eruption and magma crystallization ages of las tres virgenes baja california constrained by combined 230th 238u and u th he dating of zircon las tres virgenes volcano is a calc alkaline composite cone located near the main gulf of california escarpment on the e coast of the baja california peninsula high sensitivity ion microprobe u series 230 th 238 u ages for zircon from la virgen tephra average 121 10 12 ka 1 mswd 2 7 with discrete age peaks at 100 and 160 ka the noble gas mass spectrometric u th he zircon age corrected for disequilibrium and pre eruptive storage is 36 3 ka this result for the eruption age of la virgen tephra is significantly older than previously postulated historic or holocene ages that were based on an 18th century map reference and 14 c dating of accidental charcoal respectively the new u th he zircon age is consistent with a n26 4 ka age derived from cosmogenic he exposure dating of an overlying basaltic lava flow hausback b p and abrams m j 1996 plinian eruption of la virgen tephra volcan las tres virgenes baja california sur mexico eos transactions american geophysical union 77 46 suppl 813 814 u pb zircon analysis of ignimbrites erupted from the adjacent early pleistocene la reforma and el aguajito calderas yielded ages of 1 38 0 03 ma n 12 mswd 1 0 and 1 17 0 07 ma n 23 mswd 1 3 respectively no evidence for these ages is found among la virgen zircons whereas pre quaternary zircon xenocrysts are common the la virgen magma therefore evolved unrelated to early pleistocene magmatism in adjacent calderas but assimilated local basement rocks a gap between average th u and u th he zircon ages suggests that zircon crystallization was discontinuous in the la virgen magma chamber in addition partial resorption of zircon suggests episodic thermal rejuvenation most likely by basaltic recharge based on the zircon record the n100 ka lifetime of the thermal anomaly that sustained repeated intrusive pulses significantly exceeds the age of the last eruption this strengthens the view that tres virgenes has a potential for future eruptions

volcanic markers of the post subduction evolution of baja california and sonora mexico slab tearing versus lithospheric rupture of the gulf of california the study of the geochemical compositions and k ar or ar ar ages of ca 350 neogene and quaternary lavas from baja california the gulf of california and sonora allows us to discuss the nature of their mantle or crustal sources the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement nine petrographic geochemical groups are distinguished regular calc alkaline lavas adakites magnesian andesites and related basalts and basaltic andesites niobium enriched basalts alkali basalts and trachybasalts oceanic morb type basalts tholeiitic transitional basalts and basaltic andesites peralkaline rhyolites comendites and icelandites we show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting three successive stages are distinguished between 23 and 13 ma calc alkaline lavas linked to the subduction of the pacific farallon plate formed the comondu and central coast of the sonora volcanic arc in the extensional domain of western sonora lithospheric mantle derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the basin and range province the end of the farallon subduction was marked by the emplacement of much more complex middle to late miocene volcanic associations between 13 and 7 ma calc alkaline activity became sporadic and was replaced by unusual post subduction magma types including adakites niobium enriched basalts magnesian andesites comendites and icelandites the spatial and temporal distribution of these lavas is consistent with the development of a slab tear evolving into a 200 km wide slab window sub parallel to the trench and extending from the pacific coast of baja california to coastal sonora tholeiitic transitional and alkali basalts of subslab origin ascended through this window and adakites derived from the partial melting of its upper lip relatively close to the trench calc alkaline lavas magnesian andesites and niobium enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot pacific asthenosphere through the window during the plio quaternary the no slab regime following the sinking of the old part of the farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic transitional basalts of deep asthenospheric origin in baja california and sonora the lithospheric rupture connected with the opening of the gulf of california generated a high thermal regime associated to asthenospheric uprise and emplaced quaternary depleted morb type tholeiites this thermal regime also induced partial melting of the thinned lithospheric mantle of the gulf area generating calc alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle

statistical pattern analysis of surficial karst in the pleistocene miami oolite of south florida abstract a robust airborne light detection and ranging digital terrain model lidar dtm and select outcrops are used to examine the extent and characteristics of the surficial karst overprint of the late pleistocene miami oolite in south florida subaerial exposure of the miami oolite barrier bar and shoals to a meteoric diagenetic environment lasting ca 120 kyr from the end of the last interglacial highstand mis 5e until today has resulted in diagenetic alteration including surface and shallow subsurface dissolution producing extensive dolines and a few small stratiform caves analysis of the lidar dtm suggests that 50 of the dolines in the miami oolite have been obscured lost to urbanization though a large number of depressions remain apparent and can be examined for trends and spatial patterns the verified dolines are analyzed for their size and depth their lateral distribution and relation to depositional topography and the separation distance between them statistical pattern analysis shows that the average separation distance and average density of dolines on the strike oriented barrier bar versus dip oriented shoals is statistically inseparable doline distribution on the barrier bar is clustered because of the control exerted on dissolution by the depositional topography of the shoal system whereas patterning of dolines in the more platform ward lower relief shoals is statistically indistinguishable from random the areal extent and depth of dissolution of the dolines are well described by simple mathematical functions and the depth of the dolines increases as a function of their size the separation and density results from the miami oolite are compared to results from other carbonate terrains near surface stratiform caves in the miami oolite occur in sites where the largest and deepest dolines are present and sit at or near the top of the present water table

dendritic calcite crystals formed by calcification of algal filaments in a vadose environment abstract uncalcified and calcified algae grow on and color walls of cavities that occur in karst breccia which fills pockets in the tertiary bluff formation of grand cayman island the calcified algae are important because the calcification has been achieved through the formation of dendritic calcite crystals although common in metals such branching crystals have not been widely recognized in naturally occurring calcite the dendritic calcite crystals are exactly like most dendritic crystals in that they have a primary needle or stem with secondary and tertiary branches most dendritic calcite crystals have an open treelike form and are easy to recognize as calcification proceeds the dendritic crystals become progressively filled in and are increasingly difficult to recognize as dendritic crystals filling of the dendritic crystals can result in two dimensional sheets or three dimensional columns assuming the three dimensional columnar dendritic crystals developed planar and or curving smooth surfaces then lacking other evidence they would be indistinguishable from normal columnar calcite crystals and their dendritic origin would be hidden variables thought to control dendrite crystal formation include a creation of localized highly supersaturated solutions leading to rapid rates of nucleation and crystal growth and b impurities in the form of organic molecules and other substances algae are excellent candidates for controlling a via their metabolism and contributing to b via their decay combining these ideas with the results of this study and with the known common occurrence of algae in many speleothemic environments provides an impetus for a reexamination of the origin of subaerially formed calcite crystals

origin of the adakite high nb basalt association and its implications for postsubduction magmatism in baja california mexico constraining the origin of the adakite high nb basalt hnb association in baja california mexico is critical to a better understanding of global arc magmatism currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted farallon cocos plate thus providing support for the slab melting origin of adakites elsewhere moreover a tectono magmatic model involving the production of both adakite and hnb from slab melts offers a comprehensive explanation for the origin of the atypical arc related postsubduction magmatism in baja california this paper proposes alternative models for the origin of hnb and postsubduction magmatism in baja california wherein the unusual geologic setting of western mexico and westward movement of north america permitted the influx of pacific asthenosphere beneath the adjacent gulf of california after the cessation of subduction unlike the previous tectono magmatic model the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in baja california as tholeiites and hnb respectively fractional crystallization of some of the hnb magmas plus assimilation of tholeiitic materials produced nb enriched basalts neb the influx of pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower baja california crust producing adakite rocks and the preexist ing metasomatized mantle wedge producing bajaites and calc alkaline magmas

la purisima volcanic field baja california sur mexico miocene to quaternary volcanism related to subduction and opening of an asthenospheric window geological mapping and geochemical analyses combined with 40 k 40 ar ages for lavas from the late miocene to quaternary la purisimavolcanicfield bajacaliforniasur provideevidenceforfivevolcanicevents these inturn mayreflectplateinteractionsin the region the oldest event event 1 prior to 11 ma corresponds to the emission of normal to k rich calc alkaline lavas exposed as large mesas in the eastern part of the studied area and as pyroclastic breccias and volcaniclastic sediments to the west it is associated with the end of the comonduarc activity resulting from subduction of the farallon and guadalupe plates between 10 6 and 8 8 ma event 2 magnesian andesites and tholeiites were emplaced at 5 5 ma event 3 and 2 5 ma event 4 small volumes of magnesian andesites erupted in the central and southern parts of the volcanic field finally between 1 2 ma and holocene event 5 numerous basaltic and magnesian andesitic fissural and central emissions resulted in the formation of strombolian cones and associated lava flows mainly distributed within a nnw sse trending graben located se of the town of la purisima magmatic events 2 to 5 occurred well after the supposed end of the subduction event their geochemical characteristics are still typical of subduction modified sources and possibly indicate partial melting of hot slab and formation of an asthenospheric window due to a slab rupture event which followed ridge trench collision prior to the continental breakup of the gulf of california extensional province d 2005 elsevier b v all rights reserved

solving a cold case new occurrences reinforce juvenile callianassids as the ophiomorpha puerilis tracemakers abstract the ichnotaxon ophiomorpha puerilis was erected based on material from late pleistocene shallow marine siliciclastic deposits of southeastern brazil these tiny burrows with a distinctive lining of cylindrical pellets having rounded ends were originally interpreted as burrows of juvenile callianassid shrimp with polychaetes also considered as a possible tracemaker recent discoveries of o puerilis in late pleistocene shallow marine carbonates from two localities in the bahamas and from the miami limestone of south florida described herein have expanded significantly the geographic range of the ichnotaxon and provided numerous well preserved specimens for detailed structural analysis of its burrow form and pellets the new information strengthens the interpretation of juvenile callianassids as the tracemakers resolving the cold case of the burrow s tracemaker microscopic analyses revealed that the pellets of o puerilis are constructional not fecal furthermore larger forms of ophiomorpha are common and occur in close association with o puerilis at each locality these new data allowed close comparison of o puerilis pellets with those of sergio mirim a modern callianassid from southeastern brazilian beaches ophiomorpha nodosa from brazilian pleistocene deposits and fossil polychaete burrows filled with fecal pellets as described in the literature the combined evidence allows us to exclude polychaetes as potential o puerilis tracemakers rather o puerilis records the settling and initial burrowing behavior of callianassid decapodids the characteristics of an ophiomorpha ichnofabric with the addition of o puerilis also is described and explored an emended diagnosis for the o puerilis ichnotaxon is provided based on the new structural data for this burrow

late quaternary coastal evolution and aeolian sedimentation in the tectonically active southern atacama desert chile abstract analyses of aeolianites and associated dune surficial carbonate and marine terrace sediments from north central chile 27 54 s yield a record of environmental change for the coastal southern atacama desert spanning at least the last glacial interglacial cycle optically stimulated luminescence dating indicates phases of aeolian dune construction at around 130 111 98 77 69 and 41 28 ka thin section and stable carbon and oxygen isotope analyses suggest a predominantly marine sediment source for the three oldest dune phases aeolianites appear to have accumulated mainly from tectonically uplifted interglacial marine sediments that were deflated during windier and or stormier intervals bedding orientations indicate that sand transporting winds varied in direction from s ese during mis 5e and wnw ese during mis 5c 5a winds from the southeast quadrant are unusual today in this region of the atacama suggesting either major shifts in atmospheric circulation or topographic airflow modification thin section evidence indicates that the aeolianites were cemented by two phases of vadose carbonate tentatively linked to wetter periods around 70 and 45 ka tectonic uplift in the area has proceeded at an average rate of 305 542 mm kyr 1 the study illustrates the complexity of understanding onshore offshore sediment fluxes in the context of late quaternary sea level fluctuations for an area undergoing rapid tectonic uplift

