s wave attenuation in the coastal region of jalisco colima mexico abstract aftershock data from the october 9 1995 m 7 6 earthquake that occurred in the coastal subduction region of colima jalisco mexico are used to obtain estimates of the frequency independent quality factor q s and spectral decay parameter in the approximate frequency range 3 hz f 40 hz as functions of hypocentral distance r a regression analysis results in the relations for s waves q s 261 397 3 198r 15 536 0 009651 0 000462r 0 0012 the observed distance dependence of q s and is similar to that reported for the mexican regions of oaxaca and guerrero our q s values agree with those reported for oaxaca and northern baja ca for frequencies around 8 hz and are somewhat lower a factor of 0 8 to 0 5 for higher frequencies around 20 hz however they are significantly lower than all but the lowest values reported for the guerrero region attaining factors 0 25 and 0 13 for frequency ranges around 8 and 20 hz respectively it thus appears that the guerrero region has an anomalously low attenuation compared with the flanking coastal regions of oaxaca chiapas to the se and jalisco colima to the nw

a garnet two mica granite coyote mountains southern arizona geologic setting uranium lead isotopic systematics of zircon and nature of the granite source region the 58 m y old pan tak granite considered representative of the widespread early tertiary garnet two mica granites of south central arizona consists of biotite and muscovite biotite granite intruded by garnet muscovite and garnet biotite granite and associated pegmatite textural evidence indicates that muscovite is a primary magmatic mineral the granite was intruded into jurassic dioritic to granitic rocks and paleozoic metasedimentary rocks during a latest cretaceous and early tertiary regional metamorphic episode part of the pluton was subsequently mylonitized during an early or middle tertiary deformational episode of metamorphic core complex aspect the fabrics produced by these two metamorphic episodes differ in distribution style orientation and probably age the zircon population of the pan tak granite consists of a mixture of small euhedral crystals and larger rounded crystals with or without overgrowths uranium lead isotopic ratios of five size fractions of discordant zircon define a chord with a lower concordia intercept age of 58 2 m y and a late precambrian upper intercept 207 pb 206 pb ages range from 214 m y for the finest fraction to 434 m y for the coarsest fraction the pan tak discordia is interpreted as a mixing line between 58 m y old magmatic zircon and an older zircon component largely or entirely of late precambrian age geologic and isotopic evidence indicates that this older zircon component was not assimilated from the country rocks but rather was inherited from the source material of the granite the presence of this inherited zircon indicates that the pan tak magma was produced by melting of crustal rocks containing late precambrian zircon this source material was probably the 1 7 to 1 4 b y old crystalline basement of south central arizona generation of the pan tak and related granites by crustal anatexis was one aspect of a latest cretaceous to early tertiary orogenic episode in south central arizona

origin of mesozoic and tertiary granite in the western united states and implications for pre mesozoic crustal structure 2 nd and sr isotopic studies of unmineralized and cu and mo mineralized granite in the precambrian craton in the cordilleran region of the western united states mesozoic and tertiary peraluminous granitic rocks display regional variations in initial 143nd 144nd end end 10 to 12 in southern arizona 17 to 19 in the northern great basin ngb and 30 in the northern rocky mountains initial 87sr 86sr values are between 0 710 and 0 721 and show no regional pattern metaluminous granitic rocks have a wider range of end values extending from values similar to those of the peraluminous granites to much higher values the 87sr 86sr values are mostly fairly low between 0 705 and 0 710 except in the ngb where values as high as 0 7157 are observed no systematic differences between the end or 87sr 86sr values of cu or mo mineralized and unmineralized granite were discerned except for cu mineralized granite in eastern nevada and mo mineralized granite in colorado which have end values higher 0 and lower 10 0 respectively than unmineralized granite in the same region comparison to end values of exposed precambrian rock suggests that the peraluminous granite and the mo granite in colorado were derived exclusively from felsic precambrian basement rocks and that the regional variations in the end values reflect the regional variation in the average crustal age the nd data confirm that the precambrian basement underlying the ngb and eastern california is isotopically distinct from precambrian crust in the remainder of the western united states the similarity between the end values of peraluminous granite and precambrian crust also suggests that the high 147sm 144nd 0 13 and the low total light rare earth element lree abundances characteristic of peraluminous granite in southern arizona were imposed during the chemical evolution of the magmas metaluminous granite are interpreted to have formed via mixing of mantle derived magma and large proportions of low 87sr 86sr granulite facies lower crust except in the eastern ngb where the mantle magmas mixed with a lower crustal source with a significantly higher 87sr 86sr ratio ree abundance patterns for metaluminous granite in the ngb are characterized by extreme lree enrichment which supports the proposed origin for these rocks by mantle crust mixing or by remelting of the lower crust alone no systematic difference exists between the sources of cu or mo mineralized and unmineralized metaluminous granite but the data suggest that the cu sources are in the mantle and the mo sources are in preexisting crust overall the nd data indicate that continental interior granite in the western united states was primarily derived from preexisting crust and although changes in the thermal structure of the continental mantle may have triggered magma formation the resulting granites do not represent significant juvenile additions to the continental crust

