provenance of jurassic accretionary complex mino terrane inner zone of south west japan implications for palaeogeography of eastern asia provenance and tectonic history of the jurassic accretionary complex mino terrane located in the inner zone of south west japan were studied using sandstone framework composition and mudrock geochemistry modal analysis of sandstones shows that the tectonic setting of the source area for the studied mino terrane clastic rocks was uplifted basement largely dominated by high grade metamorphic terrain composed of quartz and feldspar especially plagioclase the textural and mineralogical immaturity extent of alkali and alkaline earth element leaching low chemical index of alteration values and depleted rare earth element ree contents suggest rapid uplift and erosion within the source terrain and a relatively weak weathering intensity factor analysis revealed that grain size effects governed compositional heterogeneity in the studied sediments provenance of the sediments is interpreted as being plagioclase enriched felsic basement rock such as granodiorite within a continental margin and evolved arc tectonic setting rather than active volcanic arc lack of a contribution from active volcanic arcs may have resulted from the cessation of volcanism during the reorganization of the subducting plate system and or erosion of arc volcanics and exposure of basement considering the previous studies on palaeogeography and palaeocurrent reconstruction the north eastern part of the yeongnam massif in the korean peninsula is interpreted as the most probable source area for the studied turbidites the results of mixing calculation for mino terrane sediments suggest that precambrian leucocratic granite and the basement rock of the cretaceous gyeongsang basin shed large amounts of sediments to the mino trench whilst precambrian granitic gneiss and the triassic pluton supplied lesser amounts the results of this study reveal that although active subduction accretion processes were occurring the mino trench was bordered by continental basement rocks this knowledge contributes to enhanced understanding of the jurassic palaeogeography of the east asia continental margin

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

kinematic setting and structural control of arc volcanism abstract the evaluation of the kinematic setting and the structural control of volcanic arcs are important in defining the tectono magmatic processes along convergent plate boundaries however our knowledge is fragmented and the available data highlight different behaviours this study analyzes the kinematic setting the structural control and the volcanic productivity of 16 arcs these arcs are characterized by predominant extensional compressional strike slip or oblique motions there is an overall coupling between the normal vs parallel motions along the arc and those of the underlying slab therefore the higher the trench normal or parallel component of the subduction rate the higher is the amount of arc normal or parallel motion this relation confirms that strain partitioning is in general feasible at many convergent settings involving also the volcanic arc portion the arc normal motion may be characterized by extension or compression as a function of the increase in the trench normal convergence or subduction velocity the lack of an evident relation between the subduction rate or convergence rate and the volcanic productivity of the arc is partly inconsistent with previous studies which highlight a relation between the subduction rate and melt production below arcs this discrepancy mainly suggests that different processes control the generation rise and eruption of magma at different depths varying the intrusive extrusive ratio along arcs the structure of the arc does not control the distribution of the volcanoes however it does control the volcanic output through different processes even though regional or local extension associated with strike slip or compressive structures is the ultimate requisite in any setting in general the higher the amount of extension the higher is the output rate along the arc

tectonomagmatic characteristics of the back arc portion of the calama olacapato el toro fault zone central andes 1 post 20 ma magmatism in the central andes is either localized in the magmatic arc or distributed east of it on the altiplano puna plateau here there is a distinct concentration of magmatic centers on nw se trending lineaments such as the calama olacapato el toro cot that extends into the eastern cordillera to the east of the puna understanding the possible genetic relationship between prominent structures and magmatic centers on these lineaments is important to elucidate the tectonomagmatic evolution of the central andes we investigated the back arc area of the cot using remote sensing geological structural and petrochemical data our study demonstrates that this portion of the cot consists of nw se striking faults formed under overall left lateral transtension that decreases in activity toward the cot termini deformation on the cot occurred during and after activity of prominent n s striking transpressive fault systems and is coeval with magmatism which is focused on the central cot the most evolved magmatic rocks with an upper crustal imprint are exposed on the central cot whereas more primitive mantle derived mafic to moderately evolved magmatic rocks are found toward the cot termini this points to a genetic relationship between upper crustal deformation and magmatic activity that led to enhanced magma storage in the central cot cot magmas may result either from slab steepening or episodic delamination of the asthenospheric mantle

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

magma rates in feeding conduits of different volcanic centres abstract a quasi stationary magma flow rate in asthenospheric and crustal conduits of central type volcanoes and volcanic centres was studied analytically under the following conditions magma rises through cylindrical channels in which the magma temperature does not change with time but the wall rocks are gradually heated the magma rates were calculated for basaltic andesitic and dacitic volcanoes using the continental and oceanic geotherms it follows from these calculations that the magma supply rate may determine the kind of activity of a volcanic centre being constant for large and very active volcanoes intermittent for usual volcanic centres of island arcs or sporadic for volcamic fields clusters of cinder cones and areal volcanism theoretical conclusions are consistent with observational data

radiometric and kinematic evidence for middle jurassic strike slip faulting in southern mexico related to the opening of the gulf of mexico one of the least known aspects of the evolution of the gulf of mexico is the nature and location of shear zones along which the relevant continental fragments were displaced the sierra de juarez mylonitic complex located in southern mexico is a polyorogenic north northwest trending structure here we report u pb mylonitization dates of 165 20 ma for igneous zircon from the syntectonic san felipe granite and an integrated 40 ar 39 ar age of 169 3 1 7 ma from synkinematic muscovite both of which indicate a middle jurassic age for the strike slip event along the sierra de juarez mylonitic complex this event therefore occurred during the opening of the gulf of mexico and we propose that the shear zone was kinematically related to the southeast displacement of the yucatan block

southward migration of volcanic activity in the central mexican volcanic belt asymmetric extension within a two layer crustal stretching model abstract magmatic activity in the miocene to present mexican volcanic belt mvb is migrating to the south volcanic activity is associated with extension which is being accommodated along a major 1000 km long e w oriented continental rift in turn the mvb is cross cut by older though active n s to nnw oriented major extensional faults along which the polygenetic volcanoes are aligned a southward migration pattern is also observed for the monogenetic volcanism which however appears to be related to e w and n60 e oriented extensional faults we suggest that the southward migration of volcanic activity can be explained in terms of a two layer crustal stretching model brittle and ductile domains the layers would be separated at an upper crustal level by a zone of simple shear decoupling at the brittle ductile transition zone bdtz the overall movement above the bdtz is southward directed the only direction to which the central mvb can extend and grow our model suggests that the magmas that feed the volcanism become stored at the bdtz evidence supporting this assumption is provided by the al in hornblende thermobarometer and the ol pl sior pseudoternary diagram which indicate average pressures between 2 5 and 4 kbar the magmas feeding the monogenetic volcanism ascend rapidly along active e w and n60 e extensional faults large strain rates i e they do not have enough time to form large magma chambers the magmas feeding the polygenetic volcanism are emplaced along n s to nnw faults lower strain rates these magmas remain stored for longer periods at the bdtz of the n s to nnw faults and therefore form large magmatic chambers shaping vertical overshoots of several kilometers of height the results from geobarometry indicating magma emplacement depth at around 8 km for the polygenetic volcanism and 12 km for the monogenetic volcanism are in good agreement with the rheological constraints of a bdtz at about 10 km of depth we envisage a feedback mechanism regarding magma storage and shallowing of the bdtz i e magma emplacement shallows the bdtz in turn this shallowing controls the new zones for magma emplacement a southward directed process

