post oligocene river incision southern sierra madre occidental mexico the sierra madre occidental of western mexico consists of a granitic basement covered by oligocene ignimbrites that define a reference surface from which to estimate late cenozoic river incision a 90 m grid digital elevation model was used to characterize contemporary topography and interpolate the late oligocene surface of the ignimbrite plateau from a surface fit to the highest points in the relatively undissected uplands between major river valleys long term river incision rates calculated from the difference between this reference surface and longitudinal profiles of 11 rivers that flow toward the tepic zacoalco rift zone range from about 0 01 to 0 2 mm year 1 river profiles of this region also show evidence of river capture driven by flexural uplift along the flank of the rift zone river profile concavity values in the sierra madre occidental range from 0 22 to 0 63 a range similar to that reported previously for a wide range of environments in contrast the empirically constrained ratio of exponents in the stream power model of river incision m n ranges from 0 44 to 0 52 close to the expected theoretical value of 0 5 the wider range of observed values may illustrate how can differ from the driving values of m n in non steady state bedrock river systems

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

myth and catastrophic reality using myth to identify cosmic impacts and massive plinian eruptions in holocene south america major natural catastrophes such as floods fire darkness and sky falling down are prominently reflected in traditional south american creation myths cosmology religion and worldview cosmogonic myths represent a rich and largely untapped data set concerning the most dramatic natural events and processes experienced by cultural groups during the past several thousand years observational details regarding specific catastrophes are encoded in myth storylines typically cast in terms of supernatural characters and actions not only are the myths amenable to scientific analysis some sets of myths encode multiple catastrophes in meaningful relative chronological order the present study considers 4259 myths including 284 universal perceived in the narratives to be worldwide catastrophe myths from 20 cultural groups east of the andes these myths are examined in light of available geological palaeoenvironmental archaeological and documentary evidence our analysis reveals three likely major plinian volcanic eruptions in columbia and the gran chaco we also identify a set of traditions that are probably linked to the well known campo del cielo iron meteorite impact in northern argentina around 4000 years ago along with a separate set of traditions alluding to a possible airburst in the brazilian highlands these impacts apparantly triggered widespread mass fires there are hints of cosmic impacts in the mythologies for other locations in south america south america is a vast and physically diverse region occupying some 17 819 000 sq km approximately 12 of the world s overall land mass south america was seemingly the last of the inhabited continents to be colonized a process beginning by at least 12 000 bp prior to european contact in the sixteenth century a wide range of societies developed and flourished throughout south america ranging from simple migratory huntergatherers of patagonia and tierra del fuego to well known state level societies of the central andes and adjacent coastal plains of peru and chile the inca the chimu and the moche continue to impress archaeologists and historians with their monumental architecture and their mastery of irrigation and general water management in between these two extremes were semi sedentary and sedentary village horticulturalists and various small independent chiefdoms relying heavily on agriculture and maritime resources the distinguished anthropologist julian steward identified a total of 178 named different tribes for south america steward 1946 1959 steward faron 1959 it is readily apparent that thousands of additional tribal names are present in historical documents and that much confusion exists due to synonymy and language uncertainties early explorers and missionaries had no systematic way to classify the various groups of people they encountered there are at least 65 known language families in south america with estimates of the numbers of individual languages ranging between 400 to as many as 3000 bierhorst 1988 p 17 this uncertainty is compounded by the fact that many of these languages and the groups who spoke them have become extinct during the past several hundred years despite these conditions south america has a rich legacy of oral traditions and mythologies south american mythology has been the source for some of the most creative and comprehensive studies of mythology performed to date e g levi strauss 1969 1973 1978 1981 bierhorst 1988 of particular value for our study is the set of 4259 myths published during the period of 1970 1992 by the university of california at los angeles ucla as part of the monumental 24volume series entitled folk literature of south american indians wilbert simoneau 1992 the distribution of the ucla collection representing 20 different cultural groups east of the andes is depicted in figure 1 we hypothesize that great or worldwide catastrophic events appearing in south american indian from piccardi l masse w b eds myth and geology geological society london special publications 273 177 202 0305 8719 07 15 00 the geological society of london 2007 fig 1 map of south america depicting countries e g paraguay geographic regions e g gran chaco and cultures with their total numbers of myths in the ucla collection e g toba with 636 myths w b masse m j masse 178

comparative proximal features of the main plinian deposits campanian ignimbrite and pomici di base of campi flegrei and vesuvius abstract the proximal plinian fall deposits of the campanian ignimbrite ci 38 ky fedele et al 2008 and pomici di base pdb 18 ky bertagnini et al 1998 have been investigated in order to understand the contribution of each part of the plume to the proximal sedimentation following houghton et al 2004b we consider three main transport regimes jet phase producing facies fb buoyant region of the plume producing facies fa and direct lateral ejection producing facies fc as well documented in medial locations sparks et al 1992 1997 ernst et al 1996 transport regimes can develop different facies even in proximal locations according to the dynamics of the eruptive column our proximal deposits show stratification and diffuse bedding allowing us to introduce two new facies stratified fa sfa and diffuse bedded fb dbfb these facies retain the transport regime previously proposed for fa buoyant plume and fb jet phase but their lithological features are influenced by near vent depositional conditions lithology and sedimentological data grain size componentry maximum clasts suggest that most of the sedimentation occurred mainly from the buoyant plume with simultaneous contribution from the other two different dynamic regimes coarse clasts falling from the lower margins of the plume strongly affected the sedimentation of the ci proximal fall deposit with a minor contribution from lithic clasts ballistically emplaced and partial collapses of the plume forming pyroclastic density currents in contrast the pdb proximal fall deposit was strongly affected by coarse clasts emplaced directly from the vent through parabolic trajectories with very little contribution of material emplaced from the lower part of the plume these differences can be attributed to different vent conduit processes acting during the eruptions

early mid pleistocene environments in the valsequillo basin central mexico a reassessment the valsequillo basin in central mexico has been of interest due to the presence of megafauna and evidence for early human occupation but research has been controversial it has been suggested that extensive and deep lakes characterised the early pleistocene environment but sediment exposure is highly fragmentary and reliable dating has been difficult here we report for the first time early pleistocene palaeoenvironmental reconstructions using stable isotopes diatoms tephra and pollen we studied several stratigraphic sections of mainly non volcanic rocks containing the 1 3 ma xalnene ash as a stratigraphic marker the isotope and other proxy data show that topographically low points in the basin were occupied by spring fed shallow water lakes during the early mid pleistocene with a trend to drier conditions the basin was a dynamic volcaniclastic environment during this period with the production of the toluquilla volcano sequence and other rhyolitic dacitic volcanic ashes interbedded with the lake sediments at the sections studied there is no evidence from the sections for extensive and deep lakes before or after the xalnene ash deposition the presence of lakes in the basin during the early pleistocene would have made it attractive for megafauna

