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

the role of oceanic forcing in mid holocene northern hemisphere climatic change previous model investigations suggested that changes in orbital forcing and feedbacks associated with northward expansion of the boreal forest were both required to explain the full magnitude of enhanced high latitude northern hemisphere summer warming at 6 ka thus implying that biospheric feedbacks decreased planetary albedo associated with forest expansion may be large in the future foley et al 1994 tempo testing earth system models with paleo observations 1996 before the magnitude of past biospheric and other feedbacks can be estimated with confidence however the role of realistic high latitude oceanic forcing i e sea surface temperature sst increases and sea ice reductions should also be considered here we review existing paleoceanographic observations that suggest portions of the north atlantic were up to 4 c warmer than today at 6 ka we then combine our estimates of mid holocene north atlantic sst and sea ice conditions with new climate model simulations to suggest that a significant portion of high latitude summer warming at 6 ka in the northern hemisphere can be accounted for by altered orbital forcing ssts and sea ice relative to today our results underscore the importance of incorporating realistic boundary conditions when using paleoenvironmental data to evaluate the climate system s sensitivity to altered forcing and suggest that high latitude oceanic feedbacks were major contributors to enhanced high latitude summer warming in the northern hemisphere at 6 ka

variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

effect of tsunamis generated in the manila trench on the gulf of thailand abstract tsunamis generated in the manila trench can be a threat to thailand besides runup of tsunamis along the eastern coast infrastructures in the gulf of thailand for example gas pipelines and platforms can be affected by tsunamis in this study the simulation of tsunamis in the gulf of thailand is conducted six cases of fault ruptures in the manila trench are considered for earthquakes with magnitudes of 8 0 8 5 and 9 0 the linear shallow water wave theory in spherical coordinate system is used for tsunami simulation in the large area covering southeast asia while the nonlinear shallow water wave theory in cartesian coordinate system is used for tsunami simulation in the gulf of thailand it is found that tsunamis reach the southern part of thailand in 13 h after an earthquake and reach bangkok in 19 h the tsunami amplitude is largest in the direction towards the philippines and vietnam the southern part of china is also severely affected the gulf of thailand is affected by the diffraction of tsunamis around the southern part of vietnam and cambodia the tsunami amplitude at the southernmost coastline is about 0 65 m for the mw 9 0 earthquake the current velocity in the gulf of thailand due to the mw 9 0 earthquake is generally less than 0 2 m s

high frequency of super cyclones along the great barrier reef over the past 5 000 years understanding long term variability in the occurrence of tropical cyclones that are of extreme intensity is important for determining their role in ecological disturbances1 2 3 4 5 for predicting present and future community vulnerability and economic loss6 and for assessing whether changes in the variability of such cyclones are induced by climate change7 our ability to accurately make these assessments has been limited by the short less than 100 years instrumented record of cyclone intensity here we determine the intensity of prehistoric tropical cyclones over the past 5 000 years from ridges of detrital coral and shell deposited above highest tide and terraces that have been eroded into coarse grained alluvial fan deposits these features occur along 1 500 km of the great barrier reef and also the gulf of carpentaria australia we infer that the deposits were formed by storms with recurrence intervals of two to three centuries8 9 10 11 and we show that the cyclones responsible must have been of extreme intensity central pressures less than 920 hpa our estimate of the frequency of such super cyclones is an order of magnitude higher than that previously estimated which was once every several millennia12 13 14 and is sufficiently high to suggest that the character of rainforests and coral reef communities were probably shaped by these events

holocene climate variability and change a data based review the holocene thermal maximum with peak temperatures prior to 7 ka bp is mostly accentuated in the northern hemisphere still visible in the southern hemisphere and possibly did not exist in the tropics between this period and the modern warming a remarkable negative temperature trend occurred in the northern hemisphere which was probably the effect of a decreasing orbital induced insolation during the boreal summer on average the northern hemisphere humidity precipitation records do not show any significant trend a mechanistic explanation for the multi decadal to century scale holocene cold relapses which mainly occurred in the atlantic european regions exists for only the early holocene cooling events which are probably the result of a collapse of the meridional overturning circulation owing to freshwater outbursts from the laurentide ice sheet possibly the late holocene cold events after c 4 ka bp are influenced by the covarying influence of major tropical volcanic eruptions and grand solar minima

geology and geochemistry of amealco caldera qro mexico amealco caldera located in the central part of the mexican volcanic belt mvb about 130 km nw of mexico city is characterized by a quasi circular 11 km diameter structure whose southern portion is partly truncated by a regional fault the caldera was formed 5 m y ago due to a voluminous eruption of 65 km3 eq magma volume of ignimbrites which covered an area of 1800 km2 this was followed by eruption of 2 km3 of andesitic and dacitic lava flows from the border of the caldera and emplacement of a total of 8 silicic rhyolitic domes 8 km3 inside the caldera finally postcaldera volcanism 4 km3 consisted of extrusion of andesitic and basaltic andesite lavas and scoriae we have analyzed 17 representative rock samples for their major and trace element contents by x ray fluorescence spectrometry and for rare earth elements by high performance liquid chromatography all samples were also analyzed for their sr and nd isotopic compositions which range as follows 87sr 86sr 0 70379 0 70451 143nd 144nd 0 51267 0 51278 the chemical and isotopic data are used to constrain the origin and evolution of magmas in the caldera no combination of altered morb and overlying sediments from the subducted slab cocos plate can produce the sr and nd isotopic compositions observed in rocks from amealco caldera further modelling reveals that the mantle derived primary magmas in amealco caldera may have assimilated up to 10 of a crustal component during their evolution from basaltic andesites to dacites but no further assimilation appears to have taken place during the evolution of the rhyolitic magmas the structure and genesis of the amealco caldera are comparable to some other calderas of the mvb