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

k rich mantle metasomatism control of localization and initiation of lithospheric strike slip faulting a conceptual model is proposed in which bulk transtension or local transtension during bulk simple shear resulting from mantle anisotropy contrasts or lithosphere rheology contrasts of heterogeneously enriched lithospheric mantle triggers localized k rich magmatism which focuses strain and causes nucleation of lithosphere scale transtensional or strike slip shear zones transtension triggered magmatism is most likely to be located at sites of maximum metasomatism of the lithospheric mantle magma generated fractures propagate upwards nucleating zones of lithospheric weakness which focus shear in narrow transcurrent faults or at basin margins in this way magmatism controls fault timing and location although volcanism will be coeval with fault development and volcanoes will appear fault controlled counterintuitively our model suggests that faults are in a sense volcanocontrolled we suggest that this new transtension k rich magmatism transcurrent faulting association represents a hitherto unrecognized genetic relationship as significant as for example the ocean island magma series

scaling laws of the size distribution of monogenetic volcanoes within the michoacan guanajuato volcanic field mexico abstract the michoacan guanajuato volcanic field displays about 1040 monogenetic volcanoes mainly composed of basaltic cinder cones this monogenetic volcanic field is the consequence of a dextral transtensive tectonic regime within the transmexican volcanic belt tmvb the largest intra continental volcanic arc around the world related to the subduction of the rivera and cocos plates underneath the north american plate we performed a statistical analysis for the size distribution of the basal diameter wco for cinder cones dataset used here was compiled by hasenaka and carmichael 1985 monogenetic volcanoes obey a power law very similar to the gutenberg richter law for earthquakes with respect to their size distribution log 10 n wco log 10 wco with 5 01 and 2 98 therefore the monogenetic volcanoes exhibit a wco size distribution empirical power law suggesting a self organized criticality phenomenon

variation of cenozoic extension and volcanism across the southern sierra madre occidental volcanic province mexico the middle to late cenozoic tectonic magmatic evolution of the sierra madre occidental volcanic province south of the tropic of cancer is summarized and analyzed for the first time based on new geologic and structural work and published information in the eastern part of the study region mesa central physiographic province silicic volcanism occurred in a short lived episode culminating at ca 30 ma and was followed by crustal scale extension between 30 and 27 ma in the western part of the study area sierra madre occidental physiographic province a voluminous episode of ignimbrite volcanism at 24 21 ma was succeeded by east west extension that produced regularly spaced grabens affecting only the upper crust in the westernmost part of the study region an andesitic to rhyolitic arc formed between 17 and 12 ma was affected by crustal scale north northwest trending extensional faulting leading to the formation of the gulf of california in the mesa central the maximum extension was oriented approximately east west and amounted to 20 in the eastern sierra madre occidental physiographic province extension was only 8 and oriented approximately east west we observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late oligocene early miocene and late miocene time comparison with the offshore tectonics indicates that the first two tectonic magmatic pulses coincide with periods of fast spreading at the pacific farallon boundary south of the shirley fracture zone we propose that increases in the spreading rate are related to periods of high subduction rate which in turn correspond to episodes of retreating subduction a retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate boundary conditions i e plate tectonics ultimately determined timing magnitude and orientation of extension whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism

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

stratigraphy geomorphology geochemistry and hazard implications of the nejapa volcanic field western managua nicaragua abstract the nejapa volcanic field nvf is located on the western outskirts of managua nicaragua it consists of at least 30 volcanic structures emplaced along the n s nejapa fault which represents the western active edge of the managua graben the study area covers the central and southern parts of the volcanic field we document the basic geomorphology stratigraphy chemistry and evolution of 17 monogenetic volcanic structures ticomo a b c d and e altos de ticomo nejapa san patricio nejapa norte motastepe el hormigon la embajada asososca satelite refineria and cuesta el plomo a and b stratigraphy aided by radiocarbon dating suggests that 23 eruptions have occurred in the area during the past 34 000 years fifteen of these eruptions originated in the volcanic field between 28 500 and 2 130 yr bp with recurrence intervals varying from 400 to 7 000 yr most of these eruptions were phreatomagmatic with minor strombolian and fissural lava flow events a future eruption along the fault might be of a phreatomagmatic type posing a serious threat to the more than 500 000 inhabitants in western managua

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

basaltic volcanism in the bering sea geochronology and volcanic evolution of st paul island pribilof islands alaska abstract the evolution of a pleistocene to holocene basaltic volcanic field in the back arc region of the aleutian subduction system is investigated at st paul island alaska one of the youngest eruptive centers in the bering sea basalt province new 40ar 39ar and 14c age determinations indicate that subaerial volcanic activity forming the island began as early as 540 ka and has continued nearly to the present the youngest eruption occurred approximately 3230 years bp magmas erupted on st paul are basaltic with mgo contents ranging from 14 to 4 wt and phenocryst assemblages of olivine clinopyroxene plagioclase all are alkalic the surface of st paul is composed mainly of numerous tephra cones surrounded by coalescing low viscosity pahoehoe lava flows a central highland spans the island from east to west and is constructed of relatively young eruptive centers where rocks show a minimum of weathering and little deformation by faulting in contrast older lava flows forming the wave eroded base of the island are gently to moderately tilted and faulted geochronologic stratigraphic and geochemical data indicate that eruptive styles on st paul evolved from early mostly effusive eruptions of chemically little evolved lavas that form the base of the island to more explosive monogenetic scoria cones to polygenetic centers forming shields by repeated effusive eruptions of evolved low viscosity lavas localization of the monogenetic and polygenetic centers appears to be related to east west and northeast southwest trending fault and fissure systems with polygenetic centers located at intersections of major structures the combined volcanic and compositional changes on st paul island suggest that the magmatic system as a whole may be trending toward eruption of more evolved magmas related to the progressive development of crustal magma chambers in which crystal fractionation and magmatic differentiation are occurring

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

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

the sanfandila earthquake sequence of 1998 queretaro mexico activation of an undocumented fault in the northern edge of central trans mexican volcanic belt abstract a sequence of small earthquakes occurred in central mexico at the northern edge of the trans mexican volcanic belt tmvb in the state of queretaro during the first 3 months of 1998 medium to large events in the continental regime of central mexico are not common but the seismic history of the region demonstrates that faults there are capable of generating destructive events the sequence was analyzed using data from a temporary network with the goals of identifying the causative fault and its relation to regional tectonics employing a waveform inversion scheme adapted from a method used for regional studies we found that the source mechanisms conform to the style of faulting i e extension in the e w direction representative of the taxco san miguel allende fault system this system has been proposed as the southernmost extension of the basin and range br province the spatial distribution of hypocenters and source mechanisms indicate that the seismogenic segment was a fault with an azimuth of approximately 334 with almost pure dip slip since events which occurred just south from this region show features which are consistent with tmvb tectonics i e extension in an n s direction the sequence may mark the boundary between the tmvb and br stress domains

exploring glacier dynamics with subglacial water pressure pulses evidence for self organized criticality 1 in order to determine whether brief excursions or pulses in subglacial water pressure inferred by kavanaugh and clarke 2000 2001 occur water pressures at the bed of trapridge glacier yukon canada were recorded using an interface board that continuously monitored a pressure transducer during the 231 day period between 16 july 2005 and 4 march 2006 more than 7000 pressure pulses were recorded with magnitudes reaching nearly 3 times the flotation value comparison of the pressure pulse record with those from a number of other instruments installed in this soft bedded glacier indicates that these pulses are generated by stress transients that compress the water within the borehole calculations suggest that these transients are as large as 75 times the nominal driving stress both the magnitudes and interevent times for these pulses are well fitted by power law distributions that are remarkably similar to those exhibited by earthquakes these similarities suggest that the ice bed interface of a soft bedded glacier behaves much like an earthquake fault and raises the possibility that such glaciers self organize to a critical state further evidence for self organized criticality soc of soft bedded glaciers is suggested by an examination of well known ice dynamical properties and the rheological properties of subglacial sediments which suggests that soc might be a natural consequence of the rate independent behavior of subglacial sediments

overview and plumbing system implications of monogenetic volcanism in the northernmost andes volcanic province abstract monogenetic volcanic fields are commonly related to rifts and or intraplate tectonic settings however they are also less commonly recognised in subduction zones including both front and back arc volcanoes e g monogenetic volcanoes associated with the llaima volcano in chile or the san agustin volcanic field in colombia here we describe monogenetic volcanic fields associated with subduction related polygenetic volcanism in the northernmost part of the andes northern volcanic zone nvz 2 s to 4 30 n these fields are associated with the main axis of the quaternary volcanoes they are linked to the polygenetic san diego cerro machin volcano tectonic province 140 km long scvtp in colombia the chain that hosts the iconic nevado del ruiz volcano presently three monogenetic volcanic fields with a typical calc alkaline signature have been identified on both sides of this province from south to north they are 1 pijaos monogenetic volcanic field pmvf 2 villamaria termales monogenetic volcanic field vtmvf and 3 samana monogenetic volcanic field smvf pmvf is located 25 km south of cerro machin volcano the southernmost active volcano of the scvtp this field was formed by at least two eruptions with both effusive and explosive eruptive styles three cones and a maar are recognised previous work has defined the volcanoes as basaltic andesitic in composition which we recognised as the most mafic expression mgo 10 11 wt in the whole scvtp its source is related to the same but deeper magma that feeds the volcanoes in the scvtp stratigraphic relationships show that the volcanoes are younger than the underlying alluvial and volcaniclastic ibague fan 5 km of the axis of the province this field is made up of at least 14 volcanoes aligned with the active villamaria termales fault system as previous work has recognised the volcanism has been mainly effusive represented by lava domes and some lava flows since the 1980s studies have framed these andesitic to dacitic volcanoes in the context of the history of the nevado del ruiz volcano it is inferred that the magmatic source is a shallow magma reservoir underneath the scvtp stratigraphic and field relationships have shown that the last eruption occurred

stress fields of the overriding plate at convergent margins and beneath active volcanic arcs tectonic stress fields in the overriding plate at convergent plate margins are complex and vary on local to regional scales volcanic arcs are a common element of overriding plates stress fields in the volcanic arc region are related to deformation generated by subduction and to magma generation and ascent processes analysis of moment tensors of shallow and intermediate depth earthquakes in volcanic arcs indicates that the seismic strain field in the arc region of many convergent margins is subhorizontal extension oriented nearly perpendicular to the arc a process capable of generating such a globally consistent strain field is induced asthenospheric corner flow below the arc region

build up and depositional dynamics of an arc front volcaniclastic complex the miocene tepoztlan formation transmexican volcanic belt central mexico volcanic terrains such as magmatic arcs are thought to display the most complex surface environments on earth ancient volcaniclastics are notoriously difficult to interpret as they describe the interplay between a single or several volcanoes and the environment the early miocene tepoztlan formation at the southern edge of the transmexican volcanic belt belongs to the few remnants of this ancestral magmatic arc and therefore is thought to represent an example of the initial phase of evolution of the transmexican volcanic belt based on geological mapping detailed logging of lithostratigraphic sections palaeocurrent data of sedimentary features and anisotropy of magnetic susceptibility mapping of two dimensional panels from outcrop to field scale and geochronological data in an area of ca 1000 km 2 three periods in the evolution of the tepoztlan formation were distinguished which lasted around 4 myr and are representative of a volcanic cycle edifice growth phases followed by collapse in a magmatic arc setting the volcaniclastic sediments accumulated in proximal to medial distances on partly coalescing aprons similar to volcanic ring plains around at least three different stratovolcanoes these volcanoes resulted from various eruptions separated by repose periods during the first phase of the evolution of the tepoztlan formation 22ae 8t o 22 ae2 ma deposition was dominated by fluvial sediments in a braided river setting pyroclastic material from small andesitic dacitic composite volcanoes in the near vicinity was mostly eroded and reworked by fluvial processes resulting in sediments ranging from cross bedded sand to an aggradational series of river gravels the second phase 22ae 2t o 21 ae3 ma was characterized by periods of strong volcanic activity resulting in voluminous accumulations of lava and tuff which temporarily overloaded and buried the original fluvial system with its detritus continuous build up of at least three major volcanic centres further accentuated the topography and in the third phase 21ae 3t o 18 ae8 ma mass flow processes represented by an increase of debris flow deposits became dominant marking a period of edifice destruction and flank failures

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