prominence of ichnologically influenced macroporosity in the karst biscayne aquifer stratiform super k zones a combination of cyclostratigraphic ichnologic and borehole geophysical analyses of continuous core holes tracer test analyses and lattice boltzmann fl ow simulations was used to quantify biogenic macroporosity and permeability of the biscayne aquifer southeastern florida biogenic macroporosity largely manifests as 1 ichnogenic macroporosity primarily related to postdepositional burrowing activity by callianassid shrimp and fossilization of components of their complex burrow systems ophiomorpha and 2 biomoldic macroporosity originating from dissolution of fossil hard parts principally mollusk shells ophiomorpha dominated ichnofabric provides the greatest contribution to hydrologic characteristics in the biscayne aquifer in a 345 km 2 study area stratiform tabular shaped units of thalassinidean associated macroporosity are commonly confi ned to the lower part of upward shallowing highfrequency cycles throughout aggradational cycles and in one case they stack vertically within the lower part of a high frequency cycle set broad continuity of many of the macroporous units concentrates groundwater fl ow in extremely permeable passageways thus making the aquifer vulnerable to long distance transport of contaminants ichnogenic macroporosity represents an alternative pathway for concentrated groundwater fl ow that differs considerably from standard karst fl ow system paradigms which describe groundwater movement through fractures and cavernous dissolution features permeabilities were calculated using lattice boltzmann methods lbms applied to computer renderings assembled from x ray computed tomography scans of various biogenic macroporous limestone samples the highest simulated lbm permeabilities were about fi ve orders of magnitude greater than standard laboratory measurements using air permeability methods which are limited in their application to extremely permeable macroporous rock samples based on their close conformance to analytical solutions for pipe fl ow lbms offer a new means of obtaining accurate permeability values for such materials we suggest that the stratiform ichnogenic groundwater fl ow zones have permeabilities even more extreme 2 5 orders of magnitude higher than the jurassic super k zones of the giant ghawar oil fi eld the fl ow zones of the pleistocene biscayne aquifer provide examples of ichnogenic macroporosity for comparative analysis of origin and evolution in other carbonate aquifers as well as petroleum reservoirs

recent volcanic events and the distribution of hydrothermal venting at the lucky strike hydrothermal field mid atlantic ridge 1 we present new high resolution bathymetry and backscatter data acquired in 2006 with the rov victor 6000 over the lucky strike hydrothermal field mid atlantic ridge as long term monitoring of the lucky strike area momar project is being implemented these new high resolution data offer an unprecedented view of the distribution of hydrothermal edifices eruptive facies and small scale tectonic features in the lucky strike vent field we show that vents located in the nw and ne correspond with wide expanses of lumpy seafloor which we interpret as primarily made of broken chimneys and sulfide edifices they are found above scarps with relief 50 m or on associated mass wasting deposits by contrast the se and sw vents correspond with small expanses of lumpy seafloor and are located near smaller scarps which we interpret as more recent faults hydrothermal edifices in the sw venting area appear very recent postdating the emplacement and faulting of the most recent lava we propose that this difference in the age of hydrothermal edifices does not mean that hydrothermal venting itself is more recent in the southern part of the lucky strike field because preexisting sulfide deposits there may have been buried by recent volcanic deposits instead the older edifices in the northern part of the hydrothermal field may have been allowed more time to grow because they are set above the level of recent lava flows the formation of a lava lake is the most recent eruptive event detected at lucky strike lava drainback is evidenced by benches and lava pillars suggesting a close connection with an underlying magma reservoir which probably corresponds to the melt body imaged by singh et al 2006 we have found no evidence that this lake was active for months to decades as lava lakes at terrestrial volcanoes it may instead have formed as a lava pond with successive lava flows covering the eruptive vents as proposed for similar features at the epr the horizontal surface of the lake is deformed only near its southwestern shore along a nne trending set of faults and fissures which appear to control the distribution of hydrothermal chimneys

an unusual syn eruptive bimodal eruption the holocene cuicuiltic member at los humeros caldera mexico abstract the cuicuiltic member cm at los humeros caldera eastern mexican volcanic belt is a holocene 6 4 ka b p succession of alternated fallout deposits of contrasting composition trachydacite pumice and basaltic andesite scoria the cm covers approximately 250 km 2 on its proximal facies and its thickness ranges from 1 5 m to 8 0 m it postdates two caldera forming ignimbrites xaltipan and zaragoza and numerous plinian successions it is subdivided in 9 units c1 to c9 according to its textural and chemical characteristics sub horizontal topography draping layers of trachydacite pumice lapilli andesitic pumice lapilli and basaltic andesite scoria lapilli with sporadic one meter blocks are common lithofacies the base is formed by coarse trachydacite pumice lapilli c1 and c2 overlain by a layer with banded pumice c3 thin layers of ash and ash tuff are intermittent on lower units whilst continuous at the base of c4 the middle units c4 and c6 are basaltic andesite pumice and scoria lapilli to blocks c5 is in between the two mafic units and it is represented by a layer of pale grey pumice lapilli units c7 and c8 are a mixture of white trachydacite pumice scoria lapilli and banded pumice the uppermost layer c9 is a brown to grey andesitic pumice lapilli extensive fieldwork allowed a close and reliable correlation of layers that helps to understand the complexity of stratigraphic relations and sources for those layers the distribution of these units is varied across the caldera with the trachydacite layers dispersal from the centre towards the nw whilst the andesitic units have maximum thicknesses over the se and ne sectors of the caldera isopach and isopleth maps combined with detailed mapping of near vent spatter facies orientation of local bomb sags and variation of mean clasts size for some layers were very useful to determine the vent location particularly for the andesitic basaltic layers the cm represents the last explosive event registered at los humeros caldera combining simultaneously both plinian and strombolian activities the eruption was fed from one central vent and at least two independent but simultaneously active vents up to 6 km apart the eruption began with a trachydacite explosive eruption depositing pristine white pumice in the centre of the caldera in two stages after a short repose period at least two andesitic to basaltic fissure vents erupted ejecting pumice and scoria over the southeast and northeast sector mainly along a weakness structural plane parallel to the potreros scarp these eruptions became less energetic with time until reaching classic strombolian style with sudden fluxes generating local and discrete pyroclastic currents trachydacite pumice kept falling intermittently accompanied by mingling at depth the end of the cuicuiltic eruption is marked by the well graded layer c9 which registers lapilli sized pumice fall clasts from a hybridized magma chamber the cm may represent a either a trachydacite magma chamber disturbed by a basaltic intrusion triggering the eruption of the evolved material first or b a heterogeneously zoned magma reservoir tapping the evolved magma first and due to contrasting density and viscosity conditions the basaltic and andesitic materials were extruded through adjacent weakness planes up to the surface

aragonite to calcite transformation during fresh water diagenesis of carbonates insights from pore water chemistry dissolved strontium and calcium concentrations in fresh water lenses fwl and associated mixing zones mz on two small holocene ooid sand islands in the schooner cays bahamas were monitored during a 1 yr period to quantitatively analyze the transformation of aragonite to calcite the observed characteristics of this mass transfer are functions of climate and hydrology aragonite to calcite transformation in all hydrologic zones is primarily associated with meteoric recharge the transformation occurs throughout the fwl and in the mz to relative salinities of 19 and 36 sea water on the two islands rates of transformation are rapid in all zones and are greatest in the fwl a limestone composed of 100 calcite should form from an aragonite precursor within 4 700 to 15 600 yr in the fwl and within 8 700 to 60 000 yr in the upper mz efficiencies of transformation can vary between hydrologic zones due to pco2 effects yet the efficiency of the entire system fwl mz is high 87 this indicates that most caco3 derived from aragonite dissolution is reprecipitated as calcite somewhere in the fresh water system or upper mixing zone co2 effects fresh water sea water mixing and the differing solubilities between aragonite and calcite all drive the mass transfer the latter is the most significant accounting for up to nine times more mass transfer than co2 effects and at least ten times more mass transfer than fresh water sea water mixing differing solubilities should also cause mass transfer to occur throughout the hydrologic cycle but it apparently becomes ineffective after the rainy season possibly due to the inhibition of calcite precipitation

el ventorrillo a paleostructure of popocatepetl volcano insights from geochronology and geochemistry volcan popocatepetl mexico was constructed over the remains of a volcanic paleostructure based on fieldwork 40ar 39ar dating u pb dating and geochemistry we have determined the age chemistry and location of this paleostructure and named it el ventorrillo most remnants of el ventorrillo are covered by deposits from subsequent activity of popocatepetl except for the el abanico scarp and the barranca de nexpayantla where the stratigraphy of el ventorrillo eruptive products can be investigated inception of volcanism at el ventorrillo occurred at 331 10 ka with emission of the nexpayantla andesitic lavas and continued with extrusion of the yoloxochitl 267 31 ka and microwave 227 6 ka domes intrusion of dikes occurred at 298 94 and 230 3 ka activity at el ventorrillo continued with the emission of lavas that built the el abanico scarp 193 29 to 96 8 ka and continued until the tutti frutti eruption destroyed the cone 14 1 kyr ago el ventorrillo magmas produced rocks divided into two mineralogical groups the first group contains biotite amphibole rich rocks and the second group consists of biotite amphibole free lavas the rocks that contain biotite and amphibole are older than 198 13 ka whereas the rocks with no hydrous phases are younger than 227 6 ka and contain skarn and granodiorite xenoliths we interpret the change to an anhydrous mineral assemblage and the occurrence of skarn and granodiorite xenoliths as evidence for the formation of a new shallower reservoir a granodiorite xenolith was chosen for 40ar 39ar dating and u pb zircon analyses the u pb method yielded an age of 540 110 ka and the 40ar 39ar an age of 109 24 ka these ages are interpreted to indicate granodiorite crystallization 540 110 ka which metamorphosed the calcareous basement beneath popocatepetl into skarn and an influx of magma 109 24 ka that reheated the granodiorite major and trace elements sr nd and pb isotopic compositions and textural analyses of plagioclase show that el ventorrillo magmas were moderately modified by mixing with more mafic magmas and that assimilation of the calcareous basement was negligible

convective upwelling in the mantle beneath the gulf of california in the past six million years baja california has rifted obliquely apart from north america opening up the gulf of california between transform faults seafloor spreading and rifting is well established in several basins other than hotspot dominated iceland the gulf of california is the only part of the world s seafloor spreading system that has been surrounded by enough seismometers to provide horizontal resolution of upper mantle structure at a scale of 100 kilometres over a distance great enough to include several spreading segments such resolution is needed to address the long standing debate about the relative importance of dynamic and passive upwelling in the shallow mantle beneath spreading centres here we use rayleigh wave tomography to image the shear velocity in the upper 200 kilometres or so of the mantle low shear velocities similar to those beneath the east pacific rise oceanic spreading centre underlie the entire length of the gulf but there are three concentrated locations of anomalously low velocities spaced about 250 kilometres apart these anomalies are 40 to 90 kilometres beneath the surface at which depths petrological studies indicate that extensive melting of passively upwelling mantle should begin we interpret these seismic velocity anomalies as indicating that partial melting triggers dynamic upwelling driven by either the buoyancy of retained melt or by the reduced density of depleted mantle

radar frequency dielectric dispersion in sandstone implications for determination of moisture and clay content 1 the relationship between dielectric permittivity water saturation and clay content for the sherwood sandstone from ne england was characterized as part of a wider study of the vadose zone moisture dynamics and pollution vulnerability of this aquifer dielectric permittivity was measured over the full range of saturation levels for various lithologies ranging from clean medium grained sandstone to fine grained sandstone containing up to 5 clay using a specially constructed dielectric cell dielectric constant kr is largely independent of frequency between 350 mhz and 1000 mhz below 350 mhz kr of fine grained clay rich sandstone shows frequency dispersion tests on physical models of the sandstone consisting of a fine fraction of ottawa sand and montmorillonite clay indicate that the clay minerals within the sandstone are responsible for its frequency dispersive behavior these tests also show that increasing pore fluid salinity increases dielectric dispersion at the lower end of the frequency range which indicates that this arises from the interfacial maxwell wagner mechanism associated with platy clay particles water saturated sand clay mixtures show very low dielectric constants at high frequencies over 650 mhz this effect is independent of salinity and probably results from the layered geometric arrangement of solids bound and free water within the swelling clay the complex refractive index method crim with a mineral dielectric constant of about 5 provides a good match to the water saturation versus dielectric constant data for all sherwood sandstone lithologies at frequencies between 350 and 1000 mhz and for clay poor sandstone at lower frequencies below 350 mhz the presence of a few percent of clay in some sherwood sandstone lithologies raises their crim best fit mineral dielectric constant substantially

petrogenesis of alkaline basalts from socorro island mexico trace element evidence for contamination of ocean island basalt in the shallow ocean crust postcaldera mafic volcanism on socorro island mexico an island located in the eastern pacific ocean on a segment of a mid ocean ridge spreading center abandoned at 3 5 ma dominantly comprises alkalic basalt hawaiite and mugearite with subordinate mildly alkalic transitional basalt most major element data are consistent with differentiation of alkalic basalt to hawaiite and mugearite by up to 50 fractionation of plagioclase clinopyroxene olivine fe ti oxides high field strength element abundances are consistent with this interpretation and the narrow range in zr nb suggests that parental magmas formed by similar degrees of partial melting of a relatively homogeneous mantle source sr and nd isotopic ratios exhibit relatively narrow ranges 0 7031 0 7032 0 5128 0 5130 respectively despite this a subset of the alkalic basalts has negative ce anomalies and abundances of p2o5 ba y and some rare earth elements in excess of those predicted by fractional crystallization models these chemical features require open system processes the observed heterogeneities are perhaps best explained by assimilation of ocean crustal components by basalt in a shallow magma reservoir regional mantle heterogeneity is evident from elemental and isotopic data for mildly alkalic transitional basalt from socorro island mugearite from san benedicto island and submarine basalts from near socorro island evidence presented here indicates that compositional variations in ocean island basalts can be introduced by ocean crustal contamination and underscores the importance of considering this source of chemical heterogeneity in basalts before characterizing associated mantle

