geochronological evidence supporting antarctic deglaciation three million years ago the response of the antarctic ice sheets to increased global temperatures is an important unresolved issue in the assessment of future climate change in particular considerable controversy exists as to whether the east antarctic ice sheet suffered extensive deglaciation during the mid pliocene epoch 3 myr ago when temperatures were only slightly warmer than today although the ice sheet is widely assumed to have existed in something like its present form for the past 14 myr ref 1 marine diatoms eroded from the antarctic interior have been found in glacial till deposits high in the transantarctic mountains2 3 and have been bio stratigraphically dated at 3 myr before present this age has been disputed4 because it implies marine deposition in the antarctic interior and hence substantial deglaciation at a time when other evidence has been marshalled for the persistence of cold polar conditions4 here we report k ar and 40ar 39 ar ages for a volcanic ash bed in diatom bearing glaciomarine strata cored in ferrar fiord east antarctica by the ciros 2 drill hole5 which confirm the age of the diatoms at 3 myr and hence also confirm the mid pliocene deglaciation

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

stress history and tectonic development of the rio grande rift new mexico paleostress determinations from radiometrically dated dikes indicate ene wsw regional extension began by about 32 ma in new mexico and perhaps earlier in the southern part of the state the extension direction maintained this orientation until sometime after 23 ma when it rotated clockwise to an e w or wnw ese direction which is the present orientation the spreading direction in the rio grande rift however has been e w since the beginning of rifting about 31 ma and reflects control of spreading by preexisting structural grain ne sw extension occurred in the grants pie town area of the colorado plateau and in the roofs of batholiths in the socorro and questa areas concurrently with e w spreading in the rio grande rift nearly orthogonal orientations of late eocene oligocene 38 32 ma paleostress indicators in southern new mexico indicate a nearly isotropic middle tertiary stress field both the western and eastern boundaries of the zone of e w spreading along the rio grande rift in northern new mexico shifted 60 200 km eastward sometime between 23 and 16 ma perhaps coeval with rotation of the direction of regional extension this eastward shift also coincided approximately with the renewal of volcanism following an early miocene 22 17 ma lull during the late miocene 9 5 ma the eastern boundary of e w spreading shifted back to the present eastern margin of the rio grande rift the n s to nne ssw orientation of the least principal horizontal stress in the southern great plains province has persisted since at least 28 ma stress province boundaries can be remarkably sharp with stress orientations changing by 90 within 30 km or less the ne trending jemez lineament transects the colorado plateau rio grande rift basin and range and southern great plains stress provinces but except for a transition zone along the present southeastern boundary of the colorado plateau both paleostress and contemporary stress reflect the respective stress provinces not the lineament chemical analyses of dated dikes reveal highly variable compositions but indicate that true basalts are more common among early rift 20 ma volcanic rocks than is generally recognized a histogram constructed from nearly 200 radiometric ages of igneous rocks in new mexico between 104 w and 108 w longitude indicates that volcanism along the rio grande rift has been episodic with peaks at 37 34 31 27 26 22 17 13 12 6 and 5 0 ma

cenozoic marine sedimentation and ice volume variation on the east antarctic craton recycled cretaceous and cenozoic marine microfossils have been recovered from samples of the pliocene sinus formation samples were collected from outcrops in the reedy beardmore and ferrar glacier areas of the transantarctic mountains between lat 77 and 86 s the glaciogene sediments contained diatoms foraminifera calcareous nannoplankton silicoflagellates radiolarians sponge spicules palynomorphs and ostracodes of late cretaceous paleocene eocene late oligocene late miocene and pliocene age this suggests the presence of open marine basins on the east antarctic craton during late mesozoic and cenozoic time the apparent absence of early oligocene and early through middle and earliest late miocene assemblages suggests either that marine regression exposed the basin floors or that ice filled the basins during these times the high elevation setting of sirius formation outcrops suggests one of two hypotheses for their origin 1 they are in situ pliocene glaciomarine deposits that were uplifted 1 750 2 500 m with the transantarctic mountains to their present elevation 2 the sirius formation deposits are a mixture of derived sediments stripped from sub ice intracratonic basins and subsequently redeposited by ice flowing up the inland slope of the transantarctic mountains we favor the second hypothesis with transport to sites sometime within the past 3 m y

