zircon a possible indicator of mass transfer in deformation zones a solution for the roffna gneiss controversy suretta nappe switzerland abstract this paper proposes a simple volume mass balancing technique as a means to assess the volume loss and tectonic flattening of mylonitic shear zones the method involves counting the number of zircon crystals in oriented polished thin sections analyzed subsequently in the electron microprobe using the bsei mode and a spectrometer setup for detection of the element zr here we present an application to the roffna gneiss eastern switzerland a porphyritic granite of permian origin which has been metamorphosed and transformed into a phengitic schist under low grade metamorphic conditions mass balances for this material have produced controversial interpretations in that a first analysis of chemical data of wall rock and mylonite suggested an isochoric mylonitic transition event while a dramatic volume loss of some 50 in the course this process was inferred after a re evaluation of the same data electron microscopy of zircon crystals that were retrieved from rocks which had undergone different degrees of deformation revealed only minor alterations and mechanical damage of single grains this stability of zircon justified the use of single crystals and in consequence of the element zr as passive markers in determining volume mass changes during deformation results obtained by this method allow us to confirm that the deformation of the roffna gneiss involved a volume loss of 29 6 accompanied by chemical alterations deformation induced mobility changes of various elements lead to small gains in mg and cr but significant losses si al na ca and sr during the metamorphic event all other elements including the ree underwent no substantial long range transport

do transcurrent faults guide volcano growth the case of nw bicol volcanic arc luzon philippines this paper shows that although volcanoes can be aligned along transcurrent faults their growth can in reality be controlled by secondary structures that have a different orientation and kinematics assessing the relationships between local structures and volcano growth can be very useful for the evaluation of volcanic hazards and for economic purposes such as geothermal and hydrogeological exploration in the north western bicol volcanic arc luzon philippines the quaternary labo and caayunan volcanoes are aligned with the nw striking transcurrent philippine fault system they are exceptional examples of composite volcanoes mostly comprising lava domes where regional tectonic structures and hydraulic radial fracturing have alternated to control volcano growth field data and analogue modelling demonstrate that apical depressions affecting the domes reflect the geometry of magma feeding fractures these data together with alignments of domes and hot springs and other structural information suggest that magma rose at depth along the nw striking transcurrent faults but that in the uppermost crust magma rose along ne striking fractures parallel to the greatest principal stress

the terrestrial alteration of saharan shergottites dar al gani 476 and 489 a case study of weathering in a hot desert environment abstract rare earth element ree and other selected trace and minor element concentrations were determined in various phases of dar al gani 476 and 489 two basaltic shergottites recently found in the libyan sahara whereas merrillite and feldspathic glass compositions do not seem to have been altered since the fall of these meteorites olivine and pyroxenes commonly show light ree enrichments sometimes accompanied by ce anomalies that are attributed to terrestrial alteration sr and ba enrichments are also observed the calcium carbonate that resides in most cracks is essentially devoid of ree but a fine grained amorphous silicate material that fills some veins and pockets has striking light ree enrichments and elevated p sr and ba it is argued that this material likely to be impact melt as well as the notoriously light ree poor olivine and pyroxenes were preferentially affected by a crustal light ree rich contaminant caution is urged when using whole rock or even mineral separate compositional and isotopic data for hot desert meteorites

indicators of aqueous alteration in cm carbonaceous chondrites microtextures of a layered mineral containing fe s o and ni a petrographic and transmission electron microscopy study of the mighei murchison and murray cm carbonaceous chondrites shows that much of the cm matrix material was probably produced by aqueous alteration of olivine pyroxene sulfide and metal the amount of cm matrix appears to be proportional to the degree of alteration as suggested by mcsween 1979 and microtextures of pcp poorly characterized phase provide evidence of the progressive alteration pcp is divided into two major types one occurs in chondrules and aggregates and consists largely of an fe ni s o phase type i and the other occurs in matrix and consists of the fe ni s o phase and cronstedtite in various proportions type ii microtextures of pcp suggest that it resulted from a three stage alteration process 1 1 type i was produced by alteration of kamacite in chondrules and aggregates presumably early in the parent body regolith 2 2 as the alteration advanced olivine and pyroxene were converted to serpentine type i pcp separated from chondrules and aggregates into the matrix during regolith gardening simultaneously the fe ni s o phase reacted with si released by alteration of olivine and pyroxene producing well formed platy cronstedtite and coherent intergrowths of the fe ni s o phase and cronstedtite the fe ni s o phase also recrystallized into platy and pod like crystals fe s ni cr and p were leached out of type i pcp and were deposited as small grains of fe ni sulfides magnetite chromite and a mineral unidentified containing fe ni cr and p as a result pcp came to consist primarily of the fe ni s o phase and cronstedtite i e type ii pcp 3 3 during continued alteration the well formed crystals of the fe ni s o phase cronstedtite and their intergrowths in type ii pcp were replaced by poorly formed fibers in comparison to other cm chondrites mighei murchison and murray are relatively unaltered their matrices retain abundant amounts of the fe ni s o phase and cronstedtite commonly as distinctive pcp grains which account for a large proportion of the fe in these meteorite matrices in more altered cm chondrites much of the fe ni s o phase was probably consumed to produce cronstedtite magnetite and sulfides with further alteration cronstedtite itself reacted with the serpentine to form ferroan serpentine thus the cm matrix was increasingly enriched in mg with alteration and pcp was increasingly degraded and intimately mixed with the magnesian phyllosilicates

fluid flow and volume loss during mylonitization an origin for phyllonite in an overthrust setting north carolina u s a abstract in the western blue ridge province of the southern appalachians north carolina granite gneisses are converted to phyllitic mylonites phyllonites along the base of the rector branch thrust sheet the mylonites show evidence for fluid infiltration and large volume losses with decreasing distance from the thrust and local shear zones feldspars undergo grain size reduction by cracking followed by chemical breakdown to muscovite sericite and zoisite the following reaction was important during mylonitization 3 k feldspar 2 h muscovite 2 k 6 sio 2 quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300 c and 500 c in the presence of a fluid the mylonites are enriched by a factor of at least 2 5 in elements such as tio 2 p 2 o 5 zr y v la ce and depleted in sio 2 in comparison to their protolith gneisses the immobile element enrichments are attributed to enrichment in residual phases such as ilmenite zircon apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60 mass balance calculations based on major element chemistry indicate the mylonites lost large amounts of sio 2 and smaller amounts of alkalies and were infiltrated by fluid rock weight ratios of between 10 2 and 10 3 phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas reaction enhanced softening was apparently important in strain localization

morphological analysis of nevado de toluca volcano mexico new insights into the structure and evolution of an andesitic to dacitic stratovolcano abstract we present a morphological analysis of nevado de toluca volcano located 80 km wsw of mexico city based on digital elevation model study where slope and aspect maps have been generated and analysed aerial photograph and satellite image observations improve the morphological analysis the synoptic view which is offered by this analysis allowed for recognition and localization of the main volcanic and tectonic features of the area on the basis of digital elevation model value distribution and surface textures five morphological domains were defined the most interesting domain south of the crater reflects the occurrence of an ancient complex volcano distinct from the adjacent areas interaction between the volcanic and volcano tectonic evolution and the basement produced the other domains single volcanic edifices like lava domes and scoria cones and eruptive fractures were recognized finally flank collapse scarps opened to the east and to the north were identified and four relevant morphostructural lineaments and their possible role in the nevado de toluca geological and structural evolution are discussed

low temperature phase decomposition in iron nickel metal of the portales valley meteorite the portales valley meteorite provides an opportunity to investigate and compare the microstructure in fe ni metal of the metallic particles in the chondritic portion and in the metal veins the low temperature phase decomposition of fe ni metal was investigated using scanning electron microscopy transmission electron microscopy and atomic force microscopy the microstructure is formed as the portales valley meteorite cooled from high temperatures and includes the outer taenite rim the cloudy zone clear taenite and martensite martensite in turn decomposes into a fine admixture of fcc rods in a bcc matrix the width of the island phase of the cloudy zone in the metal particles of the chondritic portion and the metal veins can be used to estimate a low temperature cooling rate the microstructural evidence indicates that the chondritic portions and the metal veins in the portales valley meteorite cooled together as a mixture with a cooling rate of roughly 6 5 k ma

volcanic hazard zonation of the nevado de toluca volcano mexico abstract the nevado de toluca is a quiescent volcano located 20 km southwest of the city of toluca and 70 km west of mexico city it has been quiescent since its last eruptive activity dated at 3 3 ka bp during the pleistocene and holocene it experienced several eruptive phases including five dome collapses with the emplacement of block and ash flows and four plinian eruptions including the 10 5 ka bp plinian eruption that deposited more than 10 cm of sand sized pumice in the area occupied today by mexico city a detailed geological map coupled with computer simulations flow3d titan2d laharz and hazmap softwares were used to produce the volcanic hazard assessment based on the final hazard zonation the northern and eastern sectors of nevado de toluca would be affected by a greater number of phenomena in case of reappraisal activity block and ash flows will affect deep ravines up to a distance of 15 km and associated ash clouds could blanket the toluca basin whereas ash falls from plinian events will have catastrophic effects for populated areas within a radius of 70 km including the mexico city metropolitan area inhabited by more than 20 million people independently of the activity of the volcano lahars occur every year affecting small villages settled down flow from main ravines

shear zone patterns and strain distribution at the scale of a penninic nappe the suretta nappe eastern swiss alps abstract shear zone patterns within the suretta nappe of the swiss alps are studied in order to understand finite strain and bulk tectonic displacements the suretta nappe is chosen for two main reasons the availability of late variscan granites deformed during alpine tectonics only and because of continuous outcrop from bottom to top of the nappe this alpine nappe recorded two major ductile deformation phases which developed under low grade metamorphic conditions the analysis of shear zone patterns in the roffna granite reveals that the first deformation is well developed in the entire nappe the second deformation however is more localized in the lower and upper parts the bulk asymmetry of the shear zone patterns confirms the kinematics recorded by other penninic nappes in this area first a stacking of the nappe towards the nw followed by the top to the e shearing during post thickening extensional tectonics the shear zone patterns and strain distribution allow us to deduce kinematics at the nappe scale

the relationship between quaternary volcanism in central mexico and the seismicity and structure of subducted ocean lithosphere late quaternary volcanism in central mexico is related to the subduction of young ocean lithosphere at the middle america trench along arc variations in seismicity volcano structure and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab morphological and petrographic characteristics of major volcanoes within the trans mexican volcanic belt tmvb serve to distinguish two calc alkaline subprovinces 1 a western arc averaging 60 km in width associated with aseismic subduction of the rivera plate the main cones of this region are dominated by two pyroxene andesites comprise volumes 70 km 3 and stand less than 3 000 m above sea level 2 a broad central and eastern arc related to subduction of a gently inclined segment of the cocos plate bounded by the rivera transform and the tehuantepec ridge major volcanic edifices possess summit elevations in the range 4 000 to 6 000 m have appropriately larger volumes typically 200 km 3 and are constructed with a high proportion of amphibole bearing lavas the boundary between these subprovinces is marked by a north south oriented structural depression the colima graben and it coincides with a 100 km offset in the volcanic front extensional tectonism in the colima graben accompanied by mixed calc alkaline and alkaline volcanism of potassic affinity is likely related to a hinge type transform fault which marks the cocos rivera plate juncture in the downgoing slab a third segment of ocean floor is presently interacting with continental lithosphere south of the gulf of tehuantepec where quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse cocos noam caribbean triple junction the northern limit of this triple junction is defined by the seismically active isthmus fault which may be related to alkaline volcanic activity at san andres tuxtla a tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate depth earthquakes along the arc is directly dependent upon the age of the subducted slab ocean lithosphere younger than approximately 20 m y is subducted aseismically at convergence rates approaching 9 cm yr the length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m y the tmvb overlies the aseismic extension of this young ocean lithosphere several aspects of this study have a bearing on the segmented nature of converging margins in general 1 the tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction 2 the complete record of volcanism in the tmvb over the past million years can be related to the present plate configuration 3 alkaline and calc alkaline volcanism have developed contemporaneously at a converging plate margin 4 lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab

active faults and state of stress in the central part of the trans mexican volcanic belt mexico 1 the venta de bravo fault the central part of the trans mexican volcanic belt is being deformed by seismically active west east striking normal faults that cause pronounced scarps structural and seismic along strike discontinuities allow the following segmentation 1 the western segment the area between morelia and the los azufres volcanic center is characterized by 25 km long fault scarps that define the cuitzeo graben and the santa ana maya horst which has a relief 850 m with respect to the cuitzeo graben the faults cut several quaternary scoria cones as well as alluvium 2 the central neotectonic segment extends between maravatio in the west and altamirano volcano in the east the most prominent structure of this segment is the 45 km long venta de bravo fault with a relief 300 m the february 22 1979 mb 5 3 maravatio earthquake occurred on this fault the late quaternary vertical slip rate along the easternmost part of the venta de bravo fault is estimated at 2 mm yr the lateral extension of the segment is defined by the limits of the venta de bravo fault and the eastern termination of the cuitzeo graben 3 the eastern segment is made up of the acambay graben the limit between the central and eastern segments is defined by an extensional jog between the venta de bravo and pastores faults which indicates a left lateral strike slip component the most recent rupture of this segment occurred in the november 19 1912 ms 6 9 acambay earthquake that caused vertical displacements 50 cm along the faults flanking the graben stress determinations based on quaternary scoria cone alignments the focal mechanism of the 1979 earthquake and the inversion of striation measurements indicate a stress field with sv sene snnw the fault striations as well as the focal mechanism indicate a minor but consistent left lateral strike slip component the observed stress and deformation can be explained by the superposition of two sources of stress isostatically compensated surface loads related to the high elevation of the trans mexican volcanic belt probably cause horizontal deviatoric tension and the observed crestal normal faults which strike parallel to the axis of the volcanic belt the observed left lateral strike slip component on the other hand can be explained by compressional far field stresses caused by loads at the boundary between the cocos and north american plates the orientation of the faults with respect to the direction of plate convergence is a prefered orientation for left lateral slip along the observed faults alternatively the observed left lateral strike slip component may accommodate trench parallel motion of crustal blocks in southwestern mexico

the porosities of ordinary chondrites models and interpretation densities and porosities for 285 ordinary chondrites have been assembled and analyzed measured chondrite porosities are bimodal finds have an average porosity of 30 we conclude that mild degrees of weathering fill pore spaces lowering grain densities and porosities without significantly changing the bulk size or mass of the sample by assuming an original pristine grain density as a function of the meteorite s mineralogy determined by its class we can derive model pristine porosities these model porosities cluster around an average value of 10 for all classes of ordinary chondrites ordinary chondrites do not show any correlation of porosity model or measured with petrographic grade or sample size over a range from 0 2 g to 2 kg however we do see a correlation between shock state and porosity shock blackened meteorites are less porous than other meteorites furthermore less severely shocked meteorites show a much broader range of porosities with the maximum porosity seen among meteorites of a given shock class falling linearly as a function of that shock class this is consistent with the idea that shock compresses and closes pore space analysis of meteorite porosity provides a lower bound to the fine scale porosity of asteroids our densities even with 10 primordial porosity are significantly higher than inferred densities of possible asteroid parent bodies these asteroids are probably loose piles of rubble

brittle plastic deformation in mylonites an example from the meadow fork thrust western blue ridge province southern appalachians recent conceptual models for fault related rocks imply that mylonites with internal shear surfaces may exhibit frictional characteristics and that they potentially can form within the seismogenic zone this paper describes a lower greenschist facies quartz rich mylonite zone from the western blue ridge province in the southern appalachians that displays evidence for episodic deformation by both crystal plastic and brittle mechanisms strain was largely accommodated by quartz whereas feldspar behaved rigidly and passively plastically deformed monocrystalline quartz ribbons and the assemblage chlorite quartz muscovite are consistent with temperatures of deformation between 300 and 400 c and together with fluid inclusion measurements indicate that fluid pressures fluctuated between 90 and 300 mpa if one assumes that the maximum pressure represents lithostatic conditions this corresponds to a depth of about 12 km quartz c axis fabrics display both symmetrical and asymmetrical patterns with respect to the foliation and indicate that deformation occurred under coaxial and noncoaxial conditions healed microcracks and extensional quartz veins that are cut by quartz ribbons indicate that brittle and crystal plastic deformation occurred simultaneously or cyclically by analogy with experimental studies internal zones of localized shear in the mylonites may represent frictional behavior these results are discussed within the context of recently developed conceptual continental fault zone models

recent left oblique slip faulting in the central eastern trans mexican volcanic belt seismic hazard and geodynamic implications 1 detailed structural analysis was conducted on the e w trending tenango fault system tfs at the western end of the la pera fault system a major structure within the trans mexican volcanic belt tmvb field mapping which included trenching the fault trace revealed the displacement of recent volcanic deposits the study indicates that the tfs is an active subvertical fault with transtensive left lateral movement consistent with the contemporary stress regime in the region the estimated holocene slip rate is 0 3 0 5 mm yr the tfs activity constitutes an important seismic hazard in the intra arc lerma basin several towns are located within this basin and are particularly vulnerable to seismic risk because of its underlying strata consisting of lacustrine and volcaniclastic deposits estimated maximum mw 6 8 7 the tfs is an important structural element in the complex tectonic deformation history of central eastern tmvb and influenced the behavior of recent volcanism within the lerma basin

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

feldspar influenced rock rheologies progressive fracture subsequent cataclasis and syntectonic alteration of feldspar to phyllosilicates transformed a massive granite into a quartz mica phyllonite during early tertiary deformation in a 20 30 m wide shear zone in southeastern arizona the ductile deformation proceeded at temperatures and pressures approximating middle to upper crustal levels most of the deformation was taken up by the feldspar and its alteration products quartz deformed by both brittle fracture and crystal plastic mechanisms these results show that under certain temperature pressure strain rate conditions feldspars are weaker than quartz the observed deformation mechanisms indicate that the brittle to ductile transition in fault zones is not simply a function of the onset of temperature activated plastic deformation in quartz nor of the frictional behavior of rocks as inferred in the familiar strength vs depth curves it is more likely that the transition is complex and depends on a variety of parameters one such factor delineated in this study is the fracture and fracture facilitated syntectonic alteration of feldspar under hydrated conditions

stability of zircon in a low grade ultramylonite and its utility for chemical mass balancing the shear zone at mieville switzerland at mieville switzerland granitic basement rocks of the aiguilles rouges massif are cut by a subvertical shear zone the variscan shearing event produced a low grade ultramylonite zone which is up to 50 m wide and several kilometers long investigations of accessory zircons from the undeformed wall rock and from the most highly deformed ultramylonite show only minor alterations and mechanical damage of zircon crystals even in an extreme state of deformation this outstanding stability of zircon allows one to consider the element zr as immobile and therefore to use it as a passive marker for calculations of mass and volume changes during deformation processes application of this method confirms that the deformation at mieville proceeded under simple shear constant volume and isochemical conditions this is in agreement with the results of classical geochemical mass balancing in addition uncertainties inherent in the geochemical approach can be minimized by the new method

geology and tectonics of the southwestern boundary of the unstable sector of mt etna italy abstract the etna volcano is known to be unstable and gradually moving downslope toward east and south the study of the local geology and structure of these unstable flanks and of their boundaries is important to understand the evolution of the flank instability and the stress field driving the spreading of the edifice in this paper we present the geological evolution of the lower south western sector of mount etna base on a detailed field survey at 1 10 000 scale this area located at the boundary of the unstable south eastern flank of the volcano is affected by the active ragalna fault and by distinctive eruptive fissures monte calvario formation 18 15 ka the monte calvario formation is made of viscous lavas often associated to breccia and autoclastic lava facies fed by a set of eruptive fissures ne sw oriented their emplacement occurred immediately before the ellitico caldera collapse marking a previously poorly constrained shift in the feeding system of the volcano the orientation of these eruptive fissures and lineaments and the geometry of the sedimentary substratum modeled from subsurface data indicate that the area was dominated by a local stress field consistent with the lateral spreading of mount etna since at least from the final ellittico phase about 18 ka with a nearly constant mean displacement rate these data give new insights into the geology and structure of the flank instability at etna the main geologic hazard of the studied area is not related to the opening of a new eruptive fissures but it is posed by the presence into il calvario quarry of fluoro edenite an asbestiform mineral that caused an high mortality rate for respiratory system cancer in biancavilla town in fact the monte calvario formation groups other eruptive fissures with similar volcanological and geochemical characteristics where hydrothermal alteration occurred with the likely formation of health threatening minerals

rare earth element mobility in the roffna gneiss switzerland the roffna gneiss a deformed hercynian granite porphyry within the penninic nappes of eastern switzerland underwent extreme cataclasis with the progressive development of phengite towards the margins of the nappe under conditions of the glaucophane schist to greenschist facies this resulted in the selective mobilization of major and trace elements over distances of 10 s to 100 s of meters and the resetting of the rb sr whole rock isotopic systems some 100 my ago the component ratios and compositionvolume relationships of progressively deformed gneiss samples studied here suggest that this process was essentially isovolumetric the mineralogy of the deformation sequence appears to have been controlled by a reaction involving the breakdown of microcline albite and biotite and the formation of phengite and quartz the fluids introduced mg and h2o promoting the development of phengite and removed the na being released by the breakdown of albite the fluids were most probably derived from the surrounding triassic carbonates and quartzites these relatively high fo2 and carbonate rich fluids also introduced rare earth elements ree into the gneiss the gneiss was progressively enriched in eu up to 60 y up to 40 and yb up to 100 these enrichments are associated with the development of epitaxial xenotime around zircon in the most phengite rich sample while the ree were mobile uranium and thorium were essentially immobile the formation of xenotime was suggested to explain the observed heavy ree enrichment when large differences in the ree contents were found for replicate analyses using hf and then lithium metaborate for dissolution these differences arose because xenotime like monazite can be difficult if not impossible to dissolve in hydrofluoric acid due to the possibility of incomplete sample dissolution we now recommend fusion with lithium metaborate for all ree lu hf or sm nd studies

effect of strain rate in the distribution of monogenetic and polygenetic volcanism in the transmexican volcanic belt in the transmexican volcanic belt polygenetic and monogenetic volcanism has taken place concurrently with extensional deformation since the late miocene at a regional scale the deformation is manifested by two groups of faults the dominant group consists of normal faults nearly parallel to the arc in the other group are north northwest trending normal faults that cross the arc and in places form the boundaries of crustal blocks the larger stratovolcanoes of the transmexican volcanic belt are aligned in north south volcanic chains along some of these faults whereas monogenetic volcanoes are usually located along arc parallel normal fault systems because the arc parallel faults are 15 oblique to the subduction plate boundary and assuming stretching perpendicular to the trench the extensional deformation field facilitates activation of both arc parallel and arc transverse structures the former having a higher displacement rate than the latter we observe that in the transmexican volcanic belt polygenetic volcanoes develop along faults having small strain rate and monogenetic volcanoes are emplaced along faults having higher strain rate the agreement with the theoretical model in which monogenetic or polygenetic volcanism depends on the magmatic input rate and the regional stress is true only assuming a linear relation between regional differential stress and local strain rate as in a continuous and homogeneous medium we propose that the local strain rate rather than the regional stress field controls the coexistence of both types of volcanism in the transmexican volcanic belt

neotectonics in central mexico regional neotectonic activity in central mexico was mapped through analysis of eleven landsat thematic mapper images the study area stretches from mexico city on the east to the gulf of california on the west between 17 and 21 n latitude fault scarps of plio quaternary age define several large scale tectonic features within the mexican volcanic belt dominated by rifting transtension and shear with probable sinistral components three large crustal blocks south of the mexican volcanic belt are separated from the north american plate and from each other by zones of neotectonic activity that express the relative motions between the adjacent blocks a simple vector diagram for plates and crustal blocks in central mexico has been constructed using information from the literature combined with the orientations of fault zones as mapped in this study the relative motion between the blocks varies from about 2 5 to 8 mm year 1 which is comparable to motions of crustal blocks in areas of active rifting in other parts of the world in the mexican volcanic belt there is a clear relationship between volcanism and neotectonic activity fault zones that traverse nearly the entire region have the same location and orientation as the volcanic belt this may indicate that deformation within the crust of the overriding slab plays a significant role in determining the unusual orientation of the mexican volcanic belt relative to the trench the ultimate factor controlling the location of plio quaternary volcanism and tectonism in central mexico may be large zones of crustal weakness formed during major mesozoic and cenozoic tectonic events

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

les grandes etapes d evolution d un volcan andesitique composite exemple du nevado de toluca mexique the nevado de toluca in the middle of the mexican volcanic belt has been built by two very dissimilar phases the first one that lasted more than one million years is mainly andesitic numerous massive and autobrecciated lava flows of this phase pass outwards into thick conglomeratic formations the volume of this primitive volcano represents the essential part of the nevado

terrestrial weathering of antarctic stone meteorites formation of mg carbonates on ordinary chondrites abstract white efflorescences of weathering origin occur superposed on fusion crusts or along fractures in the interiors of approximately 5 of all meteorites in the us antarctic collection efflorescences from equilibrated ordinary chondrites consist of the hydrous mg carbonates nesquehonite hydromagnesite x ray diffraction and scanning electron microscope studies of efflorescences from lew 85320 h5 show abundant elongate prismatic crystals of nesquehonite idiomorphic not pseudomorphous after lansfordite with minor local encrustations of hydromagnesite abundances of na k ca and rb in efflorescences from lew 85320 suggest that the observed contents of these elements would require only modest fractionation of chondritic composition whereas extensive fractionation would be required to derive the observed cation ratios from terrestrial sea salts therefore cations in evaporite minerals on antarctic meteorites are most likely not products of contamination by terrestrial marine salts the mg in the efflorescences probably originated from weathering of meteoritic olivine other cations in the efflorescences are also of meteoritic provenance thermodynamic analysis of the reaction forsterite water carbon dioxide nesquehonite silica at antarctic temperatures and pco 2 indicates spontaneity for all water activities greater than 0 65 compatible with the presence of liquid water as brines and or thin films

the aljibes half graben active extension at the boundary between the trans mexican volcanic belt and the basin and range province mexico the aljibes half graben is located 140 km west northwest of mexico city and is characterized by several parallel 10 km long south dipping and east striking normal faults late miocene basalt flows with 40 ar 39 ar ages of 7 7 0 1 ma and 7 1 0 5 ma are displaced along these faults with reference to this basalt the composite throw and heave of the half graben are 480 m and 270 m respectively and the relative extension is 5 whereas the vertical slip rate along the faults of the aljibes half graben after the deposition of the basalt is 0 07 mm yr the aljibes half graben is located at the northern margin of the trans mexican volcanic belt and perpendicularly intersects northstriking normal faults of the basin and range province the relative sequence of activity of the two stress fields and fault systems cannot be resolved in the study area and the two systems may have been active simultaneously we explain this structural configuration by horizontal migration of the boundary between the two stress provinces with time which requires intermittent permutations between the intermediate and least principal stresses 2 and 3 this is supported by the low stress ratio 2 3 1 3 obtained from modeling the stress tensor associated with the slip along the faults of the aljibes half graben the orientations of the fault planes and slip vectors of the aljibes half graben are practically identical to those of the nearby acambay graben and the venta de bravo fault which caused the acambay earthquake of november 19 1912 m s 6 9 and the maravatio earthquake of february 22 1979 m b 5 3 furthermore the aljibes half graben forms the extrapolated western continuation of the northern master fault of the mezquital graben which ruptured partly in the cardonal earthquake of march 26 1976 m b 5 3 this suggests the aljibes half graben may also be seismically active although no historical earthquake is known to have occurred along this structure further evidence for the likely seismic activity of the aljibes half graben are two east to eastnortheast trending epicenter alignments between the aljibes halfgraben and the mezquital graben and structural indication for stickslip behavior based on the structure of the aljibes half graben we believe the maximum credible earthquake that may occur along the faults of the aljibes half graben is on the order of 2 10 27 dyne cm m 0 6 10 25 dyne cm and 7 5 m w 6 5

quaternary intra arc extension in the central trans mexican volcanic belt the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc parallel faulting major intra arc basins within the central part of the belt between long 998w and 1028w are from west to east the cuitzeo and acambay grabens the aljibes half graben and the mezquital graben in this region 100 eastwest2striking 2 km long steeply dipping normal faults expressed by pronounced multi event fault scarps have been mapped of the 100 faults 65 displace rocks of known quaternary age younger than 1 6 ma and 22 cut rocks with a documented age younger than 750 ka known historical surface ruptures are limited to faults of the acambay graben overall the faults cause north south to north northwest2south southeast2oriented extension of 1 km which is distributed over a 30250 km wide zone that has 5210 faults in cross section and a relative extension of 3 quaternary vertical slip rates estimated for 13 of the faults have a mean of 0 07 mm yr they are highest at the southern margin of the cuitzeo graben the northern margin of the acambay graben and in the aljibes half graben where they measure between 0 16 and 0 18 mm yr the north south to north northwest2south southeast2oriented quaternary bulk extension rate of the system is likely to be 0 2 6 0 05 mm yr this fault system is in an initial stage of coalescence its western and eastern parts consist mostly of isolated fault segments whereas in the central part deformation is localized onto a few through going faults the longest of these are the venta de bravo fault 45 km and the acambay tixmadeje fault 34 km the short fault traces have a simple structure whereas the longer ones are commonly composed of two or more segments there is no obvious migratory pattern of quaternary fault activity which suggests that the entire region is tectonically active

two subduction events in a polycyclic basement alpine and pre alpine high pressure metamorphism in the suretta nappe swiss eastern alps this study is essentially based on coupling macrostructures microstructures and metamorphic petrology in polymetamorphic mafic rocks from the swiss eastern alps suretta nappe penninic domain petrographic criteria are used in conjunction with structural analysis and microprobe work to define crystallization deformation relationships and to establish a relative but precise sequence of tectono metamorphic events a first eclogite facies overprint and related exhumation occurred before emplacement of late palaeozoic intrusives during the alpine cycle the suretta nappe was part of the thinned european continental margin the tertiary burial due to subduction and collision is responsible for d1 ductile thrusting and blueschist facies metamorphism late deformation phases related to exhumation are responsible for the development of extensional structures under greenschist facies conditions quantitative metamorphic petrology based on gibbs free energy minimization domino by de capitani gives a constraint on the p t conditions during the polymetamorphic and polycyclic evolution the first high p metamorphic event related to pre alpine structures occurred at c 700 c and at least 2 0 gpa these conditions are compatible with pre alpine high p re equilibration already described in several alpine units the alpine high p metamorphism occurred under blueschist facies conditions at c 400 450 c and 1 0 gpa similar high p low t conditions have already been described in the mesozoic and permian rock types the two high p events are clearly related to two different geothermal regimes and geodynamic environments

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

strain and shape fabric variations associated with ductile shear zones abstract the foliated and compositionally banded granitic orthogneisses in the central core of the maggia nappe a lower pennine basement nappe of the central swiss alps are shown to be the sheared equivalent of late hercynian age granitic intrusions these ductile shear zones show mineral assemblages in amphibolite facies are alpine in age and form an anastomosing network enclosing remnant lozenge shaped pods of relatively undeformed rock the foliation developed within the shear zones concomitantly with a change in shape of quartz grain aggregates from initially equidimensional through tear drop shapes to ribbon like aggregates these shape changes occurred by intracrystalline glide together with intercrystalline slip on deformation induced planar surfaces

the structural architecture of the los humeros volcanic complex and geothermal field abstract the los humeros volcanic complex lhvc is a large silicic caldera complex in the trans mexican volcanic belt tmvb hosting a geothermal field currently in exploitation by the comision federal de electricidad cfe of mexico with an installed capacity of ca 95 mw of electric power understanding the structural architecture of lhvc is important to get insights into the interplay between the volcano tectonic setting and the characteristics of the geothermal resources in the area the analysis of volcanotectonic interplay in lhvc benefits from the availability of subsurface data obtained during the exploration of the geothermal reservoir that allows the achievement of a 3d structural view of the volcano system the lhvc thus represents an important natural laboratory for the development of general models of volcano tectonic interaction in calderas in this study we discuss a structural model of lhvc based on morphostructural and field analysis integrated with well logs focal mechanism solutions and magnetotelluric imaging the structural analysis suggests that inherited regional tectonic structures recognized in the basement played an important role in the evolution of the magma feeding system caldera collapses and post caldera deformations these inherited weak planes have been reactivated by resurgence faults and post caldera magma driven hydrofractures under a local radial stress field generated by the shallow lhvc magmatic hydrothermal system the local stress field induced caldera resurgence and volcanotectonic faulting the results of this study are important to better constrain the structural architecture of large caldera complexes also our study is useful to understand the structure of the los humeros geothermal field and support the exploration of deeper super hot geothermal systems shgss and engineering of enhanced geothermal systems egss for electric power production in the lhvc and other active resurgent calderas

self similar clustering of cinder cones and crust thickness in the michoacan guanajuato and sierra de chichinautzin volcanic fields trans mexican volcanic belt abstract the spatial clustering of basaltic vents in monogenetic volcanic fields has been used as a proxy for crustal thickness in extensional and back arc tectonic settings the basaltic vents have a fractal clustered distribution self similar clustering described by a power law the power law is defined over a range the size range of the distribution of values in this case the vents separation delimited by a lower and an upper cut offs here we apply the fractal clustering analysis to the two largest monogenetic volcanic fields of the trans mexican volcanic belt tmvb a continental arc built on different crustal terranes the michoacan guanajuato volcanic field mgvf located in the central western tmvb includes over 1000 vents of late pliocene to quaternary age built on attenuated crust of mesozoic to tertiary age the sierra de chichinautzin volcanic field scvf in the central eastern tmvb is composed of 220 late pleistocene to holocene vents laying above thicker crust of precambrian to tertiary age monogenetic vents in both volcanic fields show self similar clustering with fractal exponent d 1 67 in the range 1 3 38 km mgvf and d 1 56 in the range 1 5 32 km scvf the upper cut off uco for the power law distribution of the mgvf well fits the crustal thickness below the volcanic field as derived from independent geophysical data the uco value of scvf indicates a crust thickness of about 32 km this value is in agreement with new geophysical data that indicate magma underplating the crust beneath the volcanic field area

structural architecture of the colima volcanic complex 1 the colima volcanic complex cvc is currently the most active mexican volcano and is located in the western sector of the trans mexican volcanic belt inside the active colima rift a regional n s striking extensional structure the colima rift is filled by a 1 km thick sequence of quaternary lacustrine sediments alluvium and colluvium mostly underling the about 3000 m thick volcanic pile of the cvc in this work we present the results of a detailed morphostructural and field study of quaternary faults and fractures in the cvc and the surrounding area including the regional structures of the colima rift we also present a geometrical modeling of the faults inside the volcano and a numerical model of the gravity induced stress and strain fields of the cvc the study attempts to characterize the geometry kinematics and dynamics of the deformation features of the cvc and relate it with the volcano structure the geology of the substratum and the geodynamic setting of the region our model considers that the observed deformation of the cvc and the surroundings results from the interplay between the active n s trending regional extensional tectonics and the southward spreading of the volcano over its basement forming an e w oriented volcanotectonic graben the interaction between regional tectonics and previously unrecognized volcanic spreading can control magma migration and flank instability in an area where eruptions and sector failures represent a potential high risk for more than 500 000 people

strain accumulation during basal accretion in continental collision a case study from the suretta nappe eastern swiss alps abstract stacked crystalline basement nappes are a typical feature in the core of many orogens the suretta nappe exposed in eastern switzerland consists of briancon derived crustal slices which were assembled in a south dipping subduction zone during the alpine orogenic cycle the nappe contains post variscan rocks of the rofna porphyry complex rpc and permo mesozoic cover sequences which are ideal to study the strain evolution during alpine nappe formation we present new structural cross sections finite strain analyses and a retrodeformation for the frontal part of the suretta nappe the overall geometry of the suretta nappe is the result of two main deformation phases 1 eocene top to the nnw directed thrusting and folding ferrera phase which is overprinted by 2 backfolding and backshearing niemet beverin phase we suspect that this backshearing was caused by the buoyant rise of light continental crust beneath the suretta nappe and by frictional resistance at the top contact of the suretta nappe which had been accreted to the upper plate in the course of nappe stacking in the lower and interior parts of the suretta nappe weakly to undeformed boudins are generally surrounded by l tectonites indicating wsw ene stretching foliated equivalents reveal a plane strain deformation state the upper part of the suretta nappe which was strongly affected by backshearing shows flattening strain strain data were used to reconstruct the palinspastic evolution of the suretta nappe at two distinct time slots prior to niemet beverin backfolding the suretta nappe is characterized by thrust faults and large scale detachment folds before the onset of ferrera phase nappe stacking jurassic normal faults and the rpc intrusion shape trigger the localization of subsequent deformation our structural study confirms a complex polyphase evolution for the suretta nappe which might be characteristic for crystalline basement nappes in continental collision zones elsewhere

geomorphic assessment of active tectonics in the acambay graben mexican volcanic belt spatial variations of quaternary deformation and tectonic activity of faults along the acambay graben are assessed using geomorphic and morphometric approaches the acambay graben is an east west trending structure of apparent quaternary age located in the central part of the mexican volcanic belt which gives rise to pronounced scarps over a distance of about 80 km continuing tectonic activity in the acambay graben is confirmed by recent well documented seismic episodes the intensity of active tectonics has been interpreted through a detailed geomorphic study of the fault generated mountain fronts and fluvial systems the combined geomorphic and morphometric data provide evidence for relative variations in tectonic activity among the acambay graben faults geomorphic indices suggest a relatively high degree of tectonic activity along the venta de bravo and the acambay tixmadeje faults followed in order of decreasing activity by the pastores temascalcingo and tepuxtepec faults spatial variations within faults have also been identified suggesting a higher level of tectonic activity at the tips of the faults this pattern of variation in the relative degree of tectonic activity is consistent with field evidence and seismic data for the acambay graben geomorphic evaluation of the acambay graben faults suggests that the acambay tixmadeje and venta de bravo faults and specifically the tips of these faults and a central segment near the town of venta de bravo should be considered as areas of potentially high earthquake risk 1998 john wiley sons ltd

large scale landslides triggered by quaternary tectonics in the acambay graben mexico detailed analysis was conducted on large scale gravitational tectonic deformations and landslides in the acambay graben an intra arc basin in the trans mexican volcanic belt tmvb field mapping and remote sensing revealed the slope instability of the northern graben boundary induced by the acambay tixmadeje fault two major landslides of 0 1 km3 and 0 05 km3 in volume were identified and their characteristics were analyzed according to the role of tectonics mechanism of slope failure and possible triggering factors quaternary faulting played a major role in increasing the local relief and the activity of the acambay tixmadeje fault represents the main geomorphic factor conditioning the gravitational movements moreover displacements along this fault generated sliding surfaces and reduced the strength of the rock mass the two landslides are classified as large scale rotational slides involving volcanic rocks of late miocene pleistocene age since the acambay graben is a seismogenic area with a known maximum horizontal ground acceleration of 0 5 g a strong earthquake could be ascribed as the possible triggering mechanism of the landslides our work represents the first analysis of large gravitational slope movements in tectonically active regions in mexico a process that can be common in the intra arc basins of the tmvb where active tectonic seismicity weak altered volcanic rocks and heavy rains affect the slope stability copyright 2010 john wiley sons ltd

mineralogy of meteorite groups approximately 275 mineral species have been identified in meteorites reflecting diverse redox environments and in some cases unusual nebular formation conditions anhydrous ordinary carbonaceous and r chondrites contain major olivine pyroxene and plagioclase major opaque phases include metallic fe ni troilite and chromite primitive achondrites are mineralogically similar the highly reduced enstatite chondrites and achondrites contain major enstatite plagioclase free silica and kamacite as well as nitrides a silicide and ca mg mn na cr k and ti rich sulfides aqueously altered carbonaceous chondrites contain major amounts of hydrous phyllosilicates complex organic compounds magnetite various sulfates and sulfides and carbonates in addition to kamacite and taenite iron meteorites contain carbides elemental c nitrides phosphates phosphides chromite and sulfides silicate inclusions in iab iiicd and iie iron meteorites consist of mafic silicates plagioclase and various sulfides oxides and phosphates eucrites howardites and diogenites have basaltic to orthopyroxenitic compositions and consist of major pyroxene and calcic plagioclase and several accessory oxides ureilites are made up mainly of calcic chromian olivine and low ca clinopyroxene embedded in a carbonaceous matrix accessory phases include the c polymorphs graphite diamond lonsdaleite and chaoite as well as metallic fe ni troilite and halides angrites are achondrites rich in fassaitic pyroxene i e al ti diopside minor olivine with included magnesian kirschsteinite is also present martian meteorites comprise basalts lherzolites a dunite and an orthopyroxenite major phases include various pyroxenes and olivine minor to accessory phases include various sulfides magnetite chromite and ca phosphates lunar meteorites comprise mare basalts with major augite and calcic plagioclase and anorthositic breccias with major calcic plagioclase several meteoritic phases were formed by shock metamorphism martensite 2 fe ni has a distorted body centered cubic structure and formed by a shear transformation from taenite during shock reheating and rapid cooling the c polymorphs diamond lonsdaleite and chaoite formed by shock from graphite suessite formed in the north haig ureilite by reduction of fe and si possibly from olivine via reaction with carbonaceous matrix material ringwoodite the spinel form of mg fe 2sio4 and majorite a polymorph of mg fe sio3 with the garnet structure formed inside shock veins in highly shocked ordinary chondrites secondary minerals in meteorite finds that formed during terrestrial weathering include oxides and hydroxides formed directly from metallic fe ni by oxidation phosphates formed by the alteration of schreibersite and sulfates formed by alteration of troilite

the geological and structural evolution of the cerro tuzgle quaternary stratovolcano in the back arc region of the central andes argentina abstract the aim of our paper is to contribute to a better knowledge of the volcanism in the back arc region of the central andes and its relationships with the basement geology the stress field and the tectonic evolution by studying in detail the stratigraphy and the structure of the quaternary cerro tuzgle stratovolcano in the puna plateau field mapping and remote sensing analysis reveal the stratigraphic architecture the geological evolution and the volcanotectonic interactions in the cerro tuzgle area for the first time in a volcano of the puna plateau synthemic units bounded by unconformity surfaces have been defined unrevealing the temporal and spatial relationships between constructive and destructive phases of the volcano history our study indicates that after the emplacement of a small ignimbrite deposit and of few scattered lava domes the central cerro tuzgle volcano built up throughout three distinct phases of edifice construction the first of these constructive phases ended with a previously unreported destructive event consisting of 0 5 km 3 catastrophic sector collapse of the volcanic edifice whose stratigraphic position and main characteristics have been identified the study suggests that the regional stress regime and the topography of the substrata are the main non magmatic factors controlling the constructive and destructive phases of the volcano including the directions of magmatic intrusions faulting and gravitational sector failure of the volcano the integration of synthemic stratigraphy and volcanotectonic analysis in the study of volcanic edifices showed to be an effective methodological approach for the understanding of the magmatic and tectonic evolution of the puna plateau

systematics of internal zircon morphology in major variscan granitoid types the internal morphologies of zircon crystals from different types of granitoids alkaline calcalkaline and anatectic are revealed by cathodoluminescence imaging and are described in terms of growth rates of the crystal faces relative to each other zircons in the alkaline granitoids are characterized by high and constant growth rates of 010 relative to the pyramidal forms and by symmetric grwoth of 011 zircons in the calcalkaline and anatectic granitoids are characterized by fluctuating or gradually decreasing relative growth rates of 010 by asymmetric and highly variable growth of 011 and by a tendency of 110 to become grwoth inhibited corrosion events are interspersed during zircon growth in the calcalkaline magmas in the calcalkaline and anatectic magmas a discontinuity breaks the morphological evolution at late stages of crystallization the discontinuity coincides with a sharp drop in cathodoluminescence the growth behaviour of each crystal form is analysed and compared with predictions made by the periodic bond chain pbc theory it is argued that the relative growth rate of 010 depends on supersaturation that the growth rates of 011 faces are changed in response to different ratios of adsorbing cations na k al and that 110 faces become growth inhibited by the adsorption of h2o or trace elements enriched in the residual liquid morphological and chemical discontinuities at late stages of crystallization are reasonably explained by the formation of larger growth units from smaller ionic entities in the residual liquid important factors controlling the zircon morphology in different types of granitoids are high cooling rates alkaline magmas magma mixing calcalkaline magmas enrichment of h2o and trace elements in residual liquids calcalkaline and anatectic magmas and the major element chemistry of the magma possibly the ratio of na and k to al agpaicity

the density and porosity of meteorites from the vatican collection abstract we have developed a nondestructive technique to measure the densities and porosities of large 50 g to several kilogram meteorite samples and have begun a systematic measurement program starting with meteorites from the vatican observatory collection our technique utilizes a modified archimedean method 40 m glass spheres substituting for the fluid to determine bulk density and an unusually large he pycnometer to determine grain density from these the porosity is calculated we report here our first results for 89 samples nearly doubling the published database on porosities of meteorite hand samples including porosities of 35 different meteorites these include several types not previously reported eight iron meteorites six stony iron meteorites an enstatite chondrite the ci carbonaceous chondrite orgueil and the shergottite nakhlatite chassignite snc meteorite nakhla

tectonic interpretation of the trans mexicano volcanic belt abstract although some workers interpret subduction along the middle america trench to be directly associated with volcanism in the trans mexico volcanic belt geographic distribution of intermediate depth earthquakes analysis of sn propagation and a limited number of focal mechanism solutions suggest that the belt is largely independent of subduction these data in conjunction with the position of the volcanic belt with respect to known plate boundaries in the region suggest that the belt represents the incompletely developed northern boundary of a developing micro plate it is a zone of transtension that probably results from slightly different rates of absolute plate velocities in the region

evidence of recent ruptures in the central faults of the acambay graben central mexico abstract the acambay graben within the central part of the trans mexican volcanic belt is one of the major sources of continental earthquakes in mexico to date the activity and paleoseismological history of the axial faults of the graben are not well constrained we provide morphological structural and sedimentological evidence of the seismogenic nature of two of the axial structures the temascalcingo and the tepuxtepec fault systems faults consist of multiple parallel scarps with en echelon and horse splay patterns fault systems extend for 60 km and displace quaternary to upper miocene volcanic edifices and volcano sedimentary materials surface lengths of individual fault traces range between 3 and 25 km and observed throws reach a minimum of 150 200 m the long term and short term slip rate of the temascalcingo fault system in the studied section presents similar values ranging from 0 06 0 02 minimum long term to 0 12 0 02 mm y 1 maximum value of average short term only the long term slip rate of the tepuxtepec system could be constrained in 0 01 0 02 mm y being a minimum estimate the holocene fault rupture history at two sites provided evidence of six ruptures since 12 500 11 195 bce among which three ruptures should have occurred between 11 847 652 bce and 11 425 465 bce variable single event displacements seds between 6 and 77 are interpreted as the result of fault interdependences and or the interaction with the latest volcanic activity also small displacements triggered by activity on other faults probably contributed to slip variability i e faults display primary and secondary behavior

evidence for limited ree leaching from the roffna gneiss switzerland a discussion of the paper by vocke et al 1987 cmp95 145 154

breakdown of feldspar volume gain and lateral mass transfer during mylonitization of granitoid in a low metamorphic grade shear zone abstract in a low metamorphic grade shear zone from quadrilatero ferrifero southeastern brazil granitoid rocks have been transformed into phyllonites and mylonites via activation of crystal plastic processes in quartz and fluid assisted reaction softening in feldspars quartz deformed by basal a and prism a slip with concurrent subgrain rotation recrystallization breakdown of matrix plagioclase an 22 35 via mica producing softening reactions required cationic exchanges between ca na and k forming replacement perthites in the k feldspar megacrysts which occupy around 60 of the original rock volume subsequent breakdown of the perthitic plagioclase an 12 15 led to disruption of the megacrysts enabling strain accommodation that localized the mylonite and phyllonite zones domainal volume and mass balance calculations based on major and minor elements indicate only small positive changes in mass and volume around 15 in the whole system however accentuated differences ranging from mass and volume losses of 25 and 35 at the margins to gains of 85 and 95 in the centre respectively exist between the individual longitudinal subzones occupied by the different tectonite types this scenario indicates that deformation proceeded as a nearly isochemical process although with lateral mobility of major elements principally si al and k within the shear zone these results are different from those previously reported for other granitic mylonites most of them indicating volume losses in compressional tectonic settings it is suggested that the extensional tectonic framework attributed to the moeda bonfim shear zone where mass volume losses are in principle not required enhanced the access of fluid and consequent transport of components into the shear zone principally fe si and k producing the small gains of mass and volume observed

paleoseismology of the 1912 acambay earthquake and the acambay tixmadeje fault trans mexican volcanic belt the acambay graben is a major intra arc basin of the trans mexican volcanic belt trenching studies at four sites along the acambay tixmadeje fault reveal evidence for the 1912 acambay earthquake and at least four late pleistocene and holocene ground rupturing earthquakes with similar displacements to the 1912 event evidence for multiple events was recognized by a combination of upward terminating faults fissure fills erosionally truncated fissures colluvial wedge fills and downward increasing separation of stratigraphic units sites along the central part of the ese wnw trending rangefront fault indicates large dip slip separations across a multi event scarp in holocene colluvium with a penultimate event that occurred since 5230 calibrated years before present cal yr b p on the huapango plain near the eastern end of the 1912 rupture where slip was oblique the penultimate earthquake occurred after 4700 cal yr b p and there is evidence for at least four more earthquakes in the last 34 000 cal yr b p three dimensional excavation of lacustrine and channel deposits at las lomas yields a left normal slip vector of s40 5 e with 105 10 cm of slip for the last three events there this slip vector is consistent with nne oriented extension across the graben the amount and style of slip documented for prehistoric ruptures are also consistent with the 1912 rupture with a mean of 60 cm event of dip slip on the rangefront and a mean of 35 cm event of oblique slip on the n48 w trending huapango plain assuming these values represent a characteristic behavior yields an average recurrence interval of 3600 years for the last four large earthquakes since 11 570 cal yr b p and a slip rate of 0 17 mm yr across the rangefront empirical earthquake parameters calculated from our paleoseismic data support instrumental measures indicating that the 1912 acambay earthquake was of m 6 8 7 0 the 1912 event is representative of ground rupturing earthquakes on the acambay tixmadeje fault and demonstrates that large seismic events m 7 can be generated by faults in the trans mexican volcanic belt and in other intra arc settings

accumulation mechanisms and the weathering of antarctic equilibrated ordinary chondrites induced thermoluminescence tl is used to quantitatively evaluate the degree of weathering of meteorites found in antarctica we find a weak correlation between tl sensitivity and descriptions of weathering in hand specimens the highly weathered meteorites having lower tl sensitivity than unweathered meteorites analysis of samples taken throughout large meteorites shows that the heterogeneity in tl sensitivity within meteorite finds is not large relative to the range exhibited by different weathered meteorites the tl sensitivity values can be restored by minimal acid washing suggesting the lower tl sensitivities of weathered meteorites reflects thin weathering rims on mineral grains or coating of these grains by iron oxides produced by hydration and oxidation of metal and sulfides small meteorites may tend to be more highly weathered than large meteorites at the allan hills ice fields we find that meteorite fragments 150 g may take up to 300 000 years to reach the highest degrees of weathering while meteorites 150 g require 40 000 years however at other fields local environmental conditions and variability in terrestrial history are more important in determining weathering than size alone weathering correlates poorly with surface exposure duration presumably because weathering occurs primarily during interglacial periods the allan hills locality has served as a fairly stable surface over the last 100 000 years or so and has efficiently preserved both small and large meteorites meteorites from lewis cliff however have experienced extensive weathering probably because of increased surface melt water from nearby outcrops meteorites from the elephant moraine locality tend to exhibit only minor degrees of weathering but small meteorites are less weathered than large meteorites which we suggest is due to the loss of small meteorites by aeolian transport

mechanisms of weathering of meteorites recovered from hot and cold deserts and the formation of phyllosilicates abstract petrographic mineralogical and chemical analysis of naturally weathered equilibrated ordinary chondrites collected from hot deserts and antarctica has revealed striking similarities and also pronounced differences in weathering between the two environments terrestrial weathering in all meteorites studied is dominated by oxidation and hydration of fe ni metal producing fe oxides and oxyhydroxides that have partially replaced the metal grains and have also occluded primary intergranular pores to form veins troilite weathers readily in hot desert environments but undergoes very little alteration under antarctic conditions most of the primary porosity of ordinary chondrites has been occluded by the time that 15 to 25 of the initial fe 0 and fe 2 has been oxidised to fe 3 in both environments results from modelling the volume changes upon alteration of primary minerals to a range of weathering products demonstrates that the primary porosity of most meteorites is sufficient to accommodate weathering products dilation of primary pores and brecciation which has been observed in parts of some meteorites will only occur if the meteorite is especially metal rich or has a low primary porosity these weathering products are absent from recent falls but have formed in a fall after 100 yr of museum storage cl bearing akaganeite and hibbingite are common weathering products in antarctic finds but occur in abundance in only one hot desert meteorite daraj 014 the majority of fe rich weathering products in meteorites from both environments contain low but variable concentrations of si mg and ca in most meteorites a proportion of these elements are inferred to be present as a very finely crystalline mineral with a 1 0 nm lattice fringe spacing where seen within intragranular fractures this mineral has a topotactic relationship with olivine and orthopyroxene in the heavily weathered antarctic finds alha 78045 and 77002 si is concentrated in cronstedtite a fe rich phyllosilicate an unidentified hydrous si fe ni mg mineral or gel has also partially replaced taenite in alha 78045 in addition to fe rich weathering products hot desert meteorites contain sulphates ca carbonate and silica whereas such minerals are largely absent from antarctic finds the abundance of silicate weathering products in antarctic meteorites is unexpected and indicates that olivine and pyroxene undergo significant chemical weathering in these environments as preterrestrial cronstedtite is abundant in cm2 carbonaceous chondrites the antarctic environment may be a powerful analog for aqueous alteration in the asteroidal parent bodies of primitive meteorites

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

steep subduction geometry of the rivera plate beneath the jalisco block in western mexico the morphology of the rivera plate subducted beneath the jalisco block in western mexico is determined from accurately located hypocenters of locally recorded microearthquakes and from earthquakes with magnitude mb 4 5 recorded at teleseismic distances the hypocenters of these latter earthquakes are relocated and for five of them the focal depth is constrained by the inversion of long period body waves the wadati benioff zone inferred from these data indicates a steep subduction of the rivera plate that resembles the geometry of subduction of the cocos plate beneath central america it is however very different from the shallower and almost subhorizontal subduction of the cocos plate observed in southern mexico southeast of this region the rivera plate is comparable to the juan de fuca plate in terms of the small areal extent young seafloor age low relative velocity and low teleseismic activity in the subduction zone this study shows that the dip of both the juan de fuca and rivera plates are similar once they are decoupled from the overriding continental crust the downgoing rivera plate initially starts with a dip of 10 down to a depth of 20 km and then increases gradually to a constant dip of 50 below a depth of 40 km intermediate depth seismicity is low in this zone associated with the subduction of the slow 2 cm yr and young 9 m y rivera plate the maximum depth extent of earthquakes observed in the rivera subduction zone is about 130 km the andesitic calc alkaline colima volcano appears to be directly related to the subduction of the rivera plate to the nw of this volcano the observed quaternary volcanism in the jalisco block which is parallel to the trench may also be explained by the subduction of the rivera plate

tephrochronology of the toluca basin central mexico abstract the major element glass geochemistry of 10 late pleistocene and holocene tephras found in the toluca basin in the trans mexican volcanic belt is reported for the first time as a key step towards establishing a tephrochronology for the region the context for this tephrochronology is provided by a review of late quaternary volcanism of the basin new tephra samples were collected from reference profiles around the basin in order to establish major element glass geochemical data for tephras described in previous publications but not previously analysed in addition cores were taken from the basin floor to extend the known spatial distribution of the tephra layers and establish correlations with the lacustrine record improved chronological control has been provided by seven new ams and conventional radiocarbon dates on the major tephras geochemical data are presented for 10 tephras ranging in age from ca 25 000 bp lower toluca pumice to about 8500 bp tres cruces tephra based on geochemical data it is possible to correlate between sites within the toluca basin and between the toluca basin and the basin of mexico the basis for a tephrochronological framework has been established

aster watches the world s volcanoes a new paradigm for volcanological observations from orbit onboard nasa terra spacecraft is one of the most sophisticated earth observing instruments ever flown the advanced spaceborne thermal emission and reflection radiometer aster created as part of a joint project between the japanese ministry of economy trade and industry and nasa this instrument consists of three independent bore sighted telescopes with 14 channels in the visible near infrared and thermal infrared regions capable of being pointed up to 22 5 off nadir in addition there is an additional backward looking visible channel telescope for stereo photogrammetric observations the spatial resolutions are 15 m pixel in the visible 30 m pixel in the near ir and 90 m pixel at thermal wavelengths and the swath width is 60 km aster instrument offers unprecedented opportunities for observing volcanoes and their activity from low earth orbit it is the purpose of this short paper to introduce aster to the volcanological community highlight its capabilities advantages and limitations and to show some examples of aster data and related analyses we will also explain how users can readily access aster data

the acambay graben active intraarc extension in the trans mexican volcanic belt mexico the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench the central part of the belt is being deformed by the chapala tula fault zone an approximately 450 km long and 50 km wide zone of active extension the volcanic arc and the arc parallel chapala tula fault zone are superposed nearly perpendicularly on the preexisting stress and deformation province of the mexican basin and range the acambay graben about 40 km long and 15 km wide is located approximately 100 km northwest of mexico city and is one of the major troughs within the chapala tula fault zone the border faults of the acambay graben acambay tixmadeje in the north and pastores in the south are separated in the west by stepovers from range bounding faults of similar orientation epitacio huerta in the north and venta de bravo in the south the stepovers occur at the intersection of these faults with an older system of basin and range faults an early stage right lateral component of motion along the venta de bravo and pastores faults is inferred on a map scale from a left stepping en echelon array of normal fault segments the divergence of the en echelon segments from the general fault trend decreases gradually from west to east suggesting that the early extension was rotational the present relative displacement along the southern margin of the system on the other hand results in a left lateral strike slip component this is documented on a map scale from extension structures at left stepovers and on an outcrop scale from fault striations indicating left oblique slip the striations measured at the northern system margin indicate nearly pure extensional dip slip without a consistent lateral displacement component this is supported on a map scale by the structure of the right stepover between the acambay tixmadeje and epitacio huerta faults which shows no evidence of local extension or shortening the divergence between the present directions of motion at the southern and northern margins of the extended zone can be explained by a minor rotational deformation component with the pole of rotation being located to the east of the zone of deformation this could explain why no active extension has been observed to the east of the chapala tula fault zone in the eastern part of the trans mexican volcanic belt during the ms 6 9 acambay earthquake of november 19 1912 surface rupture occurred along both margins of the graben at the base of multiple event scarps along the acambay tixmadeje fault the coseismic rupture is 41 km long the vertical offset increases gradually from the eastern end of the surface rupture to its center where it is with 50 cm at a maximum furthermore the change in the vertical surface offset along the fault is approximately proportional to the change in height of the acambay tixmadeje multiple event fault scarp the easternmost part of the ground rupture passes through a plain and not at the base of a multiple event scarp as farther west it may therefore correspond partly to an increase in length of the acambay tixmadeje fault during the 1912 earthquake the slip rate along the southern border of the acambay graben can be estimated from the displacement and age of a basalt flow for which we have obtained a 40ar 39ar age of 0 4 0 1 ma this basalt may be displaced up to 15 m by the pastores fault which indicates a middle late quaternary slip rate of 0 04 mm yr furthermore based on a coseismic surface rupture of approximately 20 cm along this fault in the 1912 earthquake we estimate a recurrence interval of 5000 years for major earthquakes along the faults of the acambay graben

volume loss fluid flow and coaxial versus noncoaxial deformation in retrograde amphibolite facies shear zones northern malawi east central africa two retrograde amphibolite facies shear zones were studied to explore the relationship between retrograde mineral reactions volume strain fluid flow mylonitization and coaxial versus noncoaxial deformation the two shear zones are the contractional mafwewu hills shear zone and the transcurrently displacing mkamasa river shear zone of northern malawi in general shear zone formation is characterized by the breakdown of feldspar and biotite and the formation of sillimanite quartz and water silica alkali and alkali earth elements were mobile mass balance calculations based on major and trace element geochemistry indicate as much as 50 60 volume loss in mylonite fluid to rock ratios estimated from the calculated depletions of silica are as much as 200 400 indicating that the initiation and activity of the shear zones were accompanied by large amounts of fluids that infiltrated the shear zones these fluids caused an almost complete loss of alkalies within the shear zone leading to the development of almost dry and sheet silicate free mylonite it is proposed that the associated destabilization of micas destroyed the pathways for fluid circulation and ultimately caused the cessation of shearing in the mafwewu hills shear zone shear zone related stretching lineations are oriented parallel to tectonic transport in the mkamasa river shear zone the stretching lineations curve from a subhorizontal attitude in the wall rock into a subvertical attitude in mylonite characterized by large volume loss shear bands in the mkamasa river shear zone indicate a sinistral sense of shear a synoptic analysis of mesoscale s and z asymmetric folds indicates that the orientation of the stretching lineations in mylonite characterized by large volume loss is subperpendicular to the regional tectonic transport direction aspect ratios of feldspar and quartz in xz sections x y z principal axes of finite strain ellipsoid show oblate grain shapes and show a positive correlation with volume strain mean kinematic vorticity numbers are 0 26 0 44 in the wall rock and 0 57 0 79 in mylonite in mylonite small kinematic vorticity numbers correlate with large volume loss the transport perpendicular orientation of the stretching lineations in strongly volume deficient mylonite is thought to be the result of a pronounced component of shortening normal to the shear zone the resulting higher degree of coaxial deformation controlled the final orientation of the stretching lineations

late pleistocene flank collapse of zempoala volcano central mexico and the role of fault reactivation abstract zempoala is an extinct pleistocene 0 7 0 8 ma stratovolcano that together with la corona volcano 0 9 ma forms the southern end of the sierra de las cruces volcanic range central mexico the volcano consists of andesitic and dacitic lava flows and domes as well as pyroclastic and epiclastic sequences and has had a complex history with several flank collapses one of these collapses occurred during the late pleistocene on the s se flank of the volcano and produced the zempoala debris avalanche deposit this collapse could have been triggered by the reactivation of two normal fault systems e w and ne sw although magmatic activity cannot be absolutely excluded the debris avalanche traveled 60 km to the south covers an area of 600 km 2 and has a total volume of 6 km 3 with a calculated heim coefficient h l of 0 03 based on the textural characteristics of the deposit we recognized three zones proximal axial and lateral distal zone the proximal zone consists of debris avalanche blocks that develop a hummocky topography the axial zone corresponds with the main debris avalanche deposit made of large clasts set in a sandy matrix which transformed to a debris flow in the lateral distal portion the deposit is heterolithologic in composition with dacitic and andesitic fragments from the old edifice that decrease in volume as bulking of exotic clasts from the substratum increase several cities cuernavaca jojutla de juarez alpuyeca with associated industrial agricultural and tourism activities have been built on the deposit which pose in evidence the possible impact in case of a new event with such characteristics since the area is still tectonically active

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

macroseismic study of shallow earthquakes in the central and eastern parts of the trans mexican volcanic belt mexico we present isoseismal maps for eight shallow earthquakes 1887 pinal de amoles 1912 acambay 1920 jalapa 1950 ixmiquilpan 1976 cardonal 1979 maravatio 1987 actopan and 1989 landa 4 1 mb 6 9 that occurred in the central and eastern parts of the trans mexican volcanic belt based on their intensity distributions we can relate several of these earthquakes to east west striking quaternary normal faults known from geological observations furthermore we derive for these events by empirical curve fitting linear relations between the magnitude calculated from the instrumental record the intensity distribution and the maximum intensity the comparison of these relations with equations from the literature suggests that the area shaken with a certain intensity by an earthquake in the trans mexican volcanic belt is approximately 10 times smaller than the area shaken by a shallow earthquake of the same magnitude in the intracratonic part of north america moreover we use the derived relationships between magnitude isoseismal area and maximum intensity to estimate the intensity magnitude mi of the 1887 pinal de amoles earthquake as 5 3 0 5

structural analysis and thermal remote sensing of the los humeros volcanic complex implications for volcano structure and geothermal exploration abstract the los humeros volcanic complex lhvc is an important geothermal target in the trans mexican volcanic belt understanding the structure of the lhvc and its influence on the occurrence of thermal anomalies and hydrothermal fluids is important to get insights into the interplay between the volcano tectonic setting and the characteristics of the geothermal resources in the area in this study we present a structural analysis of the lhvc focused on quaternary tectonic and volcano tectonic features including the areal distribution of monogenetic volcanic centers morphostructural analysis and structural field mapping revealed the geometry kinematics and dynamics of the structural features in the study area also thermal infrared remote sensing analysis has been applied to the lhvc for the first time to map the main endogenous thermal anomalies these data are integrated with newly proposed unconformity bounded stratigraphic units to evaluate the implications for the structural behavior of the caldera complex and geothermal field the lhvc is characterized by a multistage formation with at least two major episodes of caldera collapse los humeros caldera 460 ka and los potreros caldera 100 ka the study suggests that the geometry of the first collapse recalls a trap door structure and impinges on a thick volcanic succession 10 5 1 55 ma now hosting the geothermal reservoir the main ring faults of the two calderas are buried and sealed by the widespread post calderas volcanic products and for this reason they probably do not have enough permeability to be the main conveyers of the hydrothermal fluid circulation an active previously unrecognized fault system of volcano tectonic origin has been identified inside the los potreros caldera this fault system is the main geothermal target probably originated by active resurgence of the caldera floor the active fault system defines three distinct structural sectors in the caldera floor where the occurrence of hydrothermal fluids is controlled by fault induced secondary permeability the resurgence of the caldera floor could be induced by an inferred magmatic intrusion representing the heat source of the geothermal system and feeding the simultaneous monogenetic volcanic activity around the deforming area the operation of the geothermal field and the plans for further exploration should focus on both the active resurgence fault system and the new endogenous thermal anomalies mapped outside the known boundaries of the geothermal field

geometry and state of stress of the downgoing cocos plate in the isthmus of tehuantepec mexico the seismicity in the isthmus of tehuantepec mexico is anomalously active compared with other parts of the middle american subduction zone the results of a microearthquake study conducted in this region show that the slab changes dip from an almost subhorizontal geometry west of longitude 96 w to a dip of about 45 to 50 to the east of this longitude the change in dip occurs gradually in a broad flexure of the subducting slab that takes place over 150 km this geometry of the slab is reminiscent of that in southern peru the change in dip however takes place more gently in tehuantepec in addition to the change in the dip of the subducted cocos plate a rapid increase is observed in the maximum depth extent of the subducted slab in the region of subhorizontal subduction focal depths are consistently shallower than about 80 km whereas earthquakes as deep as 200 km are found where the subducted slab dips at a steeper angle both the change in the dip of the slab and of the maximum depth of intermediate depth earthquakes appear to correlate with the presence of the tehuantepec fracture zone which separates two distinct provinces of the oceanic cocos plate the age and the crustal thickness of the cocos plate increase to the southeast of the tehuantepec fracture zone relative to its northwestern counterpart in tehuantepec the dip and maximum depth of the subducted slab appears to be correlated with the age of the oceanic plate as has been observed in other subduction zones of the world

the origin of a terrane u pb zircon geochronology and tectonic evolution of the xolapa complex southern mexico terrane analysis has established differences in lithology style of deformation metamorphism and radiometric ages among several basement complexes in southern mexico in this paper we identify similarities in the geologic history of the adjacent acatlan oaxaca and xolapa complexes through a u pb zircon study on crustal sources and timing of magmatic metamorphic and tectonic activity in the xolapa complex our regional tectonic scenario is based on both new chronologic and previously established structural data inherited zircon in the xolapa complex indicates the presence of a proterozoic 1 0 1 3 ga crustal component with an age range that overlaps grenville crystallization dates 1 0 1 2 ga from the oaxaca basement and an inherited grenville crustal component 1 0 1 1 ga identified in zircons from the acatlan complex the proterozoic component in the xolapa complex indicates either that it received sediments from a continental region of grenvillean age for example the oaxaca basement which is the closest exposed grenville crust in southern mexico or that the xolapa complex has a grenville basement in the latter case the xolapa complex has been modified by widespread high grade metamorphism and large scale migmatization metamorphism and migmatization occurred from 66 to 46 ma and locally continued into oligocene time magmatism in the xolapa complex terminated with crustal growth by plutonism which is characterized by a systematic pattern of eastward younging crystallization ages from 35 ma in the west west of acapulco to 27 ma in the east east of puerto angel metamorphism and migmatization in the xolapa crust may have originated from a contemporaneous change in several parameters of convergence between the farallon and north american plates e g rate and direction of subduction and the early evolution of the caribbean this plate reorganization triggered sinistral transtension distributed across the southern mexican continental margin and the eastward translation of the chortis block the eastward younging of the xolapa plutons also is consistent with the motion of the chortis block during eastward displacement of the caribbean plate this induced a shift of the magmatic arc from its cretaceous to early tertiary position along the xolapa and chortis blocks to its present mid mexican position we conclude that a single tectonic framework accounts for the mesozoic and cenozoic geologic history of the southern mexican terranes

formation of carbonates in the tatahouine meteorite the tatahouine meteorite in southern tunisia shows terrestrial contamination that developed during 63 years of exposure on earth s surface samples collected on the day of the fall in 1931 contained fractures with no secondary minerals whereas samples collected in 1994 contain calcite aggregates 70 to 150 micrometers and rod shaped forms 100 to 600 nanometers in length and 70 to 80 nanometers in diameter on the fractures carbon isotope analysis of the carbonates within the tatahouine meteorite delta13c 2 0 per mil pee dee belemnite standard pdb and the underlying ground delta13c 3 2 per mil pdb confirm their terrestrial origin

physical and chemical response of zircons to deformation an investigation of u pb isotopic systematics in zircons from mylonitized henderson gneiss sinha and glover 1978 revealed that selected zircon fractions from the mylonite zone suffered total loss of radiogenic pb at 460 m y to further investigate the relationship between pb loss u gain and grain size reduction associated with increasing strain in the shear zone we have characterized the chemistry and morphology of zircons in the mylonitic rocks using both electron microprobe analysis and scanning electron microscopy

probabilistic seismic hazard at mt etna italy the contribution of local fault activity in mid term assessment abstract in this work we tackle the problem of seismic hazard at etna deriving from the recurrent seismogenic activity of local faults by adopting two independent methods based on probabilistic approaches we assess the hazard in terms of macroseismic intensity and represent the occurrence probability calculated for different exposure times both on maps and at fault scale seismic hazard maps obtained by applying the site approach through the sasha code and a new probabilistic attenuation model indicate the eastern flank of the volcano as the most hazardous with expected intensity i exp in 50 years i e the standard exposure time adopted in the seismic regulations ranging from degrees ix to x ems in shorter exposure periods 20 10 5 years values of i exp up to ix are also reached in the same area but they are clearly determined by the earthquakes generated by the timpe fault system in order to quantify the contribution of local seismogenic sources to the hazard of the region we reconstruct the seismic history of each fault and calculate with sasha the probability that earthquakes of a given intensity may be generated in different exposure times results confirm the high level of hazard due to the s tecla moscarello and fiandaca faults especially for earthquakes of moderate intensity i e vi i 0 vii with probabilities respectively exceeding 50 and 20 in 10 years and 30 and 10 in 5 years occurrence probability of major events i 0 viii at the fault scale has also been investigated by statistics on intertimes under stationary assumptions we obtain a probability of 6 8 in 5 years for each structure by introducing the time dependency time elapsed since the last event occurred on each fault through a bpt model we identify the moscarello and s tecla faults as the most probable sources to be activated in the next 5 years 2013 2017 this result may represent a useful indication to establish priority criteria for actions aimed at reducing seismic risk at a local scale

climate and rock weathering a study of terrestrial age dated ordinary chondritic meteorites from hot desert regions abstract ordinary chondrites oc recovered from the desert areas of roosevelt county new mexico the nullarbor region of western australia and the algerian and libyan sahara for which 14c terrestrial ages have been determined were examined by 57fe mossbauer spectroscopy oc were chosen as a standard sample to investigate weathering processes as their well constrained trace and bulk element chemistry normative mineralogy and isotopic composition define a known pre weathering starting composition given that terrestrial ages are known it is possible to compare initially very similar samples that have been subsequently weathered in a range of climatic regimes from the present day to 44 ka bp in addition recently fallen equilibrated oc contain iron only as fe0 and fe2 thus the abundance of ferric iron is directly related to the level of terrestrial weathering mossbauer spectroscopy identifies two broad types of ferric alteration paramagnetic phases akaganeite lepidocrocite and goethite and magnetically ordered principally magnetite and maghemite oc finds show a range in the percentage of total fe existing as fe3 from zero to over 80 however oxidation is comparable between fragments of the same oc separated since their time of fall i e paired meteorites our results indicate several features of meteorite weathering that may result from climatic or geomorphologic conditions at the accumulation site 1 saharan samples are overall less weathered than non saharan samples which may be related to the relatively recent age ca 20 ka of the saharan accumulation surface 2 broad differences between sites in the rate of weathering arising from regional differences in climate 3 consistent differences in the weathering products between samples that fell during humid periods and those that fell during more arid periods those falling during humid periods contain a higher proportion of magnetically ordered ferric oxides 4 one region the nullarbor that shows a variation in the total amount of ferric species that closely matches the climatic record for this area of australia for the last 30 ka points 3 and 4 may be related to the identification of a rapid initial weathering phase the majority of weathering occurs in the first few hundred years after fall followed by passivation of weathering by porosity reduction porosity reduction and the associated restriction in the ability of water to penetrate the sample appears to be the mechanism whereby a weathering assemblage formed during the brief initial period of oxidation is preserved through subsequent climatic cycles over the terrestrial lifetime of the sample

why is the central trans mexican volcanic belt 102 99 w in transtensive deformation abstract using well defined focal mechanisms new poles of rotation and accurate bathymetry it will be shown that the n34 e oblique convergence between the cocos and north america plates along the n290 e middle american trench in front of central mexico 102 99 w is associated with slip partitioning in the same way it will be shown that the forearc sliver will move rigidly eastward along a trench parallel left lateral strike slip fault by reviewing the literature and analysing seismicity the e w central trans mexican volcanic belt tmvb is the only active zone in central mexico it is the best candidate to accommodate slip partitioning at the trench where mainly extensive e w trending normal active faults with left lateral component are reported fault striae inversions derived from the literature and our focal mechanisms inversion confirm that the state of stress in the central tmvb is transtensive since middle quaternary with shmin 3 trending nw se and shmax 1 or 2 trending ne sw the oblique convergence and slip partitioning at the trench induce this state of stress the n290 w trench parallel component which should be accommodated in the upper plate is however not parallel to the e w arc parallel component but differs by about 20 clockwise this implies that the former must be partitioned along the e w faults of the central tmvb if we assume that the trench parallel slip rate of 8 mm year 1 102 99 w is fully accommodated along the central tmvb we should expect maximum horizontal extensional and left lateral slip rates along e w normal faults of 2 mm year 1 and less than 7 mm year 1 respectively strain partitioning may also be accommodated partially by the accretionary wedge or and by normal faulting that affects the deeper part of the upper plate around 18 n of latitude

active fault systems and tectono topographic configuration of the central trans mexican volcanic belt abstract the central trans mexican volcanic belt tmvb reflects the interplay between three regional fault systems the nnw sse to nw se striking taxco queretaro fault system the ne sw striking system and the e w striking morelia acambay fault system the latter is the youngest and consists of fault scarps up to 500 m high whose formation caused structural and morphological reorganization of the region in this paper we investigate possible activity of the three systems within the central tmvb and assess the role that they play in controlling the tectono topographic configuration of the area our study is based on dem derived morphometric maps longitudinal river profiles geomorphologic mapping and structural field data concerning recent faulting we find that all three regional fault systems are active within the central tmvb possibly with different displacement rates and or type of motion and that nnw sse and ne sw striking faults control the major tectono topographic elements that build up the region which are being re shaped by e w striking faults we also find that tectonic information can be deciphered from the topography of the youthful volcanic arc in question regardless its complexity

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle

tectonics and volcanism of sierra chichinautzin extension at the front of the central trans mexican volcanic belt abstract because of its recent activity and position at the southern magmatic front of the trans mexican volcanic belt tmvb the sierra chichinautzin volcanic field scn is a key area for the understanding of this controversial volcanic province volcanic activity has built more than 220 monogenetic volcanoes shields scoria cones thick lava flows and hydromagmatic structures during the last 40 000 years for a total volume of about 470 km 3 the scn basalts are geochemically similar to oibs while the intermediate and felsic volcanic rocks show a calc alkaline trend and abundant evidence for magma mixing the structural analysis of this volcanic field and surrounding areas has been based on field data satellite images and a method for detecting volcanic center alignments the tectonic data together with geophysical evidence confirm active general n s extensional conditions with a strike slip component for the scn area the same structural setting that prevails in the rest of the central tmvb extensional tectonics a negative regional bouger gravity anomaly a low velocity mantle high heat flow and shallow seismicity suggest a rift type setting involving the upwelling of anomalous mantle beneath the central tmvb the combined petrological structural and geophysical arguments support that the scn volcanism is rift related and rule out processes involving the subduction of the cocos plate which casts further doubts on the standard subduction model for the tmvb volcanism