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 real southern basin and range mid to late cenozoic extension in mexico much of northern and central mexico underwent east northeast extension in the mid to late cenozoic this area constitutes a major but little recognized part of the basin and range province the extended region is bounded on the east by the laramide thrust front of the sierra madre oriental on the west the relatively unfaulted sierra madre occidental separates the extended area in central mexico from that around the gulf of california extension occurred as far south as what is now the trans mexican volcanic belt and in oaxaca south of the belt the basin and range province in mexico constitutes approximately half of the 19 x 10 5 km 2 of western north america that underwent mid to late cenozoic extension north northwest orientations of numerous epithermal vein systems indicate that east northeast extension began as early as 30 ma in areas north of the volcanic belt major episodes of faulting began at about 23 to 24 ma and 12 to 13 ma both in mexico and in the southwestern united states faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting widespread quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the trans mexican volcanic belt in contrast the tectonics in and south of the belt are now probably related to subduction of the rivera plate contemporaneity of the 12 ma episode with early extension around the gulf of california attributed to pacific north american plate boundary reorganization suggests that extension is related predominantly to plate boundary effects the beginning of extension at 30 ma may be related to initial encroachment of the east pacific rise upon the trench that lay off western north america

eruptive dynamics of the citlaltepetl pumice at citlaltepetl volcano eastern mexico abstract citlaltepetl pico de orizaba is mexico s highest 5675 m a s l potentially active volcano which is presently in a dormant state between 9 0 and 8 5 ky b p a sequence of volcanic eruptions occurred at citlaltepetl volcano as part of the most explosive holocene episode this sequence is associated with the deposition of an intercalation of pumice fallout and scoria and pumice rich pyroclastic flow deposits named here as citlaltepetl pumice c p detailed stratigraphic and petrographic correlation of over 100 measured sections in conjunction with the analysis of the physical characteristics of the juvenile and lithic portions of each main layer of the sequence provided the basis to reconstruct the eruptive episodes and to assert the eruptive dynamics of the whole sequence which was divided into eight main eruptions separated by three brief periods of quiescence the eruption sequence started with a phreatic phase that soon developed into a bread crusted bomb bearing phase this was followed by a sequence of vigorous plinian explosive eruptions separated by brief periods of repose and a short dome collapse activity it was followed by another cycle of alternated scoria pyroclastic flows forming and plinian activity this eruptive sequence shows the complexity of single eruptive events with alternated contrasting styles suggested by the compositional differences between the pyroclastic flow and fallout products

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

the plinian fallout associated with quilotoa s 800 yr bp eruption ecuadorian andes abstract large volcanic eruptions at dacitic or rhyolitic volcanoes often generate exceptional volumes of fine ash that mantles an area up to a million km 2 these eruptions are characterized by extreme fragmentation of the magma and hence extraordinary dispersal of ash and are categorized as plinian ultraplinian or phreatoplinian events large volume co ignimbrites or co plinian ashes are often produced by such eruptions high fragmentation indices of 90 are attributed to the violent eruption of silicic magma especially if augmented by fuel coolant reactions produced when abundant external water interacts with the magma the present study documents a case where the fine ash 1 mm diameter fall deposit related to the plinian phase of the eruption comprises the overwhelming bulk about 87 wt of the eruptive products this is another example demonstrating the predominance of a widespread fine grained co plinian ash which follows the initial coarser lapilli fall historical eruptions at two other andean volcanoes quizapu chile and huaynaputina peru and at santa maria guatemala and novarupta alaska produced similar ash fall sequences quilotoa s 800 yr bp eruption in the ecuadorian andes is an example of a powerful plinian eruption at a small dacitic volcano it produced a crystal rich fine grained ash layer which mantled an extrapolated 810 000 km 2 with 1 mm thickness and has a bulk magma volume of about 18 3 km 3 stratigraphic and petrographic studies as well as granulometric and density analyses were employed to investigate these eruptive products particularly the distal co plinian fall deposits the eruptive cycle consisted of a phreatomagmatic triggering the establishment of a powerful sustained plinian column and subsequently partial column collapses leading to widespread surge and ash flow activity quilotoa s 800 yr bp eruption has a vei volcano explosivity index 6 and generated a maximum column height of about 35 km and a mass discharge rate of 2 10 8 kg s in comparison pinatubo s 1991 eruption had similar column heights and discharge rates but smaller erupted volumes factors that possibly contributed to the extreme explosiveness and resulting fine grained ash production include inferred volatile overpressures in the crystal dominated dacite magma a rapid eruption onset a forceful and sustained plinian column possibly through a crater lake thus promoting extreme fragmentation of the magma but without leaving evidence of wet phreatoplinian type deposits

the 1732 surtseyan eruption of eggoya jan mayen north atlantic deposits distribution chemistry and chronology the island of jan mayen in the north atlantic ocean is home to the world s northernmost active subaerial volcano beerenberg of the five known historical eruptions on the island the locations of two eruptions 1732 and 1818 have not been accurately located it is known that the 1732 and 1818 eruptions occurred on the south flank of beerenberg and several eruption sites have been proposed for these events here we show that the tuff cone of eggoya on the sw flank of beerenberg was the site of the 1732 eruption based on interpretation of the deposits field relations and historical sources we further describe the deposits from the eruption and show that eggoya is the largest explosive eruption described from jan mayen emplacing at least at least 0 3 0 4 km3 vei 4 of basanitic tephra up to distances of at least 111 km from jan mayen and covering a minimum area of around 500 km2 within the 2 cm isopach we also present our eruption scenario and show that this was an emergent surtseyan eruption with activity shifting between tephra jetting continuous uprush and more magmatic phases

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

ni cu sulfides and their host mafic ultramafic rocks in the mt sholl intrusion pilbara region western australia the sheetlike mt sholl intrusion is one of several gabbro peridotite bodies intruding a northeast trending archean greenstone belt near roebourne this study is based on approximately 6 000 m of diamond drill cores which show that the basal margins of the intrusion consist of wedge shaped to sheetlike extensions 30 to 150 m thick consisting of a central zone of peridotite within a gabbro envelope with a chilled margin disseminated and minor massive pyrrhotite pentlanditc and chalcopyrite occur mainly in gabbros above the basal contacts locally chilled gabbros contain sulfide droplets igneous textures of silicates and sulfides have survived but are extensively modified by recrystallization average bulk sulfide compositions show a progressive increase in cu from massive to gabbroic to peridotitic sulfides this may be a magmatic fractionation trend reflecting early separation of massive sulfides and later separation of peridotitic sulfides some sulfide textures fit the predicted crystallization behavior when the bulk compositions are related to experimental data for the fe ni cu s system however cu cu ni ratios vary widely with height in drill cores and interpretation of the magmatic controls of sulfide compositions is limited by the unknown extent of metamorphic modification the chilled gabbro has the composition of an aluminum depleted basaltic komatiite and cannot represent the magma from which the peridotitc formed the parent magma is inferred to be komatiitic and was emplaced as a heterogeneous body of basaltic komatiite magma containing sulfide liquid suspended in a peridotitic crystal mush most of the sulfide liquid settled out of the peridotitic mush into the gabbro