apparent q for upper crustal rocks of the central rio grande rift spectra from 130 digitally recorded microearthquakes in the magnitude range 0 9 to 0 3 were used to compute apparent seismic quality factors q for upper crustal rocks within 45 km of socorro new mexico in this study apparent q was defined as a measure of seismic wave attenuation due to intrinsic absorption and scattering and was computed over the frequency range 0 50 hz most of the recorded energy was in the 3 30 hz band it was found that apparent q increases with event distance for the eight stations used in this study this increase was successfully modeled with a varying thickness low q low velocity layer lying above a relatively high q high velocity half space it was found that 1 near surface low q regions lying directly beneath the recording sites vary from 0 3 to 2 0 km in thickness and exhibit qp and qs values less than 50 2 the average qs for the upper crust in the central rio grande rift excluding the near surface low q region is 535 95 confidence interval 374 938 3 the observed qp qs ratio for events recorded at eight stations varied from 0 34 to 1 39 for earthquakes recorded at three stations the qp qs ratio decreased with increasing event distance more for two other stations no systematic increase or decrease in qp qs with distance was evident and for the remaining three stations events were too closely clustered to draw any conclusions these changes in the qp qs ratio with increasing event distance may reflect varying degrees of fluid saturation in upper crustal rocks 4 at least three anomalously low qs regions exist within 20 km southwest of socorro these abnormally low q regions may reflect small magmatic intrusions or high fracture densities at depth and may lie in close proximity to microearthquake swarms at depths of 7 2 10 3 km 63 references 10 figures 4 tables less

three dimensional velocity anomalies in the upper crust near socorro new mexico 262 p wave arrival times associated with 40 local events obtained from a movable array of up to eight short period seismographs were used to determine hypocentral parameters and an accurate three dimensional seismic velocity model for the rio grande rift zone near socorro new mexico the initial attempt to model these data resulted in a representative half space velocity of 5 85 0 02 1 s d km sec no azimuthal velocity variation was found that was significantly different from the half space solution the area was then subdivided at depth 4km and into blocks one tenth of a degree on a side the resulting model showed that a block in the lower layer approximately 15 km west of socorro had an average velocity of 5 17 0 11 1 s d km sec 0 68 km sec less than the half space velocity other blocks had smaller but still significant anomalies relative to the half space velocity the preferred explanation for the anomalously low velocity is that this area represents a site of magmatic intrusion into the upper crust 4 10 km deep

cenozoic thermal mechanical and tectonic evolution of the rio grande rift careful documentation of the cenozoic geologic history of the rio grande rift in new mexico reveals a complex sequence of events at least two phases of extension have been identified an early phase of extension began in the mid oligocene about 30 ma and may have continued to the early miocene about 18 ma this phase of extension was characterized by local high strain extension events locally 50 100 regionally 30 50 low angle faulting and the development of broad relatively shallow basins all indicating an approximately ne sw 25 extension direction consistent with the regional stress field at that time extension events were not synchronous during early phase extension and were often temporally and spatially associated with major magmatism a late phase of extension occurred primarily in the late miocene 10 5 ma with minor extension continuing to the present it was characterized by apparently synchronous high angle faulting giving large vertical strains with relatively minor lateral strain 5 20 which produced the modern rio grande rift morphology extension direction was approximately e w consistent with the contemporary regional stress field late phase graben or half graben basins cut and often obscure early phase broad basins early phase extensional style and basin formation indicate a ductile lithosphere and this extension occurred during the climax of paleogene magmatic activity in this zone late phase extensional style indicates a more brittle lithosphere and this extension followed a middle miocene lull in volcanism regional uplift of about 1 km appears to have accompanied late phase extension and relatively minor volcanism has continued to the present we have estimated geotherms and calculated lithospheric strength curves for the two phases of extension using geologic data to constrain earlier events and geophysical data to constrain the modern geotherm and crustal structure a high geotherm was deduced for early phase extension resulting in a shallow crustal brittle ductile transition and negligible mantle strength the lithosphere cooled after early phase extension resulting in a deeper crustal brittle ductile transition and perhaps more significantly a considerable zone of mantle strength immediately beneath the moho these results indicate that early phase extensional style was controlled by a crustal decollement near the brittle ductile transition which was prevented during late phase extension by significant strength in the uppermost mantle late cenozoic uplift of the rift zone cannot be explained by crustal thinning during extension and geotherm evolution predicted from simple cooling however this uplift does not appear to be restricted to the rift zone and pliocene to recent volcanism and heat flow data suggest that uplift may be caused by magmatic thickening of the crust perhaps unrelated to rifting the complex interrelationship among regional and local prerifting synrifting and postrifting events in the rio grande rift suggests that rifting at least in this region should not be considered in isolation of other geologic events

shallow velocity structure of the rio grande rift north of socorro new mexico a reinterpretation shallow upper 1 2 km compressional wave velocity models from line 2a of the socorro new mexico consortium for continental reflection profiling cocorp survey of the rio grande rift supplement a previous refraction study of socorro cocorp lines 1 and 1a first arrival refraction analysis and slant stacks of the refraction profiles derived from the reflection data are used to determine velocity models of socorro cocorp line 2a exposure of paleozoic and mesozoic sedimentary formations in the vicinity of socorro line 2a allow direct correlation of a unit s velocity to its geological era weathered mesozoic and paleozoic formations within the rift are inferred to have significantly lower velocities than previously believed reinterpretation of the rio grande rift structure in the survey area results in a model that is consistent with available gravity data and provenance studies the 13 km of cenozoic spreading determined from the model is at least 5 km higher than that found 80 km to the north there is evidence of minor paleozoic deformation or nondeposition

magnetotelluric results opposing magma origin of crustal conductors in the rio grande rift although conductive intracrustal layers appear to be characteristic of continental rift zones they also have been detected in stable crustal environments the low resistivity layers are often explained by free water or crustal magma however alternative explanations are possible purely geometrical effects can be mistaken as conductive layers at depth in the interpretation of magnetotelluric or geomagnetic depth soundings certain minerals such as graphite and hydrated minerals e g serpentine may be important thermally activated processes associated with ductile flow mechanisms may also enhance conductivity two dimensional analysis of a portion of 25 new magnetotelluric stations in the central rio grande rift supports the presence of a conductive zone at 10 km depth or less however very significant differences are apparent in the crustal sections at two close localities mid crustal magma lenses well defined by seismic data cannot explain these differences in fact the crust is modeled to be more conductive by at least an order of magnitude where magma is not detected compared to where it occurs at shallow and intermediate levels only the possible effects of regional three dimensional current channeling could conceivably alter this conclusion to explain the unexpected result it is suggested that a conductive horizon occurs where an impermeable ductile cap traps pore fluids beneath the cap may be nearly an order of magnitude more conductive 100 s m than the dry brittle crust above the zone of trapped pore fluids much more conductive by more than an order of magnitude 10 m where active magma injection destroys the integrity of the ductile cap trapped fluids may escape and produce an overall decrease in conductivity the final electrical signature with this dynamic concept depends on the thermal gradient the relative impermeability of the cap the extent of pore fluids beneath and the amount and frequency of magma intrusion the temporal and spatial distribution of earthquake foci in our study area supports the existence of a ductile layer at about 10 km depth and the hypothesis of magma injection

gravity models of the albuquerque basin rio grande rift new mexico as part of a hydrologic study gravity anomalies over the albuquerque basin were modeled to determine the thickness of the low density neogene basin deposits the major aquifer of the basin the models are constrained by substantial density log and well data fifteen two dimensional 2 d models include four major density units the upper unit density 2 20g cm3 corresponds to the neogene syn rifting deposits thickness of this unit averages about 1 5 km and exceeds 2 0 km in a strip along the east side of the basin the thickness is about 1 0 km between the albuquerque and the santo domingo basins at the south end of the albuquerque basin the thickness is less than 0 5 km the configuration of the remaining density units less constrained by well data raises significant questions about the history of the basin that cannot be answered with existing data

origin and development of textures and fabrics in basal ice at summit central greenland abstract detailed textural and crystallographic characteristics of the bottom 6 m of the grip core summit central greenland are presented the results are discussed in the light of present day fabric analogues ice fabrics are shown to retain the mark from the entrainment of locally formed ice by the growing ice sheet this mark is only partly overprinted by the present day stress configuration at the ice divide these results corroborate the model put forward by souchez et al 1 on stable isotopes and gas composition grounds in which the basal silty ice appears as a remnant of a growing stage of the ice sheet during this stage which probably represents the original build up ice of local origin formed at the ground surface was overridden by the main ice sheet and incorporated into its base preservation of remnants of ancient fabric patterns and occurrence of annealing recrystallization fabrics just above the first layer of silty ice are used amongst other arguments to demonstrate that this basal ice is not presently subject to large cumulative strains the implications for flow and dating models are discussed

earthquakes near albuquerque new mexico 1976 1981 the u s geological survey conducted a 66 month long study of earthquake activity near albuquerque new mexico during 1976 1981 analysis of over 1000 hypocenters with magnitudes as large as 3 2 revealed that most of the seismicity fell into three regions 1 the socorro and albuquerque areas of the rio grande rift 2 the mount taylor area of the jemez lineament and 3 the estancia basin in the rift the hypocenters are shallow and largely confined to regions beneath the basins composite fault plane solutions suggest both normal and strike slip faulting with an average t axis oriented wnw ese most of the young faults of the rift are seemingly inactive and the rift as a whole cannot be delineated on the basis of seismicity

ice dynamics and thermal regime of taylor glacier south victoria land antarctica les donnees glaciologiques recueillies sur le taylor glacier ont servi a reconstituer les principaux traits de la dynamique et du regime thermique du glacier les etudes du bilan de masse montrent que le glacier est proche de l equilibre les conditions thermiques de la glace basale sur une grande partie de la zone d ablation ont ete calculees a partir d estimation du flux de chaleur geothermique et de mesures des temperatures de la glace pres de la surface des vitesses et de l epaisseur de la glace on a trouve que jusqu a 50 de la zone d ablation la glace basale pouvait etre fondante

estimates of terrestrial heat flow from deep petroleum tests along the rio grande rift in central and southern new mexico twenty six new estimates of heat flow are calculated for the albuquerque belen basin and the basins of the southern rio grande rift in new mexico from these and previous data the following heat flow patterns may be suggested the regional or background heat flow in the albuquerque belen basin is 77 mw m 2 there is also a ne sw trend of higher heat flow 92 110 mw m 2 across the region in the southern rio grande rift between socorro and las cruces the regional heat flow appears to be 95 mw m 2 in this region there are also higher heat flows 117 135 mw m 2 at widely scattered sites south of las cruces near the international border between mexico and the united states preliminary and perhaps suspect data suggest the possibility of a transition to the intermediate heat flows previously estimated at sites throughout much of northeastern chihuahua mexico regional heat flow values in the albuquerque belen basin and in the basins of the southern rio grande rift are consistent with but do not constrain simple models of crustal thinning higher heat flows in these two areas are thought to be caused by hydrothermal fluids largely groundwaters but possibly magma in some areas transporting heat upward along crustal fracture zones

evidence of ongoing crustal deformation related to magmatic activity near socorro new mexico recent leveling conducted by the national geodetic survey in central new mexico provides new evidence of ongoing tectonic deformation associated with the rio grande rift a 1980 1981 resurvey over suspected midcrustal and shallow magma bodies near socorro new mexico indicates continued uplift averaging 0 18 cm yr between 1951 and 1980 although this rate is somewhat smaller than that reported previously 0 34 cm yr between 1912 and 1951 the new observations suggest possible systematic errors in the earlier surveys reanalysis of the earlier data gives an average rate of 0 23 cm yr from 1912 to 1951 consistent not only with the new releveling observations but also with geomorphic evidence which indicates an average uplift rate of 0 18 cm yr over the past 20 000 years thus the rate of uplift may be relatively constant the new releveling measurements also confirm prominent zones of subsidence flanking the central uplift modeling the deformation together with other geophysical observations suggests that the movements in the socorro area are likely associated with the 19 km deep socorro magma body although the details of magma movements are not well constrained plausible models include flow of magma from the periphery toward the central magma chamber and flow from sources in the lower crust into the midcrust horizontal strain calculated for these models is generally consistent with the small strain reported from repeated trilateration surveys in the socorro area the rio grande rift is clearly an important site for future monitoring of crustal deformation

cenozoic normal faulting and the shallow structure of the rio grande rift near socorro new mexico migrated versions of a deep seismic section across the albuquerque basin in the southern rio grande rift cocorp abo pass line 1 and socorro line 1 a are used to develop a detailed interpretation of the shallow structure of the rift our interpretation which is consistent with nearby drill hole and gravity data suggests that listric faulting has been the dominant extensional style of faulting in this part of the rift during the early stages of rifting blocks of paleozoic and mesozoic prerift sedimentary strata overlying precambrian crystalline basement were offset and rotated along listric normal faults rooted in the basement syntectonic deposition of thick sections of mid tertiary sedimentary and volcanic rocks filled the developing fault bounded basins these units were continuously rotated during ongoing listric faulting and subsequently buried by later tertiary and quaternary basin fill present surface fault traces may reflect continued movement on the deeper listric faults or a more recent minor episode of shallow normal faulting in contrast to models of crustal extension proposed for nearby areas in the rio grande rift and basin and range province no clear evidence is seen for two time distinct stages of faulting characterized by early closely spaced faulting with large stratal tilts followed by wide spaced high angle faulting the large listric faults flatten into basement or converge upon an apparent detachment surface at a depth of about 5 km posing intriguing questions about the connection between shallow structure and deeper extension within the crust

pliocene paleoclimate and east antarctic ice sheet history from surficial ash deposits the preservation age and stratigraphic relation of an in situ ashfall layer with an underlying desert pavement in arena valley southern victoria land indicate that a cold desert climate has persisted in arena valley during the past 4 3 million years these data indicate that the present east antarctic ice sheet has endured for this time and that average temperatures during the pliocene in arena valley were no greater than 3 c above present values one implication is that the collapse of the east antarctic ice sheet due to greenhouse warming is unlikely even if global atmospheric temperatures rise to levels last experienced during mid pliocene times

evidence from rayleigh wave data for magma in an upper crustal dike in the albuquerque belen basin of the rio grande rift new mexico we have found evidence for a dikelike structure in the middle of the albuquerque belen basin of the rio grande rift from our interpretation of two station rayleigh wave amplitude and phase velocity data two dimensional finite element modeling suggests that the structure is about 200 m thick extends vertically from a depth of about 2 1 2 km to at least 20 km and contains material characterized by a shear wave velocity of about 1 2 km s simple physical arguments suggest that the length of the structure may be at least 28 km long nearby high heat flow values historical swarmlike seismicity recent 0 14 ma volcanism and an extensive sill like magma body at midcrustal depths all suggest that the structure is a magma filled dike source characteristics and seismometer response limit our reliable data to the frequency band of 0 08 hz to 0 250 hz thus we cannot tell whether the dike is solely an upper crustal feature or whether it extends through the crust and in to the upper mantle copyright american geophysical union 1987

grain growth in polar ice ii application grain growth observed in polar ice that is not deforming rapidly can be accounted for if concentrations and distributions of extrinsic materials microparticles bubbles and dissolved impurities are characterized fully dissolved impurities segregate to grain boundaries and slow grain growth in all cold glacial ice the high concentration of soluble impurities in wisconsinan ice from the dome c antarctica ice core and perhaps other ice cores probably causes the small grain sizes observed in that ice microparticles have little effect on grain growth in ordinary ice in ice layers that appear dirty owing to concentrations of volcanic tephra such as in the byrd station antarctica ice core or of morainal material micro particles reduce grain growth rates significantly the relatively high vapor pressure of ice allows rapid growth and high mobility of intergranular necks so grain growth in firn is limited by boundary migration rather than by neck growth bubbles formed by pore close off at the firn ice transition are less mobile than grain boundaries causing bubble boundary separation whenever geometric constraints are satisfied however such separation reduces grain growth rates by only about 10 the observed linear increase of grain area with time is thus predicted by theory but the growth rate depends on soluble impurity concentrations as well as on temperature

on the building and classification of mountains physicists believe that known laws should suffice to explain the earth s behavior but the complexities of geology have defied simple explanation today three developments are helping to solve the problem first summary interpretations such as the centennial project of the geological society of america are being compiled of the upper 1 60 km of the earth s radius second new methods show the behavior at great depths in the earth third combining these studies suggests which physical laws apply at various depths in the mantle the pattern of convection by viscous fluid flow is becoming clear upwelling plumes start rifts which fragment the lithosphere subduction carries plates down in the strong lithosphere the coulomb navier laws of brittle failure apply and confine fractures to three types corresponding to plate boundaries other laws determine where island arcs form the idea that some large areas of the surface act independently of the surrounding areas is becoming recognized and simplifies analysis one large area which behaves independently lies in the southwestern united states these developments and the observation that most plate boundaries are elevated provide a means of applying laws of physics to classifying ranges including folded mountains island arcs rifts and ridges into three classes and many subclasses illustrated by examples

a survey of multi stage diagenesis and dolomitization of jurassic limestones along a regional shelf to basin transect in the ziz valley central high atlas mountains morocco abstract petrographic and geochemical data from five localities in the ziz valley of morocco indicate that jurassic limestones have undergone early diagenesis that varied with location from shelf to basinal settings burial diagenesis that was most pronounced in basinal settings and late diagenesis caused by compression and uplift of the high atlas mountains marine cements occur at all five localities from shelf to basin center although cement types vary from peloidal microcrystalline cements updip on the shelf to equant calcite in basinal settings presence of moldic grains and or mg poor fe poor blocky cements suggest that meteoric waters influenced early diagenesis at all shelf localities and on an upturned fault block in the basinal region leaving only one locality unaffected by early meteoric processes 87 sr 86 sr ratios of 0 70810 0 70895 greater than 87 sr 86 sr of coeval limestones mg contents that decrease upward from 47 5 to 43 0 mol mgco 3 presence of dolomitized marine cements and dolomite cements that postdate marine cements but predate meteoric to burial cements suggest that dolomitization and dolomite cementation at two shelf localities took place in mixed meteoric and marine waters early in diagenesis however poorer preservation of depositional fabrics lower 18 o values and larger and more anhedral crystals suggest that dolostones downdip underwent later modification during burial whereas those updip did not compaction during diagenesis generated numerous concavo convex and sutured intergranular contacts at updip shelf downdip shelf and basinal localities where earlier meteoric cementation was not extensive compaction was insignificant in more extensively cemented mid shelf settings high sr 1200 3800 ppm and fe 1000 2300 ppm contents in brachiopod grains suggest that lmc components underwent some modification during burial in basinal settings in sr rich reducing waters fe contents of late intergranular cements increase from 2000 ppm at the basin s edge to as much as 6000 ppm in the basin s center bedding parallel stylolites occur at all localities the most negative 18 o values of sparry dolomites near the tizi n firest fault 6 2 vs pdb imply diagenetic temperatures of 65 85 c assuming water 18 o values of 0 0 2 0 vs smow those temperatures are much less than previous estimates of burial temperatures in the high atlas basin an isotopic gradient extrapolating to roughly 5 km in diagenetically modified dolostones likewise suggests a geothermal gradient less than gradients previously proposed for at least parts of the area comparison of morphologies of transverse stylolites which are found at all localities with morphologies of bedding parallel stylolites suggests that transverse stylolites formed due to compression during late diagenesis uplift accompanying that compression allowed influx of low mg waters that along with other factors caused calcitization of dolomites the fe concentration of calcite that fills late fractures increases from less than 2000 ppm at the basin center to values in excess of 3000 ppm at the basin edge opposite trends in earlier cements and reflecting uplift of the high atlas mountains and resultant changes in patterns of groundwater flow

shear wave structure of the upper crust of the albuquerque belen basin from rayleigh wave phase velocities fundamental mode rayleigh wave phase velocities have been obtained for the albuquerque belen basin of the rio grande rift source and instrumental characteristics limited useful data to the period band 6 12 s inversion of the phase velocity data yielded a model for the shear velocity of the upper 20 km of the basin the model contains two velocity inversions one near a depth of 6 km the other near 18 km the velocity inversion near 18 km is attributed to the effects of a midcrustal magma body known from previous studies the cause of the low velocity zone near 6 km is not known but it may be caused by an upper crustal magma body the average of the velocities for the upper crustal model presented here ave 3 33 km s is comparable to those reported in previous studies