remnant heat released from subjacent basaltic andesite magma bodies that constitute the root zone of the tuxpan shield

stratigraphic relations and lithologic variations in the jemez volcanic field new mexico over 100 radiometric dates and recent detailed geologic mapping allow some refinements of the stratigraphic relations of major units and generalization of temporal lithologic variations in the jemez volcanic field volcanism had begun in the area by about 16 5 ma with episodic eruptions of alkaline basalts by 13 ma alkaline volcanism had been replaced with eruptions of more voluminous olivine tholeiite high silica rhyolite derived from melts of lower crust also was erupting by about 13 ma basalt and high silica rhyolite continued to be erupted until about 7 and 6 ma respectively but effusions of dominantly andesitic differentiates of basalt that began as early as about 12 ma volumetrically overshadowed all other eruptive products between 10 and 7 ma from 7 to 3 ma the dominant erupted lithology was dacite which appears to have been generated by mixing of magmas whose compositions are approximated by earlier andesites and high silica rhyolites less than 4 3 ma volcanism was dominated by eruption of rhyolitic tuffs field relations geochemistry and dates specifically indicate the following with regards to stratigraphie relations 1 distinctions among basalt of chamisa mesa paliza canyon formation basalts and lobato basalt for other than geographic reasons are artificial basaltic volcanism was continuous in volcanic field from 13 to 7 ma 2 canovas canyon and bearhead rhyolites form a continuum of high silica rhyolite volcanism from 13 to 6 ma 3 hypabyssal and volcanic rocks of the cochiti mining district probably represent the exhumed interior of a keres group volcano s 4 temporal overlaps exist among the major stratigraphie groups which may imply some genetic relations and 5 the tewa group formation cerro rubio quartz latite may more appropriately be considered part of the tschicoma formation of the polvadera group preliminary analysis of hydrothermal alteration in the context of the volcanic stratigraphy suggests at least three distinct hydrothermal events have occurred in the volcanic field s history

origin of andesite in the deep crust and eruption rates in the tancitaro nueva italia region of the central mexican arc the tancitaro nueva italia region 4400 km 2 is located within the west central mexican volcanic belt and spans 80 km of arc length along the volcanic front on the basis of 65 40 ar 39 ar ages it is shown that a total volume of 326 57 km 3 of magma has erupted from 200 vents since 1 2 ma nearly a third of this volume 103 5 km 3 was erupted as crystal rich andesite from a large stratovolcano volcan tancitaro whereas the remaining two thirds 223 52 km 3 span the complete range from 51 to 63 wt sio 2 are relatively crystal poor and were erupted from peripheral vents the total proportions of erupted lava types are 6 7 basalt 18 basaltic andesite and 74 76 andesite there is no rhyolite and only a trace of dacite there is no evidence that any significant magma differentiation occurred in the upper crust along this arc segment based on phenocryst abundances and modes major and trace element data as well as phase equilibrium experiments from the literature the following mechanisms for formation of andesite in this region can be ruled out 1 crystal fractionation of basalt basaltic andesite in the upper crust 2 magma mixing of basalt basaltic andesite with dacite rhyolite and 3 assimilation of granitoid by basalt basaltic andesite instead the same body of evidence indicates that the andesites were formed primarily in the deep crust through partial melting 20 of hornblende rich 40 gabbronorite the basaltic andesites are required owing to their elevated ni concentrations to be hybrids of the andesitic partial melts with basaltic residual liquids that have undergone variable two pyroxene fractionation in the deep crust

pleistocene to recent alkalic volcanism in the region of sanganguey volcano nayarit mexico forty five cinder cones and associated lava flows have erupted within the last 300 000 years along five parallel lines through the calc alkaline volcano sanganguey in the northwestern segment of the mexican volcanic belt lavas erupted from these cinder cones include ne and hynormative alkali basalts hawaiites mugearites and benmoreites it is unusual that this suite has erupted in a calc aikaline volcanic belt where volcanoes in the vicinity have been erupting calc alkaline andesites dacites and rhyodacites

geology of los azufres caldera mexico and its relationships with regional tectonics ferrari l gardufio v h pasquar6 g and tibaldi a 1991 geology of los azufres caldera mexico and its relationships with regional tectonics in s p verma editor calderas genesis structure and unrest j volcanol geotherm res 47 129 148 the los azufres geothermal field is one of several silicic centres of the mexican volcanic belt mvb for which a caldera structure was suggested geological and structural surveys in a wide area surrounding los azufres reveal that this complex is situated in an area of unusual concentration of acid volcanism consisting of four pyroclastic units and several dome complexes although a complete caldera structure is not morphologically evident several facts suggest a long collapse history at los a zufres the geothermal field lies at the centre of a subcircular depressed area 27 26 km in size filled by a distinctive fluviolaeustrine sequence middle to late miocene rocks bound to the south and to the north of this depression and are encountered only at depth inside it four large ignimbritic suites of latest miocene and pliocenc age outcrop outside the depression volume estimations of these pyroclastic products are comparable with the missing volume of the depressed area no alternative caldera structures exist in a radius of 200 km from los azufres dacitic to rhyolitic lavas principally extruded as dome complexes were emplaced inside this depression during the pleistocene los azufres is therefore interpreted as a nested caldera of latest miocene and pliocene age the recent regional tectonic evolution of the central sector of mvb comprises a late miocene early pliocene left lateral transcurrent phase followed by a late pliocene quatern ary transtensional one the silicic volcanism occurring between 6 1 and 2 8 ma can be linked to the first phase while in the adjacent areas of mvb a volcanic hiatus can be recognized the following transtensional phase reached the los azufres area only during the pleistocene and disrupted the caldera structure normal faults developed during this period controlled the uprising of basic magma which partly interacted with the remaining differentiated one and produced part of the recent intracaldera cycle

influence of topography on wind speed over a coastal dune and blowout system at jockey s ridge nc usa this study examines the spatial distribution of wind speed across a coastal dune system located at jockey s ridge state park north carolina the study area consists of a trough blowout through a foredune ridge and the landforms that have developed behind the foredune wind speed and direction were measured simultaneously with single sensors placed at a fixed height in 13 locations across the blowout dune complex fractional wind speed up is computed for sampling stations using data from a mast located on the beach as the reference results show that wind speeds were generally accelerated across the study site the highest speeds were recorded on the foredune ridges adjacent to the blowout wind was accelerated through the center of the blowout throat and along the downwind lateral wall further into the blowout at the base of the ramp to the depositional lobe higher wind speeds shifted to the upwind lateral wall and continued to accelerate up the ramp as air exited to the rear significant variations in the wind speed up pattern were associated with different wind approach angles with greater speed up occurring when the winds were aligned normal to the dune system the speed up decreased as the angle of approach became increasingly oblique to the ridge the patterns of wind speed up across the site point to the influence of topography on airflow to quantify the relationship measures of several topographic variables were obtained along sample transects running upwind from each sample station along flow lines representing different wind approach angles examination of correlation coefficients between wind speed up and topographic variables suggests that for groups of stations with similar topographic characteristics 30 50 of the variations in speed up may be explained by the upwind topographic variability copyright 2014 john wiley sons ltd

tectono stratigraphic terranes and mineral resource distributions in mexico about 80 of the southern part of the north american cordillera within the republic of mexico is made up of suspect terranes these terranes are suspect because their paleogeographic setting with respect to cratonic north america at various times through much of phanerozoic time is uncertain much of northeastern and southeastern mexico is underlain by basement accreted during late paleozoic time an extension of the appalachian ouachita orogeny this orogen has been considerably modified by jurassic strike slip translations related to the opening of the gulf of mexico western and southwestern mexico is largely made up of several distinct but coeval latest jurassic to late cretaceous submarine magmatic arc terranes with unknown basement that appear to have accreted against the disrupted north american margin by early tertiary time only northeastern sonora and the state of chihuahua appear to be floored by unmoved north american cratonic basement the combined effect of mesozoic accretions and translatio

rates of pedogenic processes in volcanic landscapes of late pleistocene to holocene age in central mexico abstract the transmexican volcanic belt and its many volcanic fields of different ages offer good opportunities to study soil development on volcanic tephra of intermediate to basaltic composition we studied a soil chronosequence within the sierra chichinautzin volcanic field scvf south of the basin of mexico and aimed to establish the rates of pedogenic processes this field has been active for around 50 000 years and produced 221 cinder cones with their respective lava flows we selected 11 sites located on 14 c dated lava flows of ages between 1800 and 30 500 bp at 3100 3200 m above sea level covered with pine fir forest with an ustic soil moisture regime and an isomesic soil temperature regime we also included a younger site 1000 bp and three older sites 100 000 years two at 3100 masl and one at 2600 masl from nearby volcanic fields to widen the time frame of the chronosequence soil profile samples were analysed for total organic carbon as well as for mineral neoformations related to clay contents selective chemical extractions and x ray diffraction analyses within the scvf the total soil thickness carbon accumulation and al si and fe extracted with acid ammonium oxalate increased linearly with age on surfaces up to 10 000 years old at rates of 19 cm ky 1 4 1 kg c m 2 ky 1 4 6 kg al 2 7 kg si and 2 kg fe m 2 ky 1 respectively crystalline clay and iron oxide formation reach a maximum at the oldest site located at 2600 masl of 1650 kg clay m 2 and 70 kg fe d m 2 their increase is linear up to ages of 10 000 years at rates of 22 kg clay m 2 ky 1 and 2 8 kg fe d m 2 ky 1 thereafter the rates are much slower allophane and allophane like minerals dominate in the clay fraction at all sites of the chronosequence and small amounts of halloysite can be identified in soils older than 6200 years while kaolinite was only identified at the three oldest sites 100 000 years the linear increases of all indicators of pedogenesis in the first 10 000 years are presumably driven by recurrent tephra deposition during this time frame thereafter erosion and colluviation processes seem to disturb pedogenesis changing its rates and redistributing its products in the landscape