seismic activity and stress tensor inversion at las tres virgenes volcanic and geothermal field mexico abstract we analyze local earthquakes occurring between 2003 and 2012 at the las tres virgenes volcanic and geothermal field tvvgf to establish their temporal and spatial distribution and relationships with local and regional fault systems water injection acid stimulation and steam production tests we obtained focal mechanisms and inverted data for the stress tensor to understand the local and regional stress fields we analyzed 423 local earthquakes with magnitudes between 0 1 and 2 9 mc and hypocentral depths from 0 2 to 7 4 km b s l the cutoff depth at 7 4 km possibly delineates the brittle ductile transition zone we identified seven swarms from 1 to 7 swarms 1 december 2009 2 may 2010 3 june july 2010 and 7 december 2012 are strongly correlated with injection processes whereas swarms 5 april 2012 and 6 september 2012 are correlated with local tectonic faults stress inversion showed nw se e w and ne sw extensional orientations s hmin in agreement with the local tectonic stress field while ne sw compressional orientations s hmax are correlated with the regional tectonic stress field

role of magma mixing in the pre eruptive dynamics of the aeolian islands volcanoes southern tyrrhenian sea italy abstract we combined literature and experimental data to determine the role of magma mixing in the pre eruptive dynamics of the aeolian islands volcanoes as a first step we systematically reviewed the evidence supporting the hypothesis of mixing triggered eruptions in the aeolian archipelago providing textural chemical and rheological constraints the existing data highlighted the significant role of magma mixing in many eruptions within the aeolian archipelago examples include the upper pollara and porri volcano eruptions at salina monte guardia and the ad 1230 monte pilato eruption at lipari as well as the present day activity at stromboli then we focused on vulcano island chosen as a case study because it represents one of the volcanoes posing the highest risk in the aeolian archipelago at vulcano island we highlighted the role of magma mixing in the ad 1739 and 1888 90 eruptions finally we investigated mixing to eruption timescales for the ad 1739 eruption performing mixing experiments and evaluated the progressive decay of the chemical concentration variance with time results pointed to mixing to eruption timescales of the order of 29 9 h and magma ascent rates ranging between 3 10 2 and 5 10 2 m s 1 we finally emphasized that the presented results may have significant implications in the context of volcanic hazard mitigation and planning of emergency activities

the 23 500 y 14 c bp white pumice plinian eruption and associated debris avalanche and tochimilco lava flow of popocatepetl volcano mexico abstract the white pumice wp is one of the thickest and most voluminous plinian fallouts produced by popocatepetl volcano in central mexico during the late pleistocene holocene its eruption 23 500 14 c y bp 27 800 cal bp was triggered by the catastrophic failure of the sw flank of the volcano the resulting debris avalanche was highly mobile reaching 72 km from the cone with an apparent coefficient of friction l h of 0 06 the deposit covers an area of 1200 km 2 and has a volume of 10 4 km 3 this gigantic landslide characterized by exceptionally large proximal hummocks 400 m provoked the sudden decompression of the hydrothermal and magmatic systems which produced an initial blast followed by the rise of a plinian column that reached an altitude of 33 km the isopach map allows the recognition of a dispersal axis pointing toward the south where an area of 2490 km 2 was covered by 10 cm of pumice and ash the total volume of the pumice fallout was estimated at 1 9 km 3 dre dense rock equivalent pumice clasts are dacitic 62 66 wt sio 2 anhydrous basis highly vesicular 55 88 vol and display a seriate texture with phenocrysts of plagioclase hornblende augite hypersthene oxides ti magnetite and ilmenite apatite as the eruption advanced discharge rates became more intermittent and the height of the column fluctuated and finally collapsed generating pumice and ash flows that were emplaced around the volcano this short but intense activity was followed during subsequent years by rain induced lahars that reached great distances from the volcano at the same time more degassed andesitic dacitic 61 65 wt sio 2 magma was erupted effusively 4 4 km 3 dre in the new horseshoe shaped 5 km wide crater from which the tochimilco lava flow descended toward the sse where it inundated an area of 68 km 2 and reached as far as 22 km from its source since then multiple eruptions have reconstructed the summit cone almost completely obliterating the horseshoe shaped crater during the course of this catastrophic eruption vei 5 a total volume of 6 3 km 3 dre of juvenile magma pumice and lava were emitted and at least an additional 10 km 3 of pre existing rocks debris avalanche mobilized it surpasses in magnitude most other known plinian eruptions from popocatepetl and can be envisaged as an example of a worst case scenario for hazard evaluation purposes it dramatically changed the morphology of the volcano and had profound and far reaching effects beyond its immediate vicinity along rivers draining surrounding plains as well as on the lacustrine basins e g chalco to the ne a repeat of such an eruption in this densely populated area would certainly cause a major calamity of unprecedented dimensions

megarhizoliths in pleistocene aeolian deposits from gran canaria spain ichnological and palaeoenvironmental significance the pleistocene dune field of tufia located on the east of gran canaria spain contains different stratigraphic levels of indurated pillar like structures that are interpreted as megarhizoliths the megarhizoliths occur at the top of different aeolian sets and reach 31 5 cm in diameter and over 1 m in height these scattered free standing vertical cylindrical to slightly conical columns usually appear as hollow cylinders displaying elliptical cross sections aligned with the prevailing wind on the leeward side of some specimens the external wall shows a tail of rock matrix resembling a sort of wind shadow these tails and other remains of the associated rock matrix show a texture composed of long horizontal parallel cylinders orientated with the wind internally the most complete structures show five concentrically arranged zones zone a is a central pore corresponding to the cavity originally occupied by the root zones b and c which include alveolar and laminated carbonate textures indicating that carbonate precipitation was mostly induced by the roots and their associated microorganisms and zones d and e consisting mostly of aeolian sands in d the sand grains show thin micritic coatings whereas in e vadose aragonite cements can be seen on the grain surface suggesting a less biogenic influence in their formation the degree of cementation and the time of the precipitation of carbonate around the roots controlled the preservation of these zones thus in some cases zones b c and or d are not preserved cylinders are up to 30 the diameter of the root that nucleated them the presence of the megarhizoliths at the top of the aeolianite beds indicates that aeolian sedimentation halted several times allowing soil formation and plant colonisation during slightly more humid periods the occurrence of megarhizoliths is further proof of the alternation of arid and slightly more humid climates in the north atlantic during the last glacial period it is also noted that they may be misinterpreted as animal trace fossils or tree trunk casts resulting in incorrect ichnological or palaeoenvironmental interpretations

geophysical evidence for the lateral distribution of free phase gas at the peat basin scale in a large northern peatland 1 northern peatlands contribute to the global carbon cycle by way of gas emissions in the form of biogenic carbon dioxide and methane originating in the subsurface determining how much gas is present and where it is trapped is essential to understanding the role of northern peatlands on the global carbon cycle particularly in regard to methane cycling in this study spatial variability in free phase gas content was estimated using ground penetrating radar gpr along a 1 4 km transect crossing distinct peat landforms including a bog crest midslope lawn and fen variations in gas content and distribution were observed in landforms crossed by this transect estimated gas content up to 25 by volume was observed in landforms dominated by woody surface vegetation in areas of the midslope lawn the estimated gas content was greater than 15 while estimated gas contents between 0 and 7 were found in the fen changes in estimated gas content of up to 20 were observed over distances of 100 m across a transition between a stand of 10 m tall trees and an adjacent fen the mean of the estimated errors in gas content is 1 3 suggesting that these large changes in estimated gas content between landforms can be clearly determined using geophysical methods a geophysical image of the distribution of free phase across this transect reveals evidence for gas storage in deep peat in intervals as thin as 0 35 m these data support conceptual models based on accumulation and storage of free phase gas in deep peat

the uncertainties involved in extracting amplitude and frequency of orbitally driven sea level fluctuations from shallow water carbonate cycles in outcrops shallow water carbonates often form thick vertically stacked metre scale exposure bounded depositional packages here called carbonate cycles because the facies within carbonate cycles can be tied precisely to water depths they are considered ideal for reconstructing past sea level changes there is however increasing evidence that these depositional cycles are unreliable recorders of both the frequency and the amplitude of orbitally driven sea level fluctuations carbonate cycles record only a fraction of the amplitude of a sea level cycle a portion of the rise and nothing of the fall cores through the pleistocene cycles on great bahama bank illustrate this shortcoming although the amplitudes of the last nine sea level changes are each a hundred metres or more the thickness of the cycles varies from a few metres to ca 15 m the lack of correlation between sea level amplitude and cycle thickness is not eliminated for cycles deposited during times of lower sea level amplitudes for example the cretaceous upper cretaceous cycles on the maiella platform margin document the irregularly filled accommodation space and the resultant variability in cycle thickness and frequency uncertainties in assessing the frequencies of sea level changes from shallow water carbonate cycles are caused by missed beats and metre scale oscillations of sea level within highstands that potentially produce cycles of very short duration the random amplitude variability during the last 57 glacio eustatic sea level changes illustrates the difficulty of assessing missed beats where a sea level fluctuation is not recorded because the sea level rise does not reach the platform top missed beats are also produced by the depositional topography that is created by irregularly filled accommodation space as a result variable numbers of cycles are deposited across the platform further complicating orbital frequency analyses are decametre scale oscillations of the sea level during highstands the amplitude of these sub orbital sea level oscillations up to 17 m within the last interglacial marine isotope stage 5e are sufficient to expose shallow platforms like great bahama bank and subsequently produce an additional depositional cycle with similar facies successions the combined effects of missed beats and oscillations within highstands are likely to produce cycles and hiatuses of variable duration that are difficult to extract from the rock record consequently estimates of the orbital forcing mechanisms and frequencies from ancient shallow water carbonate cycles carry large uncertainties

late miocene adakites and nb enriched basalts from vizcaino peninsula mexico indicators of east pacific rise subduction below southern baja california a typical slab melt association was emplaced from 11 to 8 ma in the santa clara volcanic field vizcaino peninsula baja california sur it includes adakitic domes and associated pyroclastic flow deposits together with lava flows of niobium enriched basalts the trace element and isotopic sr nd pb signatures of adakites are consistent with melting of altered mid ocean ridge basalts and the sources of the nb enriched basalts contain an enriched mantle wedge component such associations commonly form at depths of 70 80 km during low dip subduction of very young oceanic crust however the santa clara field is relatively close 100 km to the paleotrench which suggests that the genesis of its adakites and nb enriched basalts occurred in a very high thermal regime linked to the subduction of the then active guadalupe spreading center of the east pacific rise our data suggest that the asthenospheric window documented below northern baja california also developed beneath the south of the peninsula during the neogene this hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region

the role of skeletal porosity in aragonite neomorphism strombus and montastrea from the pleistocene key largo limestone florida abstract the neomorphism of aragonite to calcite involves complex textural and chemical alterations that are in part controlled by the degree of communication between diagenetic and ambient formation fluids two taxa montastrea annularis and strombus gigas from the vadose zone of the pleistocene key largo formation in southern florida were examined in order to evaluate the effect of initial skeletal porosity on system closure during neomorphism fabrics of replacement calcite in both taxa are similar consisting of blocky crystals largely restricted to areas of former skeletal aragonite oriented aragonite and organic inclusions result in the preservation of relic fabrics in both taxa the chemical signatures of coralline and molluscan aragonite and calcite are strongly dependent on initial skeletal porosity compared to original aragonite montastrea calcite is enriched in magnesium 1 151 64 ppm versus 2 114 619 ppm and depleted in strontium 7 397 376 ppm versus 2 510 43 ppm these data in conjunction with large calcite oxygen depletions 3 8 0 2 versus 5 7 0 3 and variable calcite carbon depletions 0 6 0 6 versus 1 3 to 7 3 demonstrate significant exchange between neomorphic front and formational fluids in contrast strombus calcite composition is largely inherited from original aragonite and is only slightly enriched in strontium and magnesium 2 299 316 ppm versus 2 519 759 ppm and 96 23 ppm versus 173 79 ppm while carbon exhibits a small depletion 5 3 0 5 versus 4 4 0 8 similarity of str mbus aragonite and calcite compositions reflects high rock water ratios in relatively closed diagenetic systems calcite oxygen depletion 0 5 0 5 versus 3 6 0 4 observed in strombus however requires some exchange between formation and neomorphic front fluids differences in the thickness of replacement calcite rims between taxa reflect variable skeletal porosities and rates of migration of neomorphic fronts neomorphism was six times faster by volume in porous montastrea than in dense strombus skeletal structures thus skeletal porosity influences degrees of system openness during aragonite neomorphism

