improved m 2 ocean tide from topex poseidon and geosat altimetry topex poseidon t p collinear differences have been used to extract errors in the m2 tide model of cartwright and ray 1991 the fluctuation of the power of these differences clearly shows the need for corrections at a period of about 60 days close to the aliased periods of the m2 and s2 tides the level of the necessary changes at this period is about 4 cm rms further detailed analysis of collinear differences in 4 4 bins allowing for determination of up to 6 luni solar and ocean weather tides annual and semiannual shows that the m2 corrections are well discriminated from the others and have a global power of 3 5 cm rms both in phase and out of phase components of the corrections have similar broad scale characteristics dominated by a few meridionally oriented bands the t p derived m2 model shows rms deviations of 3 1 cm with data at 80 selected ocean island and bottom gauges compared with 3 9 cm for the cartwright and ray 1991 model a weighted combination of geosat and t p derived models achieves residuals of 2 8 cm rms with this global tide gauge data

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

shallow and deep water global ocean tides from altimetry and numerical modeling we present here a 1 4 resolution near global barotropic tidal model designed specifically to provide more accurate tides in shallow water the model assimilates tides derived from 4 years of topex poseidon t p altimetric data procedures are used that tend to preserve the small spatial scales in the tidal structure in shallow water data from coastal tide gauges are also assimilated into the model the result is a tidal model that is useful in most of the shallow and deep areas of the global oceans some problems exist in regions where no data are available for assimilation such as the southern ocean pelagic tide gauge comparisons show that in deep water the new model is comparable in accuracy to the best of the existing t p based global tidal models comparisons to crossover differences in shallow water suggest improved performance we found that more accurate accounting of the load tides in the ocean tide model using an iterative technique based on the green s function formalism does not yield ocean tides that are significantly different from the simpler approaches used thus far by ocean tide modelers when data are assimilated it is our hope that this tidal model will not only help advance our understanding of shallow water tidal processes around the globe but also extend the utility of altimetry to waters shallower than 1000 m

prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

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

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

u th he zircon and archaeological ages for a late prehistoric eruption in the salton trough california usa u th and u th he zircon geochronology redefines the timing of volcanic activity in the salton trough southern california usa the subaerial extension of the incipiently oceanic gulf of california u series disequilibrium corrected u th he zircon analyses for a granophyre ejecta clast from the red island rhyolite dome indicate an eruption age of 2480 470 a calendric dates between 0 and 940 before common era bce error at 95 confidence this eruption age is supported by u th zircon crystallization ages for two obsidian bearing lavas red island the host for the granophyre and obsidian butte a prehistoric quarry for obsidian that is widely distributed in southern california and northern mexico archaeological sites lavas and granophyre display overlapping zircon crystallization age distributions that support field and compositional evidence that they are cogenetic and contemporaneous the u th he eruption age is younger and significantly more precise than previous ages for these volcanoes and is the first indication that the eruption of obsidian flows coincided with human presence in the region a late prehistoric eruption age agrees with the absence of the obsidian butte lithic source among early prehistoric cultural artifacts previously attributed to submergence of the quarry location during hypothesized persistent flooding by ancient lake cahuilla

arc rift transition volcanism in the puertecitos volcanic province northeastern baja california mexico the neogene puertecitos volcanic province of northeastern baja california records a transition from arc related volcanic activity to rift volcanism associated with opening of the gulf of california the eastern puertecitos volcanic province is divided into three volcanic sequences based on mapping petrology and 40 ar 39 ar geochronology the lowest sequence comprises early to middle miocene 20 16 ma arc related andesitic lava flows volcanic necks and proximal pyroclastic and epiclastic deposits up to 400 m in thickness with minor basaltic lava flows following the initiation of crustal extension in the region 11 6 ma synrift volcanism produced two rhyolitic sequences that discordantly overlie the arc related rocks the older synrift sequence 6 4 5 8 ma is composed of rhyolite domes and a series of pyroclastic flows up to 300 m thick the upper sequence 3 2 2 7 ma consists of ash flow tuffs and pumice lapilli pyroclastic flows collectively up to 200 m thick minor andesite eruptions followed each episode of silicic synrift volcanism synvolcanic faults produced topographic relief that controlled deposition of the pyroclastic flows and caused gentler dips upsection rhyolite domes are aligned parallel to the predominant north northwest to north northeast fault pattern all three volcanic sequences are calc alkaline however the synrift andesite is characterized by lower k 2 o lower incompatible element concentrations and less fractionation of light rare earth elements than the arc related basalt and andesite this suggests that the primary melts were more primitive for synrift andesite than for the arc related rocks

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

the heating duration and provenance age of rocks in the salton sea geothermal field southern california abstract 40ar 39ar step heating of single k feldspar crystals 500 g in a furnace with a low argon blank and accurate temperature control provide constraints on the thermal history of the salton sea geothermal field southern california estimates of 40ar loss from detrital k feldspars coupled with kinetic information gleaned from laboratory degassing of 39ar allow possible thermal histories to be evaluated samples from core and cuttings of sandstones from the state 2 14 landers 1 dearborn farms 1 magmamax 2 elmore 3 and sinclair 3 wells have been analyzed for this purpose no recent 40ar loss is recognized in any of the 17 crystals measured even for samples currently at temperatures greater than 275 c this observation suggests that the present temperature distributions in these wells have not been maintained for greater than 1000 years however aggregates 100 mg of two k feldspar samples do reveal recent 40ar loss from the state 2 14 well suggesting a time scale of 1000 to 5000 years for near peak thermal conditions these estimates of heating duration are broadly consistent with earlier results from thermal modelling but do not support interpretations that paleotemperatures were once hotter than present provenance ages span the tertiary through middle proterozoic but cluster at ages of 25 70 and 1250 ma these age peaks are very common k feldspar k ar ages in detachment terranes of southeastern california and southwestern arizona both drained by the colorado river this coincidence supports earlier suggestions that the sands of the salton trough were derived mainly from the colorado river unrealistically low apparent activation energies

