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

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

seismic imaging of arc magma and fluids under the central part of northeastern japan abstract a dense seismic network was temporarily deployed during 1997 1999 in the central part of northeastern japan by japanese university research groups using the local earthquake arrival time data recorded by the network we have determined high resolution three dimensional images of p wave velocity vp s wave velocity vs and vp vs ratio in this area our results show that low vp low vs and high vp vs areas are extensively distributed in the uppermost mantle along the volcanic front in the lower crust low vp low vs and high vp vs areas are visible but they are confined to the individual volcanic areas in contrast the upper crust of volcanic areas shows low vp low vs and low vp vs rather than high vp vs these observations suggest that partial melting materials exist in the uppermost mantle along the volcanic front and they spread up to the midcrust of the volcanic areas low vp low vs and low vp vs in the upper crust of the volcanic areas suggest the presence of h2o rather than melt there deep low frequency microearthquakes perhaps caused by the rapid movement of fluids are located mostly at the edge of partial melting zones particularly at their upper surface distinct s wave reflectors bright spots with large reflection coefficients are distributed in the midcrust with slightly low vp vs suggesting that most of them are filled with h2o

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

paleomagnetic evidence for the miocene counter clockwise rotation of northeast japan rifting process of the japan arc paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of cretaceous to miocene age from northeast japan the remanent magnetizations of welded tuffs with age between 32 ma and 21 ma old from 17 widely distributed sampling sites in northeast japan are fairly well grouped with a mean ofd 41 2 i 56 5 95 7 2 the remanent magnetization of welded tuffs of 14 ma and 11 ma from 4 sites shows a northerly direction d 11 7 i 60 5 95 15 3 these data imply that northeast japan was apparently subjected to a counter clockwise rotation through 47 14 about a vertical axis on the representative point of northeast japan 141 e 39 n during the past 21 ma comparing with the period of the clockwise rotation of southwest japan the differential rotation between northeast japan and southwest japan is concluded to have occurred concurrently during the period between 21 ma and 12 ma a rifting model of the japan arc is proposed northeast japan rotated counter clockwise through 50 about a northern pivot at 146 e 44 n while southwest japan rotated clockwise through 54 about a southern pivot at 129 e 34 n the differential rotation about different rotation pivots gives rise to the opening of a diamond shaped back arc basin the japan sea

seismic structure of the northeastern japan convergent margin a synthesis many studies recently made on the basis of seismic observations have revealed a detailed structure of the crust and the upper mantle beneath the northeastern japan arc and its relationship to seismic and volcanic activity spatial distributions of the depths to the conrad and the moho discontinuities estimated from shallow earthquake data and seismic explosion data show that both discontinuities are deep in the middle of the land area and shallow toward the coastlines of the japan sea and the pacific ocean the pn velocity has a lateral variation it is as low as 7 5 km s beneath the land area while that beneath the japan sea and the pacific ocean is 8 0 8 2 km s it changes abruptly at the transition zones which are located along the coastlines precise structure and location of the subducted pacific plate beneath the land area is inferred from converted or reflected seismic waves at the top or bottom of the plate the pacific plate is composed of a thin 5 km low velocity upper layer and a thick high velocity lower layer its total thickness being 80 90 km the upper plane seismicity of the double seismic zone is confined to the thin low velocity upper layer which probably corresponds to the subducted former oceanic crust the lower plane seismicity lies at the middle of the high velocity lower layer and the lower half of the plate below it is incapable of generating earthquakes the shallower portion of the upper surface of the plate beneath the pacific ocean along which major seismicity with low angle thrust faultings is actually occurring is also located by seismic observations on land and in the sea the pacific plate subducts at an extremely low angle of 5 for the first 25 km depth and then the dip steepens rather abruptly to 30 normal fault type events at the top of the plate have not been detected in the portion where the downward bending is the largest but have been ditected near the trench axis where it is rather small tomographic inversions for seismic velocity structure clearly delineate the inclined high velocity pacific plate with a thickness of 80 90 km and low velocity zones in the crust and the mantle wedge beneath active volcanoes seismic attenuation tomography also shows similar zones of low q value beneath active volcanoes although its spatial resolution is much lower the low velocity zones with 2 6 velocity lows are continuously distributed from the upper crust just beneath active volcanoes to a depth of 100 150 km in the mantle wedge and are approximately parallel to the dip of the underlying pacific plate these low velocity zones probably reflect the pathway of magma ascent from a depth in the mantle wedge to the earth s surface corresponding to a portion of the subduction induced secondary mantle wedge flow