40ar 39ar chronology of the leucite hills wyoming eruption rates erosion rates and an evolving temperature structure of the underlying mantle abstract the lamproite lavas of the leucite hills wyoming comprise an isolated volcanic field of mesas and buttes which erupted onto a thick sequence of k t shales and sandstones volcanic activity spanned the interval from 3 0 to 0 89 ma during which time 3 of magma were erupted approximately 84 of the magma was erupted within a 10 90 ky interval between 0 94 and 0 89 ma with an average eruption rate of 5 m 3 km 2 yr more than an order of magnitude lower than the background trickle eruption rate at continental arcs the eruption rate prior to this burst of activity between 3 0 and 0 94 ma was two orders of magnitude lower at 0 02 m 3 km 2 yr there is a strong correlation r 2 0 99 between the height of the volcanic mesas the volcanic cap protects the underlying sediment from erosion and their eruption age providing an average sediment erosion rate of 0 113 0 002 mm yr over the last 2 5 million years recent seismic studies indicate that the leucite hills volcanic field overlies an abrupt transition in lithospheric structure between the archean craton to the north with fast upper mantle seismic velocities and the colorado plateau to the south with a low velocity zone similar to that beneath east pacific rise spreading ridge the trigger for the leucite hills magmatism may be related therefore to the recent emplacement of asthenospheric mantle immediately to the south and not necessarily to the more distant yellowstone hotspot to the north

neogene volcanism at the front of the central mexican volcanic belt basaltic andesites to dacites with contemporaneous shoshonites and high tio2 lava as part of the continuing study of the young mexican volcanic belt designed to document the ages and types of volcanism from the gulf of california to the valley of mexico a 50 km wide segment of the central part of the belt has been mapped and five types of lava have been found pliocene 3 77 ma shoshonite lava flows k 2 o 2 wt sio 2 52 58 wt form eroded plateaus more than 50 km behind the present volcanic front but in the past 1 m y shoshonites have erupted closer to the volcanic front 300 km from the middle america trench the shoshonite lava type is the most enriched in ba sr and zr of the suite and plagioclase phenocrysts are absent presumably because of high contents of dissolved water 3 5 wt quaternary shield volcanoes and several cinder cones with small volume lava flows are composed of high tio 2 lavas 1 2 wt which have 51 57 wt sio 2 9 ppm and zr 160 230 ppm pliocene high tio 2 lava is found within the eroded plateaus located 50 km behind the present volcanic front quaternary basaltic andesite 52 57 wt sio 2 with up to 10 wt mgo is found at the volcanic front along with more siliceous andesite 57 63 wt sio 2 representatives of the siliceous andesites 57 63 wt sio 2 are free of plagioclase phenocrysts low in al 2 o 3 15 7 wt but rich in mgo 5 wt experiments reported elsewhere suggest that the magmas contained 3 7 wt dissolved water this andesite erupted along a normal fault between 0 3 and 0 005 ma and it is associated with dacite similarly lacking plagioclase phenocrysts but having comparatively abundant pyroxene other dacite in the zitacuaro area is richly porphyritic having plagioclase and hornblende this dacite forms clusters of steep sided domes or widespread pyroclastic deposits and the latest eruptions dated by radiocarbon and k ar occurred between 0 03 0 05 ma estimates of the volume of magma erupted in the zitacuaro valle de bravo region in the past 1 m y 1 8 km 3 m y 1 km 1 show that this area is somewhat less productive per 1 km of arc than those to the west 3 5 km 3 m y 1 km 1 in the michoacan guanajuato volcanic field mgvf a large volume of dacite has erupted in the zitacuaro valle de bravo region which is rare in the mgvf the cone density in the zitacuaro valle de bravo region 2 1 100 km 2 is only slightly lower than the 2 6 cones 100 km 2 found in the mgvf

basaltic volcanism and extension near the intersection of the sierra madre volcanic province and the mexican volcanic belt three groups miocene pliocene and pleistocene off oceanic type basalts associated with normal faulting have been identified in an area surrounding the city of guadalajara mexico although most basalts within these groups have compositional characteristics of an asthenospheric source each group is also associated with lavas that have subduction related traits the first group the san cristobal plateau basalts has a minimum volume of 1 800 km 3 of predominantly alkali olivine basalt and lesser basaltic andesite and fills a pre existing extensional basin of unknown configuration 10 m y ago the relatively rapid eruption of these basalts along with their volume structural association and chemistry suggest an upwelling mantle plume beneath the guadalajara region this region may be the earliest indication of the separation of the jalisco block from north america the second basalt group the guadalajara basalts consists of small volumes of porphyritic basalts and basaltic andesites 3 3 5 0 ma in age that outcrop near the northern outskirts of guadalajara and near the town of hostotip aquillo to the northwest the youngest basalts in the guadalajara area are the 0 4 1 4 ma santa rosa suite of basalts hawaiites mugearites close to the contemporary 0 2 ma andesitic central volcano v tequila these either flowed from nearby cinder cones toward the rio santiago canyon or erupted in the canyon to dam the river the flat lying tertiary 23 27 ma ash flows of the sierra madre volcanic province that lie to the north of guadalajara are faulted north northeast into horsts and grabens that in one case can be dated as being older than 21 8 ma this fault trend has been reactivated closer to guadalajara as normal faults 200 to 300 m displacement cut the miocene plateau basalts but are hidden by a cover of younger volcanic rocks close to the city also hidden by the young volcanic succession there is the north west southeast fault zone that extends to the gulf of california as the tepic zacoalco graben system faulting of this zone is well displayed in the region of the santa rosa dam in the santiago canyon northwest of guadalajara two styles of faulting are found there an older 1 ma 3 has been approximately north northeast for at least 4 m y characteristic of the mexican basin and range province

dynamic processes acting on a longitudinal seif sand dune abstract field measurements were made on a longitudinal dune in the sinai desert in order to understand its morphology and dynamics the field measurements contradicted the wind structure indicated by the helicoidal flow theory rather it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank the essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line the occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line when this angle is 40 the velocity of the deflected wind drops and deposition takes place on the lee flank the angle of incidence in each wind storm is changed intermittently between 30 and 100 along the dune because the dune meanders and because of the sinuous outline of the crest line in this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line as a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month peaks and saddles along the crest line advance at an average rate of 0 7 m per month the lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition this can explain the formation of peaks along the crest line of the dune

airflow up the stoss slope of sand dunes limitations of current understanding the windward stoss side of a sand dune acts as a streamlined obstacle in the path of the wind continuity principles necessitate compression of the flow field up the stoss slope of a dune and shear stress must progressively increase as the flow accelerates measurements in transverse flow over thirteen dunes at padre island texas the algodones california and white sands new mexico confirm that velocity profiles on the stoss slope are not log linear and that flow acceleration occurs very close to the surface within an internal boundary layer as a consequence in the overlying flow where measurements have historically been made an overall decrease in shear stress occurs up the slope the actual shape of the velocity profiles and the identification of the appropriate segment of the profile from which to derive the shear stress that drives saltation represent major problems not approachable by traditional means

jemez mountains volcanic field new mexico time term interpretation of the cardex seismic experiment and comparison with bouguer gravity the jemez mountains volcanic field jvf is centered astride the western flank of the rio grande rift in north central new mexico near the center of the volcanic field are the valles and toledo calderas the sites of two cataclysmic eruptions of silicic tephra and subsequent caldera collapses these calderas are among the largest and most recent 1 1 and 1 4 ma of the world s giant volcanic calderas the valles caldera especially has associated highly developed hydrothermal systems and may still contain residual magma within a cooling compositionally zoned pluton at shallow crustal depths the main goal of our study was to estimate the complex shallow crustal structure beneath the jemez volcanic field using data from a seismic refraction experiment that encompassed the central part of the field a time term technique was used to process first arrival time data from the network of temporary seismic stations of the caldera and rift deep seismic experiment cardex refraction experiment the data set consisting of 290 time distance pairs was processed to yield time terms at 84 locations 78 recording stations and 6 explosion sites the p wave velocity of the precambrian basement refractor was estimated to be 5 86 0 01 km s station time terms are highly correlated to bouguer gravity anomalies and thus tend to corroborate models for which the crystalline basement outcrops along the western margins of the rift and western flank of the jvf and dips generally eastward to depths of at least 600 m below sea level beneath sediment filled axial basins of the rift

discussion and reply neogene volcanism at the front of the central mexican volcanic belt basaltic andesites to dacites with contemporaneous shoshonites and high tio2 lava discussion we read with interest the contribution by blatter et al 2001 1 on magmatism in the central part of the mexican volcanic belt coexisting subalkaline and alkaline oceanic island basalt oib type magmas in both space and time have been reported all along the belt blatter et al 2001 1

stress pattern evolution in the central sector of the mexican volcanic belt abstract stratigraphic studies and detailed field analyses of brittle and plastic deformations allow the reconstruction of the stress pattern and the tectonic evolution of the central sector of the mexican volcanic belt several compressional phases affected the metamorphic basement while the last one also involved the volcanic cover of early miocene age in middle miocene early pliocene times and ene wsw extensional phase developed representing the southern protraction of the basin and range province of the western united states in pliocene quaternary times there then followed the development of a complicated deformational cycle showing the superficial evidence of a sinistral lateral shear zone at the beginning the area was affected by a ne sw compression including reverse dextral and sinistral strike slip faulting this was followed by early pleistocene sinistral transtension characterized by a nw se direction of least principal stress and sinistral normal slip faulting during late pleistocene holocene times the transtension was concentrated along e w faults tectonic control in the distribution of volcanism has also been recognized the basin and range phase controlled the deposition of the miocene volcanic and clastic units particularly the effusion of basaltic lavas connected with the latest extensional movements of the same phase the mexican volcanic belt sequence is strictly linked with the development of a large e w sinistral lateral shear zone active from the pliocene to the present which shows the extensional component progressively prevailing over the shear one

the pleistocene cinder cones surrounding volcan colima mexico re visited eruption ages and volumes oxidation states and sulfur content located at the volcanic front in the western mexican arc in the colima rift is the active volcan colima which lies on the southern end of the massive 450 km3 colima nevado volcanic complex along the margins of this andesitic volcanic complex is a group of 11 scoria cones and associated lavas which have been dated by the 40ar 39ar method nine scoria cones erupted 1 3 km3 of alkaline magma basanite leucite basanite minette between 450 and 60 ka with 99 between 240 and 60 ka two additional cones both the oldest and calc alkaline erupted 0 003 km3 of basalt 0 5 ma and 0 003 km3 of basaltic andesite 1 2 ma respectively cone and lava volumes were estimated with the aid of digital elevation models dems the eruption rate for these scoria cones and their associated lavas over the last 1 2 myr is 1 2 km3 myr which is more than 400 times smaller than that from the andesitic colima nevado edifice in addition to these alkaline colima cones two other potassic basalts erupted at the volcanic front but 200 km to the ese near the historically active volcan jorullo and were dated at 1 06 and 0 10 ma these potassic suites reflect the tendency in the west central mexican arc for magmas close to the volcanic front to be enriched in k2o relative to those farther from the trench