intercomparison of recent ocean tide models one of the tremendous achievements of the topex poseidon t p satellite mission is the release of 12 new global ocean tide models during 1994 virtually all of these models are based on the high precision altimetry obtainable from the t p satellite and all of the models are superior to both the cartwright and ray 1990 1991 model as well as to the schwiderski 1980a b model when compared to tide gauges in the present intercomparison only global ocean tide models released during 1994 have been tested as t p continues to operate new ocean tide models will continue to be released and existing ocean models will be updated hence this intercomparison can only present a snapshot of the state of the art within ocean tide modeling however the intercomparison demonstrates the high accuracy and consistency of all these recent ocean tide models from a comparison with a common 104 tide gauge data set compiled by le provost and others unpublished data the general result is that six of the new ocean tide models have rms agreement better than 3 cm the obtainable accuracy with different tide gauge types raises the question about the accuracy of ground truth as ocean tide models become increasingly more accurate the accuracy of the harmonic constants derived from the tide gauge recordings becomes significant generally the rms agreement between models based on 2 years of t p altimetry is significantly better than the agreement between models based on 1 year of t p altimetry for models based on two years of t p altimetry the rms agreement has values around 2 0 1 4 1 0 and 0 7 cm for the m2 s2 k1 and o1 constituents respectively as a preliminary evaluation these values might be taken as a measure of the consistency of ocean tides estimated from the first part of the t p mission which is extremely promising for future ocean tide models derived from t p altimetry using 3 or more years of data this also suggests that further improvement using 3 or more years of data will only be marginal the differences arise when studying the ocean tides on continental shelves surrounding the ocean major differences are seen especially on the patagonian shelf as the continental shelves are critical for tidal dissipation it will be very important to improve tide models on the shelves in future global ocean tide models

nature and significance of igneous rocks cored in the state 2 14 research borehole salton sea scientific drilling project california the state 2 14 research borehole of the salton sea scientific drilling project penetrated 3 22 km of pleistocene to recent sedimentary rocks in the salton sea geothermal system located in the salton trough of southern california and northern baja california mexico in addition three intervals of igneous rocks were recovered a silicic tuff and two sills of altered diabase the chemical composition of the silicic tuff at 1704 m depth suggests that it is correlative with the durmid hill tuff cropping out 25 km nw of the geothermal system in turn both of these tuffs may be deposits of the bishop tuff erupted from the long valley caldera of central california at 0 7 ma the diabases are similar to basaltic xenoliths found in the nearby salton buttes rhyolite domes these diabase are interpreted as hypabyssal intrusions resulting from magmatism due to rifting of the salton trough as part of the east pacific rise gulf of california transtensional system the sills apparently intruded an already developed geo thermal system and were in turn altered by it

borehole gravity measurements in the salton sea scientific drilling project well state 2 14 borehole gravity measurements over a depth range from 1737 to 1027 m and the vertical gradient of gravity above ground were measured at the salton sea scientific drilling program well state 2 14 uncorrected borehole gravimetric densities match values from gamma gamma logs indicating that the high densities seen in state 2 14 in the depth range 0 5 to 3 km extend for a few kilometers from the well the above ground gradient was found to be 4 1 micro gal m higher than expected correcting for this value increases the gravimetric density in the borehole combining the borehole gravity and estimated vertical gravity gradients on the surface they find that this densified zone coincides with much of a broad thermal anomaly that has been found to the northeast of the salton sea geothermal field

accuracy assessment of recent ocean tide models over 20 global ocean tide models have been developed since 1994 primarily as a consequence of analysis of the precise altimetric measurements from topex poseidon and as a result of parallel developments in numerical tidal modeling and data assimilation this paper provides an accuracy assessment of 10 such tide models and discusses their benefits in many fields including geodesy oceanography and geophysics a variety of tests indicate that all these tide models agree within 2 3 cm in the deep ocean and they represent a significant improvement over the classical schwiderski 1980 model by approximately 5 cm rms as a result two tide models were selected for the reprocessing of topex poseidon geophysical data records in late 1995 current ocean tide models allow an improved observation of deep ocean surface dynamic topography using satellite altimetry other significant contributions include theft applications in an improved orbit computation for topex poseidon and other geodetic satellites to yield accurate predictions of earth rotation excitations and improved estimates of ocean loading corrections for geodetic observatories and to allow better separation of astronomical tides from phenomena with meteorological and geophysical origins the largest differences between these tide models occur in shallow waters indicating that the current models are still problematic in these areas future improvement of global tide models is anticipated with additional high quality altimeter data and with advances in numerical techniques to assimilate data into high resolution hydrodynamic models

crustal and upper mantle velocity structure of the salton trough southeast california this paper presents data and modelling results from a crustal and upper mantle wide angle seismic transect across the salton trough region in southeast california the salton trough is a unique part of the basin and range province where mid ocean ridge transform spreading in the gulf of california has evolved northward into the continent in 1992 the u s geological survey usgs conducted the final leg of the pacific to arizona crustal experiment pace two perpendicular models of the crust and upper mantle were fit to wide angle reflection and refraction travel times seismic amplitudes and bouguer gravity anomalies the first profile crossed the salton trough from the southwest to the northeast and the second was a strike line that paralleled the salton sea along its western edge we found thin crust 21 22 km thick beneath the axis of the salton trough imperial valley and locally thicker crust 27 km beneath the chocolate mountains to the northeast we modelled a slight thinning of the crust further to the northeast beneath the colorado river 24 km and subsequent thickening beneath the metamorphic core complex belt northeast of the colorado river there is a deep apparently young basin 5 6 km unmetamorphosed sediments beneath the imperial valley and a shallower 2 3 km basin beneath the colorado river a regional 6 9 km s layer between 15 km depth and the moho underlies the salton trough as well as the chocolate mountains where it pinches out at the moho this lower crustal layer is spatially associated with a low velocity 7 6 7 7 km s upper mantle we found that our crustal model is locally compatible with the previously suggested notion that the crust of the salton trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust however we observe an apparently magmatically emplaced lower crust to the northeast outside of the salton trough and propose that this layer in part predates salton trough rifting it may also in part result from migration of magmatic spreading centers associated with the southern san andreas fault system these spreading centers may have existed east of their current loca tions in the past and may have influenced the lower crust and upper mantle to the east of the current salton trough