sea level changes in the tectonically stable malay thai peninsula abstract the malay thai peninsula is part of sundaland a geological region of relative crustal stability therefore the positions of old shorelines of the past 20 000 years or so can be considered to indicate positions of actual past sea levels over 200 shoreline indicators at positions below as well as above present sea level have been radiocarbon dated these dated positions indicate that prior to 6 ka the sea rose from at least 90 m below present sea level at rates that initially amounted to 15 mm year and later became 6 mm year at ca 6 ka the rising sea surpassed its current position and at ca 5 ka it reached 5 m during this maximum marine inundation the peninsula was narrower since the low coastal plains did not yet exist in the period following this maximum transgression the available shoreline data indicate that regional sea level subsided through a series of fluctuations that were characterized by amplitudes of 2 m and periods of about 2000 years the peaks and troughs of the fluctuations became progressively lower the latest peak was at about 1 7 m or 0 7 m above present high tide a few hundred years ago the cause of these fluctuations is not yet known but the trend suggests that in the near future regional sea level may subside at annual rates between 1 5 mm and 2 mm this rate of descent is 25 to 30 of the estimated rate of sea level rise that may result from global warming in other words for the malay thai peninsula the effect of sea level rise on account of manmade greenhouse gases will be partly compensated by actual subsidence of regional sea level

the sierra de mil cumbres michoacan mexico transitional volcanism between the sierra madre occidental and the trans mexican volcanic belt abstract the sierra de mil cumbres is a miocene volcanic range located in central mexico in the north eastern part of the state of michoacan near the city of morelia structurally it is a ene trending horst that covers an area of 1022 km2 approximately 20 km wide 60 km long and contains exposures of chemically bimodal volcanism in the form of ignimbrites lava domes lava flows cinder cones and related deposits the main volcanic manifestations of this range are the la escalera caldera 16 3 23 ma the garnica volcanic complex 18 3 17 9 ma the atecuaro caldera 16 3 19 4 ma and the indaparapeo volcanic complex 14 1 17 5 ma the sierra de mil cumbres stands in space and time at the intersection between the miocene recent trans mexican volcanic belt and the late cretaceous early miocene sierra madre occidental and so provides new insights into the geological evolution of central mexico arc volcanism in the sierra de mil cumbres was initiated by a massive nnw sse extension probably during the counterclockwise rotation of the sierra madre occidental new geological mapping stratigraphic analysis detailed geochemistry and 40ar 39ar geochronology demonstrates that this intra plate volcanism was emplaced between 14 and 23 ma

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

tectonics structural evolution and caldera formation on vulcano island aeolian archipelago southern tyrrhenian sea abstract a detailed structural study and a review of the eruptive history of the island of vulcano aeolian arc southern tyrrhenian sea have been carried out with the purpose of understanding the evolution of both volcanic and tectonic structures the island is composed of four main volcanoes which date from about 120 ka to historical times two calderas occur in the central sector of the island the time space evolution of the volcanism indicates a shifting of the activity from the southeastern sectors towards the northwest two main systems of nw se trending right lateral strike slip faults affect the island ne sw and n s trending normal faults are also present geophysical investigation indicate the nw se trending strike slip faults are likely to be the surface expression of a main regional crustal structure this system of discontinuity is related with the stress field acting in the southern sector of the aeolian archipelago normal n s and ne sw oriented faults represent extensional fractures the ne sw trending structures are related to a local stress field the results of the kinematic analysis as well as the geometric relationships between the faults and the constant subsidence velocities are consistent with the opening of a pull apart basin volcanological and geochronological data are also consistent with the proposed model in the general deformative style affecting the island the calderas can be explained as the consequence of different opening phases of a pull apart 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

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

hydro geochemical and isotopic fluid evolution of the los azufres geothermal field central mexico abstract hydrothermal alteration at los azufres geothermal field is mostly propylitic with a progressive dehydration with depth and temperature increase argillic and advanced argillic zones overlie the propylitic zone owing to the activity of gases in the system the deepest fluid inclusions proto fluid are liquid rich with low salinity with nacl dominant fluid type and ice melting temperatures t mi near zero 0 c and salinities of 0 8 wt nacl equivalent the homogenization temperature t h 325 5 c the boiling zone shows t h 300 c and apparent salinities between 1 and 4 9 wt nacl equivalent implying a vaporization process and a very important participation of non condensable gases ncgs mostly co 2 positive clathrate melting temperatures fusion with t h 150 c are observed in the upper part of the geothermal reservoir from 0 to 700 m depth these could well be the evidence of a high gas concentration the current water produced at the geothermal wells is nacl rich geothermal brine and is fully equilibrated with the host rock at temperatures between t 300 and 340 c the hot spring waters are acid sulfate indicating that they are derived from meteoric water heated by geothermal steam the ncgs related to the steam dominant zone are composed mostly of co 2 80 98 of all the gases the gases represent between 2 and 9 wt of the total mass of the fluid of the reservoir the authors interpret the evolution of this system as deep liquid water boiling when ascending through fractures connected to the surface boiling is caused by a drop of pressure which favors an increase in the steam phase within the brine ascending towards the surface during this ascent the fluid becomes steam dominant in the shallowest zone and mixes with meteoric water in perched aquifers stable isotope compositions 18 o d of the geothermal brine indicate mixing between meteoric water and a minor magmatic component the enrichment in 18 o is due to the rock water interaction at relatively high temperatures 13 c stable isotope data show a magmatic source with a minor meteoric contribution for co 2 the initial isotopic value 34 s res 2 3 which implies a magmatic source more negative values are observed for shallow pyrite and range from 34 s fes 2 4 to 4 9 indicating boiling the same fractionation tendencies are observed for fluids in the reservoir from results for 18 o

assessing sedimentary records of paleohurricane activity using modeled hurricane climatology patterns of overwash deposition observed within back barrier sediment archives can indicate past changes in tropical cyclone activity however it is necessary to evaluate the significance of observed trends in the context of the full range of variability under modern climate conditions here we present a method for assessing the statistical significance of patterns observed within a sedimentary hurricane overwash reconstruction to alleviate restrictions associated with the limited number of historical hurricanes affecting a specific site we apply a recently published technique for generating a large number of synthetic storms using a coupled ocean atmosphere hurricane model set to simulate modern climatology thousands of overwash records are generated for a site using a random draw of these synthetic hurricanes a prescribed threshold for overwash and a specified temporal resolution based on sedimentation rates observed at a particular site as a test case we apply this monte carlo technique to a hurricane induced overwash reconstruction developed from laguna playa grande lpg a coastal lagoon located on the island of vieques puerto rico in the northeastern caribbean apparent overwash rates in the lpg overwash record are observed to be four times lower between 2500 and 1000 years b p when compared to apparent overwash rates during the last 300 years however probability distributions based on monte carlo simulations indicate that as much as 65 of this drop can be explained by a reduction in the temporal resolution for older sediments due to a decrease in sedimentation rates periods of no apparent overwash activity at lpg between 2500 and 3600 years b p and 500 1000 years b p are exceptionally long and are unlikely to occur above 99 confidence under the current climate conditions in addition breaks in activity are difficult to produce even when the hurricane model is forced to a constant el nino state results from this study continue to support the interpretation that the western north atlantic has exhibited significant changes in hurricane climatology over the last 5500 years