reconstruction of the ad 1600 huaynaputina eruption based on the correlation of geologic evidence with early spanish chronicles the largest historical eruption vei 6 in the andes began on february 19 and continued until march 6 or 15 ad 1600 at huaynaputina a dacitic stratovolcano located on a high volcanic plateau in south peru tephra falls pyroclastic flows and surges disrupted life in an area of 4900 km2 around the volcano and ash fall was reported 200 500 km away in south peru west bolivia and north chile the aftermath of the large scale eruption was severe and protracted for the people and colonial economy of south peru by linking up the series of events inferred from spanish chronicles with the lithofacies and composition of the tephra bulk volume 11 4 12 1 km3 dense rock equivalent dre 4 6 4 95 km3 we distinguish five eruptive phases 1 during the plinian phase a sustained plinian column 27 35 km high on february 19 20 delivered a dacitic pumice fall of 3 1 km3 dre volume the plinian pumice formed a widespread lobe of 95 000 km2 within the 1 cm isopach strong winds carried fine ash 500 km to the west and west northwest into the pacific ocean the computed volumetric eruption rate was in the range of 5 4 6 6 104 to 1 105 m3 s and the mass eruption rate 1 3 1 6 108 kg s the onset and high discharge of the sustained plinian eruption was fueled by the disruption of an active hydrothermal system enclosed in the pre ad 1600 amphitheater the plinian column shut off as the vent was choked when the fragmentation focus deepened to beneath the weathered bedrock 1600 m below the vent area 2 during the second phase a dwindling column sent ash falls on proximal to medial areas and possibly pyroclastic surges on proximal slopes 3 during the third ignimbrite forming phase with interspersed hydromagmatic events pyroclastic flows 1 5 2 km3 in volume were channeled into the rio tambo canyon and tributaries the flows with interbedded base surge deposits in proximal tributaries probably produced vigorous columns over high rugged relief around the huaynaputina plateau winds winnowing the columns dispersed a widespread co ignimbrite ash probably mixed with co plinian ash over an area of 265 000 km2 4 during the fourth phase an unusual crystal ash fall was deposited when the residual magma with a crystal content as high as 80 was tapped near the end of the eruption 5 during the fifth phase ash flows produced surge deposits and lag fall breccias near vent small volume ash flow deposits in proximal catchments and a thin ash fall layer in medial to distal areas the proximal deposits were also produced by diluted flows able to surmount ridges 1000 m high as far as 15 km east from the vent the ignimbritic and hydromagmatic phases greatly modified the 400 m diameter plinian vent tapping of the crystal rich magma and ash flows towards the end of the eruption led to the formation of two youthful vents in domes geochemistry and mineralogy of the plinian and post plinian units point to an unusual zoned magma sequence the ignimbrite forming phase tapped a magma batch richer in silica than the less differentiated plinian magma the crystal rich magma of unit 4 was fed by crystal mush in a layered magma reservoir and or from two magma reservoirs at distinct depths the geochemical and mineralogical trend throughout the eruption and preliminary measurements of geobarometers suggest a complex model linking a shallow 6 7 km magma reservoir to a deeper 15 km reservoir the total dre volume 4 6 4 95 km3 of erupted tephra did not lead to caldera collapse ring fractures cutting multiple vents are associated with a dyke swarm intruding the weathered volcanic bedrock this suggests the onset of a funnel type or piecemeal collapse

three empirical methods for the calculation of distal volume of tephra fall deposits abstract three empirical methods for the calculation of distal tephra fall volumes are presented and tested the three methods allow calculation of approximate distal tephra fall volumes under different conditions and abundances of field data all methods use proximal data to extrapolate the distal situation they are based on i shape and dispersal axis of proximal isopachs method of ellipses ii break in slope distance on log t vs a diagram where one distal isopach is available method of a ip with one distal isopach and iii distal thinning rate and break in slope distance on log t vs a diagram method of k 1 vs a ip the application of the three methods to published data of 23 32 eruptions in which it is possible to define at least two segments on the log t vs a diagram yields different results the method of ellipses yields good results if the thickness of reconstructed distal isopachs is greater than about 1 cm whereas the method of a ip with one distal isopach is more accurate if the distal isopach has a thickness below the half thickness at the break in slope distance t t ip the method of distal thinning rate k 1 vs break in slope distance a ip allows calculation of distal thinning rate and break in slope distance from only proximal data in this case calculated distal volumes approximate the actual one only if the deposits have a proximal volume total volume ratio greater than 0 7 otherwise they must be considered as minimum values to be added to the proximal volumes an empirical correlation between proximal thinning rate k and column height h t 7 6 k 0 47 r 0 90 is also presented column heights calculated in this way are within 20 of the observed published values in 94 of the examined cases

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

preeruptive magma viscosity an important measure of magma eruptibility 1 using a compilation of melt compositions meltwater contents temperatures and phenocryst contents the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas the basaltic to rhyolitic magmas have preeruptive viscosities over the range 101 to 108 pa s although bulk sio2 content has often been used as a qualitative measure of preeruptive magma viscosity the bulk sio2 content shows a weak correlation with magma viscosity correlation coefficient r 0 5 because of a wide range of phenocryst contents from 0 to 50 vol andesitic magmas have viscosities ranging from 102 to 107 pa s which are lower or higher than those of phenocryst poor rhyolitic magmas with 105 to 106 pa s focusing on andesitic to rhyolitic magmas the r between bulk sio2 contents and magma viscosities changes to 0 1 in contrast the melt only sio2 content from a basaltic to rhyolitic melt shows a good linear correlation with melt only viscosity r 0 9 although most of the calculated viscosities of erupted magmas fall below 106 pa s as consistent with the previous compilation study this paper describes 20 examples of highly viscous magmas with 106 pa s in most cases composed of mixtures of high silica rhyolitic melt 75 79 wt sio2 and abundant phenocrysts 30 55 vol in these highly viscous magmas 9 examples have erupted following the precursory eruption of less viscous magma suggesting that precursory dike propagation and conduit formation by the less viscous magma with 106 pa s induced the following eruption of less eruptible highly viscous magmas