relations between dune morphology air flow and sediment flux on reversing dunes silver peak nevada relations between wind speed sediment flux and dune morphology were measured for two reversing dunes situated in the south western part of the silver peak dunefield in clayton valley west central nevada the larger dune was 120 m in length with a height of 12ae5 m and the smaller dune 80 m long and 6 m high both dunes were sharp crested aligned approximately e w perpendicular to the dominant wind direction and had slightly concave stoss profiles twenty seven rotating cup anemometers were placed 0ae3 m elevation along n s transects on each of the dunes at each anemometer site a passive wedge shaped sediment trap was used to measure sediment flux amplification of wind speed was observed towards the crest on the stoss side of both dunes with speed up factors ucrest ubase ranging from 1ae50 3ae19 with a corresponding increase in sediment flux by 1 2 orders of magnitude in general the ratio of crest flux to base flux qc qb increased with increasing incident basal wind speed on both dunes direct measurements of the stoss slope variation in sediment flux relative to the dune crest are in good agreement with owen s transport model friction speed u was approximated from near surface 0ae3 m point wind speed although not all assumptions of the owen model are upheld the modified model performance is sufficiently robust to predict short term variation in stoss sediment flux on the study dunes improved models that adequately account for variation in sediment flux under changing air flow and transport conditions are necessary for the prediction of longterm evolution of dunes in this regard further progress in model development will require increased understanding of the spatial and temporal variability of airflow and the short term response of sediment flux to these flow conditions

the los azufres caldera mexico the result of multiple nested collapses reply to a comment by robin and pradal in a recent paper ferrari et al 1991 we tried to reconstruct the geology of the los azufres silicic centre in the framework of the geo structural evolution of the surrounding region our paper was based on new geologic geochronologic and structural investigations carried out in a larger area of the central mexican volcanic belt pasquarb et al 1991 as a consequence of our results we quest ioned the late pleistocene resurgent caldera model of pradal and robin 1985 and suggested a more complex geologic scenario than the model proposed by those authors ferrari et al 1991 p 130 although the studies of robin and pradal in the los azufres area seem extensive at the moment of writing only their short 1985 paper was available so we based our critique on that preliminary presentation some new data and interpretations anticipated in their present comment partly modify their previous view we look forward to a paper by robin and pradal on los azufres which will present their data in a more complete form and contribute to the understanding of this complex volcanic structure

reworked old crust derived shoshonitic magma the guarany pluton northeastern brazil abstract the 572 ma guarany stock consists of magmatic epidote bearing hornblende monzodiorite to biotite granite that intruded paleoproterozoic orthogneisses about 10 km inland from the coast in northeastern brazil co magmatic diorite enclaves and dikes are abundant throughout the pluton the monzodiorite granite pluton and diorite enclaves are shoshonitic and display continuous trends in variation diagrams they display chemical and isotopic characteristics of crustal melts such as enrichment in incompatible elements high back calculated initial 87 sr 86 sr ratios avg 0 71253 negative end 0 57ga values avg 14 58 as well as high and variable 9 1 to 11 1 vsmow 18 o zircon values correlations between o isotope and whole rock silica contents as well as initial 87 sr 86 sr ratios with 1 sr concentrations suggest hybridization of a lower continental crustal melt with more felsic crustal rocks concomitant with fractional crystallization amphibole chemistry and whole rock zr tio 2 and p 2 o 5 contents suggest magma solidification at a pressure 7 kbar and near liquidus temperature 900 c the parental magma was likely formed by partial melting of old tdm 2 0 ga amphibolitic lower continental crustal rocks in a post collisional setting probably triggered by underplating of mantle derived mafic magma during the period of relaxation after collision

the influence of topography and approach angles on local deflections of airflow within a coastal blowout the spatial variability of air flow through complex topography is an important but not fully understood component of dune development and dynamics this study examines the spatial variability of the wind field in a linear blowout in coastal dunes at jockey s ridge state park on the outer banks of north carolina a spatial array of single height anemometers and wind vanes were placed within the blowout topography exerted a significant steering effect when onshore winds approached from directions within 50 of the blowout axis under those conditions wind flow in the blowout aligned to the axis regardless of approach angle maximizing the potential for erosion and transport in the trough in other locations aspect variations caused deflection both proportional and disproportional to changes in the approaching wind when prevailing winds approached from directions more oblique than 50 to the blowout axis topographic steering through the blowout trough was reduced and secondary flow generated by flow separation over the trough became more prominent during those approach angles wind directions and speeds within the upper blowout trough became erratic as vortices and turbulence dominated the flow minimizing transport potential the changing characteristics of airflow in the blowout relative to differing approach angles has implications on dune development and variations in transport potential under changing conditions copyright 2013 john wiley sons ltd

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

variations in wind velocity and sand transport on the windward flanks of desert sand dunes the magnitudes of increases in wind velocity or speed up factors have been measured on the windward flanks of transverse and linear dunes of varying height on transverse dunes velocity speed up varied with dune shape and height for linear dunes speed up factors varied principally with wind direction relative to the dune with dune shape and dune height the main effect of velocity speed up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas this is greatest for large dunes with winds of moderate velocity blowing at a large angle to the dune changing ratios of base to crest sand transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase on linear dunes the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi directional wind regime

contemporaneous calc alkaline and alkaline volcanism at sanganguey volcano nayarit mexico sanganguey volcano located in the northwestern segment of the mexican volcanic belt is a calc alkaline stratovolcano that has erupted a chemically continuous suite of andesites and dacites during the past 200 000 yr alkalic volcanism however appears to have been predominant in the area since the pliocene alkalic basalts and their derivatives erupted along extensional fractures trending northwest through sanganguey and continued to erupt through the flanks of the stratocone following cessation of calc alkaline activity the calc alkaline andesite dacite volcanism was focused beneath sanganguey and appears to have been controlled by crystal fractionation in a shallow magma chamber no basalts that erupted in the area have chemical compositions that could be parental to the sanganguey calc alkaline suite the most basic calc alkaline rocks basaltic andesites have incompatible trace element concentrations nearly identical to the most basic alkaline rocks implying that the two suites were derived from distinct parental magmas alternatively it is suggested that stagnation of mantle derived alkalic basalts provided heat input to mobilize lower crustal rocks resulting in the generation of calc alkaline magmas

the geomorphological significance of airflow patterns in transverse dune interdunes abstract the interdunes between aeolian dunes have been relatively ignored when compared with the research attention on the morphodynamics of the dune bodies themselves this neglect is in spite of the possible significance of interdune dynamics for the geomorphology of the sand dune system as a whole especially with regard to dune spacing this paper considers the mean airflow within four relatively simple transverse dune interdunes the study locations were chosen in order to sample interdunes with different size and surface characteristics the dynamics of which were investigated for when incident flow was normal to the upwind crest the findings confirm existing models of flow reattachment length and recovery for aeolian dune lee side flow and show a consistent pattern of increasing near surface velocity downwind of reattachment that supports a mechanism for interdunes as sand free features flow dynamics are characterised for the different types of interdune observed where two groups are recognised the flow patterns in relatively short interdunes where dunes are closely spaced with a sandy surface were accordant with those of the flow response model in extended interdunes where bounding dunes were spaced with a length well over that for flow separation evidence at the downwind edge of the interdunes suggested that flow reacted to the subsequent dune for the case of these extended interdunes a new descriptive model is presented to characterise their dynamics in this model the variation in near surface flow allowed process zones to be identified through the interdune the geomorphological significance of the processes dominating each zone is discussed and comparisons are made between the flow response case and the new interdune model from this study in a discussion on the controls of spacing between dunes where reattachment length exerts a fundamental control the role of sediment availability is also highlighted as a significant factor the presence of a sandy bed can in some circumstances determine whether dune development and therefore spacing is controlled primarily by elements of flow response

long lived magmatic phases at los azufres volcanic center mexico abstract the los azufres volcanic complex has had a long eruptive history of 1 5 myr or more less than 3 myr subsequent to the building of a large miocene and pliocene andesitic complex since about 1 5 ma its development consists of two main periods of volcanic activity the first period from 1 5 to 0 8 ma has two magmatic cycles about 200 000 years in duration characterized by acidic followed basaltic volcanism such a magmatic evolution suggests successive emptyings of a shallow reservoir in which differentiation previously occurred about 0 6 ma major deep magma supplies were probably responsible for the resurgent doming which led to uplift of the southern part of the caldera from 0 6 ma to present the second period exposes volcanic products grading from basalts to rhyolites considering the recent age of the last ignimbrites 26 000 to 29 000 years one can assume that the volcanic activity related to a voluminous differentiated magma body at shallow depth has not yet ended especially in the southern area in and around the resurgent zone occupied by the geothermal field

a model for flow over two dimensional bed forms when non cohesive sediment is set in motion by a unidirectional flow waves of sand often result moreover these waves typically are asymmetrical with steep lee faces that produce flow separation behind each wave a wake region forms which grows in height and decays in strength with distance downstream producing a near bottom acceleration the no slip condition at the bed however requires an internal boundary layer to form beneath the wake region which retards the flow there the interaction of these flow processes produces a local maximum in the boundary shear stress downstream of a bump even over an otherwise flat bed because the erosion rate is proportional to the stress divergence for bed load and weak suspended load transport erosion will occur upstream of this point and desposition will occur downstream of it thus influencing the bottom shape herein the equations of motion are solved for the accelerating internal boundary layer beneath a wake the resulting velocity and boundary shear str

dune and dune field development on padre island texas with implications for interdune deposition and water table controlled accumulation abstract portions of the back island dune fields on padre island texas undergo seasonal destructive and constructive phases in which they are reduced to a nearly planar surface during the winter and then reform during the spring and summer initial depositional sites for dry sand that we observed were associated with roughness elements that caused a lowering of the transport capacity of the flow bedforms develop through a series of morphologic and dynamic stages 1 irregular patches of dry sand 2 wind ripple protodunes with leeward flow expansion 3 grainfall protodunes with a range from leeward flow expansion to flow separation 4 barchan dunes with grainflow and 5 crescentic ridges that characterize the mature field the survival and further development of protodunes and dunes is f vored by increased size a more advanced stage of development unidirectional and moderate wind and a plentiful sand supply these factors can be viewed as the result of the interrelationship of aspects of eolian transport the nature of leeward secondary airflow patterns and sand supply dune field ordering occurs by repeated merging splitting cannibalization and lateral linking of dunes on padre island and other areas of dunes that we studied flow conditions across portions of interdune flats favor the transport of dry loose sand from interdune flats onto the dunes so that dunes grow at the expense of interdune flats dry surface deposits are therefore unlikely to accumulate on interdune flats but rather to characterize interdune depressions that may represent the minimum d stance to which an interdune flat can be closed otherwise accumulations on interdune flats most commonly should be formed on wet or damp surfaces or occur associated with some mechanism that protects the deposits from deflation padre island dune fields represent a type of eolian system where accumulations of dunes and interdune flats occur during periods of a water table rise with bypassing and deflation occurring as the water table becomes static or falls respectively