atlantic seasonal hurricane frequency part i el nino and 30 mb quasi biennial oscillation influences abstract this is the first of two papers on atlantic seasonal hurricane frequency in this paper seasonal hurricane frequency as related to e1 nino events during 1900 82 and to the equatorial quasi biennial oscillation qbo of stratospheric zonal wind from 1950 to 1982 is discussed it is shown that a substantial negative correlation is typically present between the seasonal number of hurricanes hurricane days and tropical storms and moderate or strong 15 cases el nin off the south american west coast a similar negative anomaly in hurricane activity occurs when equatorial winds at 30 mb are from an easterly direction and or are becoming more easterly with time during the hurricane season this association of atlantic hurricane activity with el nino can also be made with the southern oscillation index by contrast seasonal hurricane frequency is slightly above normal in non el nino years and substantially above normal when equatorial stratospheric winds blow from a westerly direction and or are bec

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

tectonic evolution of the central eastern sector of trans mexican volcanic belt and its influence on the eruptive history of the nevado de toluca volcano mexico abstract the nevado de toluca is an andesitic to dacitic stratovolcano of late pliocene holocene age located within the central and eastern sectors of the trans mexican volcanic belt morphostructural analysis aerial photograph and satellite image interpretation structural analysis and geological fieldwork were methods used to investigate the relationship between the evolution of the volcano and the tectonic framework of its basement the study revealed that the area of nevado de toluca is affected by three main fault systems that intersect close to the volcanic edifice these are from oldest to youngest the taxco queretaro san antonio and tenango fault systems the nnw sse taxco queretaro fault system was active in the area since early miocene and is characterized by right lateral transtensive movement its reactivation during early to middle pleistocene was responsible for the emplacement of andesitic to dacitic lava flows and domes of la cieneguilla supersynthem the ne sw san antonio fault system was active during late pliocene before the reactivation of the taxco queretaro fault system and is characterized by extensional left lateral oblique slip kinematics the youngest is the e w tenango fault system that has been active since late pleistocene this fault system is characterized by transtensive left lateral strike slip movement and partly coeval with the youngest eruptive phase the nevado supersynthem which formed the present summit cone of the nevado de toluca volcano the stress re orientation from the taxco queretaro to the tenango fault system during late pleistocene is responsible for the 1 ma hiatus in the magmatic activity between 1 15 ma and 42 ka after this period of repose the eruptive style drastically changed from effusive to explosive with the emission of dacitic products the methodology presented here furnish new data that can be used to better assess the complex structural evolution of this sector of the trans mexican volcanic belt and to understand the influence of changes in the tectonic setting on the growth of volcanic edifices

tectonics along the trans mexican volcanic belt according to palaeomagnetic data abstract subduction of the cocos plate beneath the southern mexico continental margin takes place at the middle america trench volcanism of calc alkaline character occurs along the e w trending trans mexican volcanic belt tmvb which includes several active stratovolcanoes monogenetic cinder cone fields and silicic centres however a direct association between the volcanism and plate subduction has been questioned for a number of reasons including the oblique orientation of the tmvb with respect to the trench most palaeomagnetic directions for the cretaceous to quaternary within or immediately to the south of the tmvb appear discordant the declinations are rotated anticlockwise relative to expected declinations this angular divergence may be explained by rotations of crustal blocks by different degrees in response to large scale left lateral shear across central mexico the fault system may have acted as an efficient structural control for the rise of magmas which may explain some of the peculiarities of the tmvb

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

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

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

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

lake sediment record of late holocene hurricane activities from coastal alabama coastal lake sediments contain a stratigraphically and chronologically distinct record of major hurricane strikes during late holocene time frederic a category 3 hurricane that struck the alabama coast on the gulf of mexico in 1979 left a distinct sand layer in the nearshore sediments of lake shelby as a result of storm tide overwash of beaches and dunes sediment cores taken from the center of lake shelby contain multiple sand layers suggesting that major hurricanes of category 4 or 5 intensity directly struck the alabama coast at ca 3 2 3 0 2 6 2 2 1 4 and 0 8 ka 14 c yr with an average recurrence interval of 600 yr the alabama coast is likely to be struck by a category 4 or 5 hurricane within the next century

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

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

magnitude of hurricane ike storm surge sedimentation implications for coastal marsh aggradation there is a paucity of information on the regional distribution and magnitude of hurricane storm surge sedimentation this study assesses the spatial extent and magnitude of hurricane ike s 2008 storm surge sedimentation and discusses implications for the role of hurricanes in marsh aggradation the characteristics of the storm surge deposit including thickness inland penetration volume and mass were determined for 15 transects across marshes bordering the gulf of mexico in south eastern texas and south western louisiana the deposit is up to 0 85 m thick extends up to 3 6 km inland and has an estimated volume of about 13 7 million m3 and an estimated mass of about 16 2 million metric tons this level of sedimentation is one to two orders of magnitude larger than other potential sources of marsh sedimentation including annual riverine inputs and inputs from alongshore sediment transport the study findings add support to a growing body of evidence that hurricanes may be the predominant sediment source for long term aggradation of many coastal marshes bordering the gulf of mexico copyright 2012 john wiley sons ltd

stratigraphy and structure of the canas dulces caldera costa rica marti and aguirre were funded by spanish ministry of science and innovation micinn grant cgl2010 22022 c02 02 and by universidan nacional autonoma de mexico unam grant in 112312

sedimentary evidence of hurricane strikes in western long island new york 1 evidence of historical landfalling hurricanes and prehistoric storms has been recovered from backbarrier environments in the new york city area overwash deposits correlate with landfalls of the most intense documented hurricanes in the area including the hurricanes of 1893 1821 1788 and 1693 a d there is little evidence of intense hurricane landfalls in the region for several hundred years prior to the late 17th century a d the apparent increase in intense hurricane landfalls around 300 years ago occurs during the latter half of the little ice age a time of lower tropical sea surface temperatures multiple washovers laid down between 2200 and 900 cal yr b p suggest an interval of frequent intense hurricane landfalls in the region our results provide preliminary evidence that fluctuations in intense hurricane landfall in the northeastern united states were roughly synchronous with hurricane landfall fluctuations observed for the caribbean and gulf coast suggesting north atlantic wide changes in hurricane activity