volcanic eruption eye witnessed and recorded by prehistoric humans abstract human footprints in hydrovolcanic ash near cakallar volcano kula western turkey were discovered in 1968 a nearby pictograph interpreted as depicting cakallar volcano would define it as the oldest site where humans demonstrably eye witnessed a volcanic eruption and possibly artistically recorded it despite cakallar s volcanological and cultural importance its eruption age has remained controversial here two independent dating methods cosmogenic 36cl and combined u pb and u th he zircon zdd geochronology yielded the first internally consistent eruption ages controlled by detailed volcanostratigraphic mapping concordant 36cl ages of 4 7 0 6 ka errors 1 were obtained for a cone breaching lava flow zdd ages for crustal xenoliths from scoria deposits directly overlying the footprints yielded an age of 4 7 0 7 ka this firmly places the cakallar eruption and prehistoric human footprints and plausibly the rock art into the bronze age reinforcing the notion that prehistoric artwork recorded natural events

subsurface geometry of the san andreas fault in southern california results from the salton seismic imaging project ssip and strong ground motion expectations the san andreas fault saf is one of the most studied strike slip faults in the world yet its subsurface geometry is still uncertain in most locations the salton seismic imaging project ssip was undertaken to image the structure surrounding the saf and also its subsurface geometry we present ssip studies at two locations in the coachella valley of the northern salton trough on our line 4 a fault crossing profile just north of the salton sea sedimentary basin depth reaches 4 km southwest of the saf on our line 6 a fault crossing profile at the north end of the coachella valley sedimentary basin depth is 2 3 km and centered on the central most active trace of the saf subsurface geometry of the saf and nearby faults along these two lines is determined using a new method of seismic reflection imaging combined with potential field studies and earthquakes below a 6 9 km depth range the saf dips 50 60 ne and above this depth range it dips more steeply nearby faults are also imaged in the upper 10 km many of which dip steeply and project to mapped surface fault traces these secondary faults may join the saf at depths below about 10 km to form a flower like structure in appendix d we show that rupture on a northeast dipping saf using a single plane that approximates the two dips seen in our study produces shaking that differs from shaking calculated for the great california shakeout for which the southern saf was modeled as vertical in most places shorter period t 1 s shaking is increased locally by up to a factor of 2 on the hanging wall and is decreased locally by up to a factor of 2 on the footwall compared to shaking calculated for a vertical fault

probing the accumulation history of the voluminous toba magma the age and compositional zonation in crystals from the youngest toba tuff record the prelude to earth9s largest quaternary eruption we used allanite crystals to date and decipher this zoning and found that the crystals retain a record of at least 150 000 years of magma storage and evolution the dominant subvolcanic magma was relatively homogeneous and thermally stagnant for 110 000 years in the 35 000 years before eruption the diversity of melts increased substantially as the system grew in size before erupting 75 000 years ago

the effects of secular disequilibrium on u th he systematics and dating of quaternary volcanic zircon and apatite the u th he dating method applied to u rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions however in phases with crystallization ages less than 1 ma chemical fractionation within the 238 u decay series may modify the he ingrowth rate causing he ages computed from the secular equilibrium age equation to be incorrect the resulting systematic error depends on the 230 th 238 u activity ratio of the dated phase when it is erupted and on the eruption age zircons which exclude th relative to u will likely have secular equilibrium he ages that underestimate the eruption age by up to a few tens of decreasing with increasing eruption age apatites tend to accommodate u and th with little fractionation so apatite secular equilibrium he ages will be nearly concordant with eruption age if minerals are erupted immediately after crystallization the disequilibrium effect can be reasonably accounted for based on th u systematics however crystals are likely to reside for unknown but potentially long periods in a magma chamber such that the degree of secular disequilibrium will be reduced prior to the onset of he accumulation u th he analyses of co genetic phases that fractionate the u th ratio differently like apatite and zircon can be used to better constrain eruption age as well as to provide insights into magma chamber residence time we illustrate this approach with u th he analyses of zircons and apatites of the pleistocene age rangitawa tephra new zealand

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

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

a bottom driven mechanism for distributed faulting in the gulf of california rift abstract observations of active faulting in the continent ocean transition of the northern gulf of california show multiple oblique slip faults distributed in a 200 70 km 2 area developed some time after a westward relocation of the plate boundary at 2 ma in contrast north and south of this broad pull apart structure major transform faults accommodate pacific north america plate motion here we propose that the mechanism for distributed brittle deformation results from the boundary conditions present in the northern gulf where basal shear is distributed between the cerro prieto strike slip fault southernmost fault of the san andreas fault system and the ballenas transform fault we hypothesize that in oblique extensional settings whether deformation is partitioned in a few dip slip and strike slip faults or in numerous oblique slip faults may depend on 1 bottom driven distributed extension and shear deformation of the lower crust or upper mantle and 2 the rift obliquity to test this idea we explore the effects of bottom driven shear on the deformation of a brittle elastic plastic layer with the help of pseudo three dimensional numerical models that include side forces strain localization results when the basal shear abruptly increases in a step function manner while oblique slip on numerous faults dominates when basal shear is distributed we further explore how the style of faulting varies with obliquity and demonstrate that the style of delocalized faulting observed in the northern gulf of california is reproduced in models with an obliquity of 0 7 and distributed basal shear boundary conditions consistent with the interpreted obliquity and boundary conditions of the study area

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

high precision and high resolution carbonate 230th dating by mc icp ms with sem protocols abstract to facilitate the measurement of u th isotopic compositions suitable for high precision and high resolution 230th dating of coral and speleothem carbonates secondary electron multiplier sem protocol techniques for multicollector inductively coupled plasma mass spectrometry mc icp ms have been developed the instrumental sensitivities are 1 2 with a precision of 1 2 2 for abundance determination of 50 200 fg 234u 1 4 ng 238u or 230th this method features chemistry refinements improvements to procedural and instrumental blanks spectral inference reductions and careful consideration of non linear sem behavior measurement consistency of this mc icp ms combined with previous mass spectrometric results on u th standards and a variety of carbonates demonstrates the validity of the sem protocol method for fossil corals a routine u th isotopic determination at permil level precision requires only 10 50 mg of carbonate as little as 200 mg of young coral with an age of less than 20 yr can be dated with a precision of 0 3 0 8 yr about 20 200 mg speleothem samples with sub ppm to ppm u are required to earn a 5 precision on ages from 5 to 100 kyr requirement of small sample size 10 100s mg carbonate can permit high temporal resolution to date speleothems with slow growth rates i e 1 10 mm kyr this high precision 230th chronology is critical to accurately establish age models date events and splice geochemical proxy time series records from multiple samples in the fields of paleoclimatology and paleoceanography the u th isotopic determination techniques described here can also be applied to different environmental samples such as waters rocks and sediments

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

stratigraphy and geochronology of the comondu group near loreto baja california sur mexico abstract upper oligocene to middle miocene volcanic and sedimentary rocks in the loreto region baja california sur are widely exposed and make up the comondu group as redefined here following mcfall the comondu group is part of a volcanic arc and forearc basin that formed along the northwestern margin of mexico regional to detailed scale mapping stratigraphic analysis and geochronology in a 10 20 km wide and 70 km long belt from the gulf escarpment to the coast near loreto reveal three main units in the comondu group and a composite thickness of 1 5 2 km 1 the lower clastic unit 30 19 ma contains 200 300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt flows that accumulated in a forearc basin 2 the middle breccia and lava flow unit 19 15 ma is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris flows interbedded with minor andesite lava flows 3 the upper lava flow and breccia unit 15 12 ma consists almost entirely of up to 600 meters of andesite lava flows a few kilometers west of loreto that are part of a composite volcano north and south of loreto for up to 25 km the unit is composed of andesite lava flows and massive andesite breccia that formed from thick debris flows near that and other volcanic centers the three units of the comondu group in the loreto area correlated with the salto pelones and ricason formations of the bahia concepcion area the lower clastic unit of this study is time equivalent to the marine san gregorio el cien and isidro formations on the western and southern baja california peninsula the middle and upper units of the loreto area are the proximal equivalent to the type section of the comondu group near the village of comondu during deposition of the lower part of the comondu group the arc lay to the east on mainland mexico in the sierra madre occidental and then migrated to the position of the gulf of california at about 25 ma at 19 ma the proximal part of the arc migrated abruptly 50 km westward to approximately the position of carmen island and at 15 ma the arc once again stepped west forming eruptive centers along the coast near loreto until 12 ma also at 15 ma the proximal forearc region changed from aggradational to neutral or erosional such that no rocks of the upper unit are preserved in the gulf escarpment

alteration and remelting of nascent oceanic crust during continental rupture evidence from zircon geochemistry of rhyolites and xenoliths from the salton trough california abstract rhyolite lavas and xenoliths from the salton sea geothermal field southern california provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust salton buttes rhyolite lavas contain xenoliths that include granophyres fine grained altered rhyolites felsite and amphibole bearing basalts zircon is present in lavas and xenoliths surprisingly even in the basaltic xenoliths where it occurs in plagioclase rich regions interpreted as pockets of crystallized partial melt zircons in the xenoliths are exclusively late pleistocene holocene in age and lack evidence for inheritance u th isochron ages are 20 5 1 2 1 2 ka granophyres 18 3 3 5 3 6 ka felsite 30 1 12 4 14 1 ka and 9 2 6 6 7 0 ka basalts all errors 1 the dominant zircon population in the rhyolite lavas yielded u th ages between 18 and 10 ka with few pre quaternary xenocrysts present 18ozircon values are lower than typical crustal basement values thus ruling out rhyolite genesis by melting of continental crust moreover 18ozircon values are 0 5 1 0 lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid ocean ridge basalt suggesting that basaltic crust and silicic plutons in the subsurface of the salton sea geothermal field isotopically exchanged with meteoric waters this is evidence for deep reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma

continental rupture and the creation of new crust in the salton trough rift southern california and northern mexico results from the salton seismic imaging project a refraction and wide angle reflection seismic profile along the axis of the salton trough california and mexico was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting from the northern salton sea to the southern imperial valley the crust is 17 18 km thick and approximately one dimensional the transition at depth from colorado river sediment to underlying crystalline rock is gradual and is not a depositional surface the crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow deeper felsic crystalline rock could be stretched preexisting crust or higher grade metamorphosed sediment the lower crust below 12 km depth is interpreted to be gabbro emplaced by rift related magmatic intrusion by underplating low upper mantle velocity indicates high temperature and partial melting under the coachella valley sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement mafic rock does not exist at 12 18 km depth as it does to the south and a weak reflection suggests moho at 28 km depth structure in adjacent mexico has slower midcrustal velocity and rocks with mantle velocity must be much deeper than in the imperial valley slower velocity and thicker crust in the coachella and mexicali valleys define the rift zone between them to be 100 km wide in the direction of plate motion north american lithosphere in the central salton trough has been rifted apart and is being replaced by new crust created by magmatism sedimentation and metamorphism

waning miocene subduction and arc volcanism in baja california the san luis gonzaga volcanic field abstract subduction of the guadalupe magdalena microplate beneath baja california ended in the middle miocene and the last volcanic events in the frontal arc extinguished along the present day eastern margin of the baja california peninsula the san luis gonzaga area in the north central gulf coast contains one of the younger arc related volcanic centers in northern baja california the volcanic succession contains three sequences the basal sequence group 1 is composed of stratified pyroclastic deposits up to 500 m thick and subordinate lava flows the near vent facies crop out in tilted fault blocks along the present shoreline whereas the distal facies are exposed across 12 km toward the west and includes epiclastic deposits and at least three ash flow tuffs this sequence is internally concordant and overlies smooth paleosurface developed on granitic basement and pinches out across the gulf escarpment the potrero andesite group 2 is a series of dacite to basaltic andesite lava flows from a shield volcano located 15 km west of today s coastline similar source vents also occurs further south of the san luis gonzaga area a sequence of dacite domes group 3 intrudes the near vent facies of group 1 and contains subordinate volcanic breccia and minor lava flows that overlie group 1 sequence cross cutting relationships and the abundance of volcanic breccia associated with the domes suggest that these domes were emplaced as semi rigid intrusions spines with low explosive activity the san luis gonzaga volcanic suite ranges in composition from basaltic andesite to dacite with predominant plagioclase and pyroxene and variable amounts of hornblende trace element patterns indicate calc alkaline to mildly alkaline magmas with high ba and low nb contents incompatible element ratios and mineralogical characteristics suggest different magma batches and or different amount of crustal assimilation for the three sequences that produced contrasting eruptive styles a pink dacitic tuff that lies in the upper portion of group 1 yielded an 40 ar 39 ar age of 17 2 0 3 ma the most precise of three ages from the andesitic sheet flows of group 2 is 15 4 0 5 ma the dacite domes of group 3 represent the end of miocene arc activity but they are strongly altered and no reliable ages were obtained these data fall within the range of isotopic ages reported for arc lavas 21 to 16 ma from the discontinuous series of isolated volcanic fields in northern baja california this contrasts with the continuous and more voluminous arc volcanism in southern baja california that started earlier 25 ma and ended later 11 ma these space time patterns of arc volcanism reflect the geometry of the subducted farallon slab and the history of its breakup as the rivera triple junction migrated to the south

thermal and petrologic constraints on lower crustal melt accumulation under the salton sea geothermal field abstract in the salton sea region of southern california usa concurrent magmatism extension subsidence and sedimentation over the past 0 5 to 1 0 ma have led to the creation of the salton sea geothermal field ssgf the second largest and hottest geothermal system in the continental united states and the small volume rhyolite eruptions that created the salton buttes in this study we determine the flux of mantle derived basaltic magma that would be required to produce the elevated average heat flow and sustain the magmatic roots of rhyolite volcanism observed at the surface of the salton sea region we use a 2d thermal model to show that a lower crustal partially molten mush containing 20 40 interstitial melt develops over a 10 5 yr timescale for basalt fluxes of 0 008 to 0 010 m 3 m 2 yr 0 0008 to 0 001 km 3 yr injection rate given extension rates at or below the current value of 0 01 m yr brothers et al 2009 these regions of partial melt are a natural consequence of a thermal regime that scales with average surface heat flow in the salton trough and are consistent with seismic observations our results indicate limited melting and assimilation of pre existing rocks in the lower crust instead we find that basalt fractionation in the lower crust produces derivative melts of andesitic to dacitic composition such melts are then expected to ascend and accumulate in the upper crust where they further evolve to give rise to small volume rhyolite eruptions salton buttes and fuel local spikes in surface heat flux as currently seen in the ssgf such upper crustal magma evolution with limited assimilation of hydrothermally altered material is required to explain the slight decrease in 18 o values of zircons and melts that have been measured in these rhyolites

thick deltaic sedimentation and detachment faulting delay the onset of continental rupture in the northern gulf of california analysis of seismic reflection profiles 1 the transition from distributed continental extension to the rupture of continental lithosphere is imaged in the northern gulf of california across the obliquely conjugate tiburon upper delfin basin segment structural mapping on a 5 20 km grid of seismic reflection lines of petroleos mexicanos demonstrates that 1000 extension is accommodated on a series of nne striking listric normal faults that merge at depth into a detachment fault the detachment juxtaposes a late neogene marine sequence over thinned continental crust and contains an intrabasinal divide due to footwall uplift two northwest striking dextral oblique faults bound both ends of the detachment and shear the continental crust parallel to the tectonic transport a regional unconformity in the upper 0 5 s two way travel time and crest erosion of rollover anticlines above the detachment indicates inversion and footwall uplift during the lithospheric rupture in the upper delfin and lower delfin basins the maximum length of new crust in both delfin basins is less than 40 km based on the lack of an acoustic basement and the absence of a lower sedimentary sequence beneath a wedge shaped upper sequence that reaches 5 km in thickness a fundamental difference exists between the tiburon delfin segment and the guaymas segment to the south in terms of presence of low angle normal faults and amount of new oceanic lithosphere which we attribute to thermal insulation diffuse upper plate extension and slip on low angle normal faults engendered by a thick sedimentary lid

active ore formation at a brine interface in metamorphosed deltaic lacustrine sediments the salton sea geothermal system california the salton sea geothermal system is an area of active hydrothermal metamorphism of pliocene to pleistocene deltaic lacustrine and evaporitic sediments deposited in a modern continental rift zone temperatures up to 365 degrees c hypersaline metalliferous brines with up to 26 wt percent total dissolved solids and active base metal ore formation accompany low pressure greenschist and amphibolite facies metamorphism at depths less than 3 2 km drill cores recovered by the salton sea scientific drilling project shed new light on the nature and origin of ore formation in the eastern portion of the salton sea geothermal system base metal ore mineralization occurs in vertical fractures that comprise the major form of fluid permeability at 1 to 3 km in the modern geothermal system type 1 veins are dominated by carbonate contain pyrrhotite and other sulfides are completely sealed and occur in wall rock showing minimal alteration fluid inclusion data indicate that they were deposited under a thermal gradient much higher than that now found in the eastern part of the system type 2 veins are dominated by silicates and hematite are permeable and occur in wall rocks showing pervasive chloritization and epidotization fluid inclusion data indicate that they are forming now from the geothermal brines under the modern thermal gradient fluid inclusions record an apparent steep progressive salinity increase in the vein systems in the upper 2 km of the system however fluid production data from salton sea geothermal wells indicate that reduced metalliferous hypersaline brine averaging 23 wt percent total dissolved solids is overlain by more oxidized metal poor fluid averaging 5 wt percent total dissolved solids a sharp interface exists between the fluids at 1 to 2 km paralleling the 250 degrees c isothermal surface a model is developed for type 2 vein ore formation involving mixing at the interface between the two fluids magmatic intrusion induces fracturing heating and the expulsion of reduced connate fluids to cause early type 1 vein mineralization following the rising thermal front deep hypersaline brine rises diapirically and mixes with overlying fluids in vertical fractures transecting the ascending interface forming type 2 veins the hypersaline brine rises to a level of density temperature stabilization causing pervasive greenschist metamorphism of the host sediments the diapir then begins to cool and descend shallow oxidized brine is drawn in as the hypersaline brine descends promoting late stage hematite in type 2 veins the eastern salton sea geothermal system presently appears to be in this latter mode of cooling and retrograde brine descent ore formation in the system is caused by the coincidence of transaxial entry of a major river into an active rift zone deposition of metal bearing deltaic sediments to form a closed basin sedimentary environment episodic lacustrine evaporite formation and injection of heat and elements by rift related magmatic intrusions at depth the presence of an upwelling brine diapir in the system may be analogous to conditions immediately prior to the exhalation of metalliferous fluids that formed stratiform fe zn pb cu sulfide and oxide deposits in paleorift zone environments

does slab window opening cause uplift of the overriding plate a case study from the gulf of california abstract complete subduction of an oceanic plate results in slab window opening a key uncertainty in this process is whether the higher heat flux and asthenospheric upwelling conventionally associated with slab window opening generate a detectable topographic signature in the overriding plate we focus on the baja california peninsula which incorporates the western margin of the gulf of california rift the topography and tectonics of the rift flank along the peninsula are strongly bimodal north of the puertecitos accommodation zone the primary drainage divide attains a mean elevation of ca 1600 m above sea level asl above an asthenospheric slab window opened by pacific farallon spreading ridge subduction along this section of the trench at ca 17 15 ma to the south mean topography decreases abruptly to ca 800 m asl excluding the structurally distinct los cabos block at the southern tip of the peninsula above fragments of the oceanic farallon slab which stalled following slab tear off at ca 15 14 ma along the peninsula a low relief surface established atop miocene subduction related volcaniclastic units has been incised by a west draining canyon network in response to uplift these canyons exhibit cut and fill relationships with widespread post subduction lavas here we utilise landsat and digital elevation model dem data integrated with previously published k ar and 40ar 39ar lava crystallisation ages to constrain the onset of rift flank uplift to ca 9 5 ma later than slab window formation in the north and ca 11 10 ma later in the south these greatly exceed response time estimates of ca 2 ma or less for uplift triggered by slab window opening instead uplift timing of the high elevation northern region is consistent with lower lithospheric erosion driven by rift related convective upwelling to the south stalled slab fragments likely inhibited convective return flow preventing lithospheric erosion and limiting uplift to the isostatic response to crustal unloading during rifting

errors in recent ocean tide models possible origin and cause abstract recently the topex poseidon science working team has recommended the fes95 2 1 and csr3 0 ocean tide models for reprocessing the topex poseidon geophysical data records without doubt the performance of these models especially in the deep oceans is excellent however from a comparison of these hydrodynamically consistent models with the purely empirical dw3 2 and deos96 1 models it appears that fes95 2 1 and csr3 0 are affected by basin boundary related errors which are caused by the basin wise solution procedure of the fes ocean tide model series in their turn the empirical dw3 2 and deos96 1 models seem to suffer from significant errors in the antarctic seas due to the seasonal growth and decay of antarctic sea ice also bathymetry induced differences were found between the hydrodynamically consistent models and the empirical models concerning these differences topex poseidon and ers 1 crossover statistics unfortunately do not provide conclusive results on which models are in error

a preliminary tidal analysis of topex poseidon altimetry approximately 12 months of data from the topex poseidon satellite altimeter mission are analyzed for the major short period oceanic tides a harmonic analysis is performed on data captured within bins defined on a deep ocean grid which owing to tidal aliasing considerations must have a relatively coarse spatial resolution our analysis is in terms of corrections to the schwiderski and cartwright ray models and it confirms many of the schwiderski differences previously reported by cartwright and ray our differences with respect to the geosat based cartwright and ray model form a sectorial pattern in m2 with high low differences separated roughly 180 in longitude we suggest that these sectorial errors were most likely induced by geosat s relatively large orbit error comparisons to independent data validate the improved topex poseidon solutions in situ ground truth shows m2 rms differences of 4 10cm schwiderski 3 86cm cartwright and ray 2 63cm this paper global rates of energy dissipation confirm earlier estimates for m2 and show improved agreement with satellite tracking studies for k1 and s2 these preliminary exercises confirm that topex poseidon should result in a new generation of improved global tidal models

global ocean tides from ers 1 and topex poseidon altimetry ocean tide models representing all major diurnal and semidiurnal tidal constituents with a spatial resolution of 0 75 0 75 have been estimated using the first 1 5 years of ers 1 and topex poseidon altimetry the ocean tide model was derived from the combined use of ers 1 and topex poseidon data by using a modified orthotide formulation that simultaneously solves for all diurnal and semidiurnal constituents as well as the annual signal an additional adjustment of the solar semidiurnal harmonic of the gravitational potential was applied in order to account for radiational forcing particularly in the s2 constituent topex poseidon provides excellent ocean tide estimates in the open ocean however especially in coastal regions the track spacing of topex poseidon 315 km at the equator is too coarse to determine large parts of the ocean tide signal for these regions the inclusion of data from the ers 1 35 day repeat mission provides a valuable supplement as the ers 1 satellite has a track spacing which is around 3 6 times better than that of the topex poseidon satellite the combined ers 1 and topex poseidon ocean tide solution exhibits distinct sectoral geographical pattern of highs and lows when compared with the cartwright and ray 1990 1991 ocean tide model this indicates the presence of small but fundamental orbit errors present in the cartwright and ray ocean tide solution compared with a new set of 104 tide gauge readings compiled by le provost the rms differences of the combined ers 1 and topex solution are 2 51 1 67 1 58 and 1 13 cm for the m2 s2 k1 and o1 constituents respectively the increased spatial resolution of the combined ers 1 and topex model as compared to a topex alone model is seen to reduce rms differences from 37 to 22 cm for the m2 constituent when compared to a selection of 90 pelagic and coastal tide gauges in the northwest european shelf region

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

uniformly mantle like δ18o in zircons from oceanic plagiogranites and gabbros lower ocean crust is primarily gabbroic although 1 2 felsic igneous rocks that are referred to collectively as plagiogranites occur locally recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater derived fluids and thus they may indicate early high temperature hydrothermal fluid circulation to explore seawater rock interaction prior to and during the genesis of plagiogranite and other late stage magmas oxygen isotope ratios preserved in igneous zircon have been measured by ion microprobe a total of 197 zircons from 43 plagiogranite evolved gabbro and hydrothermally altered fault rock samples have been analyzed samples originate primarily from drill core acquired during ocean drilling program and integrated ocean drilling program operations near the mid atlantic and southwest indian ridges with the exception of rare distinctively luminescent rims all zircons from ocean crust record remarkably uniform 18o with an average value of 5 2 0 5 2sd the average 18o zrc would be in magmatic equilibrium with unaltered morb 18o wr 5 6 5 7 and is consistent with the previously determined value for equilibrium with the mantle the narrow range of measured 18o values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts if plagiogranite forms by hydrous partial melting the uniform mantle like 18o zrc requires melting and zircon crystallization prior to significant amounts of water rock interactions that alter the protolith 18o zircons from ocean crust have been proposed as a tectonic analog for 3 9 ga detrital zircons from the earliest hadean earth by multiple workers however zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments many of the 3 9 ga zircons have mildly elevated 18o 6 0 7 5 but such values have not been identified in any zircons from the large sample suite examined here the difference in 18o in combination with newly acquired lithium concentrations and published trace element data clearly shows that the 3 9 ga detrital zircons did not originate by processes analogous to those in modern mid ocean ridge settings

petrology of alarcon rise lavas gulf of california nascent intracontinental ocean crust 1 the alarcon basin in the southern gulf of california originated by intracontinental rifting of the southwestern margin of north america and has evolved into an intercontinental ocean basin by 3 7 m y of seafloor spreading lava samples collected on the axial ridge along alarcon rise two of the many near axis seamounts on the northwest flank of the rise and an abyssal hill on 0 6 ma crust on the southeast flank were examined petrographically and analyzed for major and trace element and he sr nd pb isotope composition most samples are typical mid ocean ridge basalt morb but rocks at the axial ridge site closest to continent are more differentiated rocks from the axial ridge and abyssal hill are genetically related through separate episodes of fractional crystallization seamount samples are relatively more primitive than seafloor samples and may have been formed by smaller degree of melting of the same mantle source as the seafloor lavas tamayo transform fault has no apparent effect on the geochemistry of axial ridge lavas erupted close to it this is contrary to the postulated transform fault effect alarcon basin differs from the salton trough and guaymas basin in that its youngest seafloor shows no evidence for contamination with continental crust or a subduction component we propose that pacific morb source mantle underlies the gulf of california because the western north american margin has overridden the pacific suboceanic mantle miocene rifting and pliocene spreading in alarcon basin have created a window allowing the most recent morb melts to rise uncontaminated to the surface

empirical ocean tide models estimated from topex poseidon altimetry three empirical ocean tide models are determined from repeat cycles 10 to 78 of the topex poseidon t p altimeter mission the three models investigate the effects of the satellite orbit ephemeris on the ocean tides determined from t p altimetry and the effect of extracting the free core nutation resonance in the definition of the diurnal ocean tide admittance the altimetric data series use the joint gravity model jgm 2 geopotential orbit ephemeris and the preliminary jgm 3 orbit ephemeris computed at the university of texas center for space research the altimetric data from the t p mission are shown to have the precision necessary to estimate the monthly and fortnightly ocean tides in each bin inclusion of existing models of the ocean tides in the polar latitudes not sampled by the altimeter demonstrates the importance of these latitudes on spherical harmonic representations of the ocean tides the ocean tides are first estimated in bins of size 2 834 in longitude by 1 in latitude and then smoothed to 1 by 1 grids within 66 latitude the orthotide response formalism of groves and reynolds 1975 is used to represent the diurnal and semidiurnal ocean tides while a constant admittance is assumed across narrow bandwidths around each of the monthly mm fortnightly mf and termensual mt tidal components comparisons of the t p ocean tide models to tide gauge observations indicate their accuracies to be of the order of 2 3 cm the t p derived ocean tide models remove approximately 20 cm2 more of the t p measured sea surface variance than the cartwright and ray 1991 tide model and show a 19 cm2 and 15 cm2 improvement over the schwiderski 1980a b and cartwright and ray 1991 tide models respectively when compared to tide gauge estimates of the ocean tides

dating young basalt eruptions by u th he on xenolithic zircons accurate ages for young e g pleistocene volcanic eruptions are important for geomorphic tectonic climatic and hazard studies existing techniques can be time consuming and expensive when many ages are needed and in the case of k ar and 40ar 39ar dating extraneous ar often can limit precision especially for continental basalts erupted through old lithosphere we present a new technique for dating young basaltic eruptions by u th he dating of zircons zhe from crustal xenoliths single crystal zhe dates generally have lower precision than typical 40ar 39ar dates but can be determined relatively easily on multiple replicate grain aliquots we dated zircons from xenoliths from four volcanic centers in western north america little bear mountain british columbia 157 3 5 2 2 ka weighted 95 confidence interval ci mean square of weighted deviates mswd 1 7 and prindle volcano alaska 176 16 8 9 ka mswd 13 in the northern cordilleran volcanic province and fish springs 273 23 8 6 mswd 43 and oak creek 179 8 1 4 5 ka mswd 12 in the big pine volcanic field california all zhe ages are either equivalent to or younger than previously determined k ar or 40ar 39ar ages indicating the possibility of inherited 40ar in some of the previous measurements zircons from upper crustal xenoliths in the oak creek and fish springs vents show poorer reproducibility and multiple apparent age distribution peaks consistent with either intracrystalline u th zonation or 99 99 he degassing assuming ca 100 ma pre entrainment zhe ages of some zircons during magmatic entrainment removal of clear outliers in the older age distribution peaks of the upper crustal xenoliths most of which have extremely high u compared to other zircons of the same xenolith improve the reproducibilities of fish springs to 4 7 95 ci mswd 4 8 and oak creek to 3 4 95 ci mswd 6 2 coupled thermal and he diffusion modeling using appropriate xenolith sizes and magma temperatures and assuming published diffusion kinetics for zircon indicate that incomplete he degassing would require entrainment times 1 h however the observation of extremely high u in most zircons with older ages raises the possibility that zircons with high radiation dosages may have more retentive he diffusion characteristics

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

buried rhyolites within the active high temperature salton sea geothermal system abstract previously unrecognized pulses of rhyolite volcanism occurred in the salton trough between 420 8 ka and 479 38 ka 2 based on high spatial resolution u pb zircon geochronology presently these rhyolite lavas tuffs and shallow subvolcanic sills are buried to depths between 1 6 and 2 7 km at ambient temperatures between 200 and 300 c and are overprinted by propylitic to potassic hydrothermal alteration mineral assemblages consisting of finely intergrown quartz k feldspar chlorite epidote and minor pyrite alteration resistant geochemical indicators whole rock nd isotopes zircon oxygen isotopes reveal that these rhyolites are derived from remelting of morb type crust that was chilled and hydrothermally altered by deep circulating hydrothermal waters u pb zircon dating confirms the presence of bishop tuff in well state 2 14 at 1 7 km depth approximately 5 km ne of the geothermal wells that penetrated the buried rhyolites these results indicate accelerated subsidence towards the center of the salton trough increasing from 2 2 mm a to 3 8 mm a based on these results the present day salton sea geothermal field is identified as a focus zone of episodic rhyolitic volcanism intense heat flow and metamorphism that predates present day geothermal activity and holocene volcanism by at least 400 ka

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

cenozoic volcanism and two stage extension in the salton trough southern california and northern baja california pronounced differences in age and geochemistry between cenozoic volcanic rocks within the salton trough southern california and northern baja california imply their formation within two distinct tectonic settings rather than the previously proposed single phase of continental rifting the emplacement of oligocene miocene alkaline lavas overlapped with crustal extension and basin formation in the region during this time post 4 ma subalkaline magmatism related to the gulf of california east pacific rise divergent plate margin exploited crust already ruptured during miocene extension new rare earth element and sr nd isotopic data and k ar 40ar 39ar and u pb ages for basalt andesite and granophyric granite xenoliths contained within the salton buttes rhyolite domes part of the post 4 ma oceanic spreading regime indicate that they are the components of a differentiated magmatic system these data also refute the presence of crystalline basement beneath this part of the salton trough we suggest that the rupture of crystalline basement beneath the axis of the salton trough was more effective during oligocene miocene time

geochemistry of igneous rocks from the cerro prieto geothermal field northern baja california mexico abstract fractional crystallization of basaltic magma derived from an oceanic affinity source region present beneath the salton trough and emplaced into a pull apart basin of this continental rift regime produced a tholeiitic suite of hypabyssal rocks consisting of basalt andesite and dacite within the cerro prieto geothermal field located in northern baja california mexico higher light rare earth element abundances for a basalt from the cerro prieto geothermal field in comparison to basalts from the gulf of california and the east pacific rise suggest partial assimilation of crustal materials into the parental magmas generated beneath the salton trough the crustal contaminant may be present near the surface today in the form of granitoids of the peninsular ranges batholith at deeper levels as hydrothermally altered materials near the base of the salton trough or may be a relict feature of tertiary subduction contained within the upper mantle beneath the salton trough the sr isotopic compositions of dacites from the nearby cerro prieto volcano range from 0 7029 to 0 7036 indicating an oceanic affinity source for these rocks the suite of hypabyssal rocks of tholeiitic affinity present within the cerro prieto geothermal field related by fractional crystallization link the dacite volcano of cerro prieto to gabbroic plutons inferred to exist beneath the cerro prieto geothermal field

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

oceanic tides from geosat altimetry direct tidal analysis of the altimetry from the geosat exact repeat mission s first year is used to derive estimates of the diurnal and semidiurnal oceanic tides the geoid is removed by collinear differences at 34 1 day separation and the orbit error is reduced by subtracting a slowly modulated 1 cycle revolution term a sequence of independent analyses at grid areas of 1 latitude 1 5 longitude using orthotide functions ensures complete definition of the diurnal and semidiurnal species global admittance maps for m2 and s2 within the latitudes 58 n and 59 s compare well with ground truth at 66 open ocean sites maps of differences between geosat and the naval surface weapons center model show important areas of the order of 10 15 cm the integral for the global power input from the moon gives 2 51 tw m2 0 44 tw s2 lower than many previous estimates but in agreement with figures deduced from a recent analysis of satellite orbits

shallow water tides in the northwest european shelf region from topex poseidon altimetry during recent years the accurate measurements from the topex poseidon satellite have improved global ocean tide models considerably however these global ocean tide models are still not accurate enough on the continental shelves for detailed oceanographic studies the need for increasing accuracy in shelf regions calls for inclusion of more than just semidiurnal and diurnal constituents in future global ocean tide models this is because a considerable part of the tidal variability on the shelves is caused by shallow water constituents one example is the m4 constituent which exceeds 50 cm at several locations on the northwest european shelf so far topex poseidon has not been considered for resolving these constituents mainly owing to its coarse ground track resolution reliable empirical estimates of several major shallow water constituents can be obtained from topex poseidon by combining along track and crossover observations estimates of the major shallow water constituents for the northwest european shelf m4 m6 and ms4 are presented in this study and compared with tide gauges and existing hydrodynamic models

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

obsidian hydration dating of volcanic events abstract obsidian hydration dating of volcanic events had been compared with ages of the same events determined by the 14c and k ar methods at several localities the localities ranging in age from 1200 to over 1 million yr include newberry craters oregon coso hot springs california salton sea california yellowstone national park wyoming and mineral range utah in most cases the agreement is quite good a number of factors including volcanic glass composition and exposuretemperature history must be known in order to relate hydration thickness to age the effect of composition can be determined from chemical analysis or the refractive index of the glass exposure temperature history requires a number of considerations enumerated in this paper

oceanic magmatism in sedimentary basins of the northern gulf of california rift rift related magmatism in the northernmost gulf of california and the adjacent subaerial salton trough and cerro prieto basins comprises intermediate to rhyolitic surfi cial and buried lava fl ows and domes including their xenolith cargo in addition geothermal drill wells frequently penetrate subsurface gabbroic to granitic sills and dikes which intruded into colorado river delta fl uviatile and lacustrine sediments combined singlecrystal u th pb and u th he zircon ages reveal late pleistocene to holocene eruption ages for three volcanic centers in adjacent rift basins from n to s salton buttes eruption age 2 48 0 47 ka 95 confi dence cerro prieto maximum eruption age 73 7 ka and roca consag eruption age 43 6 ka u th zircon and allanite crystallization ages are close to the eruption ages with the exception of roca consag lava the zircon population of which is dominated by zircon with ca 1 ma crystallization ages a population interpreted to be recycled from an unknown crustal source underlying the wagner basin nd isotopic ratios for subsurface micro gabbros from cerro prieto end 8 9 overlap with values for midoceanic ridge basalts morb from the east pacifi c rise adjacent to the southern gulf of california cerro prieto microgranites and salton sea basaltic xenoliths have similarly elevated end values the lowest end value for late pleistocene holo cene igneous rocks from the northern gulf of california is for cerro prieto dacitic lava end 0 6 this value implies minor 20 assimilation of continental crustal rocks which however is an upper limit because of crystal scale evidence for magma contamination by unconsolidated sediment at the time of eruption zircon crystals in felsic rocks rhyolite lavas intrusive microgranites and granophyre xenoliths have trace element and submantle 18 o compositions that are robust indicators for a mafi c source that has exchanged oxygen by interacting with meteoric hydrothermal fl uids collectively these data imply that oceanic rifting has initiated in the salton trough and cerro prieto basins there morb type magmas formed mafi c intrusions within thick sedimentary basin fi ll where they became exposed to deep reaching hydrothermal fl uids diverse intermediate to high silica rhyolitic magmas that are prevalent at the surface are produced by fractional crystallization of mafi c parental magmas with minor assimilation of sediments or pre rift basement rocks and by partial melting of hydrothermally altered mafi c intrusions