minette lavas and associated leucitites from the western front of the mexican volcanic belt petrology chemistry and origin during the late pliocene k rich minette and leucitite lavas erupted in the western mexican volcanic belt near the town of los volcanes a region which is located much closer to the middle america trench than the main line of currently active andesite stratovolcanoes during this period the tectonic regime in western mexico was highly complex due to the simultaneous occurrence of active subduction of the young rivera plate and rifting caused by crustal extension most of these basic lavas contain phenocrysts of phlogopite augite and apatite along with microphenocrysts of leucite and fe ti oxide olivine is absent from all but two of the flows one an olivine leucitite and the other a felsic minette the phlogopite phenocrysts and high whole rock fe2o3 feo ratios which are characteristic of this suite record evidence of high magmatic water contents and oxygen fugacities all of these rock types are highly enriched in the large ion lithophile and light rare earth elements with sr 5100 ppm ba 4800 ppm and ce 330 ppm they are also mildly enriched in the high field strength elements e g zr 260 700 ppm and display the strong relative enrichment of the lile over the hfse that is characteristic of magmas erupted in convergent margins consideration of high pressure phase equilibria in the mg2sio4 camgsi2o6 kalsio4 sio2 system shows that the minettes from this region are not related through fractional crystallization to the more mgo rich olivinebearing minettes which have erupted in other parts of the western mexican volcanic belt during the quaternary this conclusion is consistent with both the trace element geochemistry of these lavas and with the results of fractional crystallization models instead the data suggests that these high k magmas were derived from a source region which consists predominantly of phlogopite clinopyroxene and apatite and which has formed through hydrous enrichment of the subarc mantle in response to subduction

a potassic volcanic front in western mexico the lamprophyric and related lavas of san sebastian the superposition of two tectonic regimes rifting and subduction in western mexico has led to the development of a lamprophyric volcanic front 70 km closer to the middle america trench than is the main axis of andesitic volcanism the san sebastian cindercone field represents the northernmost extent of lamprophyric activity in western mexico and lies 40 km trenchward of volcan ceboruco a historically active andesitic stratovolcano the predominant country rocks are cretaceous silicic ash flows although marine sedimentary rocks emplaced between 45 and 115 ma also occur minette absarokite and kersantite lavas were erupted between 0 48 and 0 26 ma in the san sebastian field in close association with flows of basaltic andesite and hornblende andesite there is a gradual transition observed between the two intermediate lava types basaltic andesite and kersantite as plagioclase is suppressed and hornblende and phlogopite are stabilized in melts of intermediate composition this transition reflects the increasing influence of water during crystallization and documents a wide range of hydrous conditions examination of the primary lamprophyric lava types absarokite and minette reveals a remarkable range in their oxidation states if these lavas reflect their mantle source conditions then the redox state of western mexico9s sub arc mantle spans the interval between the ni nio and magnetite hematite buffers a variation in fo 2 of five log units the oxidation and hydrous enrichment of mexico9s sub arc mantle probably formed in response to subduction the eruption of lamprophyric lavas in front of the andesitic suite of the main axis however may have depended upon rifting 1 to trigger melting of the modified mantle and 2 to provide an easy passage to the surface through crustal thinning and block faulting

potassium argon geochronology of a basalt andesite dacite arc system the mount adams volcanic field cascade range of southern washington high precision k ar dating and detailed mapping have established an eruptive chronology for a cascade stratovolcano and its surrounding array of coeval basaltic centers mount adams is a 200 km 3 andesitic cone that stands at the center of a coeval 1250 km 2 quaternary volcanic field that contains 60 discrete vents k ar ages were measured for 74 samples from 63 of the 136 volcanic units defined in the course of the mapping within analytical error there are no discrepancies between k ar ages and stratigraphic sequence major activity began in the area ca 940 ka and inception of the central stratovolcano took place at ca 520 ka a plot of cumulative volume erupted versus time shows that between 940 and 520 ka the eruptive rate was 3 k y and 80 of the products were basaltic andesites are volumetrically dominant and were emplaced in three main cone building episodes centered at 500 450 and 30 ka at eruptive rates of 1 6 5 km 3 k y at a lower rate of 0 05 0 1 km 3 k y the magmatic system remained almost continuously active between the main pulses although breaks in activity as long as 30 k y are permitted by the k ar data andesitic dacitic activity in the focal region and dominantly basaltic activity on the periphery have coexisted for 520 k y and their products are interstratified the last main episode of cone construction occurred ca 40 10 ka the oldest andesite identifiably derived from within the present day edifice having an age of 33 14 ka andesites forming the south summit rim and the true summit have ages of 13 8 ka and 15 8 ka respectively the time volume composition data bear upon several fundamental questions concerning the long term behavior of arc volcanoes stratovolcanoes commonly grow in spurts but can stay active between the widely spaced episodes of peak productivity large stratocone systems may remain active for half a million years subdivision of complex stratovolcanoes into eruptive or constructional stages without detailed mapping geochronology and compositional data should be treated skeptically discussion of volumetric eruptive rates can be misleading without an adequate time scale stratovolcanoes need never develop large upper crustal magma chambers basalt erupts peripherally but its ascent is suppressed beneath stratovolcanoes by deep crustal domains of magma storage and hybridization that form where concentrated injection of basalt amplifies crustal melting

toward a model for airflow on the lee side of aeolian dunes the interaction between dunes and the primary wind results in a complex pattern of secondary airflow on the lee side of dunes from 15 dunes studied during transverse flow conditions at padre island in texas white sands in new mexico and the algodones in california distinct flow regions can generally be recognized with the overall flow structure comparing favourably to that proposed for subaqueous bedforms downwind of dunes with flow separation is a back flow eddy that extends about four dune brink heights downwind from the brink of the dune beyond the separation cell the velocity profiles can be divided into regions based upon segments separated by kinks in the velocity profiles the interior is an area above the dunes of relative high wind speed but low velocity gradient beneath the interior is the wake which consists of two layers the upper wake exhibits an uppermost portion where the flow decelerates while the remainder exhibits accelerating flow so that the overall velocity gradient decreases downwind the lower wake exhibits low velocity gradients and wind speeds that accelerate downwind at all heights but primarily near the top of the layer thereby causing the velocity gradient to increase downwind at about eight dune heights downwind the upper and lower wakes equilibrate to a single profile with the kink between them no longer apparent the lowest recognizable region is the internal boundary layer it is recognized by a relatively steep velocity gradient below the wake and never exceeds a few tens of centimetres in height for our data set because of acceleration and increasing shear stress within this layer interdune flats are at least potentially erosional overall the wake and internal boundary layer show a downward transfer of momentum from upper regions so that the flow recovers where flow separation does not occur simple flow expansion down the lee face causes flow deceleration

three dimensional airflow and sediment transport patterns over barchan dunes abstract airflow dynamics and potential sediment transport were measured and modelled across various barchan dune topographies and incident wind conditions modification of near surface flow was recorded simultaneously in three dimensions 3d using dense arrays of high resolution 3d ultrasonic anemometers in situ measurements provided rigorous validation and calibration for computational fluid dynamics cfd modelling measured and modelled results show good agreement between flow velocity directionality and turbulence intensity modelling of characteristic airflow conditions and surface shear stress beyond the instrument locations elucidated airflow dynamics across the entire landform surface at an unprecedented level of detail emergent turbulent airflow patterns were identified in the form of two counter rotating vortices that converge at the dune centreline downwind of the dune crest integrating a sediment transport function with cfd surface airflow allows for the spatial mapping of flux patterns across the entirety of the dune and interdune surface on the stoss slope and laterally along the outer barchan arms there is strong potential sediment flux in response to increased streamwise stress in lee side locations sediment transport remains at above threshold conditions and is redirected in response to complex turbulent vortices identified in the overlying wake zone the precision of the models allows for the identification of complex flow perturbations and associated surface stresses that prove difficult to measure in the field cfd in combination with a sediment transport function is demonstrated to be a useful tool in investigating morphodynamics of mobile dune systems

alkaline and calc alkaline lavas near los volcanes jalisco mexico geochemical diversity and its significance in volcanic arcs a remarkably diverse suite of lavas erupted during the late pliocene at the volcanic front of the western mexican volcanic belt near the town of los volcanes jalisco this region is much closer to the middle america trench than the main axis of quaternary andesite dacite stratovolcanoes and volcanism occurred in a complex tectonic regime involving both subduction of the young rivera plate and transverse crustal extension of the jalisco structural block the variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the mexican volcanic belt other alkaline varieties intermediate between these extremes include absarokites trachybasalts and trachyandesites phlogopite amphibole and apatite are common phenocryst phases whole rock compositions show a wide range of alkali contents e g k2o of 1 0 8 6 wt and typically contain 5 wt mgo mgo ni v and cr show little systematic decrease with increasing sio2 suggesting that these lavas have evolved from primitive mantle derived magmas with a wide range of sio2 contents strong enrichments in incompatible trace elements are observed in all of the lavas sr 700 5100 ppm ba 470 4800 ce 22 325 ppm zr 90 700 ppm as is the relative enrichment of large ion lithophile ba sr rb and light rare earth elements la ce over the high field strength elements ti zr which is typical of magmas in volcanic arcs this enrichment pattern suggests that these magmas come from source regions which contain incompatible element rich phases such as phlogopite amphibole and apatite the petrological and geochemical features of the lavas which occur in the los volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins the complex tectonic regime in western mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub arc mantle