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

repeated recharge assimilation and hybridization in magmas erupted from el chichon as recorded by plagioclase and amphibole phenocrysts abstract compositional and isotopic zoning patterns in plagioclase and amphibole phenocrysts from el chichon record multiple cycles of country rock assimilation magma injection hybridization and mixing laser ablation icp ms and electron microprobe analyses of plagioclase crystals from 7 eruptions spanning 3100 years reveal four types of zoning these compositional and isotopic zones are often associated with textural changes observed in the crystals in thin section e g sieved or patchy regions amphiboles are frequently zoned in al and si and in two magmas may have clinopyroxene rims interestingly most plagioclase show multiple and repeated zoning patterns moreover all magmas contain all zoning patterns and textures and crystals with substantially different sequences of zones occur within mm of one another the most reasonable explanation for the origin of these textures is a frequently recharged chamber plagioclase zones with increasing anorthite contents an and decreasing 87sr 86sr ratios record injection by a hotter possibly wetter and more primitive magma lower 87sr 86sr ratio zones with decreasing an and increasing 87sr 86sr ratios record assimilation of country rock and or hybridization of the host and injected magmas injection of hot magma may provide the heat for country rock assimilation changes in an without corresponding changes in 87sr 86sr ratio likely record slight variations in pressure or temperature during crystallization or the far field thermal effects of magma injection variations in 87sr 86sr ratio unaccompanied by anzoning record assimilation of country rock amphibole zoning patterns also record periodic heating events amphibole with clinopyroxene rims record episodes where the magma was heated beyond the amphibole stability field bulk compositional homogeneity and the juxtaposition of many crystals with disparate zoning patterns in single pumice require the magmatic system to be well mixed strontium diffusion rates indicate that the plagioclase zoning patterns cannot have persisted at magmatic pressures and temperatures for more than 500 years thus cycles of injection and assimilation occur on timescales equal to or shorter than the eruption recurrence interval long term compositional and isotopic homogeneity indicate that there is a balance between recharge assimilation and crystallization

unconformities and porosity development in carbonate strata ideas from a hedberg conference the purpose of this paper is to highlight ideas from a hedberg conference concerning detection of unconformities and associated porosity in carbonate strata many types of information including cores and outcrops seismic data eustatic sea level curves wireline logs biostratigraphic data stable isotope trends cycle stacking patterns and tectonic and basin evolution models can be used to predict and or detect subaerial unconformities all methods of detecting subaerial exposure with these types of information have limits and pitfalls detailed analysis of the stratigraphy sedimentology and diagenetic history of a rock sequence from outcrop or core data is considered the most reliable means of detecting subaerial exposure though an integrated approach using all a ailable information is recommended the relationship between subaerial exposure and subsurface porosity in carbonates is not simple or easily predicted subsurface porosity is the product of many factors operating during deposition subaerial exposure and burial several concepts should be recognized when evaluating porosity associated with subaerial unconformities 1 carbonate diagenesis during subaerial exposure rearranges pore networks but usually does not increase the total porosity near surface dissolution can simply lower a topographic surface without increasing porosity within the host limestone in many cases total porosity in carbonates under unconformities is actually reduced during subaerial exposure 2 permeability is likely to change more than porosity during subaerial exposure increasing in some c ses but decreasing in others 3 pore systems evolve with time during subaerial exposure short periods of subaerial exposure 10 000 400 000 yr are often associated with greater porosity than long intervals of subaerial exposure 1 20 million yr prolonged subaerial exposure may change permeability less than porosity because high permeability karst related conduits can form quickly and persist for millions of years 4 many carbonates subjected to subaerial exposure have little or no porosity in the deep subsurface because compaction cementation and stratal collapse reduce porosity during burial 5 unconformity related diagenesis may enhance reservoir potential by creating pore systems that are resistant to compaction during deeper burial 6 vugs caverns and breccias can fo m in the deep subsurface because of dissolution by basinal fluids independent of subaerial exposure 7 lithologic changes at unconformities like shales overlying carbonates can influence the flow of subsurface fluids during end page 857 deeper burial resulting in deep burial dissolution localized along unconformities if subaerial exposure and exposure surfaces are integrated into a sequence stratigraphic paleogeographic and climatic framework systematic and hopefully predictable patterns will emerge however we do not fully understand the complex interplay between the factors critical to creating and preserving porosity in carbonates additional research is needed to quantify critical processes and products so we can reliably predict porosity associated with subaerial exposure during exploration in frontier basins and on a smaller scale during reservoir analysis

origin of nb enriched basalts and adakites in baja california mexico revisited reply maury et al 2009 present new and detailed field observations and petrologic analyses to argue that the new model that i propose as an alternative explanation for postsubduction magmatism in baja california mexico castillo 2008 1 is incorrect specifically they claim that their new

rock fabric controls on pore evolution and porosity permeability trends in oolitic grainstone reservoirs and reservoir analogs although the general influence of rock fabric on porosity and permeability k within carbonates is well documented if and how pore evolution and k scatter quantitatively relate to depositional fabric remains poorly constrained this project empirically explores this uncertainty within oolitic grainstones from a range of geologic ages and diagenetic histories to understand depositional sediment pore relationships and how they can evolve with lithification integrating data from point counting digital image analysis nuclear magnetic resonance and core analysis of holocene pleistocene pennsylvanian and mississippian oolitic grainstones reveals quantitative relations among rock fabric pores and petrophysical parameters oolitic grainstones of similar sedimentology taken from distinct diagenetic scenarios display a unique combination of pore size shape spatial distribution and k character within each scenario pore attributes and k are correlated more closely with grain size sorting and type than with cementation and compaction collectively these results are interpreted to suggest that sedimentology controls the trends or variability within an oolitic succession but that diagenesis defines the absolute values of pore attributes and petrophysical parameters these findings suggest that petrophysical variability within oolitic reservoirs may closely follow sedimentologic trends which may be predictable within a stratigraphic framework

depositional architecture and petrophysical variability of an oolitic tidal sand shoal pennsylvanian missourian kansas u s a

groundwater geochemistry fluctuations along a fresh saltwater gradient on the carbonate islands of the lower florida keys abstract climate change will have long lasting effects on the availability of fresh water on small carbonate islands that have isolated fresh groundwater lenses particularly as sea level rises and rainfall regimes shift the carbonate islands of the florida keys provide an ideal location to study the effect of variable rainfall on the aqueous geochemistry of the islands groundwater in a rainfall driven carbonate system the expectation is that limestone dissolution will occur within the vadose zone resulting in increased ions in the groundwater however geochemical processes are also affected by the salinity of groundwater and the extent of the mixing zone between fresh and salt water we chose two islands to conduct the study of the shallow groundwater the largest island in the lower florida keys big pine key bpk and a smaller island upper sugarloaf key slk from may 2011 through april 2012 monthly groundwater samples were collected from 24 shallow 1 m deep wells located along a fresh to saline gradient on both islands groundwater chemistry was compared with rainfall amounts from a weather station on bpk saturation indices for aragonite and calcite generated with geochemical modeling in phreeqc were compared to conservative mixing between gulf of mexico water and freshwater equilibrium to supersaturated conditions with respect to carbonate minerals dominated in all of the groundwater samples saturation indices varied with rainfall with the most supersaturated samples observed after a large rain event and samples approaching equilibrium after the longest period without rainfall calcium in excess of what would be expected from conservative mixing of fresh water and seawater was observed in all groundwater samples and was elevated at near shore locations especially on bpk contrary to expectations dissolution resulting from mixing of freshwater and seawater was not supported in the shallow groundwater instead dissolution within the narrow vadose zone from rain events likely resulted in the excess calcium in the groundwater seasonal fluctuations in groundwater composition were primarily observed on the smaller island and were related to the fresh water balance changing rapidly after a heavy rain event and suggest that a size threshold has been surpassed for a stable lens rising seas will further decrease lens extent and vadose zone depth reducing the potential for future limestone dissolution

seismic structure beneath the gulf of california a contribution from group velocity measurements rayleigh wave group velocity dispersion measurements from local and regional earthquakes are used to interpret the lithospheric structure in the gulf of california region we compute group velocity maps for rayleigh waves from 10 to 150 s using earthquakes recorded by broad band stations of the network of autonomously recording seismographs in baja california and mexico mainland unm in mexico bor dpp and gor in southern california and tuc in arizona the study area is gridded in 120 longitude cells by 180 latitude cells with an equal spacing of 10 10 km assuming that each gridpoint is laterally homogeneous for each period the tomographic maps are inverted to produce a 3 d lithospheric shear wave velocity model for the region near the gulf of california rift axis we found three prominent low shear wave velocity regions which are associated with mantle upwelling near the cerro prieto volcanic field the ballenas transform fault and the east pacific rise upwelling of the mantle at lithospheric and asthenospheric depths characterizes most of the gulf this more detailed finding is new when compared to previous surface wave studies in the region a low velocity zone in northcentral baja at 28on which extends east south eastwards is interpreted as an asthenospheric window in addition we also identify a well defined high velocity zone in the upper mantle beneath central western baja california which correlates with the previously interpreted location of the stalled guadalupe and magdalena microplates we interpret locations of the fossil slab and slab window in light of the distribution of unique post subduction volcanic rocks in the gulf of california and baja california we also observe a high velocity anomaly at 50 km depth extending down to 130 km near the southwestern baja coastline and beneath baja which may represent another remnant of the farallon slab

imaging rainfall drainage within the miami oolitic limestone using high resolution time lapse ground penetrating radar 1 the vadose zone of the miami limestone is capable of draining several centimeters of rainfall within a fraction of an hour once the water enters the ground little is known about the flow paths in the oolitic rock a new rotary laser positioned ground penetrating radar gpr system enables centimeter precise and rapid acquisition of time lapse surveys in the field two dimensional 2 d gpr time lapse surveying at a 3 min interval before during and after rainfall shows how buried sand filled dissolution sinks efficiently drain the bulk of the rainwater hourly repeated 3 d imaging of a dissolution sink in response to surface infiltration shows how the wetting front propagates at a rate of 0 6 1 2 m h traversing the 5 m thick vadose zone within hours at the same time some of the water migrates laterally into the host rock guided by stratigraphic unit boundaries average lateral propagation measured over a 28 hour period was of the order of 0 1 m h on a seasonal time frame redistribution involves the entire rock volume comparing 3 d surveys acquired after wet summer and dry winter conditions shows good gpr event correspondence but also time shifts up to 20 ns caused by the change of overall water content within the vadose zone high precision time lapse gpr imaging can therefore be used to noninvasively characterize natural drainage inside the vadose zone ranging from transient loading to seasonal variation

quantitative interrogation of a fossilized carbonate sand body the pleistocene miami oolite of south florida exposures of the pleistocene miami oolite in south florida provide excellent examples of preserved primary sedimentary features and subsequent diagenetic changes of a fossilized ooid sand body that has been subaerially exposed in a tropical climate since its deposition during the last interglacial highstand marine isotope stage 5e using a robust airborne light detection and ranging digital terrain model and select outcrops a detailed analysis of the morphologies and dimensions of the different portions of the miami oolite was conducted the sand body extending 95 km north to south and approximately 15 km wide consists of shoals or bars separated by tidal channels and is partly bounded on the ocean facing side by a prograding barrier bar that collectively cover an area of approximately 1000 sq km results of the quantitative interrogation of the miami oolite are compared with those of previously published work from modern sand bodies on great bahama bank including the exumas schooners cays and tongue of the ocean digital terrain models developed for these modern sand bodies on the great bahama bank and the ancient miami oolite were used as the basis for the definition of shoals shoal crests peaks and channels which were examined with respect to their size shape and orientation the exumas sand body is the best morphometric match when analysing the bars and the channels and the joulter cays area is an analogue for the formation of the barrier bar and for burrow reworking of cross bedding within stabilized sand shoals the exumas is a particularly compelling analogue for the miami oolite with respect to length and overall visual comparison between the morphologies of the sand bodies shoal or bar shape number of tidal channels channel length and width and high areas parallel to the strike trend of the sand body islands in the exumas and possible islands as part of the miami oolite barrier bar despite ca 115 kyr of subaerial exposure and meteoric diagenesis including karst the morphology of the miami oolite is still relatively intact to the point where it can be easily analysed and meaningful comparisons to modern systems can be made the miami oolite serves as a key reference example for comparison to holocene sand units in the bahamas it validates the concept of comparative sedimentology and in particular emphasizes how results from the modern can improve the interpretation of a fossilized example

