depleted mantle source for the ulungur river a type granites from north xinjiang china geochemistry and nd sr isotopic evidence and implications for phanerozoic crustal growth abstract the ulungur river granites from north xinjiang china display two petrological trends 1 a peralkaline trend composed of alkali amphibole and alkali pyroxene bearing granites and dykes and 2 an aluminous trend represented by alkali granites monzogranites and aluminous granite dykes within the peralkaline granites rb sr whole rock isochron dating yielded an age of 300 ma for the emplacement of the peralkaline granites and 270 ma for the aluminous granites the ulungur river granites share all the geochemical features common to a type granites of the world the ulungur river granites are post orogenic and emplaced in a post collisional extensional environment the peralkaline and aluminous rocks have indistinguishable nd isotopic compositions they have unusually positive and relatively uniform nd t values ranging from 5 1 to 6 7 for the peralkaline rocks at 300 ma and from 5 5 to 5 9 for the aluminous rocks at 270 ma the initial 87sr 86sr ratios cannot be determined precisely by the whole rock isochron technique mainly because most samples are highly radiogenic and have high rb sr ratios in addition the large variation in sr isotopic compositions can partly be explained by crustal contamination during magma differentiation the nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long lived depleted mantle reservoir the formation of the ulungur river a type granites is hypothesized as follows 1 a depleted mantle was first metasomatised then subjected to partial melting leading to the generation of basic magmas now represented by the kalatongke mafic ultramafic complex in the ulungur river area 2 the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite hornblende plagioclase potassium feldspar and other accessory phases such as apatite and ilmenite with the residual magmas forming the peralkaline granites during the magmatic differentiation the liquids were probably contaminated by crustal materials as evidenced by nb depletion and nd sr isotopic systematics on the other hand the formation of aluminous granites involved a replenishment of new mantle derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases hornblende and biotite the petrological evolution of the ulungur river granites is thus from peralkaline to calc alkaline and not from calc alkaline to peralkaline as more often observed elsewhere the ulungur river granites in common with the voluminous granitoids occurring in the central asian orogenic belt represent a significant juvenile addition of the continental crust in the phanerozoic

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

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

relation between permeability and effective stress along a plate boundary fault barbados accretionary complex in situ bulk permeability was measured in a borehole that intersected the decollement zone a low angle detachment fault between the north american and caribbean plates permeability measurements were made at a variety of fluid pressure conditions defining a quantitative relation between bulk permeability and effective stress for this plate boundary fault zone the bulk permeability in this zone changed by several orders of magnitude as a consistent function of fluid pressure this relation may help to explain the dynamics of fluid fault interactions and the transient nature of hydrologic processes during deformation at convergent margins

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

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

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

gold butte crustal section south virgin mountains nevada the gold butte block consists of a complex of proterozoic medium to high grade metamorphic and plutonic rocks unconformably overlain by steeply east dipping paleozoic sedimentary rocks on its east side this complex is one of the largest continuous exposures of crystalline rocks in the southwestern united states that lie structurally beneath a steeply tilted section of phanerozoic strata and it may represent part of an intact proterozoic miocene crustal section reaching paleodepths of approximately 15 km the section was exposed during tectonic denudation in miocene time at about 15 ma and lies directly adjacent to the colorado plateau to the east proterozoic and tertiary geological features support the hypothesis that the block represents a relatively intact crustal section a large exposure of proterozoic rapakivi granite the gold butte granite circa 1 45 ga crops out over most of the crystalline complex and displays strong zoning in phenocryst abundance which increases toward the top of a 10 to 12 km thick magma chamber the wallrocks of the granite are predominantly garnet bearing gneisses circa 1 7 ga which display increasing pressures of final equilibration westward away from the basal phanerozoic unconformity thermobarometry of gneisses immediately beneath the unconformity indicate pressures of 2 3 kbar whereas those 12 13 km west of the unconformity yield pressures of 5 6 kbar suggesting the crustal section was established by 1 7 ga the style of deformation displayed by denudation structures also varies from east to west with brittle faulting only in eastern portions of the block and retrograde metamorphism mylonitization and chlorite brecciation of the garnet gneiss observed in western areas