mean flow and turbulence structure over fixed two dimensional dunes implications for sediment transport and bedform stability detailed measurements of flow velocity and its turbulent fluctuation were obtained over fixed two dimensional dunes in a laboratory channel laser doppler anemometry was used to measure the downstream and vertical components of velocity at more than 1800 points over one dune wavelength the density of the sampling grid allowed construction of a unique set of contour maps for all mean flow and turbulence parameters which are assessed using higher moment measures and quadrant analysis these flow field maps illustrate that 1 the time averaged downstream and vertical velocities agree well with previous studies of quasi equilibrium flow over fixed and mobile bedforms and show a remarkable symmetry from crest to crest 2 the maximum root mean square rms of the downstream velocity values occur at and just downstream of flow reattachment and within the flow separation cell 3 the maximum vertical rms values occur within and above the zone of flow separation along the shear layer and this zone advects and diffuses downstream extending almost to the next crest 4 positive downstream skewness values occur within the separation cell whereas positive vertical skewness values are restricted to the shear layer 5 the highest reynolds stresses are located within the zone of flow separation and along the shear layer 6 high magnitude high frequency quadrant 2 events ejections are concentrated along the shear layer kelvin helmholtz instabilities and dominate the contribution to the local reynolds stress and 7 high magnitude high frequency quadrant 4 events occur bounding the separation zone near reattachment and close to the dune crest and are significant contributors to the local reynolds stress at each location these data demonstrate that the turbulence structure associated with dunes is controlled intrinsically by the formation magnitude and downstream extent of the flow separation zone and resultant shear layer furthermore the origin of dune related macroturbulence lies in the dynamics of the shear layer rather than classical turbulent boundary layer bursting the fluid dynamic distinction between dunes and ripples is reasoned to be linked to the velocity differential across the shear layer and hence the magnitude of the kelvin helmholtz instabilities which are both greater for dunes than ripples these instabilities control the local flow and turbulence structure and dictate the modes of sediment entrainment and their transport rates

volcanic stratigraphy of the los azufres geothermal area mexico abstract the los azufres volcanic center located 200 km northwest of mexico city is one of a number of pleistocene silicic volcanic centers with active geothermal systems that lie north of the axis of the mexican neovolcanic belt silicic volcanism began approximately 1 m y ago with eruption of the agua fria rhyodacite to high silica rhyolite lava domes and flows they are covered by rhyodacite and dacite lava domes and flows of the san andres volcano dated at 0 3 m y a north to northeast trending set of high angle normal faults is offset by a younger and more prominent east trending set that parallels the principal regional structural trend these faults cut all but the youngest volcanic rocks of the center the yerbabuena rhyodacites to high silica rhyolites these morphologically well preserved lava domes are approximately 0 15 m y old and are probably associated with air fall tuffs that blanket the western portion of the volcanic center two episodes of high heat flow suggested by studies of drill core samples and fluids from the los azufres geothermal field may correspond to two different episodes of magma injection to high levels the most recent one resulting in eruption of the yerbabuena lavas

hawaiites and related lavas in the atenguillo graben western mexican volcanic belt hawaiite and mugearite alkali olivine basalt and basaltic andesite have erupted from three shield volcanoes in the atenguillo graben jalisco mexico the youngest shield volcano la laja 0 66 ma is the focus of this study basal pyroclastic surge deposits of basaltic ash lapilli and blocks are overlain by seven to eight 5 to 10 m thick hawaiite flows pillow lavas along a 4 km stretch on the southeast flank of the volcano and lake bed sediments define a shoreline and lake bottom southward to the town of atenguillo eruption of la laja apparently dammed the northward flowing atenguillo river and formed a lake of as much as 100 m depth subsequent fluvial erosion has destroyed the lava dam exposing the shield succession discharge rates for the atenguillo river and rainfall estimates for the atenguillo basin both suggest that this lake formed within 20 to 40 yr eruption of nearly 10 km 3 of material within 10 to 100 yr indicates an output rate of 0 1 to 1 km 3 yr which is larger than any rate calculated for volcanoes in the mexican volcanic belt mvb but similar in magnitude to that of mauna loa and kilauea volcanoes hawaii the older volcanoes el vigia 2 7 ma and tin cleto 3 6 ma are composed of basaltic andesite and alkali olivine basalt respectively the composition of these lavas is unusual for a continental arc setting total feo tio 2 and alkali concentrations are higher than those of basic calc alkaline lavas from the western mvb alkaline earth light rare earth and light rare earth high field strength element ratios in these lavas are low relative to those of calc alkaline lavas but similar to those of oceanic island basalt values such compositional traits coupled with a high magma output rate are characteristic of oceanic island volcanoes both hydrous fluid enriched sub arc mantle and oceanic island type mantle components are present beneath western mexico contemporaneous subduction and extension of this region of the western mvb allow tapping of both sources and thus the eruption of compositionally diverse lavas

the tholeiite to alkalic basalt transition at haleakala volcano maui hawaii previous studies of alkalic lavas erupted during the waning growth stages 0 9 ma to present of haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios these geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components we studied lavas from a 250 m thick stratigraphic sequence in honomanu gulch that includes the oldest 1 1 ma subaerial basalts exposed at haleakaka the lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic sr nd pb and incompatible element abundance ratios e g nb la la ce la sr hf sm ti eu these lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous clinopyroxene rich garnet lherzolite source the intercalated tholeiitic and alkalic honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume and we infer that the tholeiitic lavas reflect a melting range of 10 to 15 while the intercalated alkalic lavas reflect a range of 6 5 to 8 melting however within the uppermost 50 m of section 87sr 86sr decreases from 0 70371 to 0 70328 as eruption age decreased from 0 97 ma to 0 78 ma we infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at 0 93 ma the mixing proportions of source components changed with a morb related mantle component becoming increasingly important as eruption age decreased

evidence for geomagnetic excursions recorded in brunhes and matuyama chron lavas from the trans mexican volcanic belt 1 this study presents paleomagnetic data from 59 independent lava flows from the trans mexican volcanic belt tmvb with ages from 6 4 ma to recent 52 being younger than 1 ma and 11 new 40ar 39ar age determinations most remanence carriers are ti poor titanomagnetite of pseudosingle domain magnetic structure nine lavas contain small amounts of titanomaghemite and four lavas additional titano hematite paleosecular variation of lava flows younger than 1 7 ma is consistent with latitude dependent model g and also in agreement with other pleistocene paleomagnetic data from the tmvb the directional record of brunhes and matuyama chrons lavas was correlated to the geomagnetic polarity timescale and there is evidence for at least four geomagnetic excursions one lava flow dated at 592 20 ka has a fully reversed paleodirection and most likely erupted during the big lost excursion another fully reversed flow dated at 671 12 ka gives new volcanic evidence for the delta stage 17 excursion this excursion is supported by a reversed intermediate direction of another flow from a different volcanic field but of very close age of 673 10 ka from the matuyama age lavas one flow with normal polarity magnetization dated at 949 37 ka could either be related to the kamikatsura or the santa rosa excursion and a normal polarity flow dated at 1628 56 ka could have been emplaced during the gilsa excursion the results presented here confirm in one case but disagree in four cases with results presented in two previous studies of the same lava flows and interpreted as geomagnetic excursions

fluid sources for the la guitarra epithermal deposit temascaltepec district mexico volatile and helium isotope analyses in fluid inclusions the la guitarra deposit temascaltepec district south central mexico belongs to the low intermediate sulfidation epithermal type has a polymetallic character although it is currently being mined for ag and au the mineralization shows a polyphasic character and formed through several stages and sub stages named i iia iib iic iid and iii the previous structural mineralogical fluid inclusion and stable isotope studies were used to constrain the selection of samples for volatile and helium isotope analyses portrayed in this study the n2 ar overall range obtained from analytical runs on fluid inclusion volatiles by means of quadrupole mass spectrometry qms is 0 to 2526 and it ranges 0 to 2526 for stage i 0 to 1264 for stage iia 0 to 1369 for stage iib 11 to 2401 for stage iic 19 to 324 for stage iid and 0 to 2526 for stage iii these values combined with the co2 ch4 ratios and n2 he ar and n2 ch4 ar relationships suggest the occurrence of fluids from magmatic crustal and shallow meteoric sources in the forming epithermal vein deposit the helium isotope analyses obtained by means of noble gas mass spectrometry display r ra average values between 0 5 and 2 pointing to the occurrence of mantle derived helium that was relatively diluted or contaminated by crustal helium these volatile analyses when correlated with the stable isotope data from previous works and he isotope data show the same distribution of data concerning sources for mineralizing fluids especially those corresponding to magmatic and crustal sources thus the overall geochemical data from mineralizing fluids are revealed as intrinsically consistent when compared to each other the three main sources for mineralizing fluids magmatic and both deep and shallow meteoric fluids are accountable at any scale from stages of mineralization down to specific mineral associations the volatile and helium isotope data obtained in this paper suggest that the precious metal bearing mineral associations formed after hydrothermal pulses of predominantly oxidized

an empirical model of aeolian dune lee face airflow airflow data gathered over dunes ranging from 60 m tall complex crescentic dunes to 2 m tall simplecrescentic dunes were used to develop an empirical model of dune lee face airflow for straight crested dunes the nature of lee face flow varies and was found to be controlled by the interaction of at least three factors dune shape the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability three types of lee face flow separated attached and deflected along slope or attached and undeflected were found to occur separated flows characterized by a zone of low speed 0 3o of crestal speed back eddy flow typically occur leeward of steep sided dunes in transverse flow conditions unstable atmospheric thermal stability also favours flow separation attached flows characterized by higher flow speeds up to 84 of crestal speed that are a cosine function of the incidence angle typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions depending on the slope of the lee face attached flow may be either deflected along slope lee slopes greater than about 20 or have the same direction as the primary flow lee slopes less than about 20 neutral atmospheric thermal stability also favours flow attachment as each of the three types of lee face flow is defined by a range of wind speeds and directions the nature of lee face flow is intimately tied to the type of aeolian depositional process i e wind ripple or superimposed dune migration grainflow or grainfall that occurs on the lee slope and the resulting pattern of dune deposits therefore the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross strata