energy constrained recharge assimilation and fractional crystallization ec raχfc a visual basic computer code for calculating trace element and isotope variations of open system magmatic systems 1 volcanic and plutonic rocks provide abundant evidence for complex processes that occur in magma storage and transport systems the fingerprint of these processes which include fractional crystallization assimilation and magma recharge is captured in petrologic and geochemical characteristics of suites of cogenetic rocks quantitatively evaluating the relative contributions of each process requires integration of mass species and energy constraints applied in a self consistent way the energy constrained model energy constrained recharge assimilation and fractional crystallization ec ra fc tracks the trace element and isotopic evolution of a magmatic system melt solids undergoing simultaneous fractional crystallization recharge and assimilation mass thermal and compositional trace element and isotope output is provided for melt in the magma body cumulates enclaves and anatectic i e country rock melt theory of the ec computational method has been presented by spera and bohrson 2001 2002 2004 and applications to natural systems have been elucidated by bohrson and spera 2001 2003 and fowler et al 2004 the purpose of this contribution is to make the final version of the ec ra fc computer code available and to provide instructions for code implementation description of input and output parameters and estimates of typical values for some input parameters a brief discussion highlights measures by which the user may evaluate the quality of the output and also provides some guidelines for implementing nonlinear productivity functions the ec ra fc computer code is written in visual basic the programming language of excel the code therefore launches in excel and is compatible with both pc and mac platforms the code is available on the authors web sites http magma geol ucsb edu and http www geology cwu edu ecrafc as well as in the auxiliary material

late quaternary vadose carbonate diagenesis in coastal and desert dune and beach sands is there a palaeoclimatic signal a study of the variation in cement amounts in aeolianite and sandy beach calcarenites that range in age from holocene to last interglacial is used to assess whether a palaeoclimatic control on vadose diagenesis can be identi ed examples are taken from modern day arid to subhumid settings and represent a geographical distribution ranging from libya and oman to the mediterranean the caribbean and mexico the results indicate that a palaeoclimatic signal can be identi ed in some deposits however in other sediments there is substantial variability in mean cement abundance within deposits as well as between these sand bodies moreover many of the better cemented sediments are located in areas where there is accelerated diagenesis such as in the sea spray zone close to the groundwater table or near to a palaeosurface rather than in climatically wet regions the inference is that palaeoclimatic interpretations are substantially complicated by other factors that affect diagenetic processes and change therefore caution is needed when studying the role of climate in vadose diagenesis in the light of the effects of other intrinsic and extrinsic controls this paper does not aim to provide a de nitive comparison of sites from different climatic zones the approach taken here is a to see if there is an overall palaeoclimatic signal in the samples studied b to use examples to illustrate how explanations controls other than those relating to climate can account for the variabilities observed and c if there is a climatic effect to see if it is the main over riding control on vadose diagenesis the conclusions drawn from this research highlight that it is possible to misinterpret evidence if the study is approached with preconceived notions of simplistic relationships between diagenesis and climate copyright 2004 john wiley sons ltd

genesis and evolution of the cerro prieto volcanic complex baja california mexico the cerro prieto volcanic complex cpvc located in northwestern mexico is the only surface manifestation of the cerro prieto geothermal field the third largest producer of geothermal energy in the world this geothermal field and the salton sea in the usa sit in a pull apart basin that belongs to the trans tensional tectonic zone that includes the san andreas fault system and the salton trough basin to the nw and the east pacific rise to the se in spite of its strategic importance in the generation of geothermal energy the origin of cerro prieto and its relationship with the geothermal reservoir were unknown in this contribution we discuss the origin evolution and mechanisms of formation of this small monogenetic volcano and the magmas that fed the system the volcanic complex is located on top of the cerro prieto left lateral fault to the northwest of the cerro prieto geothermal field the complex consists of a lava cone and a series of domes 0 15 km3 protruding from tertiary sandstones and recent unconsolidated sediments of the alluvial plain of the colorado river the cerro prieto volcanic complex consists of seven stratigraphic units emplaced in a brief time span around 78 81 ka its activity began with the extrusion of a dacitic lava that came into contact with water saturated sediments causing brecciation of the lava the activity continued with the emplacement of dacitic domes and a dyke that were destroyed by a phreatic explosion emplacing a lithic rich breccia this phreatic explosion formed a 300 m wide and 40 m deep circular crater the activity then migrated 650 m to the sw where three dacitic lava domes were extruded and ended with the emplacement of a fissure fed lava flow subsequent remobilization of the rocks in the complex has generated debris and hyperconcentrated flow deposits interbedded with fluviatile sediments in the surrounding terrain all rocks of the cpvc are dacites with phenocrysts of plagioclase orthopyroxene and fe ti oxides and minor quartz gabbro dioritic dykes intruded in tertiary sandstones and siltstones of the local basement contain phenocrysts of plagioclase clinopyroxene fe ti oxides and olivine all cpvc rocks are chemically homogeneous suggesting discrete modifications by crustal assimilation fractional crystallization or magma mixing processes during upper crustal residence aeromagnetic results suggest that the depth of the magmatic source in the area is located below 3 5 km cpvc rocks contain chemical anomalies typical of subduction zones which suggests that all cpvc magmas could have been generated by partial melting of the remains of the subducted farallon plate

application of high frequency ground penetrating radar to the reconstruction of 3d sedimentary architecture in a flume model of a fluvial system abstract ground penetrating radar gpr offers a fast and efficient method for visualizing three dimensional 3d images of shallow subsurface structures we investigated the application of gpr to nondestructive imaging of deposits formed in a small experimental flume using a high frequency gpr antenna the gpr measurements made with a 1 6 ghz antenna were conducted on flume models of sandy fluvial systems we conducted two experiments one to reveal the penetration depth and resolution of the gpr antenna and to reconstruct the 3d architecture of fluvial deposits created in a flume and the other to examine the characteristics of reflection boundaries the experimental materials were mainly loamy soil the first experiment showed that a 1 6 ghz antenna was suited to laboratory use offering a depth resolution of 2 3 cm in full penetration of a bed 15 24 cm thick gpr scanning yielded an accurate reconstruction of the 3d architecture of a fluvial channel using magnetite marker beds the second experiment also successfully imaged a 3d architecture using quartz sand instead of magnetite and loamy soil which have different relative permittivities this research shows that gpr is a feasible technique for 3d analysis of experimental flume deposits

rates of vadose diagenesis in quaternary dune and shallow marine calcarenites warrnambool victoria australia abstract progressively younger cycles of quaternary dune limestones and shallow marine calcarenites infill a coastal embayment near warrnambool in southeastern australia the formations have been dated using u th and c 14 methods or have had their ages estimated by comparison with adjacent deposits they provide a well dated framework for the quantification of diagenetic rates in the fresh water vadose zone the sequence as a whole shows progressive increase in diagenetic alteration with age related to increasing duration of subaerial exposure diagenesis includes alteration of high mg calcite to low mg calcite dissolution of aragonite and precipitation of increasing amounts of void filling low mg calcite spar these changes are reflected in the progression of bulk rock isotope values from the youngest to the oldest strata the rocks at warrnambool pass through 3 main stages of diagenetic evolution the end of stage 1 is marked by the disappearance of high mg calcite this occurs in just less than 90 000 years the end of stage 2 occurs between 400 000 and 700 000 years and is marked by the disappearance of aragonite the end of stage 3 and the beginning of stage 4 is marked by the complete infilling of all pore space by low mg calcite spar this is not generally seen in this sequence but is restricted to specific diagenetic sites such as immediately below paleosol horizons

volcano structural analysis of la garrotxa volcanic field ne iberia implications for the plumbing system abstract the garrotxa volcanic field is related to the neogene quaternary european rift system and is the youngest representation of monogenetic volcanism in the iberian peninsula it encompasses over 50 eruptive vents most of them well preserved cones in an area of about 600 km2 lying between the cities of olot and girona ne spain in this paper we investigate the relationship between the neogene extensional tectonics and the spatial distribution of the volcanoes in the area the analysis includes the distribution of faults fissures and vents as well as morphostructural lineaments and the morphometrical analysis of volcanic cones and craters in addition we use the location of the regional seismicity recorded since 1978 and the sites of freshwater springs and mantle derived gases as indicators of active faults and open fractures finally we consider the location of ultramafic xenoliths within volcanic deposits as a way of identifying the deepest fractures in the zone and estimating magma ascent velocities the results obtained show that this volcanic area consists of an extensional basin delimited by two principal nw se faults that favoured the ascent of magma from either the source region or from shallower reservoirs located at the base of the crust towards the upper part of the crust magma transport was captured by shallow secondary extensional nnw sse striking faults whose formation is linked to the slight transtensional movement of the main bounding faults our study provides evidence of how the local stress field and contrasts in substrate stratigraphy could have controlled magma migration which suggests that precise knowledge of the stress configuration substrate geology and structural discontinuities in such regions is crucial in the forecasting of monogenetic volcanism

origin of the adakite high nb basalt association and its implications for postsubduction magmatism in baja california mexico discussion constraining the origin of the adakite high nb basalt hnb association in baja california mexico is critical to a better understanding of global arc magmatism currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted farallon cocos plate thus providing support for the slab melting origin of adakites elsewhere moreover a tectono magmatic model involving the production of both adakite and hnb from slab melts offers a comprehensive explanation for the origin of the atypical arc related postsubduction magmatism in baja california this paper proposes alternative models for the origin of hnb and postsubduction magmatism in baja california wherein the unusual geologic setting of western mexico and westward movement of north america permitted the influx of pacific asthenosphere beneath the adjacent gulf of california after the cessation of subduction unlike the previous tectono magmatic model the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in baja california as tholeiites and hnb respectively fractional crystallization of some of the hnb magmas plus assimilation of tholeiitic materials produced nb enriched basalts neb the influx of pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower baja california crust producing adakite rocks and the preexist ing metasomatized mantle wedge producing bajaites and calc alkaline magmas

relationship between proterozoic dykes and associated volcanic sequences evidence from the harp swarm and seal lake group labrador canada abstract geochemical studies of the harp dyke swarm and extrusive intrusive igneous rocks of the seal lake group of labrador canada show that each can be subdivided into three distinct chemical groups two of the chemical groups within the seal lake group show strong similarities to those present within the harp dykes but with more restricted and generally more primitive compositions chemical variation within individual harp dykes suggests a dominant role for phenocryst differentiation processes with little or no influence by crustal contamination however fractional crystallisation processes cannot account for the variation observed within each of the harp dyke chemical groups which instead is likely to be dominated by in situ crystallisation processes the greater range of incompatible trace element concentrations within many harp dykes compared to seal lake group igneous rocks is most likely a result of such mechanisms rather than by fractional crystallisation or crustal contamination processes although dykes that do not chemically correspond with sampled lavas may have fed flows which are now eroded it is more likely that they were non emergent hence the chemical similarities and differences between the dykes and lavas can be linked to progressive mantle melting processes this is consistent with spatial and temporal evidence that the harp and seal lake igneous rocks are not strictly coeval and hence most harp dykes did not act as feeders to seal lake group lavas a model of heterogeneous lithosphere extension with a progressively increasing pure shear component with time can satisfactorily explain the harp seal lake magma relationships

isotopic evidence for the origin of cenozoic volcanic rocks in the pinacate volcanic field northwestern mexico six volcanic rocks reconnaissance samples representing most of the temporal and compositional variation in the pinacate volcanic field of sonora and arizona are characterized for major element and nd sr isotopic compositions the samples consist of basanite through trachyte of an early shield volcano and alkali basalts and a tholeiite from later craters and cinder cones with the exception of the trachyte sample which has increased 87sr 86sr due to crustal effects all 87sr 86sr values fall between 0 70312 and 0 70342 while nd values are all between 5 0 and 5 7 clinopyroxene in a rare spinel lherzolite nodule derived from the uppermost mantle beneath the field has 87sr 86sr of 0 70320 but nd of 8 8 three nd units higher than the volcanic rocks both the volcanic rocks and the nodule record the presence of asthenospheric rather than enriched lithospheric mantle beneath pinacate this is consistent with one or both of a proximity of pinacate to the gulf of california spreading center and b presence of similar asthenospheric mantle signatures in volcanic rocks over a wide contiguous area of the southwestern usa we consider the comparison to other southwestern usa magma sources as the more relevant alternative although a definite conclusion is not possible at this stage