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

the initiation and development of a caldera forming plinian eruption 172 ka lower pumice 2 eruption santorini greece the rhyodacitic 172 ka lower pumice 2 lp2 eruption terminated the first magmatic cycle at santorini greece producing a proximal 50 m thick succession of pyroclastic fall deposits diffusely stratified to massive ignimbrites and multiple lithic breccias the eruption commenced with the development of a short lived precursory eruption column depositing a 15 cm blanket of 1 2 cm sized pumice fragments at near vent localities lp2 a1 the precursor deposits are conformably overlain by a 30 m thick sequence of reversely graded ungraded pumice fall deposits that reflect opening and widening of a point source vent increasing mass discharge rates up to 108 kg s 1 and the development of a 36 km high plinian eruption column lp2 a2 a3 the progressive increase in maximum vesicle number density nvf in rhyodacitic pumice from 3 2 109 cm 3 in the basal fall unit of lp2 a2 1 to 9 2 109 cm 3 in lp2 a3 translates to an increase in magma decompression rate from 18 to 29 mpa s 1 over the course of the initial plinian phase this is interpreted to be a consequence of progressive vent widening and a deepening of the fragmentation surface such interpretations are supported by the increase in lithic clast abundance vertically through lp2 a and the occurrence of basement derived deep lithic components in lp2 a3 the increasing lithic clast content and the inability to effectively entrain air into the eruption column due to vent widening resulted in column collapse and the development of pyroclastic density currents pdcs lp2 b a major vent excavation event or the opening of new vents possibly associated with incipient caldera collapse facilitated the ingress of water into the magmatic system the development of widespread pdcs and the deposition of a 20m thick massive phreatomagmatic tuff lp2 c the eruption cumulated in catastrophic caldera collapse the enlargement of a pre existing flooded caldera and the discharge of lithic rich pdcs depositing proximal 9 m thick lithic lag breccias lp2 d

analysis of digital elevation data for palaeosurfaces in south western sweden a 3873 km large rectangular area of the precambrian basement in south western sweden has been investigated regarding gross morphology of palaeosurfaces gis and digital elevation data were used to

event stratigraphy and emplacement mechanisms of the last major caldera eruption on sete cidades volcano sao miguel azores the 16 ka santa barbara formation sete cidades is an active yet dormant central volcano located in the westernmost part of sao miguel island azores archipelago characterized by a 5 km wide multi stage caldera formed in the last 36 ka in this work we present new stratigraphic grain size petrographic and geochemical data of the 16 ka santa barbara formation related to the last stage of caldera formation we define the lithostratigraphy of this pyroclastic sequence into three members based on stratigraphic position lithofacies and distribution of the deposits the preserved deposits outcrop in a narrow area over 5 km between the caldera rim and the coastal cliffs and include ash beds with dispersed lapilli lower member coarse grained pumice and lithic rich lapilli tuffs middle member and pumice fall deposits upper member the minimum bulk volume preserved on land is about 0 03 km3 however the majority of the pyroclastic products were deposited in the ocean the presence of magmatic mafic enclaves 48 wt sio2 within the juvenile pumice clasts 63 wt sio2 is testimony to the injection of trachybasaltic magma into the trachytic magma reservoir shortly prior to eruption the mingling of these two magmas acted as the eruption trigger the eruption history can be divided into three main phases each characterized by different eruption styles and deposit emplacement mechanisms accompanied by enlargement of the caldera the eruption started with a sequence of short lived explosions generating unstable columns that produced ash fallout and generated dilute pyroclastic density currents mostly deposited in the sw sector of the volcano the climax of the eruption was marked by successive low fountaining events that generated high particle concentration pyroclastic density currents emplaced on the sw and ne flanks the final phase of the eruption was characterized by establishment of a sub plinian eruption column that produced pumice fall deposits on the ne flank of the volcano our study allows delineation of eruptive scenarios for future major explosive eruptions at sete cidades a volcano which is located on the most populated island of the azores sao miguel island and can provide a framework for the assessment of volcanic hazards on oceanic islands elsewhere

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

causes of complexity in a fallout dominated plinian eruption sequence 312 ka fasnia member diego hernandez formation tenerife spain the 312 ka fasnia eruption from the las canadas caldera on tenerife canary islands spain produced a complex sequence of twenty two intercalated units including 7 pumice fall 7 ignimbrite and 8 ash surge and fall deposits that define two distinct eruption sequences lower and upper fasnia sequences the fallout units themselves are internally complex reflecting waxing and waning of the eruption column while many of the ignimbrites reflect multiple intra plinian partial column collapse events associated with the injection of lithic clasts into the eruption column the lower and upper fasnia eruption phases were each terminated by caldera collapse and complete column collapse events probable blockage of the conduit and vent system during lower fasnia caldera collapse event briefly terminated the eruption resulting in a short lived period of erosion and sedimentation prior to the onset of the upper fasnia phase the transition to the upper fasnia eruption phase coincided with the eruption of more geochemically homogeneous pyroclasts in total 62 km3 of tephra were erupted including 49 km3 of juvenile clasts and n12 km3 of lithic clasts the dre volume of magma erupted was 13 km3 lower fasnia n 5 km3 upper fasnia n 8 km3 two thirds of which 9 10 km3 was deposited purely by fallout the fasnia member is one of the most complex plinian sequences known

generation of oceanic island basalt type volcanism in the western trans mexican volcanic belt by slab rollback asthenosphere infiltration and variable flux melting mantle plumes or decompression melting of a heterogeneous mantle have been proposed to explain oceanic island basalt oib type lavas found in the western trans mexican volcanic belt we show that mantle plumes cannot account for several geologic observations whereas decompression melting is unrealistic given the low extension rate in the volcanic arc on the basis of new geologic and geochemical studies we propose an alternative model that reconciles geologic data plate tectonic history and petrology since 8 5 ma the volcanic front has migrated trenchward 80 km and oib began to be emplaced in the rear half of the arc after a period of stalled subduction of the rivera plate between 7 2 and 4 8 ma as a whole oib accounts for 5 of the total volume of volcanism their trace element signatures require an enriched mantle source akin to the deep asthenosphere but also indicate a minor involvement of a subduction component we propose that during the period of very low convergence at the end of the miocene the leading edge of the rivera slab started to sink into the mantle and continued rolling back until it reached the present dip of 45 the sinking of the slab may have induced small amounts of asthenosphere to flow laterally into the opening mantle wedge from the gulf of california rift on the west and through the subducted rivera cocos plate boundary on the east this mechanism would produce veins of relatively enriched mantle which would be able to produce melts with an oib composition once the flux of fluids released from the subducted slab resumed

the use of high resolution elevation histograms for mapping submerged terraces tests from the eastern tyrrhenian sea and the eastern atlantic ocean abstract histogram plots can be used as a cost and time efficient tool to detect horizontal or gently dipping geomorphic surfaces within a background regional topography one of the main classes of these surfaces is typified by terraces formed during marine still stands this contribution uses elevation histogram plots as morphometric indicators for detecting the possible existence of concealed terrace orders in digital elevation models dems derived by the processing of bathymetric data the approach is particularly useful when dealing with submerged terraces and platforms because of the obvious difficulties in mapping those surfaces the advantage of using this indicator consists in the rapid identification of dem recurrent bathymetric values this tool is most powerful when dealing with shortly developed terraces and or multiple terrace orders whose detection could have been missed by direct observation two case studies characterized by the presence of a single terracing event are tested the southern slope of the ischia island and the top of the palinuro seamount both located in the eastern tyrrhenian sea margin elevation histograms are used for terrace order computation in a more complex setting the gorringe bank located offshore sw portugal in the eastern atlantic ocean the results show that a joint use of elevation histogram computation and geomorphologic analysis can be particularly efficient in the study of planar surfaces

morphostructural analysis and cenozoic evolution of elephant island southern scotia arc antarctica elephant island located in the vicinity of the present day active boundary of the south shetland block and the antarctic and scotia plates is a region of particular interest for understanding the past and present geodynamic evolution of the southern scotia arc lineament from different data sources field measured fractures and geomorphological evidences have been analysed in this context the lineaments extracted from aerial photographs 1 624 from a dem 348 and from radarsat 2 satellite data 1 365 indicate four dominant lineament sets with ne sw nw se n s and w e strikes all data sources identified similar lineament families but differences in the frequency distributions and subsequently on the dominant orientations were observed the measurements direct of fractures were obtained from 23 sites in the field at which 278 planes were measured fracture planes indicate main modes trending in the nne ssw and nnw sse directions and a secondary mode in the e w the major trends of the fracture measurements and the lineaments display a good correlation in the e w direction however there is an angular variation in the azimuth values of the nne sse and nnw sse fractures with respect to the n s ne sw and nw se orientations of the lineaments of approximately 20 this trend deviation may be due to the fact that mapped lineaments are composed of small fracture sets that may be related to shear fractures that cannot be distinguished at the aerial photograph or radar satellite data scales submerged sea floor morphological feature orientations match the studied morphostructures on the island and the main tectonic structures in this part of the scotia arc a linkage of the main lineament families to the tectonic stages from the oligocene to the present has been proposed taking into account the information of the orientation and sense of movement of the fractures and stresses in the elephant island region

cinder cone clustering in the transmexican volcanic belt implications for structural and petrologic models cinder cone distributions have most often been characterized using univariate statistics here a new technique to volcano distribution studies cluster analysis is applied to cinder cone distribution in the central transmexican volcanic belt tmvb a total of 1016 cinder cones are identified over an area of approximately 60 000 km2 application of cluster analysis reveals structure in cinder cone distribution using a search radius parameter of 16 km 75 of the cinder cones within the central tmvb are found within eight clusters of 45 159 cones each these clusters are each 2000 to 5000 km2 in area only 22 cones are found within clusters of three or fewer cinder cones indicating that clustering is a pervasive phenomenon some petrologic variation is evident among clusters low mg alkaline cinder cones are found within a single cluster 360 400 km from the trench application of alignment analysis techniques including the two point azimuth method the hough transform and two dimensional fourier analysis demonstrates that cinder cone alignments have common orientations on a regional scale within three clusters all located in the southernmost part of the michoacan guanajuato volcanic field in the western portion of the study area these alignments consist of tens of cinder cones are 20 50 km long and are all oriented with azimuths of 020 040 parallel to the direction of plate convergence high mg lavas which last fractionated at pressures in excess of 8 kbar are only found associated with these alignments indicating that magma transport is significantly enhanced in these areas although local alignments of three to six cinder cones occur within mapped fault zones that transect the area regional cinder cone alignment patterns and the distribution of clusters themselves do not appear to be affected by the presence of these fault zones the cluster analysis the alignment analysis and some petrologic data support the hypothesis that the tmvb is segmented near 101 w

palaeoenvironmental significance of diatom and vertebrate fossils from late cenozoic tectonic basins in west central mexico a review abstract pronounced lacustrine sedimentation developed in west central mexico during the late miocene between approximately 11 and 7 ma this was in response to tectonic extension associated with the initial emplacement of the late miocene substrata of the trans mexican volcanic belt climatic conditions in west central mexico during this interval were relatively warm and humid based on the widespread distribution of interpreted lacustrine beds following a latest miocene 8 0 5 4 ma stage of arid conditions and greatly reduced deposition of fine grained lacustrine sediments extensive east west oriented relatively deep perennial lakes ensued they mark the early pliocene 5 3 4 0 ma lower pliocene diatomites contain the same diatom species e g stephanodiscus carconensis and tertiarius aff baikalensis found in rocks of this age in the western united states the relatively warm and humid conditions that characterized this interval in central mexico coincide with a period of high latitude warming higher global sea level and a reduction in size of the antarctic ice sheets because the central american seaway persisted until at least the latest miocene it might have acted to increase precipitation in central mexico this could have continued into the earliest pliocene mexican pliocene mammalian faunas also support a savanna setting with moist and warm conditions prevailing at the time shallow lakes and fluvial conditions dominate after 4 0 ma until the end of pleistocene a combination of reduced precipitation due to general global cooling and drying as well as volcanic and tectonic processes are presumed to have been the cause for this mid pliocene reduction in lake size and extent in central mexico

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

cross stratigraphies from a seismically active mud lens off peru indicate horizontal extensions of laminae missing sequences and a need for multiple cores for high resolution records abstract marine laminated sediments in dysoxic areas of the ocean floor are an excellent archive for high resolution climate reconstructions while the existence of discontinuities produced by natural events such as underwater landslides slumps strong bottom currents and or bioturbation is usually acknowledged for long records the extent of their influence on high resolution sequences is usually not considered in the present work we show strong evidence for multiple stratigraphic discontinuities in different gravity and box cores retrieved off pisco peru covering the last 600 years chronostratigraphies are largely based on cross correlation of distinct sedimentary structures determined by x ray image analysis and validated using 210 pb 241 am and 14 c profiles as well as proxy records the cross correlation of distinct stratigraphic layers allows for chronostratigraphic tie points and clearly shows that some sedimentary sequences are continuous across scales of tens of kilometers indicating that regional processes often determine laminae formation some differences in laminae thickness were found among cores which could be explained by different sedimentation rates spatially variable deposition of diatom blooms changes in silica dissolution and partial deposition erosion caused by bottom currents using multiple stratigraphic tie points provides clear evidence for laminated sequences present in some cores to be missing in other cores moreover instantaneous depositions from upslope were identified in all the cores disrupting the continuity of the sediment records these discontinuities instantaneous deposits and missing sequences may be due to slumps possibly triggered by earthquakes and or erosion by strong bottom currents in spite of the missing sequences in some cores a continuous composite record of the last six centuries was reconstructed from spliced sequences of the different cores which provides a well constrained temporal framework to develop further high resolution proxies in this region the present work shows that paleoreconstructions developed from single cores particularly in areas with strong seismic activity and or strong bottom currents are subject to both temporal gaps and instantaneous depositions from upslope both of which could be misinterpreted as abrupt climate changes or anomalous climate events we stress the need for multiple cores to determine the stratigraphic continuity and chronologies for high resolution records

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

plate interactions control middle late miocene proto gulf and basin and range extension in the southern basin and range middle late miocene proto gulf 12 6 ma extension around the gulf of california gulf extensional province is commonly interpreted as resulting from partitioning of oblique pacific north american plate motion into strike slip displacement along the margin and east northeast extension perpendicular to the margin within the north american plate we propose that this mechanism also applies to kinematically similar predominantly east northeast extension that occurred at the same time throughout the southern basin and range province from southern arizona and new mexico to the trans mexican volcanic belt new field and 40ar 39ar data in sinaloa and durango confirm that this episode of extension occurred on the mainland side of the gulf and in the basin and range east of the sierra madre occidental which is generally considered the eastern margin of the gulf extensional province published data indicate the middle late miocene episode also occurred across the northern and southern ends of the sierra madre where the gulf extensional province connects with the basin and range 1 from central sonora into southern arizona and new mexico and 2 from nayarit into central mexico north of the trans mexican volcanic belt this episode appears to have avected an area that continues to the eastern edge of the basin and range province in texas and san luis potosi recognition that this episode of extension avected the entire southern basin and range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the gulf extensional province alone published plate reconstructions require 160 to 110 km of east northeast extension between 12 and 6 ma if taken up solely within the gulf extensional province this would have generated 66 to 78 extension which is much greater than observed spread across the entire southern basin and range it requires only 20 total extension which is more consistent with observations of cumulative extension between 12 and 6 ma extension was partitioned into the gulf extensional province because 1 it lies between two stable batholith belts mesozoic peninsular ranges on the west and mid tertiary sierra madre occidental on the east that resisted extension and 2 the gulf was thermally weakened by immediately preceding arc magmatism extension in the main basin and range province in part probably avoided the relatively strong batholithic crust of the sierra madre occidental 2000 elsevier science b v all rights reserved

paleomagnetism and tectonics from the late pliocene to late pleistocene in the xalapa monogenetic volcanic field veracruz mexico

the physical volcanology of the 1600 eruption of huaynaputina southern peru volcan huaynaputina is a group of four vents located at 16 36 s 70 51 w in southern peru that produced one of the largest eruptions of historical times when 11 km3 of magma was erupted during the period 19 february to 6 march 1600 the main eruptive vents are located at 4200 m within an erosion modified amphitheater of a significantly older stratovolcano the eruption proceeded in three stages stage i was an 20 h sustained plinian eruption on 19 20 february that produced an extensive dacite pumice fall deposit magma volume 2 6 km3 throughout medial distal and distal parts of the dispersal area a fine grained plinian ashfall unit overlies the pumice fall deposit this very widespread ash magma volume 6 2 km3 has been recognized in antarctic ice cores a short period of quiescence allowed local erosion of the uppermost stage i deposits and was followed by renewed but intermittent explosive activity between 22 and 26 february stage ii this activity resulted in intercalated pyroclastic flow and pumice fall deposits 1 km3 the flow deposits are valley confined whereas associated co ignimbrite ash fall is found overlying the plinian ash deposit following another period of quiescence vulcanian type explosions of stage iii commenced on 28 february and produced crudely bedded ash lapilli and bombs of dense dacite 1 km3 activity ceased on 6 march compositions erupted are predominantly high k dacites with a phenocryst assemblage of plagioclase hornblende biotite fe ti oxides apatite major elements are broadly similar in all three stages but there are a few important differences stage i pumice has less evolved glass compositions 73 sio2 lower crystal contents 17 20 lower density 1 0 1 3 g cm3 and phase equilibria suggest higher temperature and volatile contents stage ii and stage iii juvenile clasts have more evolved glass 76 sio2 compositions higher crystal contents 25 35 higher densities up to 2 2 g cm3 and lower temperature and volatile contents all juvenile clasts show mineralogical evidence for thermal disequilibrium inflections on a plot of log thickness vs area1 2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system the ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper level winds whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower altitude wind field thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24 32 km and a total column height of 34 46 km the estimated mass discharge rate for the 20 h long stage i eruption is 2 4 108 kg s and the volumetric discharge rate is 3 6 105 m3 s the pumice fall deposit has a dispersal index hildreth and drake 1992 of 4 4 and its index of fragmentation is at least 89 reflecting the dominant volume of fines produced of the 11 km3 total volume of dacite magma erupted in 1600 approximately 85 was evacuated during stage 1 the three main vents range in size from 70 to 400 m alignment of these vents and a late stage dyke parallel to the nnw sse trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre existing weaknesses during stage i this fissure evolved into a large flared vent vent 2 with a diameter of approximately 400 m this vent was active throughout stage ii at the end of which a dome was emplaced within it during stage iii this dome was eviscerated forming the youngest vent in the group vent 3 a minor extra amphitheater vent was produced during the final event of the eruptive sequence recharge may have induced magma to rise away from a deep zone of magma generation and storage subsequently vesiculation in the rising magma batch possibly enhanced by interaction with an ancient hydrothermal system triggered and fueled the sustained plinian eruption of stage i a lower volatile content in the stage ii and stage iii magma led to transitional column behavior and pyroclastic flow generation in stage ii continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage iii no caldera collapse occurred because no shallow magma chamber developed beneath this volcano

mineralogical and geochemical constraints for the origin and evolution of magmas in sierra chichinautzin central mexican volcanic belt abstract the sierra chichinautzin volcanic field is a key area for understanding the origin of the controversial mexican volcanic belt mvb this is due to its recent volcanic activity which is located at the front of the central mvb although a direct genetic relationship between the volcanism of the mvb and the subduction of the cocos plate is accepted by most authors geological geophysical and geochemical key features of the mvb depart from those of typical magmatic arcs this fact has promoted a debate on the nature of the complex mantle beneath the mvb 1 slab induced convection in the mantle wedge that causes advection of asthenospheric mantle and 2 anomalously hot mantle related to the eastward channeling of plume related material this is a petrological study of the sierra chichinautzin volcanism using mineralogical data from selected representative samples together with their geochemistry to refine the existing models of magmatic processes the integrated petrological analyses have been used to define four main groups of rocks in sierra chichinautzin 1 ol mafic rocks 2 opx intermediate rocks 3 opx felsic rocks and 4 qtz intermediate felsic rocks moreover two different types of mantle derived primitive mafic magmas have been described based on their mineralogical and geochemical features 1 oib like mafic magmas presence of olivine plagioclase phenocrysts higher incompatible element contents with no hfse lile negative anomalies and 2 mafic magmas derived from a metasomatized mantle no plagioclase phenocrysts and more forsteritic olivine hfse lile negative anomalies we prove that the diversity of magmas basalts to dacites in sierra chichinautzin is un related to fractional crystallization processes from the mafic magmas rather magma mixing phenomena between mafic magmas and two different crustal derived felsic magma types opx dacite and qtz bt amp pl cpx opx dacite explain the textural mineralogical and geochemical signatures of most of the sierra chichinautzin rocks there is a strong genetic relationship between the mafic and felsic end members of each mixing trend the relatively anhydrous oib like mafic magmas promote partial melting of a basaltic lower crust under low water fugacity conditions generating opx bearing felsic rocks on the contrary hydrous mafic magmas derived from a metasomatized mantle promote basaltic crustal partial melting under high water fugacities yielding magmas with a complex hydrous phenocryst assemblage several important geodynamical implications for the origin of the mvb magmatism can be extracted from this petrological study 1 the existence of two different primitive mafic magmas implies the presence of an heterogeneous mantle source beneath sierra chichinautzin 2 present data do not offer enough evidence to constrain the nature of the metasomatizing fluids beneath sierra chichinautzin and 3 the negative nb anomaly of most of the sierra chichinautzin rocks andesites and dacites is due to magma mixing and therefore is not a signature of the role of fluids from the subducting cocos plate these geochemical features together with the geophysical and tectonic scenario of the sierra chichinautzin indicate that volcanism in this area is mainly related to the upwelling of an anomalously hot oib like mantle and cast further doubts on a significant role of the subducting cocos plate in the origin of this magmatism

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

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

an integrated archeomagnetic and c14 study on pre columbian potsherds and associated charcoals intercalated between holocene lacustrine sediments in western mexico geomagnetic implications 1 the firm relationship between archeomagnetic samples and their absolute ages which is usually determined or estimated using a variety of methods archaeological context thermoluminiscense c14 etc is crucial in any archaeomagnetic investigation in this paper we report 18 successful from 65 analyzed archaeointensity determinations of high technical quality performed on specimens obtained from three fragments of pre columbian potsherds these are unambiguously correlated with charcoal samples as they were found together in three thin lacustrine sedimentary layers in western mexico moreover new radiometric results were obtained in this study from all charcoal samples beta analytic yielding ages of 1490 40 1510 40 and 1580 40 bp in addition detailed magnetic measurements were carried out on all studied ceramic fragments these rock magnetic experiments which included low field susceptibility versus temperature hysteresis and first order reversal curve measurements indicate that the main magnetic carrier is ti poor titanomagnetite the average archeointensity values per ceramic fragments obtained in this study are very similar in all three cases 35 2 1 3 t n 5 36 8 1 6 t n 6 and 37 2 3 4 t n 7 this corresponds to an average intensity of 36 4 1 1 t and a virtual axial dipole moment value of 8 1 0 2 1022 am2 which is slightly lower than the present field strength corresponding at an age interval between 640 and 430 a d although we detected some relationship between the earth s magnetic field intensity and multidecadal climatic events such observations can only be considered tentative at this stage

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

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

deposition and generation of multiple widespread fall units from the c ad 1314 kaharoa rhyolitic eruption tarawera new zealand the c ad 1314 kaharoa eruption from tarawera volcano north island new zealand was a complex long duration multi episode rhyolitic event up to 13 discrete sub plinian to plinian style phases occurred in two stages from at least two different vents along an 8 km long fissure the early explosive stage produced seven pumice lapilli fall units dispersed solely to the southeast of the volcano while a later stage produced six fall units dispersed almost entirely to the north and northwest the duration of fallout for these 13 lapilli units is estimated to have totaled about 1 5 days the presence of massive textured ash layers between many of the lapilli fall units suggests that longer duration pyroclastic density current producing phases and more quiescent fallout intervals occurred between the intense explosive phases of the eruption total duration of ash fallout 7 5 days the ash deposition extended the minimum duration of the explosive episodes to at least 9 days based on settling rates of the ash possibly the explosive phase of eruptive activity lasted for 2 weeks including some brief hiatuses and pyroclastic density current deposition and was followed by 4 years of mainly lava dome forming activity at least 15 km3 7 3 km3 dense rock equivalent dre of tephra was erupted during the main explosive episodes and at least 9 1 km3 dre during the whole eruption more than doubling the previous estimate this may be an underestimate by up to 50 of the fallout volume eruption column heights for the two most widely dispersed fall units are estimated to have ranged from 25 28 km above the vents and durations of individual lapilli fall unit forming phases ranged from 1 5 to 4 h but the ash producing phases lasted much longer individually up to 30 h and represent a longer lived shut down to each eruptive episode time averaged mass discharge rates are estimated to have varied by more than one order of magnitude during the main plinian style phases from 107 to 108 kg s however the explosive phases were unsteady as indicated by fluctuating grain size with stratigraphic height in the lapilli fall units and common capping ash beds the north northwesterly dispersal of the later stage fall units and the possible duration of 4 days for these episodes indicate an unusual period of southerly to southeasterly winds therefore the main explosive part of the kaharoa eruption is most likely to have occurred during the austral winter to spring months the kaharoa eruption style is also shared by the three previous rhyolite dominated eruptions that have built tarawera volcano over the past 22 000 years knowledge of the widespread pumice and ash fallout from the kaharoa eruption is important in quantifying the hazard from this type of extended duration explosive activity and has implications for future similar rhyolitic activity in new zealand

active tectonics of northeastern sonora mexico southern basin and range province and the 3 may 1887 mw 7 4 earthquake north south striking and west dipping basin and range province normal faults form the western edge of the sierra madre occidental plateau in northeastern sonora these faults and associated half grabens extend over a distance of more than 300 km between the san bernardino basin in the north and the sahuaripa basin in the south an earthquake in 1887 ruptured three neighboring segments of this major fault zone our field mapping in this region indicates that the surface rupture of the 1887 earthquake extends farther to the south and is considerably longer 101 4 km end to end length than previously reported a compilation of the seismicity in the epicentral region of the 1887 earthquake shows the epicenters to be distributed in well defined clusters at the northern end of the 1887 surface rupture in the step overs between the three rupture segments on a neighboring fault in the west fronteras fault and on fault segments farther south along the same fault zone granados region the distribution of seismicity correlates well with calculated changes in coulomb failure stress resulting from the 1887 earthquake

absence of cocos plate subduction related basic volcanism in southern mexico a unique case on earth the relationship between volcanism and subduction of the cocos plate is examined on the basis of new as well as published sr nd and pb isotopic and geochemical data on late miocene to holocene ca 9 0 ma basic volcanic rocks from southern mexico and central america basic rocks with sio 2 52 were chosen in order to minimize the effects of crustal assimilation processes and therefore to test the contribution from deeper sources the subducted cocos plate and the mantle wedge by using fluid mobile to relatively fluid immobile elements and radiogenic isotope ratios for such rocks i show that the subduction of the cocos plate does not contribute to the basic volcanism in all of southern mexico as opposed to that in central america from guatemala to northwestern costa rica south mexican volcanism is related to ongoing rifting processes inferred from field geology seismology gravity tectonics and volcano alignments this lack of subduction relationship probably represents the first case on earth where the ongoing subduction of an oceanic plate does not give rise to basic volcanism such as is present throughout southern mexico