nd isotope systematics of coarse and fine grained sediments examples from the middle proterozoic belt purcell supergroup sm nd isotopic compositions are reported for 20 fine and 5 coarse grained samples of the belt purcell supergroup of western north america varepsilon nd and sm nd ratios for fine grained belt purcell sediments are remarkably uniform indicating crustal residence ages ranging only from 1 6 to 2 1 ga varepsilon nd and sm nd ratios for coarse grained sediments however are much more varied and contain evidence of sm nd fractionation during sedimentary recycling assuming that this ree fractionation occurred between the time source areas were eroded and when sediment was deposited a new method of calculating crustal residence age intervals is derived for the coarse grained samples which serves to place constraints on the average time since source materials were extracted from the mantle the nd data from both sample groups provide an average age estimate of 2 0 pm 0 3 ga for the middle proterozoic crust from which most of the belt purcell supergroup was derived the isotopic data toge

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

high stress drop events in the victoria baja california earthquake swarm of 1978 march summary the victoria earthquake swarm of 1978 march with magnitudes up to ml 4 8 occurred near the northern end of the cerro prieto fault in northern baja california mexico accurate epicentre locations of a number of earthquakes in the swarm reveal that the activity concentrated in a zone of about 6 km radius projected on the horizontal with earthquakes occuring mostly at depths of around 12 km a composite fault plane solution prepared with data from the larger earthquakes of the swarm indicates right lateral strike slip motion along a vertical plane extending parallel to the cerro prieto fault seismic moment source radius and stress drop are calculated from strong motion records and digital seismograph records obtained at short epicentral distances in most cases less than 10 km from calculated displacement spectra and brune s model stress drops between 1 bar and 1 kbar were estimated for earthquakes with seismic moments in the range 1 4 1019 2 9 1023 dyne cm eighteen events of 44 studied here had stress drops higher than 90 bar a result in striking contrast to lower stress drop results from previous distant station studies of earthquakes in the region results of comparing near source and large distance spectral shapes for two earthquakes in the victoria swarm region indicate that scattering and attenuation are a likely cause of the severe reduction of high frequency energy clearly shown by the larger distance spectra and thus of the resulting lower stress drop determinations at large distances if these events are characteristic of other large events in the region then the low stress drop determinations of thatcher and thatcher hanks for this region may be in doubt very high peak horizontal ground accelerations up to 0 6 g were measured from the near source records which were characterized by short duration bursts of high frequency energy it is suggested here that these high peak accelerations resulted from a combined effect of high stress drop and sediment amplification using a crustal model derived from mcmechan mooney a sedimentary amplification factor of 3 4 is estimated and used as a correction in calculation of the earthquake source parameters four other high stress drop up to 2 5 kbar earthquakes in this area were also analysed the high stress drop values obtained from seismic spectra are corroborated by using the rms root mean square acceleration formulation introduced by hanks