fossil slabs attached to unsubducted fragments of the farallon plate as the pacific farallon spreading center approached north america the farallon plate fragmented into a number of small plates some of the microplate fragments ceased subducting before the spreading center reached the trench most tectonic models have assumed that the subducting oceanic slab detached from these microplates close to the trench but recent seismic tomography studies have revealed a high velocity anomaly beneath baja california that appears to be a fossil slab still attached to the guadalupe and magdalena microplates here using surface wave tomography we establish the lateral extent of this fossil slab and show that it is correlated with the distribution of high mg andesites thought to derive from partial melting of the subducted oceanic crust we also reinterpret the high seismic velocity anomaly beneath the southern central valley of california as another fossil slab extending to a depth of 200 km or more that is attached to the former monterey microplate the existence of these fossil slabs may force a reexamination of models of the tectonic evolution of western north america over the last 30 my

estimating porosity and solid dielectric permittivity in the miami limestone using high frequency ground penetrating radar gpr measurements at the laboratory scale subsurface water flow in south florida is largely controlled by the heterogeneous nature of the karst limestone in the biscayne aquifer and its upper formation the miami limestone these heterogeneities are amplified by dissolution structures that induce changes in the aquifer s material and physical properties i e porosity and dielectric permittivity and create preferential flow paths understanding such patterns are critical for the development of realistic groundwater flow models particularly in the everglades where restoration of hydrological conditions is intended in this work we used noninvasive ground penetrating radar gpr to estimate the spatial variability in porosity and the dielectric permittivity of the solid phase of the limestone at centimeter scale resolution to evaluate the potential for field based gpr studies a laboratory setup that included high frequency gpr measurements under completely unsaturated and saturated conditions was used to estimate changes in electromagnetic wave velocity through miami limestone samples the complex refractive index model was used to derive estimates of porosity and dielectric permittivity of the solid phase of the limestone porosity estimates of the samples ranged between 45 2 and 66 0 and showed good correspondence with estimates of porosity using analytical and digital image techniques solid dielectric permittivity values ranged between 7 0 and 13 0 this study shows the ability of gpr to image the spatial variability of porosity and dielectric permittivity in the miami limestone and shows potential for expanding these results to larger scales and other karst aquifers

depositional environments and diagenesis of the kuldhar and keera dome carbonates late bathonian early callovian of western india abstract this paper describes 11 microfacies types in late bathonian early callovian carbonates of the kuldhar member of the jaisalmer formation rajasthan and the keera golden oolite member of the chari formation kachchh mainland western india the different microfacies associations reported in this study reflect an ideal shallowing upward sequence representing a system of bioclastic bars developed on the lower ramp evolving into an oolitic bar to bank system separating restricted lagoonal from lower ramp environment four main types of cements i e bladed fibrous syntaxial overgrowth and blocky cement characterized in a few cases by ferroan calcite and anhydrite ii occur in these carbonates the study also reveals that chemical compaction followed the two phases of early mechanical compaction that largely governed porosity of these limestones however micritization and neomorphism also contributed significantly in this respect diagenetic signatures in these carbonates suggest that marine phreatic and fresh water phreatic environments dominated but deep burial diagenesis also played its role in shaping these rocks the early and late diagenetic changes have been controlled by the depositional facies evolving in a basin riddled with rifting in an extensional tectonic regime forcing regional scale sea level fluctuations

geochemistry of holocene volcanic rocks associated with ridge subduction in baja california mexico although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood1 6 the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized the baja california peninsula in mexico provides an ideal scenario to study the latter process geophysical models indicate that in response to oblique collision of the ancestral east pacific rise epr with the western seaboard of north america at 29 myr subduction of the farallon plate was succeeded by the progressive southwards development of a transform fault system7 9 at 12 5 myr a considerable length of the epr was simultaneously subducted beneath baja thus terminating subduction processes along this segment10 by 3 5 myr the locus of transcurrent faulting had switched from the west of baja to within the developing gulf of california coupling baja to the pacific plate11 13 we present here data for holocene volcanic rocks from baja california norte which show that post ridge subduction volcanism has a distinctive geochemistry these results have important consequences for the interpretation of ancient orogenic belts

the evolution of the mantle s chemical structure abstract the geochemistry of flood basalts and their associated picrites and of komatiites and their associated basalts combined with a theoretical model for the structure of mantle starting plumes can be used to decipher key elements of the geochemical structure of the deep mantle and show how it has varied through time we argue that the thermal boundary layer above the core consisted mainly of depleted mantle similar to the present morb source during the archaean and this was largely replaced between 2 7 and 2 0 billion years ago by enriched mantle to form the oib source we suggest that this change in the nature of the hotspot source reflects a fundamental change in the dominant component of downward convection from cold plumes breaking away from beneath a stable lithosphere during the pre archaean to subduction of lithosphere in the archaean and post archaean mantles

subduction control on the curie isotherm around the pacific north america plate boundary in northwestern mexico gulf of california preliminary results abstract spectral analysis of namag magnetic data enabled us to estimate depths to the curie isotherm along the pacific north america plate boundary the estimated depths for the gulf of california and southern baja california peninsula correlate quite well with the depths to the moho available from previous studies based on receiver function and seismic refraction in northernmost baja california and southwestern usa no correlation is observed between depths to the curie isotherm and depths to the mantle obtained from receiver function analysis depths to the curie isotherm are shallower along the coastal plains of sonora and sinaloa differences between receiver function based mantle depths and curie depths are equally large however there is more consistency with depths established by seismic refraction studies four zones with shallow curie isotherm are identified in southern gulf of california gs1 gs2 gs3 and gs4 gs1 is located at the gulf of california mouth the alarcon rise is located between gs1 and gs2 pescaderos basin is located northwest of gs2 gs3 stretches from the carmen basin to the farallon basin gs4 extends from tiburon island up to northern guaymas basin northernmost shallow curie depth zone gn1 comprises upper and lower delfin basins as well as the consag and wagner basins a northern prolongation also covers pinacate volcanic center the southern shallow curie isotherm zones are associated with lithospheric asthenospheric material upwelling zones gn1 also includes a convective component related to the younger magmatism of the pinacate volcanic field deeper curie depths are found at southern baja california peninsula interpreted as associated with thicker continental or stretched crust it is possible that the crust is underlain or underplated by magnetic oceanic crust as suggested by satellite magnetic anomalies centered at the magdalena shelf a contrasting pattern features northern baja california deep curie zones to the west contrasting with relative shallower curie isotherm areas to the east the prolongation towards the peninsula of the extinct shirley transform fracture zone roughly separates the two domains observed along the peninsula and suggests that the curie isotherm along the peninsula is controlled by differences subduction of the guadalupe and magdalena microplates major differences pointed out between northern and southern gulf of california might indicate that subduction also played a key role at the beginning of the opening of the gulf of california however currently the curie pattern is controlled by the rifting processes shallow curie zones correlating quite well with the rift basins and the lithosphere asthenosphere upwelling material

integration of electrical resistivity imaging and ground penetrating radar to investigate solution features in the biscayne aquifer summary the limestone composing the biscayne aquifer in southeast florida is characterized by cavities and solution features that are difficult to detect and quantify accurately because of their heterogeneous spatial distribution such heterogeneities have been shown by previous studies to exert a strong influence in the direction of groundwater flow in this study we use an integrated array of geophysical methods to detect the lateral extent and distribution of solution features as indicative of anisotropy in the biscayne aquifer geophysical methods included azimuthal resistivity measurements electrical resistivity imaging eri and ground penetrating radar gpr and were constrained with direct borehole information from nearby wells the geophysical measurements suggest the presence of a zone of low electrical resistivity from eri and low electromagnetic wave velocity from gpr below the water table at depths of 4 9 m that corresponds to the depth of solution conduits seen in digital borehole images azimuthal electrical measurements at the site reported coefficients of electrical anisotropy as high as 1 36 suggesting the presence of an area of high porosity most likely comprising different types of porosity oriented in the e w direction this study shows how integrated geophysical methods can help detect the presence of areas of enhanced porosity which may influence the direction of groundwater flow in a complex anisotropic and heterogeneous karst system like the biscayne aquifer

genesis of magmas from the tres virgenes volcanic complex baja california sur mexico abstract the late pleistocene tres virgenes volcanic complex tvvc baja california sur mexico is formed by 3 volcanic edifices el viejo el azufre and la virgen although the tvvc is emplaced in a tectonic environment where subduction was terminated its volcanic products are calc alkaline in addition to its typical arc calc alkaline nature we classified the tvvc rocks as adakitic typical arc calc alkaline rocks are widely described in baja california so we focus on the adakitic rocks relatively high silica adakitic rocks 57 64 sio2wt sr y 30 49 la yb 4 8 from the tvvc are constrained between 110 90 ka and could be produced when typical arc calc alkaline magmas partially melt the lower crust alternatively these high silica adakitic rocks could be formed by mixing between typical arc calc alkaline magmas and low silica adakitic rocks low silica adakitic rocks 50 55 sio2wt sr y 50 65 la yb 3 7 are produced when the metasomatized mantle wedge is partially melted with the exception of la virgen tefra a plinian eruption where typical arc calc alkaline magmas are involved mineral composition and lack of xenocrysts suggest mixing and assimilation are not dominant processes at the tvvc upper crust reservoir in contrast mineral textures in some rocks suggest that magma reservoirs were heated we propose that the low silica adakitic magmas are the heating mixing elements modifying the reservoir plumbing system

tectonic controls on magmatism associated with continental break up an example from the parana etendeka province the high and low ti basalts of the parana etendeka province were primarily derived from old trace element enriched source regions in the lithospheric mantle and they are associated with dyke swarms of different orientations these swarms appear to reflect different amounts of extension and it is inferred that the high and low ti magma types were characterised by different melt generation rates of 0 15 km3 yr 1 and 0 4 km3 yr 1 respectively stewart et al earth planet sci lett 143 1996 95 109 there is probably a gap of 2 myr between the end of the main phase of cfb magmatism and the oldest rocks on the adjacent ocean floor a simple numerical model has been used to constrain the amounts and rates of melt generated from the continental lithosphere and asthenosphere under finite duration extension melting in the mantle is assumed to be controlled by the dry peridotite solidus in the asthenosphere and the hydrous 0 2 h2o peridotite solidus in the lithosphere for a maximum of 4 and a duration of extension of 10 myr the derivation of melt from the asthenosphere by dry peridotite melting depends primarily on potential temperature tp and is relatively insensitive to the thickness of the mbl while the converse is the case for melt derived from the lithosphere by hydrous peridotite melting for a tp of 1450 50 c inferred from the crustal thickness estimates along the rio grande rise and walvis ridge the model successfully generates 2 4 km of lithosphere derived melt before producing significant volumes of asthenosphere derived melt it is concluded that increases of melt volume with time can be generated by decompression melting of the mantle lithosphere critically in areas of significant melt generation within the mantle lithosphere during extension and break up there is likely to be a gap in the volcanic record between the end of melt generation in the lithosphere and the onset of melting in the underlying asthenosphere no such gap is present if all melts are generated within the mantle plume and thus these models may in principle be tested in the geologic record

carbonate petrography as an indicator of climate and sea level changes new data from bahamian quaternary units coeval stratigraphic units of similar petrology occur throughout the northern bahamas islands the petrographic composition of these limestones provides clues about regional sea level and climate changes during the late quaternary at least eight fossil shoreline units which are linked to transgressive episodes between the middle pleistocene and the late holocene are recognized in the bahamas the petrographic composition of these units is either dominated by ooids and peloids or by bioclasts sedimentological observations demonstrate that oolitic peloidal units were formed when sea level was higher than today whereas skeletal units were deposited at or below modern ordnance datum skeletal units may reflect times of partial or modest platform flooding when the bulk of sediments brought to islands originates from bank margin reefs in contrast oolitic peloidal units correspond to major flooding events and active water circulation on the bank top cement fabrics further show that the early diagenesis of oolitic units took place during warm and humid climatic conditions whereas skeletal rock bodies underwent subaerial diagenesis during drier climatic conditions characterized by marked seasonal changes this example from the bahamas suggests that compositional analysis of limestone from fossil carbonate platforms could be used for resolving ancient climate and sea level changes

holocene plinian eruption of la virgen volcano baja california mexico a plinian eruption occurred approximately 6500 yr ago at la virgen volcano the youngest volcano of the tres virgenes volcanic complex tvvc located in baja california mexico deposits of the eruption suggest a sequence of events that started with the opening of the volcanic conduit and development of a plinian eruption column up to 18 km in height this eruption column produced a fallout deposit with a dispersal axis toward the southwest an areal extent of about 500 km 2 and a minimum volume of 1 14 km 3 vulcanian activity hydromagmatic followed the plinian phase producing pyroclastic surge and fallout deposits the eruptive activity ceased after a basaltic andesite lava flow was emplaced closing the eruptive activity petrological and geochemical evidence indicates that the eruption was triggered by magma mixing processes our studies confirm that la virgen is a dormant volcano with the potential for future violent eruptions the present study provides important information for the construction of a volcanic hazards map significant hazards are presented to the population living within a distance of 30 km from the volcano together with the interstate road connecting the entire peninsula of baja california which runs at a distance of only 3 km from the volcano