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

counterclockwise rotations in the late eocene oligocene volcanic fields of san luis potosi and sierra de guanajuato eastern mesa central mexico abstract we used paleomagnetic and structural data to investigate the late eocene oligocene tectonic evolution of the mesa central area in mexico the mesa central was affected by nw trending faults tepehuanes san luis fault system coeval with a late eocene oligocene ignimbrite flare up and by post 27 ma nne trending grabens related to the basin and range we obtained reliable paleomagnetic directions from 61 sites within the late eocene oligocene volcanic series 30 to 27 ma of the san luis potosi volcanic field and sierra de guanajuato for each site we also measured the anisotropy of magnetic susceptibility ams tilt corrections were made using ams data for 33 sites where in situ bedding measurements were not available paleomagnetic directions indicate counterclockwise rotations of about 10 with respect to stable north america after 30 25 ma structural data suggest that the volcanic succession was mainly affected by normal faults however we also found evidences for oblique or horizontal striae showing a left lateral component along nw trending faults and a right lateral component along ne trending faults both motions are consistent with a n s extension oblique to the tepehuanes san luis fault system previous paleomagnetic studies in northern and southern mexico show the prevalence of minor left lateral shear components along regional scale transpressional and transtensional lineaments our paleomagnetic data may reflect thus small vertical axis rotations related to a minor shear component coeval with the oligocene intra arc extension in central mexico

effects of wall rock elasticity on magma flow in dykes during explosive eruptions abstract magma flow during explosive volcanic eruptions has been described assuming rigid conduits with simple cylindrical or planar geometries here we study the dynamics of explosive volcanic flows to take account of the role of elastic deformation of the conduit influenced by local magmatic pressure three cases are investigated a dyke with elliptical cross section a cylindrical conduit and a deep dyke connected to a shallow cylinder the model cpiuc macedonio et al 2005 was used for simulations and generalized to account for elastic deformations of the conduit cross section area due to magmatic overpressure fragmentation level is typically deeper in a dyke than in a cylinder for flows in wide dykes pressure at the fragmentation depth can be lower than the surrounding lithostatic pressure by several tens of mpa indicating that the wall rocks of the dyke will be unstable constraining the dyke width and eventually blocking the eruption on the other hand when the fragmentation level is shallow the corresponding lithostatic pressure is not large enough to close the dyke and eruptions from wide dykes are possible the behaviour changes drastically when we assume the conduit is a dyke at depth that evolves to a cylinder near the surface in this case even very wide dykes can be stable because the fragmentation level moves into the cylindrical region where deformation is negligible

enigmatic clastogenic rhyolitic volcanism the corral de coquena spatter ring north chile we report on the unusual occurrence of the products of lava fountaining in a pliocene calc alkaline rhyolitic monogenetic center from northern chile corral de coquena is a discontinuous ring of lava located in the moat of la pacana caldera 23 27 s 67 23 5 w part of the altiplano puna volcanic complex of the central andes the volcanic structure is composed of a maar like crater with an associated pyroclastic possibly phreatomagmatic unit that is overlain by rhyolitic glassy lava ramparts in which evidence of spatter agglutinate and clastogenic material is found typical explanations for the unusual textures in a rhyolitic lava such as peralkaline composition high volatile content or superheated magma are untenable in this case we propose that the most likely explanation for this extreme style of rhyolitic volcanism is a combination of moderately high eruption rate and efficient degassing prior to eruption in the light of reports of several other bodies of fountain fed silicic magma from the uk us and japan we propose that corral de coquena is a rhyolitic spatter ring superimposed upon a maar like crater we further propose that pyroclastic fountaining should be considered an end member of the spectrum of eruptive styles of calc alkaline silicic magmas and that corral de coquena is a rare example preserved because of the hyper arid climate in the altiplano puna volcanic complex

chemical and petrographic fingerprinting of volcanic ashes as a tool for high resolution stratigraphy of the upper miocene pisco formation peru tephra layers are a unique tool for stratigraphy their geologically instantaneous deposition across wide areas makes them a powerful instrument for dating and correlating distant localities in this paper we apply tephra fingerprinting for high resolution stratigraphic purposes in the upper miocene portion of the pisco formation which crops out along the southern peruvian coast the pisco formation diatomaceous strata host an important marine vertebrate fossil lagerstatte whose palaeontological relevance has entailed the necessity of reconstructing a detailed chronostratigraphic framework using 39 ar 40 ar dating and tephra correlations distal ashes from the central andes volcanoes are frequent in the pisco formation but their similar glass composition and mineral assemblage together with the shallow marine environment limiting tephra preservation could be unpromising for tephra based correlations in this study tephra layers from measured stratigraphic sections were fingerprinted with a combined petrographic and microanalytical approach including glass shard morphology and granulometric analyses based on the obtained results we correlated sections several kilometres apart greatly increasing the chronostratigraphic detail major element composition of biotite proved a valuable tool to discriminate the central andes metaluminous to peraluminous tephra deposited in the peruvian forearc basins this study highlights the applicability of tephra fingerprinting in unfavourable shallow marine environments supplementary material a compilation of tephra data field observations granulometric parameters petrographic data and epma analyses on glass epma single analyses on glass and biotite 39 ar 40 ar step heating results measured stratigraphic logs and biotite chemical diagrams are available at https doi org 10 6084 m9 figshare c 4201484

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

geotechnical modeling of fractures and cavities that are associated with geotechnical engineering problems in kuala lumpur limestone malaysia this study presents a modified geotechnical model of subsurface cavities and fractures that are associated with foundation and pilling problems the topographical model was used to automatically extract minimum surface curvature slope and pits the image processing shows that more than fourteen regional faults trending ne sw and wnw affect the kuala lumpur limestone bedrock and the surface topography from south to north these fractures often show higher probability of piling and constructions problems the faults are tensional low lying and wrench of length 10 20 km opencast ex mining pond floors bathymetry are good indicators of subsurface fractures and cavities that affect the limestone bedrock the landsat image band one shows that the fractures of opencast ex mining pond floors have the same trend as the regional faults these techniques can help geotechnical engineers to predict subsurface fractures and cavities especially in areas adjacent to ex mining ponds in brief most of the subsurface cavities and ex mining ponds for example the phong fatt pond are located on and along fault zones

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