geologic records of holocene typhoon strikes on the gulf of thailand coast abstract washover sedimentation resulting from modern typhoon strikes on the gulf of thailand coast forms anomalous sand layers in low energy coastal environments including marshes ponds and swales the primary diagnostic for recognizing prehistoric typhoon deposited sand layers in the geologic record are sharp upper and lower contacts between coarser grained transported sand layers and finer grained in situ sediments in this first paleotempestology study in thailand cores from two low energy settings on the gulf of thailand coast a coastal marsh near cha am and beach ridge plain swales near kui buri reveal geologic evidence of up to 19 typhoon strikes within the last 8000 years the sand layers have sharp upper and lower contacts with enclosing finer sediments some sand layers also contain other evidence of a sudden powerful landward directed surge of ocean water including gravel sized clasts offshore foraminifera abundant shell fragments plant debris and mud rip up clasts sand layers record eleven typhoon strikes at cha am ranging in age from ad 1952 to 7575 cal yr bp and eight typhoon strikes at kui buri ranging in age from 4075 to 7740 cal yr bp bayesian age depth models derived from eight ams radiocarbon dates suggest that the frequency of typhoon strikes was 2 5 times greater from 3900 to 7800 cal yr bp compared to 0 3900 cal yr bp possible explanations for this variability in the typhoon record are that typhoons were more frequent and or more intense in southeast asia in the mid holocene because of climatic changes associated with the mid holocene warm period or that the record reflects site sensitivity changes resulting from a mid holocene sea level highstand the preliminary finding of a possible link between warmer conditions and a greater frequency of intense typhoon strikes could have important societal implications given possible consequences of ongoing global warming

phase segregation boiling and reinjection at the los azufres geothermal field mexico monitored by water stable isotopes chloride and enthalpy abstract monitoring the impact of brine reinjection is crucial to the sustainable exploitation of geothermal resources reinjection monitoring at the los azufres geothermal field mexico has historically been carried out by investigating the temporal and spatial evolution of chloride and the stable isotopic composition of water here we review the evolution of the stable isotopic composition of the total fluid discharge td since the earliest measurements in 1980 integrating new data obtained in 2014 based on dtd and 18otd values measured in 2014 two groups of fluids were identified the first group is characterized by dtd and 18otd similar to those measured in previous survey years 1980 1982 1983 1986 and 2000 the second group shows a large positive shift in the hydrogen isotopic composition dtd of 10 compared to the first group the evolution of reinjected brines and production wells over time between 1980 and 2014 shows 18otd and dtd enrichment until 2000 this is followed between 2000 and 2014 by a slight decrease in 18otd values and a net increase in dtd explained by extensive phase segregation and boiling at temperatures of over 280 c the observed trend suggests that a steam phase caused by boiling is invading the reservoir and the phenomenon is observed in both production and reinjected fluids a comparison of the spatially kriged 2000 and 2014 18otd and dtd datasets reveals that reinjected brine is spreading progressively to the eastern part of the field mainly in the southern production zone spz and through e w corridors delimited by the e w fault systems which underlie the secondary permeability of the reservoir however reinjection is still not able to suppress boiling in the main production zone this situation needs to be taken into account in developing future reinjection strategies to counterbalance the substantial fluid extraction from the field

lacustrine record in a volcanic intra arc setting the evolution of the late neogene cuitzeo basin system central western mexico michoacan abstract late miocene to quaternary lacustrine and alluvial lacustrine sequences were deposited in the charo morelia sub basin to the south of the recent cuitzeo lacustrine depocentre central western mexico where deposition was largely controlled by widespread faulting and block tilting moreover pyroclastic and epiclastic deposits occur throughout the terrigenous and biogenic sequence which indicates intense synchronous volcanic activity directly linked to the faulting the charo alluvial lacustrine sequences crop out extensively to the south of the present cuitzeo lake and their exposures record the development and spreading of late miocene and pliocene alluvial lacustrine zones the resulting sequences are up to 40 m thick and record four major evolutionary stages related with specific tectonic and volcanic events which caused changes in the basin morphometry and the palaeodrainage conditions the first of these stages late miocene caused the generation of small ne sw oriented shallow lacustrine and swampy zones which resulted from the activity of strike slip faults with the same orientation alluvial deposits and shallow lacustrine diatomaceous horizons were deposited during this stage the second stage early pliocene gave rise to the formation of ne sw trending basins related to the extensional reactivation of the earlier ne sw strike slip faults within this second stage the charo morelia lacustrine depocentre became a larger deeper tropical lake the planktonic and planktonic facultative lacustrine diatom floral assemblages record the development during this stage of a major lake transgression which could be caused both by wetter climate and tectonic watershed and drainage rearrangement the third and fourth stages quaternary were related to widespread block tilting processes which were linked to the generation of e w oriented extensional faults and which resulted in the shifting and restriction of the larger perennial steadier lacustrine zones to the present cuitzeo lake to the south of this lake the sedimentary record of this stage started with widespread fluvial sequences which erosively overlaid the diatomitic deposits the lacustrine zones developed during this alluvial dominated stage were restricted to small depressions and consisted of small lakes and pond related environments which were fed by ephemeral fluvial currents these minor water bodies were infilled and overlain by acidic pyroclastic fall deposits developed during a major volcanic episode which affected the charo morelia sub basin fourth stage the cuitzeo and other ancient to recent lacustrine systems i e chapala lake evolved in the trans mexican volcanic belt as a result of its tectono volcanic evolution which largely influenced the highly sensitive lacustrine systems therefore the analysis of the development sedimentary evolution and shifting of the lacustrine zones may help us to understand the structural evolution of the central mexican igneous arc complex and the ongoing processes developed along the related major subduction zone which is oblique to the middle american trench in the pacific margin