slab tearing following ridge trench collision evidence from miocene volcanism in baja california mexico abstract neogene magmatic activity in central baja california underwent a major change at ca 12 5 ma when the pacific farallon active oceanic ridge collided with the trench east of vizcaino peninsula the calc alkaline magmatism which built the comondu volcanic arc vanished and was replaced by unusual volcanic associations which were erupted within six late miocene to quaternary volcanic fields jaraguay san borja san ignacio santa rosalia santa clara la purisima delineating a 600 km array along the baja california peninsula new fieldwork k ar datings and geochemical analyses on jaraguay and san borja lavas combined with previous data allow us to show that these associations include 1 adakites emplaced between 12 5 and 8 2 ma in the santa clara santa rosalia and jaraguay volcanic fields 2 niobium enriched basalts neb in santa clara and santa rosalia 11 2 to 7 4 ma 3 tholeiitic basalts and basaltic andesites 11 3 to 7 2 ma displaying a very weak subduction imprint in la purisima and san ignacio 4 alkali trachybasalts in mesas san carlos and santa catarina northwest of jaraguay 9 3 to 7 5 ma and finally 5 basalts and associated magnesian basaltic andesites and andesites locally referred to as bajaites the latter lavas display very specific geochemical characteristics including very high sr and ba contents they were emplaced in all the above mentioned volcanic fields between 14 6 and 5 3 ma the origin of these late miocene volcanics has been considered linked either to the opening of an asthenospheric window through which they ascended tholeiites and alkali trachybasalts or to the melting of its edges due to thermal erosion adakites and the subsequent reaction between adakitic melts and the supraslab mantle neb and bajaites however the identification of the remnants of the pacific farallon fossil ridge south of vizcaino peninsula indicates that active ridge subduction was not responsible for the opening of the asthenospheric window we propose that the emplacement of slab window related volcanic rocks between 13 and 7 ma from ca 25 n to 30 n was due to a process of slab tearing which started when the active ridge collided with the trench east of vizcaino peninsula the oldest part of the downgoing plate sunk into the mantle leading to the opening of a tear in the slab which likely started from the southern limit of the already existing southern california slab window it propagated parallel to the trench over about 600 km and developed into an asthenospheric window

preliminary geologic studies of sierra el aguajito baja california mexico a resurgent type caldera abstract geologic field studies conducted in the quaternary volcanic field of tres virgenes state of baja california sur mexico revealed the existence of a resurgent caldera the caldera s eruptive products the aguajito sequence overlie the products of the nearby reforma caldera reforma sequence whose youngest products have already been dated as quaternary the rim of the aguajito caldera is inferred by the existence of an arcuate alignment of rhyolitic domes the mean diameter of this subcircular feature is 10 km the volume of its mapped acidic products is a minimum of 10 km3 several horizons within the sequence contain shells k ar dates of the ignimbrites and domes of el aguajito formation confirm that the unit are pleistocene the detailed stratigraphy also shows the evolution of a marine regression partly related to the caldera

fabric selective diagenesis in the late pleistocene miami limestone abstract the miami limestone is a late pleistocene ooid sand body which has undergone 130 000 years of meteoric diagenesis the resultant limestone has a generally high but variable 15 60 porosity which is indirectly related to the amount of primary aragonite remaining in the limestone 0 45 the predominant textural theme in porosity development is the role of the primary sediment fabric which controls early cementation and porosity development on the scale of individual grains and sedimentary structures and is still influential during late development of channel and cavernous porosity the porosity development within the miami limestone reflects a change from an early system wherein fabric selective diagenesis occurred primarily as a function of the ability of the sediment to retain pore water to a system wherein the cemented sediments channelize water flow and effectively isolate portions of metastable aragonitic limestone from meteoric waters

the recent isla san luis volcanic centre petrology of a rift related volcanic suite in the northern gulf of california mexico abstract isla san luis one of the most recent eruptive centres in the gulf of california presents a complete magma evolution trend from basaltic andesites to rhyolites the less evolved lavas are palagonite tuffs related to surtseyan type activity which characterized the emergent stage of the island subaerial lava flows and later high energy hydromagmatic eruptions are dacites which make up the tuff rings of the southeastern corner of the island rhyolites are the latest erupted products and the development of the younger dome in the centre of the island was preceded by ash and pumice fallout containing quenched bombs with an obsidian crust and a pumiceous core basaltic andesites contain olivine fo87 80 calcium rich plagioclase and sparse clinopyroxene microphenocrysts the typical mineralogy of dacitic and rhyolitic lavas is plagioclase two pyroxenes major and trace elements vary regularly with mgo taken as a differentiation index isla san luis lavas are enriched in lile and depleted in nb ta and ti their characteristics are intermediate between tholeiitic and calc alkaline lavas they are also lree enriched and the la yb n ratios steepen slightly with differentiation such an evolution is better explained by combined fractional crystallization and assimilation processes

variability in early vadose carbonate diagenesis in sandstones abstract early carbonate diagenesis in calcareous and siliciclastic sandstones in the vadose environment is reviewed and shown to be more extensive than generally recognized and more importantly highly variable in character both spatially and temporally spatial variability is classed into three hierarchical scales macro regional meso sediment body and micro individual and groups of pores dependent and independent variables relating to the diagenesis are shown to vary according to scale at the macro scale the independent variables are tectonic activity climate sea level and lithofacies at the meso scale they include location of the sediment body relative to the watertable and sea level and exposure to sea spray plant cover water balance relationships texture and allochem content micro scale independent variables are the chemistry amounts and mobility of the pore waters and individual particle microstructures and mineralogies dependent variables at one scale are shown to exist as independent variables at the next more detailed scale the infinitely complex combinations of variables operating over different time scales explain the observed and characteristic variability in diagenetic processes and products within the vadose zone

deep mantle plume osmium isotope signature from west greenland tertiary picrites picrites from nuussuaq peninsula and qeqertarssuaq disko island west greenland preserve trace element and isotopic signatures reflecting the composition of the icelandic plume head os isotope ratios are low 187os 188os i 0 1272 0 1371 in terms of global plume related magmatism and this is coupled with anomalously high os abundances and radiogenic 143nd 144nd i isotopes crustally contaminated basalts within the west greenland sequences possess os and nd isotopic signatures consistent with mixing between initial plume head compositions and two discrete types of west greenland continental crust one crustal component is of a local sedimentary origin and the other is typical of ancient felsic crust the low os isotopic signatures of the picritic units are considered to be those of the initial mantle plume the fact that such low os isotope ratios occur in sequences with high 3he 4he and the non systematic variation in os isotopes with indices of fractionation accumulation and pb isotopes argue against mixing with depleted morb mantle or ancient subcontinental lithospheric mantle the high os concentrations in the picrites are attributed to high degrees of partial melting 25 of mantle containing no residual sulphide this is consistent with models for plume heads in which anomalous mantle temperatures initiate melting at high pressures generating large degrees of partial melting unradiogenic os and radiogenic nd components in plume related cfb magmatism may preserve contributions from a reservoir which is sampled only occasionally in young oceanic basalts such a reservoir shares os isotopic features with the primitive upper mantle pum which may also be manifest in the kea component of hawaiian magmatism therefore portions of the west greenland continental flood basalt province arguably represent the first direct sampling of unmodified plume head material derived from the lower mantle or lower portions of the upper mantle

crustal thickness variations in the margins of the gulf of california from receiver functions receiver functions rfs from teleseismic events recorded by the nars baja array were used to map crustal thickness in the continental margins of the gulf of california a newly forming ocean basin although the upper crust is known to have split apart simultaneously along the entire length of the gulf little is known about the behaviour of the lower crust in this region the rfs show clear p to s wave conversions from the moho beneath the stations the delay times between the direct p and p to s waves indicate thinner crust closer to the gulf along the entire baja california peninsula the thinner crust is associated with the eastern peninsular ranges batholith prb crustal thickness is uncorrelated with topography in the prb and the moho is not flat suggesting mantle compensation by a weaker than normal mantle based on seismological evidence the approximately w e shallowing in moho depths is significant with extremes in crustal thickness of 21 and 37 km similar results have been obtained at the northern end of the gulf by lewis et al who proposed a mechanism of lower crustal flow associated with rifting in the gulf extensional province for thinning of the crust based on the amount of pre pliocene extension possible in the continental margins if the lower crust did thin in concert with the upper crust it is possible that the crust was thinned during the early stages of rifting before the opening of the ocean basin in this case we suggest that when breakup occurred the lower crust in the margins of the gulf was still behaving ductilely alternatively the lower crust may have thinned after the gulf opened the implications of these mechanisms are discussed

fault tectonics of the baja california peninsula and the opening of the sea of cortez mexico abstract a detailed field analysis of neogene and quaternary faults in baja california has enabled us to reconstruct the stress pattern and the tectonic evolution of the central and southern parts of the peninsula the deformation which is related to the opening of the gulf affects the whole peninsula but decreases from east to west most observed faults normal and or dextral strike nnw sse to wnw ese their mechanisms include both strike slip and dip slip as well as intermediate motions compressional events have occurred since late neogene times but were probably of minor quantitative importance because reverse faults are rare and small the principal fault pattern includes dextral nnw sse riedel shears and n s tension faults induced by dextral strike slip along two main nw se fault zones bordering the peninsula the gulf of california to the east which is the most important and the tosco abreojos fault to the west the resulting pattern of deformation shows that the eastern part has been a complex transform extensional zone since late miocene early pliocene times

post subduction volcanism in the baja california peninsula mexico the effects of tectonic reconfiguration in volcanic systems abstract late cenozoic volcanism in the baja california peninsula records the effects of cessation of subduction at a previously convergent plate margin prior to 12 5 ma the predominant volcanic activity had a calc alkaline signature however after 12 5 ma during the period of tectonic transition from a convergent to a strike slip boundary the style and composition of the magmatic products changed dramatically it has been proposed that the origin of the post subduction anomalous slab melt related magmas i e adakites niobium enriched basalts magnesian andesites is linked to regional tectonic events such as the subduction of an active ridge the breakoff of the subducted slab or the tearing of the stalled slab here we compile already published data on this volcanism and critically examine the tectonic models proposed to explain its origin an alternative approach based on a physical model of volcanic systems suggests that post subduction volcanism is strongly influenced by more local aspects including the stress field and the tensile strength of the rock overlying the zones where partial melting occurred this interpretation allows us to reconcile the apparent contradictory observations of the temporal and spatial distributions of post subduction activity providing a more adequate explanation of many characteristics of the baja california post subduction volcanism

chemically diverse sporadic volcanism at seamounts offshore southern and baja california compositions of lavas from seven small to medium sized seamounts between lat 34 0n25 n are all highly differentiated trachyte and trachyandesite based on 40 ar 39 ar laser fusion techniques morb like lava from one of the northernedificesisasoldastheunderlying oceanic crust 20 ma indicating that it originatedataspreadingcenter otherseamountlavaagesaremuchyoungerthanthe oceanic crust on which they reside ranging from 16 8 6 0 3 to 7 ma for some of the northern seamounts to 270 616 ka for the trachyte from rocas alijos similar highly evolved lavas cap fossil spreading centers like guadalupe and socorro islands but rocas alijos based on magnetic anomalies is not an abandoned spreading center but may instead have formed on a leaky transform fault some of the seamounts with transitional and alkalic lavas may have formed as part of a short age progressive chain formed by a short lived mantle plume many others aligned along abandoned spreading centers orfaultsandfracturezoneswhichareabundant in the tectonically complex region offshore southern and peninsular california may have resulted from upwelling mantle diapirs in response to localized extension someoftheepisodesofvolcanismappearto havebeencontemporaneouswithvolcanism in the continental borderland and coastal southern california suggesting linkage between extension along the continental margin and the seamount province farther offshore the data available for the abundant volcanic edifices of varying sizes shapes and orientations in this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism tappingvariablyenriched heterogeneousmantle which occurred sporadically from early miocene to late pleistocene