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

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

structure of an island arc wide angle seismic studies in the eastern aleutian islands alaska new seismic wide angle data from the eastern aleutian islands show a mafic composition and a 30 km thick island arc crust traveltimes of p and s refracted arrivals and prominent crustal and mantle reflectors observed to offsets of over 300 km were used to derive velocity models for the eastern aleutian arc between the islands of atka and unimak using a three dimensional finite difference modeling and tomography code we interpret the highest crustal p wave velocities of 7 2 7 4 km s between about 12 and about 22 km depth to the south and north of the main volcanic line as remainders of preexisting oceanic crust into which arc magma is intruded apart from the pieces of oceanic crust the velocities suggest an overall mafic composition for the arc composed mainly of metabasalts diorite and diabase in the upper crust and garnet granulite or amphibolite to hornblendite in the lower crust reflected arrivals from the subducting pacific plate at depths of 45 55 km beneath the fore arc together with pn show a mantle wedge with p wave velocities as low as 7 4 km s increasing with depth to about 8 1 km s with an average of about 7 7 km s a mantle composition that grades from mainly pyroxenite probably ultramafic cumulates near the moho to dunite at greater depths best explains these observed velocities

heterogeneous stress state of island arc crust in northeastern japan affected by hot mantle fingers by considering a thermal structure based on dense geothermal observations we model the stress state of the crust beneath the northeastern japan island arc under a compressional tectonic regime using a finite element method with viscoelasticity and elastoplasticity we consider a three layer structure upper crust lower crust and uppermost mantle to define flow properties numerical results show that the brittle viscous transition becomes shallower beneath the ou backbone range compared with areas near the margins of the pacific ocean and the japan sea moreover several elongate regions with a shallow brittle viscous transition are oriented transverse to the arc and these regions correspond to hot fingers i e high temperature regions in the mantle wedge the stress level is low in these regions due to viscous deformation areas of seismicity roughly correspond to zones of stress accumulation where many intraplate earthquakes occur our model produces regions with high uplift rates that largely coincide with regions of high elevation e g the ou backbone range the stress state fault development and uplift around the ou backbone range can all be explained by our model the results also suggest the existence of low viscosity regions corresponding to hot fingers in the island arc crust these low viscosity regions have possibly affected viscous relaxation processes following the 2011 tohoku oki earthquake

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

compositional and metamorphic controls on velocity and reflectivity in the continental crust an example from the grenville province of eastern quebec we present velocity and density data for a suite of gabbros and metagabbros over a range of metamorphic grades from subgreenschist to the eclogite facies most of these rocks have a mesocratic protolith with overlapping bulk chemistry ranges providing an opportunity to investigate the systematics of mineralogy and physical properties across a wide range of metamorphic grades with minimal compositional variation in addition more mafic gabbros in the eclogite facies were also investigated in order to take into account variations in the extent of transformation of gabbro to eclogite controlled by bulk chemistry the physical properties data are used as the basis for a first order crustal model composed of granitic anorthositic and gabbroic components the objective of constructing the crustal model is to evaluate the relative contribution of compositional variation and metamorphism to the velocity density and reflectivity structure of the continental crust the model suggests that in the upper crust if fracture porosity is neglected composition dominates the velocity and density structure while in the middle and lower crust composition and metamorphism play equally important roles simulations designed to predict potential reflection coefficient size distributions for the various bulk chemistries and mineralogies in the crustal model indicate step increases in the potential for large reflection coefficients that are dominantly controlled by metamorphic grade step increases in reflectivity are predicted between the lower and upper amphibolite facies and between incompletely and completely eclogitized metagabbros in the eclogite facies we also consider several alternative mixing models which account for the effects of one of the three lithologic components occurring as isolated bodies rather than perfectly mixed with the other two components these models indicate a reduction of the potential for large reflection coefficients by as much as 50 emphasizing the importance of the degree of mixing for crustal reflectivity

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

three dimensional structure of vp vs and vp vs beneath northeastern japan implications for arc magmatism and fluids we estimated both p v p and s wave velocity v s structures beneath northeastern japan by applying a tomographic method to 169 712 p and 103 993 s wave arrival time data from 4338 local events the average value of v p v s ratio is 1 69 in the upper crust 1 75 in the lower crust and 1 77 in the uppermost mantle these differences in v p v s ratio may mainly reflect the lithological variations with depth low v p low v s and high v p v s zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge here the low v p low v s and high v p v s values are interpreted because of partial melting materials which imply the presence of a vast amount of melt in the uppermost mantle in the lower crust low v p low v s and high v p v s zones are not continuously visible along the volcanic front but are confined to individual volcanic areas which suggests that melting of lower crustal materials occurs just beneath each active volcano in contrast the upper crust beneath active volcanoes exhibits low v p low v s and low v p v s these results suggest that the partial melting zones under northeastern japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes the low v p low v s and low v p v s features in the upper crust suggest the existence of h 2 o rather than melt right beneath active volcanoes

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

effect of pore geometry on vp vs from equilibrium geometry to crack seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes including oblate spheroids tubes cracks and an equilibrium geometry controlled by a dihedral angle the relative role of liquid compressibility and pore geometry on the v p v s velocity ratio is clarified the result clearly indicates that p and s velocity structures determined by seismic tomography can be used to verify whether interfacial energy controlled melt or fluid geometry equilibrium geometry is achieved relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio as a function of the aspect ratio a significant effect of pore geometry on d in v s d in v p the ratio of the fractional changes in v s and v p is shown equilibrium geometry of the partially molten rocks characterized by a dihedral angle of 20 40 corresponds to an aspect ratio of 0 1 0 15 the value of d in v s d in v p expected for the texturally equilibrated partially molten rocks is shown to be 1 1 5 which is much smaller than that expected for cracks and dikes with an aspect ratio of 10 2 10 3 in the upper mantle low velocity regions the seismologically obtained value of d in v s d in v p is within this range beneath the bolivian andes 1 1 1 4 but is as high as 2 beneath iceland 1 7 2 3 and beneath northeastern japan 2 0 the former region can be regarded as a region where equilibrium geometry is achieved and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate

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

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

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

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

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

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

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

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

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

continental crust crustal underplating and low q upper mantle beneath an oceanic island arc a detailed structural model of the crust subducting slab and underlying upper mantle across the northern izu ogasawara bonin island arc system is derived from a marine seismic reflection and ocean bottom seismographic refraction survey and subsequent forward modeling combined with tomographic inversion the model indicates that the crust is thickest beneath the presently active rift zone and a granitic crust may have formed in the mid crust a highly attenuative mantle that is one with low quality q seems to be confined mainly beneath the presently active rift zone in contrast high p wave velocity persists in the lower crust between the forearc and eastern margin of the back arc basin suggesting a large scale magma input responsible for the arc formation

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

seismotectonics beneath the tokyo metropolitan area japan effect of slab slab contact and overlap on seismicity 1 we first determine the configuration of the upper surface of the pacific pac slab beneath kanto japan from the distribution of interplate earthquakes relocated by an appropriate 1 d velocity model then traveltime tomography is carried out to estimate three dimensional seismic velocity structures around kanto using 735 520 p wave and 444 049 s wave arrival times from 6508 local earthquakes the obtained results suggest that the philippine sea phs slab is subducting to depths of 130 140 km without a gap even to the northwest of the izu collision zone we subsequently define the lateral extent of the contact zone between the bottom of the phs slab and the upper surface of the pac slab phs pac interface and reveal that the slab contact zone underlies a wider area beneath kanto in harmony with the kanto plain the downdip limit of interplate thrust type earthquakes on the pac slab is deepened by 30 km locally under the slab contact zone this deepening is probably caused by a lower temperature environment in the pac slab resulting from the overlap with the phs slab subducting above and consequent thermal shielding by the phs slab from the hot mantle wedge we detect an extremely low velocity anomaly in the easternmost portion of the phs slab which is probably attributable to serpentinization of mantle peridotite interplate earthquakes are almost absent along the phs pac interface overlain by the serpentinized mantle in the phs slab suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine

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

opening of the sea of japan back arc basin by asthenospheric injection abstract the position of a volcanic front on an arc is governed by dehydration reactions of amphibole chlorite in down dragged hydrated peridotite at the base of the mantle wedge as these reactions are essentially pressure dependent ca 3 5 gpa the paleo position of a volcanic front offers an estimate for the depth of the subducted oceanic lithosphere beneath it this paper evaluates volcanism in the northeast japan arc since the oligocene and discusses the mass transportation in the mantle wedge during the episode of back arc extension in the sea of japan with reference to the migration of the volcanic front the volcanic front during the oligocene ca 30 ma was located along the western coast of the present northeast japan are and migrated about 200 km trenchwards at 23 ma before the opening of the sea of japan back arc basin it follows that the angle of subduction became steeper during the period of 30 23 ma triggered by the injection of the asthenosphere into the mantle wedge the back arc extension of the sea of japan between 21 and 14 ma is a manifestation of the dynamic process of the lithosphere due to the asthenospheric injection the temperature of the injecting asthenospheric mantle was higher than that of the surrounding upper mantle or lithosphere this caused the characteristic volcanism at 23 22 ma 1 high magnesian andesite volcanism took place in the near trench region 2 across arc compositional variation was not observed 3 the volume of volcanic materials increases towards the back arc side of volcanic arc the volcanic front has migrated over a short distance inwards less than 100 km since 15 ma possible as a result of cooling in the mantle wedge because both amphibole and chlorite break down at slightly higher pressures at lower temperatures the upper mantle materials which injected into the mantle wedge and caused back arc spreading are identical to those of the mid oceanic ridge basalt source in sr nd isotopic compositions because volcanic rocks in the back arc region of the northeast japan arc show secular variation in isotopic compositions from enriched to depleted isotopic characteristics during the last 30 ma

discontinuous change in temperature derivative of vp in lower crustal rocks 1 compressional wave velocities vp of gabbronorite pyroxene granulite wehrlite and websterite were measured up to 1000 c at 1 gpa during heating and cooling the wehrlite and websterite which contain secondary hydrous minerals show irreversible change in temperature derivative of vp vp t during heating these rocks show sudden decrease in vp when dehydration reactions took place during cooling the rocks show a linear increase in vp in contrast the gabbronorite and pyroxene granulite which are nearly dry crystalline assemblage show reversible and discontinuous change in vp t at around 400 c it changes from 2 4 and 2 1 10 4 km s 1 c 1 to 4 7 and 4 5 10 4 km s 1 c 1 this reversible and discontinuous change in vp t is difficult to explain by the previously proposed mechanisms such as thermal cracking dehydration reaction and or partial melting the data suggest that this reversible and discontinuous change would be associated with phase transition in plagioclase

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

aeromagnetic evidence for a buried early cretaceous magmatic arc northeast japan positive aeromagnetic anomalies recent drilling and models constructed from these data delineate the plutonic roots of the early cretaceous kitakami magmatic arc in northeast japan buried plutons mostly offshore produce belts of positive magnetic anomalies these anomalies and magnetotelluric data suggest that the plutons form a batholith 70 120 km wide nearly 800 km long and 10 15 km thick the batholith may mark the location of the main kitakami arc most of the exposed kitakami plutons are 2 20 km in diameter some are 3 km thick the small plutons line up along nw trending faults some may have been satellite vents that tapped into the magma supply of the main arc the batholithic roots of the main arc now compose almost half of the modern japan forearc basement steep magnetic gradients offset anomalies and basin stratigraphy portray extensive faulting of the kitakami batholith during oblique subduction in the late cretaceous and rifting in the miocene the eastern boundary of the kitakami batholith lies between 90 and 140 km west of the modern trench much closer than the 300 km distance between the active arc and trench the early cretaceous forearc basin and accretionary prism may underlie the modern forearc basin east of the batholith but clear evidence is lacking much of the early cretaceous margin including most of its forearc therefore is missing how the material was removed is unknown it could have been strike slip faulted eroded by subduction related processes or both

p wave tomography anisotropy and seismotectonics in the eastern margin of japan sea abstract to understand the seismotectonics in the eastern margin of the japan sea we determined the first high resolution 3 d p wave velocity structure and azimuthal anisotropy under the japan sea off northeast japan using 175 425 high precision p wave arrival times from 2833 local earthquakes recorded by 330 seismograph stations p wave arrival times from 145 suboceanic earthquakes relocated with sp depth phase are crucial to determine the structure of the crust and uppermost mantle under the japan sea our results show that strong velocity variations exist in the crust and uppermost mantle under the eastern margin of the japan sea many large crustal earthquakes occurred in or around low velocity zones which may represent weak sections of the seismogenic crust the p wave azimuthal anisotropy is complex under the japan sea which may also indicate the complex crustal structures there in the eastern margin of the japan sea the strong heterogeneities in the crust and upper mantle revealed by seismic tomography may reflect the complicated geologic structures such as the alternate rift zones ridges basins horsts grabens volcanics and continental fragments which were produced during the back arc spreading opening of the japan sea and the present compressional stage of the honshu arc