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

holocene sea levels and palaeoenvironments malay thai peninsula southeast asia sedimentological and palynological investigations of great songkhla lakes east coast of the malay thai peninsula southeast asia reveal sedimentary sequences rich in palynomorph assemblages dominated by pollen of mangroves and freshwater swamps compared with other regions in southeast asia the assemblages are of relatively low diversity geochronological data indicate that the great songkhla lakes record one of the earliest mangrove environments in southeast asia 8420 8190 cal yr bp which are subsequently replaced by a freshwater swamp at 7880 7680 cal yr bp owing to the decline of marine influence sea level observations from great songkhla lakes and other areas of the malay thai peninsula reveal an upward trend of holocene relative sea level from a minimum of 22 m at 9700 9250 cal yr bp to a mid holocene high stand of 4850 4450 cal yr bp which equates to a rise of c 5 5 mm yr the sea level fall from the high stand is steady at c 1 1 mm yr geophysical modelling shows that hydroisostasy

600 year sedimentary archive of hurricane strikes in a prograding beach ridge plain southwestern louisiana abstract this study tests a novel approach to paleotempestology substituting the lateral time depth of a prograding beach ridge plain for the vertical time depth of an aggrading coastal lake pond lagoon swamp or marsh trenching and coring of a beach ridge plain in southwestern louisiana revealed six sand beds and one shell bed interpreted as hurricane washover deposits the sand beds are readily distinguished from enclosing marsh sediments by their higher sand content lower organic content and contrasting microfossil assemblages based on the modern analogs of hurricanes audrey 1957 rita 2005 and ike 2008 and consideration of nearby landfalling hurricanes in the historical record it was determined that the storm intensity threshold of the study site is equivalent to a category 3 hurricane there is a wide variation in hurricane strike frequency between this site and other sites bordering the gulf of mexico this variation results from differences in site sensitivity and the nature of geologic or geochemical proxies used to derive hurricane strike records the approach demonstrated by this study could have applicability at other sites around the world where beach ridge plains are subject to tropical cyclone strikes

the importance of quaternary records in reducing risk from tropical cyclones numerous late holocene records of tropical cyclones have been collected from north east queensland australia they are in the form of multiple sedimentary ridges paralleling the shore the ridges are composed of coral fragments or shell and sand or pure sand a method to determine the intensity of the tropical cyclone responsible for deposition of the ridges at each site has been applied and the results suggest that in the majority of cases these features were deposited by very high magnitude events the results suggest that extrapolations of long term cyclone magnitude and frequencies from the 30 to 40 year long instrumental record which is the method commonly used to assess risk from this hazard substantially underestimate the risk from this hazard this is confirmed for the cairns region by an 800 year long high resolution isotope record of tropical cyclones preserved in a limestone stalagmite together the sedimentary and isotope records suggest that tropical cyclone activity in north east queensland has been in a phase of quiescence since before european settlement of the region in approximately ad 1870 it is suggested that the incorporation of the palaeo record into risk assessments will provide a more realistic guide to the magnitude frequency relationship of tropical cyclones and help reduce risk from this hazard as a consequence

coastal geomorphic conditions and styles of storm surge washover deposits from southern thailand abstract the characteristics of tropical storm washover deposits laid down during the years 2007 to 2011 along the southern peninsular coast of the gulf of thailand got were described in relation to their different geomorphic conditions including perched fan washover terrace and sheetwash lineations preserved behind the beach zone within 100 m of the shoreline as a result washover terrace and sheetwash lineations were found where the beach configuration was uniform and promoted an unconfined flow non uniform beach configurations that promoted a confined flow resulted in a perched fan deposit washover sediments were differentiated into two types based on sedimentary characteristics including i a thick bedded sand of multiple reverse grading layers and ii a medium bedded sand of multiple normal grading layers in the case of thick bedded washover deposits the internal sedimentary structures were characterized by the presence of sub horizontal bedding reverse grading lamination foreset bedding and wavy bedding whereas horizontal bedding normal grading and dunes were the dominant structures in the medium bedded washover sand rip up clasts were rare and recognized only in the washover deposits in the bottom unit which reflects the condition when a mud supply was available all washover successions were found in the landward inclined bedding with a basal sharp contact a high elevated beach ridge associated with a large swale at the backshore proved suitable for a thick bedded washover type whereas a small beach ridge with uniformly flat backshore topography promoted a medium bedded washover sediment

sedimentary evidence of intense hurricane strikes from new jersey nine vibracores from the backbarrier marsh at whale beach new jersey reveal three large scale overwash deposits associated with historic and prehistoric storms the uppermost and smallest overwash fan was deposited in the ash wednesday northeaster of march 5 8 1962 a second more substantial overwash fan between 100 and 35 cm depth dates to the late eighteenth or early nineteenth century this fan was most likely deposited during the 1821 hurricane the only intense hurricane to make landfall in new jersey in the past 350 yr a third larger overwash fan between 200 and 300 cm depth was deposited between a d 1278 and 1438 and is likely the result of a prehistoric intense hurricane strike the combination of historical and stratigraphic evidence indicates that two intense hurricanes winds 50 ms 21 have likely made landfall on the southern new jersey coast in the past 700 yr resulting in an annual landfall probability of 0 3

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

los azufres silicic center mexico inference of caldera structural elements from gravity aeromagnetic and geoelectric data los azufres geothermal field is located within a silicic volcanic complex in central mexico the complex is one of the major silicic centers in the trans mexican volcanic belt tmvb pradal and robin 1985 first suggested the existence of the los azufres caldera and ferrari et al 1991 recognized the existence of a collapse structure according to pradal and robin this is a caldera of resurgent type this geophysical study aims to contribute to the knowledge of the structure of the los azufres area gravity aeromagnetic magnetotelluric mt and d c vertical electric resistivity soundings were analyzed results show that los azufres is a very structurally complex setting with relatively thin crust caused by the extensional tectonics characterizing this central sector of the tmvb faults belonging to the e w to ne sw extensional neotectonics and nw se basin and range province systems are observed to affect the geologic units of los azufres according to our study the los azufres geothermal field is located in a structural high located in the middle of a sub circular depression delimited to the north northeast by the santa ines range and to the southwest by the mil cumbres formation the larger depression consists of two narrow deep depressions that correspond to la venta and to the valley of juarez they are separated by the above mentioned structural high these sub depressions are believed to be the sites of a maximum caldera collapse and the structural high is interpreted to be at least in part the caldera s resurgent dome geoelectric structure of the caldera derived from d c resistivity indicates that the brines of the los azufres geothermal system ascend along faults both bounding and internally disrupting the structural high resurgent dome a reasonable correlation is observed between gravity and aeromagnetic data