geochemical and physical controls on vadose zone hydrology of holocene carbonate sands grand bahama island this paper explores the relationship between vadose zone hydrology and geochemical changes in mixed mineralogy carbonate sands from a bahamian coastal dune of holocene age cores were taken from two sites at site a a shallow humic entisol is developed beneath open scrub vegetation while at site b a deeper more organic rich inceptisol has formed beneath a mature hardwood coppice x ray diffraction analysis reveals significant contrasts in mineralogy both within and between the two sites with partial stabilization of high mg calcite and aragonite to low mg calcite stabilization is greater at site b and is accompanied by a significant increase in total porosity diagenetic changes in pore size distribution have implications for residence times of percolating water as determined using measurements of moisture retention characteristics using pressure plate apparatus and hydrological models of unsaturated zone moisture flux the diagenetically more mature sands from site b have a 50 100 per cent higher moisture retention although unsaturated hydraulic conductivity is also higher particularly at greater suctions the increase in water retention is likely to enhance further rates of mineral controlled reactions while development of an organic rich soil also enhances the geochemical drive for dissolution carbonate diagenesis thus appears to be strongly linked to vadose zone hydrology and the interactions identified here have important consequences for the nature and long term rates of mineral stabilization

a 13 3 ka age determination of a tholeiite pinacate volcanic field mexico and improved methods for 40ar 39ar dating of young basaltic rocks abstract among the youngest lava flows of the pinacate volcanic field sonora mexico is a large outpouring of tholeiite the ives flow this tube fed pahoehoe flow contrasts sharply with other pinacate lavas in its great volume alkali poor composition and morphologic features which include novel small structures named spatter tubes despite its k poor character young age and the presence of excess 40 ar we determined a 40 ar 39 ar age on samples of this flow at 13 3 ka such an age determination is made possible via careful monitoring of the mass discrimination of the mass spectrometer and by stacking results from multiple incremental heating experiments into a single composite isochron this age is among the youngest ever to be determined with such precision by the 40 ar 39 ar method on a k poor tholeiite

geochemical diversity of late miocene volcanism in southern baja california mexico implication of mantle and crustal sources during the opening of an asthenospheric window abstract five main petrologic and geochemical groups can be identified among the middle to late miocene lavas from the western part of southern baja california 1 calc alkaline and k rich andesites emplaced between 15 5 and 11 7 ma 2 adakites and 3 associated niobium rich basalts erupted between 11 7 and 8 5 ma in the santa clara volcanic field vizcaino peninsula 4 10 6 9 2 ma tholeiitic basalts and basaltic andesites that form large tabular plateaus near san ignacio and 5 magnesian and basaltic andesites of adakitic affinity whose emplacement started at 11 7 ma south of san ignacio and between 9 7 and 8 8 ma near la purisima these lavas although spatially and temporally related display very different geochemical signatures their trace elements and isotopic characteristics suggest that three different magma sources were involved in their genesis partial melts of subducting altered oceanic crust produced the adakites when erupted directly at the surface these magmas were eventually trap

provenance of proterozoic basal aravalli mafic volcanic rocks from rajasthan northwestern india nd isotopes evidence for enriched mantle reservoirs abstract komatiites komatiitic basalts and tholeiites from nathdwara rajasthan have been investigated for their nd isotopic characteristics 143nd 144nd vary broadly in the komatiites 0 51131 0 51252 and show a restricted range in komatiitic basalts and tholeiites 0 51120 0 51179 and 0 51141 0 51202 respectively the corresponding epsilon nd nd values indicate an enriched source for these rocks the model ages of komatiitic basalts and tholeiites cluster between 2 2 and 3 0 ga the komatiites show older model ages between 3 5 and 3 8 ga if the correlations found between nd and sm isotopic ratios can be interpreted as isochrones then the komatiites point to the age of 2300 ma and the komatiitic basalts and tholeiites constrain the age of 1800 2000 ma the age of 2300 ma for the komatiite samples is similar to the range estimated for the komatiitic basalt and tholeiite samples 1800 2000 ma considering the errors involved collectively the data suggest an age of 2300 1800 ma for the basal aravalli volcanism the variation of nd isotopic ratios between the komatiites and the associated basalts is too large to reflect analytical bias or open system behavior for sm and nd it may be due to significant mantle heterogeneity in the source region isotopically distinct sources for the komatiites and komatiitic basalts are possible but simple mixing between a depleted mantle component and a crustal component in the manner of magmatic assimilation of wall rock material is inconsistent with the observed isotopic ratios the inference of an enriched mantle component appears inevitable as the alternative model of mixing would have generated more siliceous high magnesian boninitic rocks with much lower fe and ti than are observed this enriched material may have been part of the sub continental lithospheric mantle sclm under the aravalli craton which could have generated the basal aravalli magmatic rocks we suggest that during early middle proterozoic the sclm under cratonic regions consisted of relatively fertile mantle capable of generating komatiites tholeiites and alkalic magmas

the cerro mencenares volcanic center baja california sur source and tectonic control on postsubduction magmatism within the gulf rift at the end of active subduction along the north american plate 12 5 ma in the baja california peninsula arc magmatism was replaced mainly by alkali rich high mg basaltic andesites and andesites with peculiar and unusually high k rb sr rb and lree hree ratios defined bajaite by rogers et al 1985 and baja alkalic suite by sawlan 1991 two interesting features of this magmatic change are represented by i emplacement of transitional high k calc alkaline andesites that preceded in places the baja alkalic suite magmatism and ii subduction unrelated medium k calc alkaline magmatism in baja california sur active in areas within the gulf of california rift for example tres virgenes reforma caldera isla coronado a previously unknown example of this calc alkaline activity was studied in the area of cerro mencenares located north of loreto baja california sur the mencenares volcanic complex is located along the gulf coast east of the main gulf escarpment activity of the mencenares volcanic complex occurred in three stages i an initial phase of rhyolitic dome emplacement ii andesitic to dacitic volcanism that produced the mencenares stratovolcano iii late stage emplacement of silicic domes and flows mostly dacite and rhyolite along north south trending normal faults tephra and lava flows interfinger with marine sediments of the pliocene loreto basin and the youngest volcanic products were probably emplaced during quaternary times this volcanism is coeval with the baja alkalic suite magmatism west of the gulf escarpment in the latter areas baja alkalic suite magmas ascended rapidly with only limited differentiation in the mencenares volcanic complex a complex tectonic framework favored the formation of crustal magma chambers where more complex phenomena such as extensive differentiation and mixing could take place a change from medium k calc alkaline toward baja alkalic suite like chemical characteristics is present in the younger products of the mencenares volcanic complex and mixing possibly occurred between medium k calc alkaline and baja alkalic suite magmas magma evolution in the loreto area suggests that i baja alkalic suite and medium k calc alkaline magmas can coexist in time and interact in the same volcanic field ii if a relic subduction related component is the main cause of medium k or high k calc alkaline volcanism then this component appears to decrease from east to west and iii although prior calc alkaline magmatism may not be unquestionably considered a necessary condition for the generation of baja alkalic suite magmas episodes of medium k or high k calc alkaline melt extraction from mantle sources precede or accompany baja alkalic suite magmatism in many instances

is holocene storm generated stratification in florida bay a reflection of solar storm cycles a descriptive analysis of surficial sediments of crane key florida bay showed that the sediments consist of storm layers winter storm and hurricane deposits and algal laminated sediments storm layers are riddled with the following types of cavities gas escape vugs dissolution vugs burrows rootholes and cryptalgal vugs structures and sediment types are arranged into a 15 cm thick thickening upward storm generated sequence which formed in approximately 100 yr under a deepening trend periodograms of sea level variations match the frequency distribution of strong intensity storms which occurred in south florida since the beginning of this century the calculated recurrence time of strong storms 10 3 yr and the time interval of formation of the sequence 100 25 yr are probably a response of climatic parameters to short period 11 yr and longer period 90 110 yr cycles of solar activity comparison with the ancient record shows analogous dm thick storm generated sequences probably linked to solar cycles and 102 yr sea level rises

eruptive chronology and tectonic context of the late pleistocene tres virgenes volcanic complex baja california sur mexico abstract based on a detailed geological map and stratigraphy aided by new 230th 238u and 206p 238u age determinations we established the eruptive chronology of the tres virgenes volcanic complex tvvc the complex consists of three northeast southwest aligned stratovolcanoes which from older to younger are el viejo el azufre and la virgen and associated cinder cones and domes the alignment was fed from fissures that mark the southern trace of the left lateral cimarron fault the tvvc was built upon the basement rocks of the peninsular ranges batholith 99 ma the santa lucia formation 21 6 ma the esperanza basalt 7 6 ma and the 1 1 ma ignimbrite of el aguajito caldera the tvvc began its formation at ca 300 ka mainly with dacitic lava flows and domes that formed el viejo volcano 4 2 km3 at around 173 ka activity migrated to the southwest to build el azufre volcano 3 8 km3 with the emission of dacitic lavas and several phases of dome construction and destruction with the generation of pyroclastic density currents el azufre s activity ended 128 ka with the extrusion of the summit dome at about 112 ka volcanism migrated 4 2 km to the southwest to begin the construction of la virgen volcano through the emission of andesitic and dacitic lavas that built the largest cone of the complex until 22 ka 31 2 km3 this volcano was built upon several stages of cone construction with the emission of lava flows and peripheral dacitic domes and cinder cones between 128 and 112 ka six cinder cones vented along the cimarron fault between the la virgen and el azufre volcanoes by assuming a constant emission of magma the total volume of the tvvc 39 km3 was erupted at an average rate of 0 13 km3 kyr of this volume effusive eruptions dominated the evolution of the complex 89 of which 62 are dacites and 38 andesites the tvvc rocks have compositions of 50 67 wt in silica and low medium k 0 5 2 5 wt k2o trace element signatures of the tvvc rocks sr y 2 km within the peninsular ranges batholith as confirmed by geobarometry aeromagnetic data and the drill holes of the geothermal field today the tres virgenes geothermal field generates 10 mw of electricity although future exploration of several nearby areas may increase such capacity

source and petrogenesis of paleoproterozoic meta mafic rocks intruding into the north liaohe group implications for back arc extension prior to the formation of the jiao liao ji belt north china craton abstract meta mafic rocks as sills dykes and veins are widely distributed within the north liaohe group liaodong peninsula which consist mainly of meta diabase gabbro and garnet amphibolite with minor felsic clastic rocks as enclaves although a long standing debate existed about the petrogenesis for the meta mafic rocks it is a critical judgement for using geochemistry and geochronology of these rocks to reveal the complicated tectonic evolution of the paleoproterozoic jiao liao ji belt jljb in this paper a combined study of zircon u pb isotopic data whole rock geochemistry and sm nd isotope analyses are reported for these rocks to investigate their sources petrogenesis and tectonic settings abundant zircon u pb la icp ms age data reveal that the meta mafic rocks have a magmatic emplacement age of ca 2130 ma with a metamorphic age of ca 1878 ma these meta mafic rocks have characteristic geochemical compositions with moderate sio2 46 68 53 90 wt low tio2 0 56 2 33 wt and variable total fe2o3 9 65 17 58 wt mgo 2 77 11 49 wt cr 1 44 1580 ppm and ni 6 4 168 ppm similar to evolved tholeiitic basalt they exhibit enriched mid ocean ridge basalt e morb like geochemical features e g trace element patterns with highly variable end t values from 1 5 to 3 1 relative enrichment of light rare earth elements lrees and depletion of high field strength elements hfses e g nb ta zr hf and ti the geochemical trends of major and trace elements and nd isotopes suggest that the primary magma for these meta mafic rocks were probably derived from partial melting of a depleted asthenospheric mantle in the spinel stability field with extensive fractional crystallization of a three phase assemblage of olivine clinopyroxene and plagioclase crustal assimilation e g continental detritus and limited metasomatism from subduction related fluids and or melts in view of the widespread distribution of paleoproterozoic magmatism including the liaoji granites and the voluminous sedimentary rocks in the liaodong peninsula a newly tectonic model has been proposed that a ca 2 1 ga oceanic plate subduction event induced a back arc basin opening which closed to form the jljb in the period of ca 1 9 ga

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

partial melting of slab window margins genesis of adakitic and non adakitic magmas abstract when a mid ocean spreading ridge subducts it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window we examine the fate of the slab edges using simplified slab window geometries specific thermal parameters and assumptions regarding shear stress and slab hydration six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions non adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5 65 km whereas adakitic melts form proximal to the edges at depths of 25 90 km as anatexis proceeds the subducted crust is converted to a migmatite of slab melt and eclogitic restite much of the migmatite may shear away from the slab and become incorporated into the mantle the melts will rise and leave behind fragments of restite within mantle peridotite if the peridotite is part of the overriding plate then the restite fragments may become long term residents of the continental lithospheric mantle however if the restite becomes entrained in the asthenosphere it may undergo upwelling and melting or flow away as ductile streaks to become long term mantle heterogeneities slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle