oceanic equatorial waves and the 1991 93 el nino abstract equatorial kelvin and rossby waves associated with the 1991 93 el nino warm event were detected in temperature observations made by the tropical atmosphere ocean tao buoy array intraseasonal kelvin waves were a prominent part of equatorial thermocline depth variability and were well represented by a simple model consisting only of first and second mode baroclinic kelvin waves the second mode was essential to properly represent the observed amplitude thermocline depth variability at 5 n and 5 s was dominated by annual and interannual rossby waves which were found to have been largely wind forced in midbasin with little if any signal associated with eastern boundary reflection an evaluation of the wyrtki buildup hypothesis and the delayed oscillator hypothesis in connection with the 1991 93 events showed that a long lag about two years occurred between the arrival of the downwelling signal in the west and the subsequent initiation of el nino this was considerably longer than suggested

structure and holocene rupture of the morelia fault trans mexican volcanic belt and their significance for seismic hazard assessment abstract the trans mexican volcanic belt is an active volcanic arc being deformed by an intra arc extensional fault network the morelia fault with a length of 14 km and a maximum throw of 260 m is a major normal fault in the central part of this volcanic arc and forms part of a 36 km long fault array slickenlines measured on the primary fault surface indicate north south extensional dip slip with a minor left lateral component in a well exposed cross section in the hanging wall of the morelia fault some few meters from the master fault trace two buried soil layers containing ceramic shards and stone tools yielded radiocarbon ages of a d 420 660 prehispanic classic period chupicuaro culture and a d 1290 1435 postclassic period respectively the underlying bedrock atecuaro ignimbrite is cut in a staircase pattern by three synthetic secondary normal faults that delimit two colluvial wedges three slip events with displacements ranging between 46 and 185 cm average 109 cm can be inferred from this section the latest of these events must have occurred after a d 1290 1435 during the postclassic period or possibly during early novohispanic times the morelia fault which is capable of generating m w 6 7 earthquakes poses a major seismic ground shaking hazard and locally a surface fault rupture hazard within the metropolitan morelia region with a population of more than 900 000 habitants the foci of a sequence of seven small 2007 earthquakes on the morelia fault were located below the center of town at a depth of 11 km and the fault trace crosses urban residential areas moreover the town repeatedly experienced severe ground shaking during historical earthquakes the best documented is the 19 june 1858 earthquake which caused morelia extensive damage corresponding to a macroseismic intensity of viii ix on the modified mercalli scale online material radiocarbon analysis reports local 1894 1904 and 1909 1911 earthquake catalogs

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

nature and timing of faulting and synextensional magmatism in the southern basin and range central eastern durango mexico the nazas area in the central eastern portion of the state of durango is within the poorly studied southern half of the basin and range province in mexico at nazas high angle normal faults that cut part of the mid tertiary volcanic sequence strike between n 20 and 70 w with 40 of them between n 40 and 50 w tilting of faulted blocks varies from 5 to 35 to the northeast most commonly around 25 ne a few blocks are tilted to the southwest as much as 25 fault offsets range from 40 m to nearly 300 m the earliest faulting occurred between 31 and 29 ma before the emplacement of mid tertiary ash flow tuffs had ended the santa ines formation a widespread fanglomerate was deposited after the earliest faulting episode and is overlain by 24 to 20 ma alkalic basalt flows although not cut by faults the flows are adjacent to or cover normal faults some of the mapped faults could have been coeval with basalt eruption as is the case in trans pecos texas where alkalic basalt having similar age and composition to that in the nazas area erupted contemporaneously with normal faulting the nazas alkalic basalt also has similar age and stratigraphic position as but is compositionally distinct from the southern cordilleran basaltic andesite scorba a widespread mafic suite in southwestern north america that has been linked to regional extension

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

building stones used in the architectural heritage of morelia mexico quarries location rock durability and stone compatibility in the monument four different rocks have been used in the architectural heritage of morelia city michoacan state mexico piedra vieja pv is the original building rock whilst tlalpujahua cointzio and jabalina stones tl co and ja respectively are the replacement varieties used in the restoration works during the last decades all of them correspond to rhyolitic ignimbrites quarried close to morelia the new varieties were selected exclusively after aesthetical similarity and no petrophysical criterion was considered in this paper a deep analysis of the weathering process of these rocks during wet dry and salt crystallization cycles is carried out rock decay is defined in terms of linearity and homogeneity ja and tl show nonlinear decay mode and heterogeneous behaviours whilst samples of co and pv weather after linear and homogeneous modes moreover each rock variety shows different decay patterns such as differential erosion pv and tl fractures tl scaling ja and granular disintegration co all these obtained results are discussed according to 1 petrographic factors 2 hydric and mechanical properties and 3 the partial effective pressures reached inside the porous system of each rock during salt crystallization finally a review about the quality of previously published durability estimators is carried out in general terms results reveal that theoretical estimators best fit the visual weathering suffered by the rocks than its mass loss concluding this paper a petrophysical and aesthetical evaluation of the compatibility between both original and replacement building stones used in the architectural heritage of morelia is carried out in order to offer technical recommendations for future restoration works co ignimbrite offers the highest chromatic compatibility with the original building rock pv but its durability is extraordinarily low and consequently its use is not recommended ja and tl are advisable replacement stones although from a petrophysical point of view tl results the most convenient these results highlight the importance of carrying out the selection of building stones for restoration works according to petrophysical criteria instead of using exclusively an aesthetic valuation

14c and 40ar 39ar radiometric dating and geologic setting of young lavas of rancho seco and mazcuta volcanoes hosting archaeological sites at the margins of the patzcuaro and zacapu lake basins central michoacan mexico abstract geologic mapping of two neighbouring areas totalling 770 km2 centred around the rancho seco and mazcuta monogenetic scoria cones and situated between the patzcuaro and zacapu lake basins in the michoacan guanajuato volcanic field central portion of the trans mexican volcanic belt allowed the recognition of 62 quaternary monogenetic volcanic structures which include cinder cones with associated lava flows domes and small to medium sized shields 40ar 39ar and 14c radiometric dating petrography and whole rock chemical analyses of volcanic products provide a stratigraphic framework and insight into different eruptive styles and erupted volumes 45 km3 of calc alkaline magma pliocene to holocene monogenetic activity has been frequent in this region forming structures which are generally aligned along an ene wsw direction associated to the morelia cuitzeo acambay normal fault system the young rancho seco and mazcuta volcanoes are of particular interest because their distal lava flows host the pre hispanic archaeological urban centres of angamuco and cortijo viejo respectively the rancho seco scoria cone was radiocarbon dated at 27 845 445 425 yr bp and emitted at least six andesitic lava flows which were emplaced toward the sw lake patzcuaro covering an area of 21 3 km2 with a volume of 0 64 km3 the mazcuta scoria cone dated at 7970 135 yr bp produced five basaltic andesite andesite lava flows toward the nw lake zacapu where they cover an area of 28 85 km2 with a volume of 0 57 km3 the recently studied archaeological site of angamuco became an important urban centre during the rise of the tarascan empire in the post classic period 900 1521 ce while the cortijo viejo site remains largely unexplored but its occupation might fall within the same period although the young lava flows on which these sites are built seem inhospitable and barren with a rugged topography and rocky substrate unfit for agriculture early inhabitants adapted and modified its surface according to their needs hence these archaeological sites exemplify how a seemingly hostile landscape was ingeniously modified to establish population centres with patterns similar to those observed on lava flows elsewhere

water mass and transport variability at 110 w in the equatorial pacific abstract an analysis of nine hydrographic sections collected in 1979 81 along 110 w in the equatorial pacific ocean is presented sections typically sampled the upper 500 m of the water column from 10 n to 3 s analysis concentrated on the repeated sections north of the equator examination of the variability of eastward transport indicates that the north equatorial countercurrent necc and the northern subsurface countercurrent nscc cannot be distinguished solely on the basis of water mass structure however using a potential density surface 25 0 as a current boundary we find that on average the nscc transports 13 7 106 m3 s 1 compared to only 8 3 106 m3 s 1 for the necc the nscc flow is sufficiently stable so that meridional surface dynamic height gradient remains a good index of zonal transport fluctuations variations in surface dynamic height observed in our data and in the eastropac data indicate a seasonal cycle to the surface topography with large values for the equatorial and coun

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

petrology magnetostratigraphy and geochronology of the miocene volcaniclastic tepoztlan formation implications for the initiation of the transmexican volcanic belt central mexico the volcaniclastic tepoztlan formation tf represents an important rock record to unravel the early evolution of the transmexican volcanic belt tmvb here a depositional model together with a chronostratigraphy of this formation is presented based on detailed field observations together with new geochronological paleomagnetic and petrological data the tf consists predominantly of deposits from pyroclastic density currents and extensive epiclastic products such as tuffaceous sandstones conglomerates and breccias originating from fluvial and mass flow processes respectively within these sediments fall deposits and lavas are sparsely intercalated the clastic material is almost exclusively of volcanic origin ranging in composition from andesite to rhyolite thick gravity driven deposits and large scale alluvial fan environments document the buildup of steep volcanic edifices k ar and ar ar dates in addition to eight magnetostratigraphic sections and lithological correlations served to construct a chronostratigraphy for the entire tepoztlan formation correlation of the 577 m composite magnetostratigraphic section with the cande and kent 1995 geomagnetic polarity time scale gpts suggests that this section represents the time intervall 22 8 18 8 ma 6bn 1n 5er aquitanian burdigalian lower miocene this correlation implies a deposition of the tf predating the extensive effusive activity in the tmvb at 12 ma and is therefore interpreted to represent its initial phase with predominantly explosive activity additionally three subdivisions of the tf were established according to the dominant mode of deposition 1 the fluvial dominated malinalco member 22 8 22 2 ma 2 the volcanic dominated san andres member 22 2 21 3 ma and 3 the mass flow dominated tepozteco member 21 3 18 8 ma

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

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

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

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

multivariate analysis of co2 h2s and ch4 diffuse degassing and correlation with fault systems in agua caliente tzitzio michoacan mexico abstract the quantitative relationships between soil gas flux anomalies and faults systems in agua caliente tzitzio geothermal system were analyzed statistically the main degassing features are related with thermal springs the data used in the multivariate analysis are geothermal gas fluxes and spatial structural data i e local fault strike exploratory data analysis eda and multivariate analysis of variance manova were implemented for the first time to statistically describe the relation between the location of degassing features and the presence of some local fault systems discriminating the direction of the main discharge channels and pointing to higher permeability faults the results show that there is a statistically significant correlation between diffuse degassing of co2 and ch4 and the n s and e w fault systems conversely h2s degassing does not show any correlation with the fault systems which may be related with the higher h2s reactivity compared with that of co2 and ch4 it is highly recommended to use eda and manova techniques in order to establish the main degassing directions in geothermal and volcanic areas

the basin and range province origin and tectonic significance the basin and range province is a vast arid tract of regionally corrugated angular topography of high relief in the western cordillera it is characterized by evenly spaced parallel mountain ranges and intervening desert basins fig ure la the range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward interrupted here and there by low fault scarps that paral lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges thermal springs located at or near range bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting in the southern part of the province especially in southeastern california and southwestern arizona range fronts have been worn back by erosion leaving a thin veneer of gravel on an erosion cut bedrock surface that slopes gently outward the range bounding faults of these mountain blocks are buried at the outer edge of such pediments often at considerble distances from the erosional remnants of the ranges themselves the american physiographer n m fenneman 1928 1931 named the basin and range province and defined its general boundaries as thus circum scribed the province includes some 800 000 km2 of area in eight western states later students pardee 1950 lawrence 1976 reynolds 1979 eaton 1979b have observed that many of the fundamental geological and geo physical characteristics of the province are found well beyond the boundaries drawn by fenneman which were based on physiography alone as a tec tonophysical entity its areal extent is greater than 1 million km2 more than 10 of the area of the united states figure 1 fenneman 1931 subdivided the province into five physiographic sections the largest of which is the great basin see figure 1b it is not as its name implies a single regional depression with a common topographic center but is characterized instead by isolated networks of interior drainage divisible into

geomechanical characterization of the miocene cuitzeo ignimbrites michoacan central mexico abstract volcanic rocks especially ignimbrites exhibit complex mechanical behaviours due to their large variation in welding porosity textural and granulometrical characteristics moreover there is a lack of knowledge of how alteration processes affect their intrinsic properties main objective of this research is to characterize the physical mechanical and hydraulic behaviour of different lithofacies of the cuitzeo ignimbrites mexico these volcaniclastic mixtures of lithics and pumice supported by a fine rich matrix are characterized by different textural and granulometric features a general lithostratigraphic section and geological mapping of the study area were accomplished to attain the temporal and spatial distribution of the ignimbrites strength deformation and the failure mode of selected lithofacies were directly related to micro textural and granulometric characteristics by a series of uniaxial and pre and post failure non destructive analyses e g ultrasonic pulse velocity and x ray image tomographies results for four different lithofacies were compared allowing us to explore the variation in the mechanical properties associated with characteristics derived from diagenesis petrophysical properties show that there is a large dependence on the textural characteristics and particularly on the pumice fragments content results derived from laboratory observations and x ray image reconstruction analysis show that the main differences in the average values of porosity are associated to the geometry and morphology of the grains and pore network these are closely related to the stress strength relationship and the mode of failure of each specimen in general the strength for all lithofacies tends to increase linearly with several of the physical properties e g ultrasonic pulse velocity bulk density slake durability tests demonstrate that a large part of the mechanical degradation could be attributed to swelling during wetting drying cycles this decay is significant in lithofacies with groundmass rich in expansible clays

early miocene arc volcanism in the mexico city basin inception of the trans mexican volcanic belt abstract in central mexico remnants of the early volcanic activity in the trans mexican volcanic belt tmvb are exposed at sierra de guadalupe in the northern part of the mexico city basin and to the south in the tepoztlan malinalco tenancingo and chiltepec areas a few published studies indicate a general early to middle miocene age for some of these volcanic centers we present new geologic geochronologic and geochemical data that refine the timing of this early volcanic activity and its geochemical character shedding light on the geodynamic evolution during this period the sierra de guadalupe is a volcanic complex with effusive and explosive activity that generated andesitic lavas with plagioclase ortho and clinopyroxene plus amphibole phenocrysts as well as dacitic lava flows and domes with plagioclase orthopyroxene amphibole and or biotite phenocrysts pyroclastic and epiclastic lahars deposits are also observed in the northern slopes of this volcanic complex the chemical composition of the sierra de guadalupe rocks is calc alkaline andesitic to dacitic new 40ar 39ar and u pb zircon ages demonstrate that volcanic activity at sierra de guadalupe lasted ca 5 0 ma from ca 20 1 ma to 14 8 ma remnants of volcanic centers found in the tepoztlan malinalco tenancingo and chiltepec areas 80 km to the south show similar compositions and eruptive styles with 40ar 39ar ages ranging between 21 and 16 ma rocks with similar ages and lithologies were also described in deep wells drilled in the mexico city basin as a whole the age distribution shows that early to middle miocene volcanism making up the initial activity of the tmvb migrated toward the north from tepoztlan 310 km from the trench up to pachuca 445 km from the trench and is likely associated with a shallowing the subducting cocos slab underneath this region our results confirm that magmatism in the tmvb began in the early miocene by dehydration of the cocos slab as attested by high ba la and th yb ratios

a linear stratified ocean model of the equatorial undercurrent a linear stratified ocean model is used to study the wind driven response of the equatorial ocean the model is an extension of the lighthill 1969 model that allows the diffusion of heat and momentum into the deeper ocean and so can develop non trivial steady solutions to retain the ability to expand solutions into sums of vertical normal modes mixing coefficients must be inversely proportional to the square of the back ground vaisala frequency the model is also similar to the earlier homogeneous ocean model of stommel i960 he extended ekman dynamics to the equator by allowing his model to generate a barotropic pressure field the present model differs in that the presence of stratification allows the generation of a baroclinic pressure field as well the most important result of this paper is that linear theory can produce a realistic equatorial current structure the model undercurrent has a reasonable width and depth scale there is westward flow both above and below the undercurrent the meridional circulation conforms to the classical picture suggested by cromwell 1953 unlike the stommel solution the response here is less sensitive to variations of parameters ocean boundaries are not necessary for the existence of the undercurrent but are necessary for the existence of the deeper equatorial intermediate current the radiation of equatorially trapped rossby and kelvin waves is essential to the development of a realistic undercurrent because the system supports the existence of these waves low order vertical modes can very nearly adjust to sverdrup balance defined below which in a bounded ocean and for winds without curl is a state of rest as a result higher order vertical modes are much more visible in the total solution this property accounts for the surface trapping and narrow width scale of the equatorial currents the high order modes tend to be in yoshida balance defined below and generate the characteristic meridional circulation pattern associated with equatorial ekman pumping

post laramide and pre basin and range deformation and implications for paleogene 55 25 ma volcanism in central mexico a geological basis for a volcano tectonic stress model at central eastern mexico in the mesa central province there are several ranges that were formed after the k t laramide compression but before the basin and range peak extensional episodes at middle late oligocene two important volcano tectonic events happened during this time interval 1 uplift of crustal blocks exhuming the triassic jurassic metamorphic sequence and formation of basins that were filled with red beds and volcanic sequences and 2 normal faulting and tilting to the ne of these blocks and fanglomerate filling of graben and half graben structures the first event from late paleocene to early eocene was related to nne and nnw oriented dextral strike slip faults these faults were combined with nw se en echelon faulting in these blocks through which plutonism and volcanism occurred the second event lasted from early oligocene to early miocene and coincided with basin and range extension intense volcanic activity occurred synchronously with the newly formed or reactivated old fault systems producing thick sequences of silicic pyroclastic rocks and large domes volcano tectonic peaks occurred in three main episodes during the middle late oligocene in this part of mexico at about 32 30 ma 30 28 ma and 26 25 ma the objectives of this work is to summarize the volcano tectonic events that occurred after the end of the laramide orogeny and before the peak episodes of basin and range faulting and sierra madre occidental oligocene volcanism and to discuss the influence of these events on the following oligocene miocene volcano tectonic peak episodes that formed the voluminous silicic volcanism in the mesa central and hence in the sierra madre occidental a model based upon geological observations summarizes the volcanic tectonic evolution of this part of mexico from the late paleocene to the early miocene

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

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

geochemical and isotopic composition of volcanic rocks of the heterogeneous miocene 23 19 ma tepoztlan formation early transmexican volcanic belt mexico abstract we present the first geochemical data major and trace elements as well as sr nd and pb isotopes on volcanic rocks from the tepoztlan formation in the central transmexican volcanic belt the tepoztlan formation is up to 800 m thick and comprises a wide range of primary volcanic rocks lavas pyroclastic density current deposits pyroclastic fall deposits and their secondary reworked products due to mass flow lahars and fluvial processes magnetostratigraphy combined with k ar and ar ar geochronology suggests an age of early miocene 23 19 ma for this formation lava flows pyroclastic rocks dykes and volcanic clasts range from basaltic andesite to rhyolite with a clear dominance of andesites and dacites all samples are subalkaline and hy normative these rocks show homogeneous ree patterns with lree enrichment and higher lile concentrations with respect to hfse notably the typical negative anomaly of nb ta and ti suggesting a subduction related magma genesis major and trace element concentrations show that either assimilation of heterogeneous continental crust or crustal recycling by subduction erosion and fractional crystallization are important processes in the evolution of the tepoztlan formation magmas isotopic compositions of the tepoztlan formation samples range from 87sr 86sr t 0 703693 to 0 704355 and 143nd 144nd t 0 512751 to 0 512882 falling within the mantle array all geochemical characteristics indicate that these rocks originated from a heterogeneous mantle modified and evolved through assimilation of country rock and fractional crystallization in the upper crust

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

a terrane interpretation of the archaean limpopo belt the limpopo belt is a zone of thickened archaean crust whose origin is currently explained by a late archaean continent continent collision between the kaapvaal and zimbabwe cratons this review shows that the two cratons have fundamentally different geological histories and that the zimbabwe craton was unlikely to have behaved as a stable cratonic block at the time of the limpopo belt collision the geological histories of the zimbabwe craton the north marginal central and south marginal zones of the limpopo belt and the kaapvaal craton are shown to be sufficiently different from one another to warrant their consideration as discrete terranes the boundaries between the five units outlined above are all major shear zones further supporting a terrane model for the limpopo belt the five units were all intruded by late to syn tectonic granites c 2 6 ga constraining the accretion event to c 2 6 ga

late pleistocene alberca de guadalupe maar volcano zacapu basin michoacan stratigraphy tectonic setting and paleo hydrogeological environment abstract the late pleistocene 21 000 yr bp alberca de guadalupe maar is one of the few phreatomagmatic volcanoes occurring within the scoria cone dominated plio quaternary michoacan guanajuato volcanic field the scarcity of this type of volcano implies that conditions favoring their formation are rarely met in this region we identify these factors by implementing current methods of investigation with emphasis on hydrogeological conditions we present the stratigraphy including a set of 40 ar 39 ar and 14 c dates of the se margin of the zacapu intermontane tectonic basin where the maar just forms a hole 1 km in diameter 150 m deep bearing a 9 m deep lake in the otherwise planar topography of the underlying early pleistocene lava flows of the cerro pelon scoria cone the maar is composed of typical phreatomagmatic surge deposits that are poorly sorted and rich in accidental lithics 60 wt of the deposit with few juveniles basaltic andesite sio 2 54 58 wt the entire structure is cut by an ene wsw trending normal fault and is underlain by andesite lavas and silicic ignimbrites partly inferred from xenoliths encountered in the maar deposits that are miocene to early pleistocene in age the crater is at the center of a n s elongated drainage basin surrounded by topographic highlands that channel water with high hydraulic pressure from most directions towards the location of the maar this geometric configuration was already in existence at the time of the maar forming eruption but the climate was different colder and more humid conditions during the last glacial maximum cw2 climate type annual precipitation of 1000 mm favored the saturation of the fractured shallow aquifer system hydraulic conductivity of 10 8 to 10 7 m s that supplied sufficient groundwater at a high flow rate directed towards the center of the basin upon contact near surface 150 m deep crater with steep inner walls and an underlying shallow diatreme

fissure ignimbrites fissure source origin for voluminous ignimbrites of the sierra madre occidental and its relationship with basin and range faulting the sierra madre occidental is mostly composed of middle tertiary large volume ignimbrites from the united states mexico border 318n the sierra madre occidental extends southward to its intersection with the mexican volcanic belt 218n ignimbrites of equivalent age extend into southern mexico as discontinuous outcrops considering the average thickness of 1000 m for these ignimbrites based on representative measured sections a conservative estimate of their total volume is 393 000 km 3 fewer than 15 calderas have been identified in this province and the source of most of these ignimbrites has been an unsolved problem we present geologic evidence indicating that fissures most of them with the regional trend of basin and range faults served as conduits for the ignimbrites these fissures can be several kilometers long and are represented by pyroclastic ignimbrite dikes rhyolitic lava dikes linearly aligned lava domes and elongated coignimbrite lithic lag breccias adjacent to basin and range faults considering that the basin and range extension overlapped in time and space with the ignimbrite flare up we propose a model in which batholith sized magma chambers reached shallow crustal levels and erupted their contents when they reached basin and range normal faults the faults acted as vents and caused fast decompression when the system was opened and large volumes of silicic magmas were explosively erupted finally devolatilized rhyolitic magmas were emplaced as domes or dikes we propose the term fissure ignimbrites for ignimbrites formed in this way

sedimentary and botanical factors influencing peat accumulation in the mississippi delta the holocene mississippi delta system consists of six cyclic deltaic packages associated peat deposits are planar and eutrophic and accumulate in areas abandoned by deltaic sedimentation peats that formed in large scale interlobe basins are the subject of this paper true peats 75 organic matter average 81 7 organic matter on a dry weight basis controls on quality and lateral distribution of peat deposits are botanical parent material the balance between subsidence and accretion rates detrital clastic influx and marine inundation botanical parent material inferred from 13 c signatures and microtome sections is always freshwater originating either in forested swamps or under herbaceous floating mats sphagnum spp is essentially absent in order for peat to accumulate rates of accretion and subsidence must balance long term subsidence rates in the study area increase from about 0 10 to 0 30 cm a 1 over a distance of 40 km from upper to lower delta plain subsurface accretion rates for true peats at a depth of 2 3 m however average about 0 05 cm a 1 renewed coarse detrital clastic influx terminates peat accumulation the largest quantities of true peat are found together with relatively large amounts of organic poor clastic sediments mostly clays delta lobe abandonment and consequent coastal erosion causes salt water intrusion into freshwater environments resulting saline marsh sediments have maximum organic matter contents of 35 in a transgressive setting these may overlie older high quality low ash freshwater peats leaching experiments reveal the presence of water soluble salts in both transgressed and fresh peats leaching reduces ash values by 1 5 to 1 3 suggesting that such losses may occur during early diagenesis it is possible that some commercial coal seams originated in analogous interlobe environments where leaching and possibly silica mobilization reduced the originally high ash contents to acceptable levels

geological and environmental controls on the change of eruptive style phreatomagmatic to strombolian effusive of late pleistocene el caracol tuff cone and its comparison with adjacent volcanoes around the zacapu basin michoacan mexico abstract the 28 300 year bp cal 32 300 bp el caracol tuff cone complex is one of the few phreatomagmatic volcanoes in the scoria cone dominated plio quaternary michoacan guanajuato volcanic field mgvf it displays a shallow circular crater of 1 km in diameter that is filled with several meter thick lava flows and is located between two ne sw trending normal faults dipping nw it lies directly on top of pliocene lavas of the san lorenzo shield volcano that forms part of a tectonic horst topographic high separating the zacapu lake basin 1980 m in the south from the rio angulo river valley 1760 m to the north detailed study of the tephra sequence indicates that the eruption occurred in two stages 1 weak phreatomagmatic in which about 0 1 0 5 km 3 dense rock equivalent dre of magma was issued within 1 to 3 months at the rate of 4 40 m 3 s and 2 purely magmatic strombolian effusive during which the vent shifted slightly its position toward the nw forming a small scoria cone 100 m high on the crater rim of the tuff cone from this scoria cone lava flows were issued first into the tuff cone crater occupying its bottom and subsequently toward the nw down the outer flank of the tuff cone and into the plain where they reached a distance of 3 5 km during this stage 0 6 km 3 dre of magma was produced at the rate of 4 m 3 s in a period of 5 months although el caracol displays many features that are characteristic for a phreatomagmatic vent its morphology types of deposits and its complex process of formation makes it strikingly different from the more typical case of the 21 000 year bp cal 25 300 bp alberca de guadalupe maar volcano situated not far at the se margin of the zacapu basin the latter was solely phreatomagmatic during the course of its eruption and is formed in its entirety by surge and fallout breccias consisting largely of xenolithic material in contrast at el caracol the hydrogeological environment namely the low groundwater flow gradient did not allow for sustained water magma interactions at ideal conditions ratio of 0 2 hence the system soon dried out due to insufficient groundwater flow toward the eruption site and the eruption turned purely magmatic this study shows that despite of its proximity to a large water reservoir zacapu lake basin and similarities with alberca de guadalupe maar in regard to age climate magma composition and tectonic environment several other specific parameters play a crucial role in shaping a monogenetic phreatomagmatic volcano the case of el caracol phreatomagmatic style followed by magmatic during the course of a monogenetic eruption has been reported from other volcanic fields around the globe where it might have similar causes furthermore the general scarcity of this type of volcanoes in the michoacan guanajuato volcanic field implies that the conditions required for their formation are rarely met in this region