mid cenozoic stress evolution and magmatism in the southern cordillera texas and mexico transition from continental arc to intraplate extension orientations of dikes veins faults and slickenlines reveal the evolution of stress during eocene to miocene magmatism in the southern cordillera where most thoroughly studied by us in trans pecos texas magmatism began at about 48 ma shortly after the cessation of laramide folding dikes and veins that formed from then until about 32 ma strike dominantly east northeast this indicates that the least principal stress 3 was north northwest additional data suggest that the maximum principal stress 1 was east northeast the stress field changed to 1 vertical and 3 east northeast i e east northeast extension at least by 28 ma and probably by 31 ma dikes and veins that formed between 31 and 17 ma when all magmatism ceased in texas strike north northwest this change marks the beginning of regional basin and range extension however major normal faulting exclusively of high angle type did not begin until about 24 ma a similar stress change marked by a similar change in dike and vein orientations occurred throughout the southwestern united states and northern mexico the time of change is not well constrained in texas but available information allows it to have occurred at the same time thoughout the southern cordillera we suggest the earlier stress field is related to east northeast convergence between the farallon and north american plates the change in stress is approximately coincident with collision of the east pacific rise and paleotrench extension may be related to the change from a convergent to a transform margin along the western edge of north america the changes in the stress field are accompanied by changes in the sources and compositions of magmas erupted in texas contemporaneity of the changes in stress and magmatism indicates that they are related combined with regional age patterns paleostress and geochemical data indicate that pre 31 ma magmatism in the southern cordillera occurred in a subduction related continental volcanic arc subsequent magmatism occurred in an environment of intraplate extension of the basin and range province

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

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

latest cretaceous a 2 type granites in the sakarya zone ne turkey partial melting of mafic lower crust in response to roll back of neo tethyan oceanic lithosphere abstract an integrated study of comprehensive geochronological geochemical and sr nd hf isotopic data was undertaken for the a type topcam pluton that intruded within the sakarya zone ne turkey with the aims of elucidating its origin and tectonic significance and gaining new insights into the generation of aluminous a type granites new la icp ms zircon u pb crystallization ages of 72 and 73 ma indicate emplacement in the late cretaceous time just after extensive metaluminous i type magmatism in the area the pluton consists mainly of alkali feldspar quartz plagioclase amphibole and biotite with accessory minerals such as magnetite apatite and zircon the outcrop is composed of granite syenite monzonite and quartz monzonite and possesses a wide range of sio2 content 57 70 wt with elevated ga al ratios and low mg mostly

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

contrasting sr and nd isotopic behaviour during magma mingling new insights from the mannum a type granite abstract the mannum granite in south australia is a classic a type granite which displays good evidence for magma mingling such as numerous syn plutonic mafic enclaves and rapakivi feldspars in both the granite and enclave study of the sr and nd isotopes show that the systems are dominantly controlled by plagioclase and titanite respectively the contrasting behaviour of these minerals during the development of a bimodal magmatic system has led to a well defined 482 ma isochron across a granite enclave boundary in the sr isotope system but not in the nd isotope system in situ techniques involving the development and production of a new titanite glass standard were developed to resolve this dichotomy inferred repeated interaction with mafic magmas has resulted in the destabilization and restabilization of plagioclase this has produced rapakivi textures and provides an effective mechanism for efficient sr isotope equilibration across the range of bulk rock compositions despite this interaction titanite in the host granite retain a range of initial nd isotope ratios indicating multiple parental magmas were assembled to produce the final pluton decoupling of the sr and nd isotope systems in other magmatic systems could be indicative of magma mixing as opposed to mingling where physical evidence e g enclaves xenocrysts etc is absent

can crystallization of olivine tholeiite give rise to potassic rhyolites an experimental investigation experiments were conducted to determine whether the rhyolites and basalts of the intraplate silica saturated potassic suites could be genetically related through crystallization extreme crystallization 96 97 of a high mgo 10 62 wt olivine tholeiite from the snake river plain with an initial bulk water content of 0 4 wt at a mid crustal pressure of 4 3 kbar generated potassic rhyolitic liquids similar in major element chemistry to those found in the quaternary rhyolite domes of the snake river plain and their plutonic equivalents in the proterozoic laramie anorthosite complex residual liquids comparable in composition to the bulk rock compositions of intermediate rocks found at the craters of the moon and cedar butte eruptive centers in the snake river plain are also generated along this crystallization path