simultaneous high p t measurements of ultrasonic compressional and shear wave velocities in ichino megata mafic xenoliths their bearings on seismic velocity perturbations in lower crust of northeast japan arc 1 simultaneous measurements of ultrasonic compressional vp and shear wave velocities vs were conducted under pressures and temperatures representative of the lower crust on five mafic xenoliths derived from the northeast japan arc crust for comparison with the observed seismic heterogeneity of the lower crust of the northeast japan arc the velocity deviations dvp and dvs of the xenoliths were calculated using the average reference vp 6 61 0 1 km s and vs 3 76 0 1 km s the dvp and dvs values of the xenoliths at 700 c and 0 8 gpa are 0 8 and 1 2 for hornblende gabbro 4 8 and 4 0 for hornblende pyroxene gabbro 0 9 and 1 5 for amphibolite 2 5 and 1 7 for pyroxene amphibolite and 0 9 and 5 6 for hornblendite respectively we compared the velocity perturbations in the lower crust determined by seismic tomography with dvp and dvs of the xenoliths using vp vs vp vs deviation diagrams on the basis of our measurements we suggest that 1 the seismically high vp and vs regions beneath the tobishima basin consist of hornblende pyroxene gabbro 2 hornblende gabbro is a predominant rock type beneath the dewa hills and ou backbone range 3 the low velocity anomalies beneath the active volcano areas may be caused by the existence of partial melts of hornblende gabbro and 4 the low vp and high vs regions beneath the kitakami mountains consist of quartz plagioclase bearing rocks our data demonstrate that the seismic heterogeneity in the lower crust of the northeast japan arc reflects variations in rock composition and temperature that are related to the regional geological history

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

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

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

timing of plutonism in the gallivare area implications for proterozoic crustal development in the northern norrbotten ore district sweden zircon ion probe secondary ion mass spectrometry or sims data from a set of intrusive rocks emplaced in the vicinity of major ore bodies as well as from large igneous intrusions in the gallivare

experimental study on diamond dissolution in kimberlitic and lamproitic melts at 1300 1420 c and 1 gpa with controlled oxygen partial pressure to evaluate the dissolution processes of diamond crystals in kimberlitic and lamproitic magmas a series of diamond dissolution experiments were carried out in the graphite stability field at 1300 1420 c and 1 gpa under the wi mw and hm buffers dissolution agents used include an aphanitic kimberlite from wesselton mine south africa and a lamproite from mount north west kimberley australia with increasing run duration diamond morphology changed from a sharp octahedron through combinations of octahedron and tetrahexahedroidal forms to spherical tetrahexahedroid having rounded faces negatively oriented trigons were formed on the octahedral 111 face as the degree of diamond dissolution increased the trigons changed from smaller shallow types to larger deep types the dissolution rate in the kimberlitic solvent at 1300 c was 0 12 mm h under the hm buffer 0 0034 mm h under the mw buffer and 0 0017 mm h under the wi buffer whereas that at 1420 c was 0 014 mm h under the wi buffer in the lamproitic solvent the dissolution rate was 0 0024 mm h at 1420 c under the wi buffer the data indicate that diamond dissolves in silicate melts as carbonate ions through an oxidizing reaction the degree of dissolution strongly depends on temperature oxidation state and the compositional dependence of co 2 solubility in the melts

petrological model of the northern izu bonin mariana arc crust constraints from high pressure measurements of elastic wave velocities of the tanzawa plutonic rocks central japan abstract ultrasonic compressional wave velocities vp and shear wave velocities vs were measured with varying pressure up to 1 0 gpa in a temperature range from 25 to 400 c for a suite of tonalitic gabbroic rocks of the miocene tanzawa plutonic complex central japan which has been interpreted as uplifted and exposed deep crust of the northern izu bonin mariana ibm arc the vp values of the tonalitic gabbroic rocks increase rapidly at low pressures from 0 1 to 0 4 gpa and then become nearly constant at higher pressures above 0 4 gpa the vp values at 1 0 gpa and 25 c are 6 3 6 6 km s for tonalites 56 4 71 1 wt sio2 6 8 km s for a quartz gabbro 53 8 wt sio2 and 7 1 7 3 km s for a hornblende gabbro 43 2 47 7 wt sio2 combining the present data with the p wave velocity profile of the northern ibm arc we infer that 6 km thick tonalitic crust exists at mid crustal depth 6 1 6 3 km s vp overlying 2 km thick hornblende gabbroic crust 6 8 km s vp our model shows large differences in acoustic impedance between the tonalite and hornblende gabbro layers being consistent with the strong reflector observed at 12 km depth in the ibm arc the measured vp of tanzawa hornblende bearing gabbroic rocks 7 1 7 3 km s is significantly lower than that vp modeled for the lowermost crustal layer of the northern ibm arc 7 3 7 7 km s at 15 22 km depth we propose that the ibm arc consists of a thick tonalitic middle crust and a mafic lower crust

extensional structure in northern honshu arc as inferred from seismic refraction wide angle reflection profiling a recent extensive seismic wide angle experiment revealed a new image of crustal and uppermantle structure across northern honshu arc japan the western part of the arc recorded the crustal deformation by the miocene back arc spreading of the sea of japan the crust is composed of highly deformed tertiary sedimentary layers a relatively low velocity 5 75 5 9 km s crystalline basement and a 15 km thick lower crust with a velocity of 6 6 7 0 km s clear westward crustal thinning from 32 to 27 km represents the extensional deformation by the backarc spreading the crust attains the maximum thickness 32 35km east of the backbone range for which the magmatic intrusion underplating since 10 15 ma is a predominant factor the eastern part of the arc has a less deformed upper crust and a reflective middle lower crust probably remaining a stable block since the time of the backarc spreading

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

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

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

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

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

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

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

the relationship between late cenozoic tectonic events and stress field and basin development in northeast japan the regional tectonic stress field basin development and crustal deformation of the ne japan arc in the interval between 32 ma to the quaternary can be synthesized based on dike vein and fault orientation data as well as on the compilation of the regional geology an extensional stress field became prevalent from 32 ma and major normal faulting started at 25 20 ma which resulted from back arc rifting normal faulting trending nearly parallel to the arc propagated from the present japan sea coast to the forearc side following the trenchward migration of the main volcanic field from 20 to 15 ma normal faults with an oblique trend to the arc developed due to the counterclockwise rotation of ne japan the rapid clockwise rotation of sw japan since 16 ma produced a nw se directed transtensional stress regime in the ne japan arc due to crustal stretching associated with this pull apart movement the back arc side of the ne japan arc subsided rapidly to middle bathyal environments after the termination of the opening of the japan sea at about 14 ma a neutral stress regime prevailed which included phases of both weak extension and compression lithospheric cooling eventually led to thermal subsidence of the back arc region and igneous underplating caused uplift of the axial zone of the volcanic arc the increase in velocity of the westward motion of the pacific plate at around 4 ma produced strong compression across the arc reactivating most of the miocene normal faults and uplifting the volcanic arc the greatest crustal shortening occurred in areas that were stretched the most in the miocene within the volcanic arc which implies tectonic inversion

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

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

seismic structure and seismogenesis off sanriku region northeastern japan summary in 1997 a seismic experiment using an airgun array and ocean bottom seismographs obss was performed in the forearc region of the northeastern japan nej arc the objectives of this experiment were to clarify seismic structures of the forearc region off sanriku japan using airgun obs data and to understand the nature of the japan trench seismogenic zone fundamental features of the structure are as follows i the subduction angle becomes steeper from east to west from 3 to 8 to 11 with the top of the plate located at depths of 11 km 40 km west from the trench 21 km 120 km west from the trench and 28 km 140 km west from the trench respectively ii the nej moho beneath the forearc is defined at a depth of approximately 20 km iii p wave velocity of the island arc upper mantle is approximately 8 km s 1 iv the p wave velocity of the cretaceous sedimentary layer exhibits significant lateral variability 4 0 5 5 km s 1 i nthe e w direction v the upper crust of nej forearc also has the significant lateral heterogeneity 5 0 6 3 km s 1 i nthe e w direction but not in the n s direction vi the middle crust defined on land extends in some locations 40 km east of the coastline vii lateral variability in the p wave velocity of the lower crust 6 7 7 0 km s 1 i ssmall compared with that of the upper crust according to these results it is suggested that the rupture areas of large earthquakes m7 in this region are limited within the contact zone between the nej arc crust and the subducting oceanic crust although a thin layer with low velocity is probably present within this contact zone the serpentinization of uppermost mantle wedge of the nej arc by reaction between the mantle materials and the water from the thin layer beneath forearc region is limited this might indicate that a significant volume of water and sediments subduct with the slab to depths of over 20 km

determination of the fluid absent solidus and supersolidus phase relationships of morb derived amphibolites in the range 4 14 kbar experiments on a morb mid ocean ridge basalt derived amphibolite have been conducted in order to determine fluid absent solidus and supersolidus phase relationships the solidus has been determined in the range 4 14 kbar by using the incremental heating technique tested against classical procedures peritectic clinopyroxene was produced below 10 kbar and epidote was the stable peritectic phase at pressures greater than 10 kbar for temperatures near the solidus garnet was the peritectic phase for temperatures above 800 c and pressures of 10 to 14 kbar the shape of the solidus was similar to previous estimations but the temperature interval between the high and low pressure regions was strongly reduced with these new estimations the possibilities for melting of the oceanic crust at p below 10 kbar are increased and consequently the number of tectonic scenarios needed for magma generation from amphibolite sources is enlarged in the absence of free water the solidus in the pressure region above 10 kbar is at higher t than previous estimations

structure and composition of the aleutian island arc and implications for continental crustal growth we present results of a seismic reflection and refraction investigation of the aleutian island arc designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust the aleutian arc has the requisite thickness 30 km to build continental crust but it differs strongly from continental crust in its composition and reflectivity structure seismic velocities and the compositions of erupted lavas suggest that the aleutian crust has a mafic bulk composition in contrast to the andesitic bulk composition of continents the silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the aleutian intraoceanic arc therefore if island arcs form a significant source of continental crust the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin the pervasive deformation intracrustal melting and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc continent collision or through subsequent establishment of a continental arc the volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two

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

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

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

high pressure high temperature seismic velocity structure of the midcrustal and lower crustal rocks of the ivrea verbano zone and serie dei laghi nw italy the ivrea verbano zone and the adjacent serie dei laghi in the inner arc of the western alps nw italy display an upended cross section through much of the thickness of the continental crust as it existed in that region in triassic jurassic time we collected a suite of oriented rock samples that represent most of the volume of the rocks of the region p and s wave velocity measurements were made in three orthogonal directions related to the mesoscopic fabric of the rocks at room temperature and up to 550 mpa confining pressure combined high temperature and high pressure measurements were made up to 700 c on a subset of the samples the lithologic units were divided into 23 different principal rock types and we present velocity data averaged in terms of these groups vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section these show that vp and vs both increase and anisotropy decreases systematically with depth of burial reflecting variations in rock type and metamorphic grade with depth vp and vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding with the slowest direction tending to be normal to the foliation or banding the vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust these correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution

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

compressional wave velocity of granite and amphibolite up to melting temperatures at 1 gpa abstract compressional wave velocities vp at above solidus temperatures and at 1 gpa were obtained for a granite and amphibolite which are considered to be major constituents of the continental crust the temperature variation of velocities showed that the vp values of granite decreased with rising temperature but substantially increased beyond the melting temperature 850 900 c such an increase may be caused by the transition of quartz the velocities of amphibolite decreased linearly with increasing temperature and dropped sharply at temperatures above the solidus 700 c indicating that partial melting of amphibolite acts to significantly lower the seismic velocities

lower crustal seismic velocities from lofoten vesteralen north norway abstract the archipelago of lofoten vesteralen comprises archaean and proterozoic rocks in granulite or amphibolite facies the area is characterised by a large gravity anomaly 120 mgal and thin crust c 25 km and it has been suggested that it represents a deep section into the continental crust compressional and shear wave velocities have been measured in the laboratory on over 70 samples from the area most of them to 0 4 gpa confining pressure velocity profiles are then deduced for sections of the crust assuming a restored horizontal moho and it is shown that upper and lower crustal divisions of approximately 5 9 6 2 km s 1 and 6 5 6 6 km s 1 can be recognised which are similar to those found on seismic refraction lines in scandinavia the velocities of the lofoten granulites are however higher than the velocities observed on the lofoten seismic refraction experiment possible reasons for this including the opening of cracks at low pressure retrograde metamorphism the interpretation of the refraction data or major tectonic dislocation are explored

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

relationship between anisotropy of p and s wave velocities and anisotropy of attenuation in serpentinite and amphibolite velocities and q values of p and s waves as functions of pressure and temperature at 100 and 600 mpa are presented for a serpentinite and an amphibolite both rocks exhibit a strong lattice preferred orientation lpo of the major mineral phases antigorite and hornblende respectively velocities and q values increase with pressure the rate of increase is different in the three orthogonal directions normal and parallel to foliation and lineation and closely related to progressive closure of microcracks increasing temperature decreases velocities and q values only slightly as long as thermal cracking is prevented by the applied confining pressure substantial anisotropy of velocities and q in p and s waves is observed in both rocks but is found to be different in origin anisotropy of p and s wave velocities is highest at low pressure and basically caused by constructive interference of effects related to oriented microcracks and to the lpo of major minerals increasing confining pressure decreases velocity anisotropy at a smaller and smaller rate the residual anisotropy of p and s wave velocities shear wave splitting at high confining pressure is mainly a result of preferred mineral orientation by contrast anisotropy of q is very low at low confining pressure and markedly enhanced as pressure is increased at high confining pressure substantial anisotropy of q in p waves is apparent but reversed from that of p wave velocities qp is highest in the direction normal to the foliation plane whereas vp and vs is lowest in this direction the generation of a pronounced anisotropy of qp by increasing pressure is due to a directionally dependent increase of contact areas on the oriented grain boundaries of the platy minerals defining the foliation the increase of q with pressure in the direction normal to foliation is mainly caused by the decrease of energy loss due to compressive strain relative to shear strain the reverse is true for the x and y directions serpentinite and x direction amphibolite parallel to the foliation plane

deep structure of the northeastern japan arc and its implications for crustal deformation and shallow seismic activity abstract seismic tomography studies in the northeastern japan arc have revealed the existence of an inclined sheet like seismic low velocity and high attenuation zone in the mantle wedge at depths shallower than about 150 km this sheet like low velocity high attenuation zone is oriented sub parallel to the subducted slab and is considered to correspond to the upwelling flow portion of the subduction induced convection the low velocity high attenuation zone reaches the moho immediately beneath the volcanic front or the ou backbone range running through the middle of the arc nearly parallel to the trench axis which suggests that the volcanic front is formed by this hot upwelling flow aqueous fluids supplied by the subducted slab are probably transported upward through this upwelling flow to reach shallow levels beneath the backbone range where they are expelled from solidified magma and migrate further upward the existence of aqueous fluids may weaken the surrounding crustal rocks resulting in local contractive deformation and uplift along the backbone range under the compressional stress field of the volcanic arc a strain rate distribution map generated from gps data reveals a notable concentration of east west contraction along the backbone range consistent with this interpretation shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone based on these observations a simple model is proposed to explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern japan arc

contrasting subduction structures within the philippine sea plate hydrous oceanic crust and anhydrous volcanic arc crust we show contrasting subduction structures within the philippine sea plate inferred from active source wide angle reflection data previous studies showed that large amplitude reflections from the slab are observed in southwest japan and indicated that a thin low velocity layer with a high fluid content is formed along the top of the subducting oceanic crust on the contrary we found that the slab reflections have smaller amplitudes in the izu collision zone central japan where the izu bonin volcanic arc has been colliding subducting suggesting that such a low velocity layer does not exist beneath the collision zone this structural difference is also supported by p wave and s wave velocity anomalies by passive source tomography and electrical conductivity and correlates with the regional distribution of deep tremors and intraslab earthquakes both of which are induced by dehydration processes within the downgoing slab based on these comparisons we suggest that the original structure of the incoming plate controls the contrasting subducting systems typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and or seawater infiltration at outer rises whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable anhydrous forms before subduction

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

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

laboratory measurement of p wave velocity in crustal and upper mantle xenoliths from ichino megata ne japan ultrabasic hydrous lower crust beneath the ne honshu arc abstract ultrasonic laboratory measurements of p wave velocity vp were carried out up to 1 0 gpa in a temperature range of 25 400 c for crustal and mantle xenoliths of ichino megata northeast japan the rocks used in the present study cover a nearly entire range of lithological variation of the ichino megata xenoliths and are considered as representative rock samples of the lower crust and upper mantle of the back arc side of the northeast ne honshu arc the vp values measured at 25 c and 1 0 gpa are 6 7 7 2 km s for the hornblende gabbros 38 6 46 9 wt sio2 7 2 km s for the hornblende pyroxene gabbro 43 8 wt sio2 6 9 7 3 km s for the amphibolites 36 1 44 3 wt sio2 8 0 8 1 km s for the spinel lherzolites 46 2 47 2 wt sio2 and 6 30 km s for the biotite granite 72 1 wt sio2 combining the present data with the vp profile of the ne honshu arc iwasaki t kato w moriya t hasemi a umino n okada t miyashita k mizogami t takeda t sekine s matsushima t tashiro k miyamachi h 2001 extensional structure in northern honshu arc as inferred from seismic refraction wide angle reflection profiling geophys res lett 28 12 2329 2332 we infer that the 15 km thick lower crust of the ne honshu arc is composed of amphibolite and or hornblende pyroxene gabbro with ultrabasic composition the present study suggests that the vp range of the lower crustal layer 6 6 7 0 km s in the ne honshu arc which is significantly lower than that obtained from various seismic measurements e g the northern izu bonin mariana arc 7 1 7 3 km s is due to the thick hydrous lower crustal layer where hornblende plagioclase and magnetite are dominant

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