reconstruction of prehistoric landfall frequencies of catastrophic hurricanes in northwestern florida from lake sediment records abstract sediment cores from western lake provide a 7000 yr record of coastal environmental changes and catastrophic hurricane landfalls along the gulf coast of the florida panhandle using hurricane opal as a modern analog we infer that overwash sand layers occurring near the center of the lake were caused by catastrophic hurricanes of category 4 or 5 intensity few catastrophic hurricanes struck the western lake area during two quiescent periods 3400 5000 and 0 1000 14 c yr b p the landfall probabilities increased dramatically to ca 0 5 per yr during an hyperactive period from 1000 3400 14 c yr b p especially in the first millennium a d the millennial scale variability in catastrophic hurricane landfalls along the gulf coast is probably controlled by shifts in the position of the jet stream and the bermuda high

boiling and depth calculations in active and fossil hydrothermal systems a comparative approach based on fluid inclusion case studies from mexico abstract a boiling model that considers the increase of salinity due to the steam loss and uses a combined density of the coexisting vapor and liquid phases was applied to fluid inclusion data from los azufres geothermal zone and from an eocene epithermal vein of taxco these case studies are taken as examples of active and fossil hydrothermal systems respectively in los azufres high temperatures of homogenization 300 c are commonly attained at depths between 1500 and 2000 m whereas salinity values above 2 0 wt nacl eq occur within the upper 500 m of the system suggesting that the geothermal zone is largely affected by boiling the depths calculated with the boiling model are close to real depths with accuracy greater than 99 for one case however considerably large error 30 was obtained to the top of the geothermal system due to enhanced co2 concentrations contrastingly the depths estimated by plotting microthermometric data on boiling point curves of constant salinity and discarding the effect of vapor on hydrostatic pressure were systematically shallower than real ones implying an underestimation of depth of up to 50 for the application case of the taxco epithermal deposit microthermometric data describe a boiling evolution path in the temperature salinity space although some values deviate from it thus likely reflecting local mixing with fluids of contrasting salinity according to our model boiling occurred from a paleo depth of 360 m which corresponds to a current sampling depth of about 200 m this level in the hydrothermal system coincides with the boundary between a lower base metal zone and an upper silver rich zone these results suggest that the descriptive models for epithermal deposits could be incorrectly calibrated in terms of depth therefore they could be revised and corrected by applying the boiling model used in this paper

holocene sea levels along the andaman sea coast of thailand for the malay thai peninsula several sea level curves for the younger holocene based on field evidence as well as on hydro isostatic modelling of a far field site have been published the general assumption is a rapid rise to a mid holocene maximum up to 5 m above present sea level followed by a constant or oscillating regression however from the andaman sea coast of thailand which was affected by the 2004 tsunami only isolated observations are available regarding holocene sea levels thus the timing and magnitude of the holocene highstand as well as the course of the regression remain to be defined as several palaeotsunamis could be detected in the meantime it is important to know the related sea levels as exactly as possible to judge the energy inundation width and potential wave height of these events therefore fixed biological indicators from the rocky coasts of the phang nga bay and phuket as well as morphological indicators from beach ridge and swale sequences along the exposed west coast ko phra thong were studied to gain information about the holocene sea level development in this region while oyster and coral data from the phang nga bay and phuket document a holocene maximum of 2 6 m at 5700 cal bp the ridge crests and swale bases in the northwest of the study area point to maximum heights of 1 5 2 0 m above the present level around 5300 years ago during the last 3000 years to when the largest part of the holocene palaeotsunami deposits from thailand was dated relative sea levels rsl in both areas did not exceed 1 5 m

criteria for the recognition of metamorphosed or altered lamprophyres a case study from the archaean of kambalda western australia abstract the ability to distinguish ancient lamprophyres from other mafic and ultramafic intrusive rock types has become increasingly important lamprophyres have been used as marker horizons in the mapping of several precambrian terranes they have been recognized for decades as possible indicators of gold mineralization and their presence has implications for the tectonic interpretation of ancient greenstone belts whilst fresh undeformed lamprophyres are readily recognized metamorphosed altered and or deformed examples are more problematical the intrusive ages of kambalda lamprophyres are constrained by available field and geochronological evidence including direct dating of one kersantite to the period 2684 2660 ma these rocks therefore belong to the world s oldest group of alkaline rocks and lamprophyres so far recognized which also includes contemporaneous examples from canada zimbabwe and probably tanzania despite their extreme age and low to medium grade regional metamorphic history these rocks still preserve characteristic lamprophyric textures chemistry and mineralogy only where metamorphism has been overprinted by gold related alteration and deformation do the lamprophyres cease to be recognizable petrographically however their unique chemistry persists and despite losses of the most mobile elements from presumed magmatic compositions altered and deformed specimens still retain far higher contents of sr k ba th nb p zr and ce than basalts of the local greenstone succession even lamprophyres which have been converted to biotite plagioclase carbonate pyrite schists retain this distinctive chemical signature during both alteration and metamorphism mobile elements k rb sr ba display far larger changes than archetypal immobile elements ce nb zr p ti clearly sanctioning traditional preference for the latter in assessing altered rocks however the surprising loss of nb during biotitization of hornblende indicates that negative nb anomalies conventionally regarded as indicators of subduction related magmatism can also be induced purely by alteration processes

tropical cyclone activity in the northwest pacific in relation to the el nino southern oscillation phenomenon abstract the interannual variations in tropical cyclone activity in the northwest pacific nwpac and their relationships with the el nino southern oscillation enso phenomenon were studied using the method of spectral analyses time series of a southern oscillation index soi defined as the sea level pressure difference between easter island and darwin and tropical cyclone activity in the entire nwpac ocean basin as well as in different regions of the nwpac were analyzed two spectral peaks are apparent in all these time series one corresponds to the generally accepted southern oscillation with a period of 3 to 3 5 years and another at the quasi biennial oscillation qbo frequency cross spectral analyses between the soi and tropical cyclone activity show significant coherence in these two spectral peaks the dominant peak is at the southern oscillation frequency with the soi leading typhoon activity by almost a year at the qbo frequency the two series are almost in phase cyclone activity in t

700 yr sedimentary record of intense hurricane landfalls in southern new england five intense category 3 or greater hurricanes occurring in 1635 1638 1815 1869 and 1938 have made landfall on the new england coast since european settlement historical records indicate that four of these hurricanes 1635 1638 1815 and 1938 and hurricane carol a strong category 2 storm in 1954 produced significant storm surges 3 m in southern rhode island storm surges of this magnitude can overtop barrier islands removing sediments from the beach and nearshore environment and depositing overwash fans across back barrier marshes lakes and lagoons in a regime of rising sea level accumulation of marsh lake or lagoon sediments on top of overwash deposits will preserve a record of overwash deposition we examined the record of overwash deposition at succotash salt marsh in east matunuck rhode island and tested the correlation with historical records of intense storms aerial photographs taken after hurricanes in 1954 and 1938 show overwash fans deposited at the site analysis of 14 sediment cores from the back barrier marsh confirmed the presence of these fans and revealed that 4 additional large scale overwash fans were deposited within the marsh sediments the four overwash fans deposited since the early seventeenth century at succotash marsh matches the historical record of significant hurricane induced storm surge these fans were most likely deposited by hurricanes in 1954 1938 1815 and either 1638 or 1635 radiocarbon dating of two prehistoric overwash fans indicated that these were deposited between a d 1295 1407 and 1404 1446 and probably represent intense hurricane strikes in the past 700 yr at least 7 intense hurricanes struck the southern rhode island coast and produced a storm surge that overtopped the barrier at succotash marsh

circular features in the trans mexican volcanic belt one hundred and ninety one circular or elliptical features have been located on landsat imagery of the trans mexican volcanic belt tmvb the origin of most of these features is unknown nine have been recognized as collapse calderas clearly visible on landsat imagery and studied in detail while an equally small number have been tentatively identified as such but not thoroughly investigated on the basis of the identification of at least five of the nine calderas through their detection on landsat images it is proposed that the present inventory is a reliable base to extend the census of the tmvb confirmed calderas now clearly too small for a population of roughly 8000 volcanic centers many of which emitted large volumes of felsic pyroclastic products

evaluation of the evolving stress field of the yellowstone volcanic plateau 1988 to 2010 from earthquake first motion inversions abstract although the last rhyolite eruption occurred around 70 ka ago the silicic yellowstone volcanic field is still considered active due to high hydrothermal and seismic activity and possible recent magma intrusions geodetic measurements document complex deformation patterns in crustal strain and seismic activity likewise reveal spatial and temporal variations in the stress field we use earthquake data recorded between 1988 and 2010 to investigate these variations and their possible causes in more detail earthquake relocations and a set of 369 well constrained double couple focal mechanism solutions were computed events were grouped according to location and time to investigate trends in faulting the majority of the events have normal faulting solutions subordinate strike slip kinematics and very rarely reverse motions the dominant direction of extension throughout the 0 64 ma yellowstone caldera is nearly ene consistent with the perpendicular direction of alignments of volcanic vents within the caldera but our study also reveals spatial and temporal variations stress field solutions for different areas and time periods were calculated from earthquake focal mechanism inversion a well resolved rotation of 3 was found from nne ssw near the hebgen lake fault zone to ene wsw near norris junction in particular the 3 direction changed throughout the years around norris geyser basin from being ene wsw as calculated in the study by waite and smith 2004 to nne ssw while the other 3 directions are mostly unchanged over time the presence of chocolate tablet structures with two sets of nearly perpendicular normal faults was identified in many stages of the deformation history both in the norris geyser basin area and inside the caldera

application of the gauss theorem to the study of silicic calderas the calderas of la primavera los azufres and los humeros mexico abstract we explored applications including limitations of gauss s theorem to the study of silicic calderas first it enables us to determine the mass deficiency from calderas mass deficiency itself has also other potential applications it enables to make qualitative comparisons between calderas we can use the mass deficiency to test in a quick way and as a preliminary step to a formal gravity inversion for the feasibility of caldera types of simple geometry i e piston subsidence and funnel models this application can be done in a straightforward way once the mass deficiency has been determined for this purpose the mass deficiency is converted to the volume of material missing at the caldera subsequently for example this volume and the respective caldera diameter enable us to estimate the height of the cylinder fitting the piston subsidence model if the obtained parameters are congruent with the known geology and geophysical information then the model may be considered further in the inversion of the gravity data for the detailed structure other simple models i e the funnel model can also be analyzed in this way in particular when working with a piston subsidence caldera type the gauss theorem enables us to estimate the caldera collapse very difficult to obtain based on geologic information alone these possible uses of gauss s theorem are illustrated with the calderas of la primavera los azufres and los humeros caldera mexico the obtained mass deficiency from these calderas follow the linear mass deficiency diameter trend observed for other calderas in particular because of their diameters and mass deficiencies la primavera and krakatau calderas can be considered equiparable this comparison is of the most importance considering that la primavera is located in the neighbourhood of a metropolis guadalajara city since geophysical studies have already established a piston subsidence model for these calderas we assessed gauss s theorem used to estimate their collapses the respective estimated subsidences for la primavera 600 m los humeros 500 m and los azufres calderas 800 and 1200 m respectively are supported by borehole data these relatively small subsidences in relation to many calderas in western us can be explained by the presence of cylindrical to sub spherical magma chambers of smaller size than those underlying the western us calderas of yellowstone valles and long valley for example in an epicontinental setting of young quaternary rocks against precambrian cratonic basement in western us a reassessment of all the available geological and geophysical information was also done in particular the caldera limits were checked and corrected when necessary based on available 2 d gravity modeling gravity gradients and the location of the volcanic vents regional tectonic lineaments might explain many of the particular details of their structures la primavera caldera might have formed through a piecemeal subsidence controlled by the pre caldera tectonics los humeros caldera is an example of a caldera that evolved by incremental growth as the vulsiniani calderas in italy the los azufres caldera comprises two contiguous calderas emplaced in a regional tectonic depression

a decadally resolved paleohurricane record archived in the late holocene sediments of a florida sinkhole article i nfo a 4500 year record of hurricane induced storm surges is developed from sediment cores collected from a coastal sinkhole near apalachee bay florida recent deposition of sand layers in the upper sediments of the pond was found to be contemporaneous with significant historic storm surges at the site modeled using slosh and the best track post 1851 a d dataset using the historic portion of the record for calibration paleohurricane deposits were identified by sand content and dated using radiocarbon based age models marine indicative foraminifera some originating at least 5 km offshore were present in several modern and ancient storm deposits the presence and long term preservation of offshore foraminifera suggest that this site and others like it may yield promising microfossil based paleohurricane reconstructions in the future due to the sub decadal 7 years resolution of the record and the site s high susceptibility to hurricane generated storm surges the average local frequency of recorded events approximately 3 9 storms per century is greater than that of previously published paleohurricane records from the region the high incidence of recorded events permitted a time series of local hurricane frequency during the last five millennia to be constructed variability in the frequency of the largest storm layers was found to be greater than what would likely occur by chance alone with intervals of both anomalously high and low storm frequency identified however the rate at which smaller layers were deposited was relatively constant over the last five millennia this may suggest that significant variability in hurricane frequency has occurred only in the highest magnitude events the frequency of high magnitude events peaked near 6 storms per century between 2800 and 2300 years ago high magnitude events were relatively rare with about 0 3 storms per century occurring between 1900 and 1600 years ago and between 400 and 150 years ago a marked decline in the number of large storm deposits which began around 600 years ago has persisted through present with below average frequency over the last 150 years when compared to the preceding five millennia

the los azufres michoacan mexico geothermal field abstract fifteen deep exploratory and production wells in a 30 km2 area have permitted the study of both the primary igneous petrology and the secondary geothermal system and its associated mineralization the los azufres field lies on a complex plio pleistocene succession of basalts andesites dacites and rhyolites each with its associated tuffs recently alkali basalt magmas have intruded the area extruding minor lava flows geothermal fluids have altered all the above rocks from a minimum of 20 of the total rock volume to a maximum of 100 with an average 75 alteration multiple distinct zones of oxides clay minerals chlorites calcite quartz and epidote are discernible together with minor amphiboles zeolites and pyrite there appear to have been two distinct heat sources the first associated with the youngest rhyolitic extrusives whilst the second current heat source at a considerably greater depth may be due to a new more basic magmatic cycle this downward displacement is associated with a lateral northeasterly migration of the heat source both geothermal systems were accompanied by temporary escapes of hot fluids along fault planes at different localities at different times which produced localized anomalous heat fluxes with associated mineralization superimposed onto the regional geothermal pattern

numerical recognition of alignments in monogenetic volcanic areas examples from the michoacan guanajuato volcanic field in mexico and calatrava in spain abstract identification of geological lineaments using numerical methods is a useful tool to reveal structures that may not be evident to the naked eye in this sense monogenetic volcanic fields represent an especially suitable case for the application of such techniques since eruptive vents can be considered as point like features application of a two point azimuth method to the michoacan guanajuato volcanic field mexico and the calatrava volcanic province spain demonstrates that the main lineaments controlling the distributions of volcanic vents 322 in calatrava and 30 in michoacan approach the respective main compressional axes that dominate in the area i e the cocos north america plates convergence and the main betics compressional direction respectively considering the stress fields that are present in each volcanic area and their respective geodynamic history it seems that although volcanism may be a consequence of contemporaneous extensional regimes the distribution of the volcanic vents in these kinds of monogenetic fields is actually controlled by reactivation of older fractures which then become more favourable for producing space for magma ascent at near surface levels

reconstruction of storm tsunami records over the last 4000 years using transported coral blocks and lagoon sediments in the southern south china sea large transported coral blocks on reef flats and elevated sedimentation rates in atoll lagoons were demonstrated to be excellent proxies for past strong storms tsunamis in the southern south china sea here we introduce another important proxy coarse fraction contents of lagoon sediments this proxy is based on the principle that coarse fractions of enclosed lagoon sediments are usually controlled by strong sedimentary dynamics on the surrounding reef flat and thus increases in the weight percentages of coarse grained fractions of the lagoon sediments should also reflect strong storm tsunami events or increases in storminess with the size of the content peak as the proxy for the degree of storminess bearing this in mind we measured the contents of the 1 mm grain size fractions referred to as coarse fraction content in an undisturbed lagoon core precisely dated to cover the last 4000 years the decadal resolution data show a systematic variation with some peaks strongly correlated with in timing with strong storm tsunami events dated at ad 1872 15 1685 8 similar to 1680 6 1443 9 1336 9 1210 5 similar to 1201 4 and 1064 30 using wave transported coral blocks using the mean coarsefraction content 9 2 for the entire core as a reference line a total of 77 peaks were identified for the past 4000 years twenty of these peaks are higher than 23 the mean value for peaks that are correlated in age with the six strong storm events identified using independently dated coral blocks among these 23 peaks 13 occurred within the last 1000 years the concentration of contentpeaks in the last millennium could be partially due to higher deposition rates and sampling resolution over the last similar to 4000 years there appears to be a weakly increasing trend in coarse fraction contents towards present time but this trend is not obvious within the last millennium this long term variation may be related to changes in both storminess and reef lagoon morphology however the data do suggest three extremely stormy periods centring around ad similar to 1200 similar to 400 bc and similar to 1200 bc respectively overall this study demonstrates that combined use of transported coral blocks and coarse fraction contents of lagoon sediments provides an excellent means for comprehensive reconstruction of past storm tsunami activity c 2008 elsevier ltd and inqua all rights reserved

foraminiferal biodiversity and holocene evolution of the phetchaburi coastal area thailand gulf abstract twenty three surface sediments were collected in may 2002 in the phetchaburi coastal area thailand gulf starting from the outer margin of the vegetated zone to a depth of about 20 m covering an area of about 800 km 2 one core was taken in the north eastern area of the mae khlong delta plain to investigate the recent paleoenvironmental evolution of this zone thirteen core levels were collected for micropaleontological foraminifers investigation this highly populated area is characterized by the inflow of the mae khlong the second major river in thailand and the khlong ban labun river empty tests pertaining to the thanathocoenosis are dominant and as for the few living specimens are most frequently represented by ammonia spp the collected species are typically of brackish water settings foraminiferal diversity is rather low and increases from the northern part to the central southern part of the studied area where taxa typical of the indo pacific realm such as miliolids and schackoinella globosa are more abundant than the cosmopolitan species such as ammonia tepida and gaudryina exilis the geographic distribution of the four foraminiferal assemblages defined on the basis of the cluster analysis indicates that the northern sector is affected by severe environmental conditions since this area is affected by the mae khlong river input the salinity fluctuation is probably a controlling factor on the foraminifers distribution ammonia tepida a inflata murrayinella murrayi and miliolinella sp 1 are the most opportunistic species able to tolerate wide environmental variability as regards to the holocene data the study carried out on the core located on the mae khlong delta plain indicates that the tidal flat paleoenvironment has been subject to different fresh water influence with a general prograding trend