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

annual reversal of the equatorial intermediate current in the pacific observations and model diagnostics abstract a large reversal of zonal transport below the thermocline was observed over a period of 6 months in the western pacific ocean between 2 s and the equator from 26 2 sv 1 sv 106 m3 s 1 eastward in october 1999 to 28 6 sv westward in april 2000 to document this reversal and assess its origin an unprecedented collection of adcp observations of zonal currents 2004 06 together with a realistic ogcm simulation of the tropical pacific was analyzed the results of this study indicate that this reversal is the signature of intense annual variability in the subsurface zonal circulation at the equator at the level of the equatorial intermediate current eic and the lower equatorial intermediate current l eic in this study the eic and the l eic are both shown to reverse seasonally to eastward currents in boreal spring and winter for the l eic over a large depth range extending from 300 m to at least 1200 m the peak to peak amplitude of the annual cycle of subthermocline zonal currents at

motion of the rivera plate since 10 ma relative to the pacific and north american plates and the mantle abstract to better understand the influence of rivera plate kinematics on the geodynamic evolution of western mexico we use more than 1400 crossings of seafloor spreading magnetic lineations along the pacific rivera rise and northern mathematician ridge to solve for rotations of the rivera plate relative to the underlying mantle and the pacific and north american plates at 14 times since 9 9 ma our comparison of magnetic anomaly crossings from the undeformed pacific plate to their counterparts on the rivera plate indicates that significant areas of the rivera plate have deformed since 9 9 ma dextral shear along the southern edge of the plate from 3 3 2 2 ma during a regional plate boundary reorganization deformed the rivera plate farther into its interior than previously recognized in addition seafloor located north of two rupture zones within the rivera plate sutured to north america after 1 5 ma anomaly crossings from these two deformed regions thus cannot be used to reconstruct motion of the rivera plate finite rotations that best reconstruct pacific plate anomaly crossings onto their undeformed counterparts on the rivera plate yield stage spreading rates that decrease gradually by 10 between 10 and 3 6 ma decrease rapidly by 20 after 3 6 ma and recover after 1 ma the slowdown in pacific rivera seafloor spreading at 3 6 ma coincided with the onset of dextral shear across the then incipient southern boundary of the rivera plate with the pacific plate the available evidence indicates that the rivera plate has been an independent microplate since at least 10 ma contrary to published assertions that it fragmented from the cocos plate at 5 ma motion of the rivera plate relative to north america has changed significantly since 10 ma in concert with significant changes in pacific rivera motion a significant and robust feature of rivera north america motion not previously recognized is the cessation of margin normal convergence and thus subduction from 2 6 to 1 0 ma along the entire plate boundary followed by a resumption of trench normal subduction along the southern half of the rivera north america plate boundary after 1 0 ma motion of the rivera plate relative to the underlying mantle since 10 ma has oscillated between periods of landward motion and seaward motion the evidence suggests that the torque exerted by slab pull on this young and hot oceanic plate is either minimal or is effectively counterbalanced by forces that resist its motion

creation of the cocos and nazca plates by fission of the farallon plate abstract throughout the early tertiary the area of the farallon oceanic plate was episodically diminished by detachment of large and small northern regions which became independently moving plates and microplates the nature and history of farallon plate fragmentation has been inferred mainly from structural patterns on the western pacific plate flank of the east pacific rise because the fragmented eastern flank has been subducted the final episode of plate fragmentation occurred at the beginning of the miocene when the cocos plate was split off leaving the much reduced farallon plate to be renamed the nazca plate and initiating cocos nazca spreading some oligocene farallon plate with rifted margins that are a direct record of this plate splitting event has survived in the eastern tropical pacific most extensively off northern peru and ecuador small remnants of the conjugate northern rifted margin are exposed off costa rica and perhaps south of panama marine geophysical profiles bathymetric magnetic and seismic reflection and multibeam sonar swaths across these rifted oceanic margins combined with surveys of 30 20 ma crust on the western rise flank indicate that i localized lithospheric rupture to create a new plate boundary was preceded by plate stretching and fracturing in a belt several hundred km wide fissural volcanism along some of these fractures built volcanic ridges e g alvarado and sarmiento ridges that are 1 2 km high and parallel to absolute farallon plate motion they closely resemble fissural ridges described from the young western flank of the present pacific nazca rise ii for 1 2 m y prior to final rupture of the farallon plate perhaps coinciding with the period of lithospheric stretching the entire plate changed direction to a more easterly nazca like course after the split the northern cocos part reverted to a northeasterly absolute motion iii the plate splitting fracture that became the site of initial cocos nazca spreading was a linear feature that at least through the 680 km of ruptured oligocene lithosphere known to have avoided subduction did not follow any pre existing feature on the farallon plate e g a fracture zone trail of a transform fault iv the margins of surviving parts of the plate splitting fracture have narrow shoulders raised by uplift of unloaded footwalls and partially buried by fissural volcanism v cocos nazca spreading began at 23 ma reports of older cocos nazca crust in the eastern panama basin were based on misidentified magnetic anomalies there is increased evidence that the driving force for the 23 ma fission of the farallon plate was the divergence of slab pull stresses at the middle america and south america subduction zones the timing and location of the split may have been influenced by i the increasingly divergent northeast slab pull at the middle america subduction zone which lengthened and reoriented because of motion between the north america and caribbean plates ii the slightly earlier detachment of a northern part of the plate that had been entering the california subduction zone contributing a less divergent plate driving stress and iii weakening of older parts of the plate by the galapagos hotspot which had come to underlie the equatorial region midway between the risecrest and the two subduction zones by the late oligocene

eocene to quaternary mafic intermediate volcanism in san luis potosi central mexico the transition from farallon plate subduction to intra plate continental magmatism abstract the san luis potosi volcanic field slpvf of central mexico includes volcanic sequences of felsic intermediate and basic compositions that were erupted as discrete episodes from the eocene to the pleistocene volcanism was dominated by widespread and voluminous rhyolitic ignimbrites of the mid tertiary ignimbrite flare up however the complete volcanic history must consider basaltic and andesitic eocene pleistocene volcanic successions that provide key evidence for understanding the geochemical evolution of the volcanism in the slpvf during this time span five sequences are recognized according to their geochemical characteristics each comprising a volcano tectonic episode the first episode comprises basaltic andesites and andesites erupted during three intervals 45 42 ma 36 31 ma and 31 30 ma the oldest was derived from subduction magmatism whereas the youngest has an intra plate magmatic signature and this represents the transition from the end of a long lasting subduction regime of the farallon plate to the initiation of intra plate continental extension in the north american plate the second episode at 29 5 28 ma comprises a bimodal succession of high silica rhyolites and alkaline basalts hawaiites that are interpreted as magmatism generated in an intra plate continental extension regime during the basin and range faulting the third episode at 21 ma is characterized by trachybasalts and trachyandesites that represent mantle basaltic melts that were contaminated through assimilation of the lower crust during advanced stage of intra plate extension that started at oligocene the fourth episode includes 12 ma alkaline basalts and andesites that were erupted from fissures these mantle derived magmas evolved to andesites by crustal anatexis and crystal fractionation within a continued extensional intra plate regime lastly the fifth episode comprises 5 0 to 0 6 ma alkaline basalts basanites containing mantle xenoliths that were erupted from maars and tuff cones which are the youngest manifestations of mantle derived intra plate extensional events based upon this volcanic record the last subduction manifestations of the extinct farallon plate occurred at about 42 ma this was followed by a transition to intra plate magmatism between 42 and 31 ma and an extensional intra plate tectonic setting from 31 ma to almost present

the eruptive history of the patzcuaro lake area in the michoacan guanajuato volcanic field central mexico field mapping c 14 and 40ar 39ar geochronology abstract monogenetic volcanic fields occur in many tectonic settings representing an unpredictable and uncertain volcanic hazard the patzcuaro lake is located in the central part of the michoacan guanajuato volcanic field mgvf field mapping of an area of 560 km2 stratigraphy and new radiometric ages 40ar 39ar and 14c aided in the identification of 45 individual volcanic landforms formed during the past 3 9 ma these landforms comprise five medium size shield volcanoes 25 scoria cones 10 lava domes two lavas associated to scoria cones one fissural lava flow and two volcanic complexes these volcanic edifices erupted a minimum volume of 20 km3 since pliocene with an output rate of 0 02 km3 ka chemical composition of magmas ranges from 47 6 to 65 3 wt silica being dominantly andesites 76 vol followed by basaltic andesites 12 vol dacites 9 vol and basalts 3 vol volcanic products vary from mainly andesitic during the pliocene basaltic to dacitic during the pleistocene and basaltic andesitic to andesitic during the holocene all these rocks are the result of low dipping slab subduction related magmatism alignment of cones and structures follow ne sw n s and e w directions the youngest faults are associated with the e w morelia acambay fault system with historic earthquake activity the volcanic complexes el estribo and la muela have cone collapses to the north that correspond with the direction of maximum regional extension associated with the e w fault system the 3 9 ma history of monogenetic volcanism around the patzcuaro lake demonstrates that monogenic fields can be long lived volcanic features