field measurements of mean and turbulent airflow over a barchan sand dune advances in our knowledge of the aeolian processes governing sand dune dynamics have been restricted by a reliance on measures of time averaged airflow such as shear velocity u it has become clear that such measures are incapable of explaining the complete dynamics of sediment transport across dune surfaces past evidence from wind tunnel and modelling studies has suggested that in some regions on a dune s surface the sediment transport might be better explained through investigations of the turbulent nature of the airflow however to date there have been no field studies providing data on the turbulent characteristics of the airflow around dunes with which to support or refute such hypotheses the field investigation presented here provides mean and turbulent airflow measurements across the centre line of a barchan sand dune in namibia data were collected using arrays of sonic anemometers and were compared with sand flux data measured using wedge shaped traps results support previously published data derived from wind tunnels and numerical models the decline in mean wind velocity at the upwind toe of the dune is shown to coincide with a rise in turbulence whilst mean velocity acceleration on the upper slope corresponds with a general decline in measured turbulence analysis of the components of reynold shear stress u w and normal stresses u 2 and w 2 supports the notion that the development of flow turbulence along the dune centre line is likely to be associated with the interplay between streamline curvature and mean flow deceleration acceleration it is suggested that due to the nature of its calculation turbulence intensity is a measure of less practical use than direct assessments of the individual components of reynolds stress particularly the instantaneous horizontal streamwise component u 2 and shear stress u w whilst increases in reynolds shear stress and the horizontal streamwise component of stress in the toe region of the dune may effectively explain the maintenance of sand flux in a region of declining mean velocity they have much less explanatory power for sand flux on the upper windward slope and in the crestal region of the dune here it is suggested that mean flow acceleration is likely to provide the most significant driving force on sand flux possibly augmented by a rise in the horizontal streamwise component of reynolds stress u 2 in the crest brink region therefore although wind turbulence is considered to be of fundamental importance in explaining the sediment transport dynamics across the dune s surface it is recognised that the interaction between mean flow deceleration acceleration streamline curvature and individual components of reynolds stress is complex and the identification of a single element of flow that offers a panacea for accounting for sand flux and dune dynamics is difficult to find

explosive rhyolitic volcanism in the jemez mountains vent locations caldera development and relation to regional structure the jemez volcanic field straddles the western margin of the rio grande rift where the rift is intersected by the jemez lineament in north central new mexico the field has a record of volcanism extending back to before 13 ma initial basaltic activity was related to active rifting with minor rhyolitic eruptions occurring along n s rift bounding faults between 10 and 7 ma voluminous andesitic volcanism took place in the central jemez mountains area overwhelming contemporaneous basaltic and rhyolitic magmatism an apparent tectonic lull took place from 7 to 4 ma accompanied by lower eruption rates during this interval dacitic magmas were erupted to form the tschicoma volcanic center but mafic and rhyolitic volcanism virtually ceased since 4 ma accompanying resumption of rifting a growing silicic magma system has been present under the central part of the jemez mountains ultimately evolving to the magma body that produced the voluminous rhyolitic bandelier tuffs explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 ma early eruptions 3 6 2 8 ma produced high silica rhyolite ignimbrites restricted to the southwest part of the jemez mountains formation of these units may have been accompanied by caldera collapse these events were followed by the two caldera forming bandelier tuff ignimbrite eruptions 1 45 and 1 12 ma post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both bandelier events vent and caldera locations for the rhyolitic eruptions during 3 4 ma have been inferred from grain size characteristics dispersal patterns and facies variations in the plinian deposits and ignimbrites lithic breccia zones of the pre bandelier ignimbrites indicate possible caldera sources in the southwest part of the present valles caldera during the eruption of both bandelier tuffs initial plinian falls and early pyroclastic flows emanated from vents centrally located in the jemez mountains in the lower bandelier tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows but there is no strong evidence to suggest such a transition occurred during the upper bandelier eruption calderas associated with the lower and upper bandelier tuffs toledo and valles respectively are almost identical in location as are the plinian vent sites for these two large eruptions the toledo embayment northeast of the valles caldera contains lava domes from up to 3 6 ma and may be a caldera or crater associated with early explosive dacitic volcanism in the tschicoma volcanic center post 1 4 ma lava domes also fill this depression the main volcanic features of the jemez mountains field including the valles caldera complex eruption vents and the apical graben of the post valles redondo resurgent block appear to be aligned along the ne sw trending jemez fault zone this zone the local expression of a precambrian basement feature the jemez lineament has exerted strong control on the location and style of eruptions from the bandelier rhyolitic magma system

a wind tunnel simulation of the effects of stoss slope on the lee airflow pattern over a two dimensional transverse dune 1 secondary airflow plays an important role in dune formation and development the lee airflow pattern over transverse dunes is important in determining the shape alignment and spacing of dunes and is influenced significantly by the lee slope angle in this paper we present the results of scaled wind tunnel simulations of the effects of stoss slope on the mean lee airflow patterns of transverse dunes the leeward velocity field was measured nonintrusively using particle image velocimetry to assess the airflow pattern we found that the flow separated over the dunes with a leeward angle close to the slip face angle of natural transverse dunes and was insensitive to stoss angle separation cells were characterized by a back eddy reattachment distance height area and aspect ratio were used to characterize the separation cell the first three parameters increased with increasing stoss angle for angles less than 15 but remained constant or decreased slightly with angles greater than 15 on the basis of the leeward velocity profiles six flow regions corresponding to those in previous models were recognized these regions are initially bounded by kinks but gradually transition downwind and merge to recover a uniform velocity profile after a significant distance we suggest that the separation cell is most developed when the angle of stoss slope reaches 15 but the significance of this angle in the evolution of transverse dunes needs further exploration

secondary airflow and sediment transport in the lee of a reversing dune lee side windspeed and sediment transport were measured over a small 1 2 m transverse ridge in the silver peak dunefield west central nevada usa using an intensive array of 25 cup anemometers and seven total flux traps during crest transverse and transporting flow conditions u0 3crest 8 4 m s 1 windspeed near the surface of the lee slope averaged half 48 per cent that of crest speeds dimensionless speeds in the separation zone ranged from 0 2 to 0 8 that of the outer flow u12 along the boundary of the separation cell windspeed increased by 10 per cent of the crest speed before separation equilibrium of upper and lower wake regions was not observed by the documented eight dune heights suggesting that wake recovery may not occur over closely spaced dunes sediment transport measured directly on both the lee slope and interdune surfaces averaged approximately 15 per cent of crest inputs this suggests that a significant amount c 70 95 per cent of sediment transported over the crest moved as fallout for this data set flux was approximately proportional to the cube of the near surface windspeed u0 3 and in general there was an order of magnitude difference between flux measured at the crest and that measured within the separation zone transport direction in the separation zone was acutely oblique to the incident direction owing to secondary flow deflection beyond the interdune transport direction progressed from oblique to crest transverse this indicates that an appreciable amount of sediment may move laterally along the lee slope and interdune corridor under crest transverse flows regarding the grain size and sorting properties of transported sediment there was no significant difference in mean grain size over the dune although in general particles were finer and more poorly sorted in the lee copyright 1999 john wiley sons ltd

the los azufres caldera mexico comment on the paper by l ferrari v h garduno g pasquare and a tibaldi or an attempt to understand the volcanic structure

sediment flux and airflow on the stoss slope of a barchan dune abstract measurements of sediment flux on the windward slope of an isolated barchan using an array of 30 sand traps provide new data that can constrain models of dune dynamics the data show that at low wind incident speeds flux increases up the dune exponentially whereas at higher wind speeds the increase with distance approaches linearity wind profile measurements conducted at the same time as the flux measurements indicate that although wind speed at a given height increases by 1 2 times from dune toe to brinkline wind shear velocity derived from the profile data decreases up the dune and is in many cases below transport threshold values this demonstrates that conventional wind profiles derived from anemometry on dunes do not measure the part of the boundary layer that is significant for sediment transport

vibroseis recording techniques and data reduction from the jemez tomography experiment abstract the jemez tomography experiment jtex is a multidisciplinary study focused on the valles caldera and the jemez mountains new mexico the objectives of the project are to create a high resolution crustal model of the subsurface structure of this silicic volcanic system and to develop an interpretation of its volcanic evolution use of vibroseis sources in the acquisition of refraction wide angle reflection seismic data provided challenges beyond conventional explosive source data processing of the jtex vibroseis data is an involved procedure consisting of sorting cross correlating filtering and stacking numerous individual seismograms in the production of final record sections however excellent results high signal to noise seismograms at relatively small spacings are obtainable with coherent arrivals at source receiver distances of more than 60 km the primary drawback in this approach lies in the massive volume of data that is necessary to produce record sections one benefit of the vibroseis source used during jtex was a method to decrease effective seismogram spacing this technique dubbed a source offset technique provides smaller overall seismograph station spacing by moving the vibroseis sources during acquisition and leaving deployed seismographs stationary after station corrections this method effectively decreases station spacings and increases detail in resulting record sections various shallow crustal heterogeneities create travel time advances and delays that affect the source offset data differently than single source data synthetic modeling demonstrates small travel time discrepancies associated with the source offset technique however the addition of traces with smaller station intervals clarifies secondary arrivals within record sections and aids in interpretation of these arrivals with a minimum amount of field effort required

phase equilibria of phlogopite lamprophyres from western mexico biotite liquid equilibria and p t estimates for biotite bearing igneous rocks olivine and augite minette powders have been equilibrated from one bar to nearly 2 0 kbar water saturated and from 900 to 1300 c and then quenched rapidly at oxygen fugacities controlled between the nickel nickel oxide nno and hematite magnetite hm oxygen buffers the liquidus of both samples is suppressed 100 c at water saturated conditions and 1500 bar both lavas contained between 3 and 4 wt water at the stage of phenocryst precipitation the partitioning of ferric and ferrous iron between phlogopite and liquid has been determined on eight samples across 3 log f o2 units when these determinations are combined with previous studies fe2o3 feo total of mg rich biotite can be calculated knowing log fo2 t and xfe thermodynamic modelling of biotite liquid equilibria results in two expressions for calculating activity coefficients for annite and phlogopite in natural biotites based on the partitioning of bao and tio2 between biotite and liquid we have formulated a thermometer and barometer over the range of 400 c tio2 partitioning between phlogopite and liquid is a function of temperature 50 c and is insensitive to pressure and h2o and o2 activities bao partitioning between phlogopite and liquid is a function of both temperature and pressure 4 kbar the latter being most important applying the tio2 and bao partitioning expressions to lamprophyre and lamproite suites shows that mexican minettes equilibrated at low pressures 5 to 15 kbar 4 kbar and temperatures 1090 to 1160 c 50 c while australian lamproites equilibrated at higher p up to 30 kbar 4 kbar and t 1125 to 1400 c 50 c experimental glass compositions and phenocryst fractionation calculations together with the bao and tio2 based pressure calculations indicate that felsic minettes from the mexican suite of lavas can be generated by simple fractionation of a more mafic parent minette at mid to lower crustal pressures

quaternary volcanism near the valley of mexico implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions the valley of mexico and surrounding regions of mexico and morelos states in central mexico contain more than 250 quaternary eruptive vents in addition to the large composite volcanoes of popocatepetl iztaccihuatl and nevado de toluca the eruptive vents include cinder and lava cones shield volcanoes and isolated andesitic and dacitic lava flows and are most numerous in the sierra chichinautzin that forms the southern terminus of the valley of mexico the chichinautzin volcanic field cvf is part of the e w trending mexican volcanic belt mvb a subduction related volcanic arc that extends across mexico the crustal thickness beneath the cvf 50 km is the greatest of any region in the mvb and one of the greatest found in any arc worldwide lavas and scoriae erupted from vents in the cvf include alkaline basalts and calc alkaline basaltic andesites andesites and dacites both alkaline and calc alkaline groups contain primitive varieties that have whole rock mg mgo and ni contents and liquidus olivine compositions fo90 that are close to those expected of partial melts from mantle peridotite primitive varieties also show a wide range of incompatible trace element abundances e g ba 210 1080 ppm ce 25 100 ppm zr 130 280 ppm data for primitive calc alkaline rocks from both the cvf and other regions of the mvb to the west are consistent with magma generation in an underlying mantle wedge that is depleted in ti zr and nb and enriched in large ion lithophile k ba rb and light rare earth la ce elements extents of partial melting estimated from ti and zr data are lower for primitive calc alkaline magmas in the cvf than for those from the regions of the mvb to the west where the crust is thinner the distinctive major element compositions low cao and al2o3 high sio2 of the primitive calc alkaline magmas in the cvf indicate a more refractory mantle source beneath this region of thick crust in contrast primitive alkaline magmas from the cvf and other regions of the mvb show compositional similarities to intraplate type alkali basalts erupted behind the arc in the mexican basin and range province these similarities are consistent with the hypothesis that slab induced convection in the mantle wedge beneath the mvb causes advection of asthenospheric mantle from behind the arc to the region of magma generation trace element systematics of primitive magmas in the mvb reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub alkaline basaltic melts

application of the multispecimen palaeointensity method to pleistocene lava flows from the trans mexican volcanic belt abstract despite the many advances recently made in palaeointensity pi studies more high quality pi data are required for a better understanding of the full vector evolution of the ancient geomagnetic field in this study we applied the recently proposed multispecimen parallel differential ptrm ms method for absolute pi determination to rocks older than those studied hitherto with this method fifty one lava flows from within the trans mexican volcanic belt which are generally younger than 1 ma returned 32 new pi estimates rock magnetic investigations revealed that in most cases the remanence was carried by ti poor titanomagnetites of pseudo single domain magnetic structure although titanomagnetites with higher ti contents and additional contributions of titano haematite were also occasionally present comparisons with reconstructions of the global dipole moment reveal that the pi estimates from this study were on average around 30 higher than expected other mainly thellier type pi data from mexico were also observed to be high relative to global records which could indicate that persistent non dipole features might be responsible for the higher than expected results however the paucity of available data obscures the significance of this observation and the balance of evidence rather suggests an artificial biasing of most measurements towards high values our results seem to corroborate results from previous studies on historical lava flows and synthetic samples in which domain state effects were found to cause overestimates of the pi by up to 30 in the ms method we expect that the degree of the overestimate in the majority of these new ms results is no larger than what might be expected from thellier experiments performed on samples with a similar given degree of multidomain behaviour however seven of the lava flows which were studied here using the ms method were subjected to a preceding demagnetisation step of 200 c in order to erase viscous remanences and these may have produced results which were further biased towards higher intensities this additional bias is expected from theoretical considerations and may represent a challenge to future application of the ms method to older rock units it could however potentially be avoided in future studies by performing measurements of the sample moment at temperatures higher than the blocking temperature of any overprint

the volcanic section at nazas durango mexico and the possibility of widespread eocene volcanism within the sierra madre occidental descriptions of volcanic rocks in the sierra madre occidental of western mexico have thus far emphasized the widespread and nearly continuous cover of ash flow tuffs and other units attributed to the oligocene ignimbrite flare up however much less attention has been given to the pre oligocene volcanic sequence beneath this ash flow blanket at nazas durango on the eastern flank of the sierra madre occidental a well exposed section includes voluminous felsic volcanic rocks of both eocene and oligocene age the oldest igneous rocks at nazas include a small exposure of intermediate volcanic breccias and lava flows that overlie cretaceous limestones and apparently were deformed with them during laramide tectonism more commonly the limestones are overlain by tertiary volcanic rocks and continental clastic deposits the tertiary volcanic section is about 800 m thick not including intercalated fanglomerates the section records three distinct magmatic episodes from 51 to 40 ma comprising felsic ash flow tuffs and intermediate lava flows and domes a pulse at about 30 ma comprising voluminous felsic ash flow tuffs with an aggregate thickness of about 500 m and from 24 to 20 ma comprising alkalic basalts the ahuichila formation a molasse type conglomerate up to 200 m thick that underlies the tertiary volcanic section was deposited during or just after the laramide deformation the santa ines formation is a widespread fanglomerate up to 110 m thick that underlies the basalts and apparently accumulated during normal faulting eocene volcanism in the nazas area is characterized by interfingering felsic ash flow tuffs and intermediate lava flows and domes a similar sequence of eocene volcanic rocks has been mapped in central chihuahua state about 350 km to the northwest of nazas felsic tuffs interbedded with intermediate volcanic rocks older than 40 ma are also exposed at tayoltita durango about 200 km southwest of nazas these occurrences suggest that a wider spectrum of compositions and volcanic styles existed during the eocene than during the oligocene in the sierra madre occidental eocene volcanism was more typical of orogenic magmatic belts developed at continental margins whereas the oligocene activity was dominated by voluminous felsic ash flow tuffs erupted during a transition of tectonic setting from subduction along a continental margin to intraplate extension in addition to these three areas there are several other localities in western mexico having volcanic rocks with reported ages between 40 and 53 ma it is possible that the volcanic field during the eocene in mexico was comparable in extent to that of the oligocene the eocene volcanism in western mexico was in part contemporaneous with the challis volcanic episode of northwestern united states and its extension into western canada however eocene magmatism in the pacific northwest apparently developed in an extensional tectonic setting whereas in mexico no evidence for eocene extensional deformation is known

reconstruction of the volcanic history of the tacambaro puruaran area michoacan mexico reveals high frequency of holocene monogenetic eruptions the 690 km2 tacambaro puruaran area located at the arc front part of the michoacan guanajuato volcanic field in the trans mexican volcanic belt tmvb records a protracted history of volcanism that culminated with intense monogenetic activity in the holocene geologic mapping 40ar 39ar and 14c radiometric dating and whole rock chemical analyses of volcanic products provide insights to that history eocene volcanics 55 40 ma exposed at uplifted blocks are related to a magmatic arc that preceded the tmvb early tmvb products are represented by poorly exposed pliocene silicic domes 5 2 ma quaternary 30 km may be related to high rates of magma production at depth and a favorable tectonic setting

geomorphology of desert sand dunes a review of recent progress through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes as developments with equipment allowed field measurements became more sophisticated however by the mid 1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported most importantly the attempt to measure the stresses imposed by the wind on the sand surface proved impossible and the use of shear or friction velocity as a surrogate for shear stress also failed to deliver at the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux in a development paralleled in fluvial geomorphology aeolian geomorphologists have attempted to measure and model turbulent structures over dunes progress has recently been made through the use of more complex numerical models based on computational fluid dynamics cfd some of the modelling work has also suggested that notions of dune equilibrium form may not be particularly helpful this range of recent developments has not meant that field studies are now redundant for linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground penetrating radar images of the internal structure of the dunes the paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form at least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies more recently attention has shifted although not completely to modelling approaches and very considerable progress has been made in developing models of dune development it is clear however that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology

shape of the subducted rivera and cocos plates in southern mexico seismic and tectonic implications the geometry of the subducted rivera and cocos plates beneath the north american plate in southern mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes the hypocenters of the teleseisms were relocated and the focal depths of 21 events were constrained using a body wave inversion scheme the suduction in southern mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the cocos plate beneath the caribbean plate to the east and of the rivera plate beneath north america to the west the dip of the interplate contact geometry is constant to a depth of 30 km and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate on the basis of the seismicity the focal mechanisms and the geometry of the downgoing slab southern mexico may be segmented into four regions 1 the jalisco region to the west where the rivera plate subducts at a steep angle that resembles the geometry of the cocos plate beneath the caribbean plate in central america 2 the michoacan region where the dip angle of the cocos plate decreases gradually toward the southeast 3 the guerrero oaxaca region bounded approximately by the onshore projection of the orozco and o gorman fracture zones where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km and 4 the southern oaxaca and chiapas region in southeastern mexico where the dip of the subduction gradually increases to a steeper subduction in central america these drastic changes in dip do not appear to take place on tear faults suggesting that smooth contortions accommodate these changes in geometry the inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the trans mexican volcanic belt suggesting it is directly related to the subduction thus the observed nonparallelism with the middle american trench is apparently due to the changing geometry of the rivera and cocos plates beneath the north american plate in southern mexico and not to zones of weakness in the crust of the north american plate as some authors have suggested

ages of epithermal deposits in mexico regional significance and links with the evolution of tertiary volcanism we present an analysis leading to a new space time classification of the mexican epithermal deposits based on new data and interpretations from seven deposits in central mexico and a new age for the la guitarra deposit temascaltepec district from the latter adularia from a barren stage of mineralization and from quartz bands of the main ore stage together with orthoclase phenocrysts from host monzogranites with a known late laramide age were analyzed by the 40ar 39ar method the ages obtained from adularia samples are 32 9 0 1 ma and 33 3 0 1 ma corresponding to the middle part of lower oligocene these ages are slightly younger than that of the ignimbrite succession exposed next to the study area and they suggest that the deposit is related to the crystallization of comagmatic intrusive rocks contrary to previous assumptions of a miocene age for several epithermal deposits in southern mexico the ages from la guitarra show that south of the trans mexican volcanic belt these deposits may be oligocene or older in addition the interpretation of a genetic link between intrusive rocks and epithermal mineralization is consistent with previous work on fluid geochemistry of the deposit to evaluate the obtained ages in a regional context we analyzed the geographic distribution and age of tertiary epithermal deposits in central mexico our study shows that mineral deposits coincide with the main volcanic pulses of the sierra madre occidental the distribution of epithermal deposits can be thus divided into three main groups 1 between 48 and 40 ma 2 between 36 and 27 ma and 3 between 23 and 18 ma within the first group are the batopilas and topia deposits that record the oldest ages found for epithermal deposits in mexico deposits of this age are hosted by the lower volcanic supergroup of the sierra madre occidental and are related to the laramide magmatism of the sierra madre occidental many of the epithermal deposits in mexico occur within the second time span in a northwest southeast belt from chihuahua to the mexico and guerrero states these deposits are related to the main episode of ignimbrite flare up of the sierra madre occidental the third group includes the bolanos san martin de bolanos and pachuca real del monte deposits plus possibly seven other epithermal deposits analyzed in this work these deposits are related to the last ignimbrite flare up of the sierra madre occidental with a general west northwest east southeast distribution recently documented in the southern sierra madre occidental

velocity profiles measured on the windward slope of a transverse dune velocity gradients measured on the windward slope of a transverse dune show that flow acceleration strongly affects the turbulence structure of wind wind profiles measured below a height of 1 6m are nonlogarithmic and the altered nature of velocity profiles varies over the windward slope of a dune these findings suggest that calculated values of shear velocity are largely dependent upon the height and location of wind speed measurements as well as the strength of the prevailing wind moreover if bagnold type transport equations are used to calculate rates of sand transport on a dune wind speeds must be measured within 0 2 m of the dune surface if wind speeds are measured above this height the resulting errors in calculated shear velocity will manifest themselves as errors in calculated rates of sand transport if rates of sand transport are derived from wind speeds measured above 0 2 m the calculated rates may underestimate sand transport on the windward face of a dune as a result of flow acceleration in contrast if a dune has a separate crest upwind of the brink the expansion and subsequent deceleration of flow downwind from the crest may result in an overestimation of sand transport

alkalic ocean island basalt type and calc alkalic volcanism in the mexican volcanic belt a case for plume related magmatism and propagating rifting at an active margin the mexican volcanic belt has been traditionally regarded as a classic case of subduction related calc alkalic volcanism however a series of geologic geophysical and petrological arguments makes this simple relationship doubtful a seismic gap beneath the belt a large scale mantle anomaly a graben triple junction domain and the presence of volumetrically important oceanic island basalt oib volcanism throughout the belt suggest a more complex tectonic scenario involving plume and subduction related processes we here propose a model involving the development of a propagating rift opening from west to east in response to plume activity the process started in miocene time within the western sector of the belt guadalajara and gave rise to a graben triple junction and oib type and calc alkalic volcanism extension and volcanism proceeded to the east giving rise to progressively younger ages for the initiation of oib type volcanism 1 miocene in the west e g guadalajara 2 pliocene in the central zone e g michoacan guanajuato and 3 quaternary farther east e g chichinautzin geochemical evidence suggests that part of the modern calc alkalic volcanism e g chichinautzin may be derived from magma mixing between the oib mafic magmas and silicic crust derived magmas however we do not preclude some influence of the subducting slab in the generation of other e g jorullo calc alkalic volcanic rocks our model suggests a currently unrooted upper plume attached to the subcontinental lithosphere which defines a hot zone beneath the mexican volcanic belt

the role of streamline curvature in sand dune dynamics evidence from field and wind tunnel measurements abstract field measurements on an unvegetated 10 m high barchan dune in oman are compared with measurements over a 1 200 scale fixed model in a wind tunnel both the field and wind tunnel data demonstrate similar patterns of wind and shear velocity over the dune confirming significant flow deceleration upwind of and at the toe of the dune acceleration of flow up the windward slope and deceleration between the crest and brink this pattern including the widely reported upwind reduction in shear velocity reflects observations of previous studies such a reduction in shear velocity upwind of the dune should result in a reduction in sand transport and subsequent sand deposition this is not observed in the field wind tunnel modelling using a near surface pulse wire probe suggests that the field method of shear velocity derivation is inadequate the wind tunnel results exhibit no reduction in shear velocity upwind of or at the toe of the dune evidence provided by reynolds stress profiles and turbulence intensities measured in the wind tunnel suggest that this maintenance of upwind shear stress may be a result of concave unstable streamline curvature these additional surface stresses are not recorded by the techniques used in the field measurements using the occurrence of streamline curvature as a starting point a new 2 d model of dune dynamics is deduced this model relies on the establishment of an equilibrium between windward slope morphology surface stresses induced by streamline curvature and streamwise acceleration adopting the criteria that concave streamline curvature and streamwise acceleration both increase surface shear stress whereas convex streamline curvature and deceleration have the opposite effect the relationships between form and process are investigated in each of three morphologically distinct zones the upwind interdune and concave toe region of the dune the convex portion of the windward slope and the crest brink region the applicability of the model is supported by measurements of the rate of sand transport and the change of the dune surface in the field

the effect of unsteady winds on sediment transport on the stoss slope of a transverse dune silver peak nv usa rapid 10 s measurements of sediment transport and wind speed on the stoss slope of a transverse dune indicate that the majority of sand transported is associated with fluctuations in wind speed with a periodicity of 5 20 min duration increases in the sediment transport rate towards the dune crest are associated with a small degree of flow acceleration the increase in wind speed is sufficient however to greatly increase values of the intermittency index so that the duration of saltation is extended in crestal regions of the dune the pattern of sediment transport on the stoss slope and therefore the locus of areas of erosion and deposition change with the regional wind speed erosion of the crest occurs during wind speed events just above transport threshold whereas periods of higher magnitude winds result in deposition of sand upwind of the crest thereby increasing dune height although short term temporal and spatial relations between sand transport and wind speed on the stoss slope are well understood it is not clear how these relations affect dune morphology over longer periods of time

turbulence fields in the lee of two dimensional transverse dunes simulated in a wind tunnel the turbulence field of airflow in the lee of a dune has significant impacts on dune dynamics and related processes we used particle image velocimetry in a wind tunnel simulation to obtain detailed velocity measurements in the lee of two dimensional transverse dune models then used the results to analyse their turbulence fields the dune models used in this study had a single lee angle of 30 and a total of six stoss angles 3 5 10 15 20 and 25 we used vorticity turbulence intensity reynolds stress and turbulent kinetic energy to characterize the turbulence fields these parameters were functions of stoss angle wind velocity distance from the dune crest and height above the ground surface the stoss angles could generally be divided into two groups based on the profiles of mean velocity turbulence and reynolds stress stoss angles of 3 and 5 usually had similar profiles and angles of 15 20 and 25 formed a second group with similar profiles the profiles for the stoss angle of 10 were usually transitional and were intermediate between the two groups vorticity reynolds stress and turbulent kinetic energy increased monotonically with increasing free stream wind velocity but their variations with respect to the stoss angle were complex the stoss angles of 15 and 20 had the maximum values of these three parameters thus these angles may have special significance in dune development given the characteristics of the mean velocity fields and turbulence fields they produce within the lee airflow it is the streamwise velocity component and its turbulence that determine the surface shear stress copyright 2008 john wiley and sons ltd

the german bank pluton offshore sw nova scotia age petrology and regional significance for alleghanian plutonism permian plutonism is widespread in the u s appalachians but not in the canadian sector of the appalachians the german bank pluton sampled in two boreholes offshore southwestern nova scotia gave a u pb age of 300 1 ma determined by isotope dilution thermal ionization mass spectrometry id tims on thermally annealed and chemically etched zircon grains electron microprobe dating of monazite yielded an age of 308 13 ma late paleozoic plutonism in the canadian appalachians was dominated by northwestward subduction of the rheic ocean whereas the rheic ocean in the u s appalachians closed by southeasterly subduction a slab tear therefore developed at the southwestern margin of the rheic ocean slab beneath the canadian appalachians the high sr and ba metaluminous magnetite granite of the german bank pluton resulted from melting of underplated lower crust and or lithospheric mantle by a rising asthenospheric diapir related to this slab tear geochemically the german bank pluton is similar to most 300 265 ma plutons of the southern appalachians except in its strongly negative nd isotopic composition that results from the underthrusting of a ca 2 ga lower crustal block of saharan craton

teleseismic p wave image of crust and upper mantle structure beneath the valles caldera new mexico initial results from the 1993 jtex passive array teleseismic p wave relative arrival time data collected from a temporary array during the 1993 jemez tomography experiment jtex have been inverted to image velocity anomalies beneath the valles caldera in northern new mexico instruments were deployed in two 30 km long profiles one of 8 and one of 9 stations these profiles crossed the caldera trending at azimuths of n46 w and n60 e respectively two dimensional teleseismic relative arrival time inversion of the 1993 data set supplemented with data from an overlapping 1987 profile confirms the existence of a mid crustal low velocity region 30 beneath the valles caldera in the depth range of 8 to 13 km below sea level with about a 6 km horizontal extent this feature is interpreted to be the seismic expression of the remnant magma chamber a shallow low velocity anomaly beneath san antonio mountain coincides with the region of highest thermal gradient values in the caldera a lower crust upper mantle low velocity anomaly is imaged but is not as well constrained due to the limited length of the profile we tentatively correlate this anomaly with the thermal effects of basaltic magmas ponded at the crust mantle boundary

late paleozoic calc alkaline to shoshonitic magmatism and its geodynamic implications yuximolegai area western tianshan xinjiang article i nfo continuous late paleozoic magmatic activity in the western tianshan xinjiang china temporally overlapped the late carboniferous collision between the tarim and the kazakhstan yili plates isotopic dating whole rock geochemistry and isotopic characteristics of a suite of calc alkaline to shoshonitic rocks from yuximo legai area in the eastern end of the awulale mountains help define the tectonic evolution of this part of the tianshan zircon u pb ages of a quartz diorite and a quartz syenite porphyry are 310 8 2 1 ma and 284 4 3 6 ma respectively the geochemical and isotopic characteristics of late carboniferous igneous rocks including the quartz diorite as well as basaltic andesite and k rich trachyandesite indicate an evolu tion from calc alkaline towards shoshonitic series this was generated by the partial melting of the hydrated mantle wedge induced by changing subduction angles the younger quartz syenite porphyry however was more likely derived from the partial melting of a thickened crust in a post collisional setting or the partial melting of hot juvenile basalt in the lower crust during early permian delamination of the subcontinental lithospheric mantle therefore the generation of the calc alkaline to shoshonitic igneous rocks in yuximole gai area is genetically linked to the geodynamic evolution of the western tianshan from convergent subduc tion to collision during the late paleozoic

40ar 39ar dating into the historical realm calibration against pliny the younger laser incremental heating of sanidine from the pumice deposited by the plinian eruption of vesuvius in 79 a d yielded a40ar 39ar isochron age of 1925 94 years ago close agreement with the gregorian calendar based age of 1918 years ago demonstrates that the 40ar 39ar method can be reliably extended into the temporal range of recorded history excess 40ar is present in the sanidine in concentrations that would cause significant errors if ignored in dating holocene samples