geochronology and zircon hf isotope geochemistry of granites in the giant chalukou mo deposit ne china implications for tectonic setting abstract in this contribution the largest porphyry mo deposit in ne china chalukou is selected to provide insights into the tectonic evolution of ne china the deposit was considered to be rift related or climax type porphyry mo deposit developed in an extensional setting while other researchers attributed its formation to the subduction of the paleo pacific ocean in view of different granite petrogenesis between rift type and subduction type porphyry mo deposits geochronological and zircon lu hf isotope studies have been carried out on granitic rocks associated with the chalukou deposit zircon grains from the ore host monzogranite and ore causative granite porphyry yield weighted mean 206pb 238u ages of 166 2 ma and 152 2 ma with molybdenite samples yielding a re os isochron age of 150 2 ma indicating a jurassic age for both granitic intrusion and mo mineralization zircon ehf t values of monzogranite and granite porphyry range from 0 9 to 3 0 and from 1 4 to 0 5 respectively when integrated with published zircon lu hf isotopic data our data suggest that the barren granites both pre and post ore were mainly derived from mantle derived juvenile rocks whereas the syn ore granite requires additional involvement of old continental crust thus a contribution of continental crust to mo mineralization is suggested the petrogenesis and geochemical data reveal that the chalukou deposit is a subduction related porphyry mo deposits and not rift related integrated evidence links its formation to the southeastward subduction of the mongol okhotsk ocean instead of the westward subduction of the paleo pacific ocean

the role of magmatic sulfides and mafic alkaline magmas in the bingham and tintic mining districts utah this paper reports the existence of abundant magmatic sulfides in introduction latite lava vitrophyres and vent facies dikes from the bingham and the eruption of sulfur rich magmas from el chichon tintic districts in central utah these dikes and flows appear to luhr et al 1984 and mt pinatubo westrich gerlach be comagmatic with the ore related monzonite intrusions in addition 1992 in recent years has attracted attention to the limited analytical data suggest that the magmatic sulfides contain most of solubility of sulfur in intermediate magmas and the factors the cu and ag present in the latites magmatic sulfides are preserved potentially responsible for the formation of magmatic only in the least oxidized and least degassed lavas and vitrophyres sulfides and anhydrite however the occurrence of magpetrography of comagmatic dikes and intrusions suggests that matic sulfides in intermediate magmas is not universal resorption and oxidation s degassing of the globules may make many volcanic suites appear to be sulfide unsaturated the metal content available to a magmatic ore fluid minette and inasmuch as magmatic sulfides may sequester chalcophile shoshonite lavas occur in subordinate amounts in the latitic volcanic metals what would their presence signify in magmatic fields associated with latite monzonite intrusions and hydrothermal systems that formed porphyry cu and mesothermal vein ore deposits of the tintic ag pb zn cu au and bingham deposits do they sequester a significant portion of the porphyry cu au mo districts utah the ages chemical and magmatic cu pb zn ag and au thereby restricting modal compositions and field relations of the mafic alkaline lavas metal availability of subsequent ore forming fluids suggest that such magmas mixed and fractionated at shallow crustal we have examined the volcanic rocks from the tintic levels to form the ore related latites and monzonites the unaltered and bingham mining districts fig 1 utah and found mafic alkaline lavas in these districts are some of the most cu rich that almost all of the volcanic units contain magmatic lavas in the great basin province sulfides some quenched dikes and intrusion margins also have preserved magmatic sulfides comparable with those in the volcanic rocks several lines of evidence indicate that the volcanic and intrusive rocks are comagmatic keith et al 1989 1991 waite 1996 the tintic district and east tintic sub district in utah have recorded over

the november 29 1978 oaxaca mexico earthquake a large simple event the recent oaxaca mexico earthquake november 29 1978 mw 7 6 ms 7 8 seismic moment m0 3 2 1027 dyne cm is of special interest because of its location within a predetermined seismic gap the event excited long period 100 200 s multiple rayleigh and love waves which were well recorded by the wwssn these data along with p wave first motion data and p waveforms were used to constrain the source mechanism the results indicate an oblique thrust mechanism consistent with subduction of the cocos plate to the northeast beneath mexico dip 14 strike n90 w rake 54 hence this event is indeed of the type anticipated by ohtake et al 1977 a local network of stations installed in a joint university of mexico california institute of technology program began operation 20 days prior to the mainshock gonzalez 1979 mcnally et al 1979 mcnally 1981 ponce et al 1980 the 43 foreshocks of magnitude m 2 8 were recorded by the network in a period of 20 days prior to the mainshock these events show an interesting spatial and temporal pattern which culminates in the last 1 8 days of the sequence with an apparent migration of activity toward the epicenter of the impending earthquake this pattern can be interpreted as a buildup of stress or migration of stress toward a fault zone asperity with supplemental stations the network continued operation until december 12 1978 singh et al 1980 and provided good hypocentral control for the more than 169 aftershocks of magnitude m 2 8 the area of the aftershock zone determined from these events is 5525 km2 85 km by 65 km in spite of the large size of the event p waves for the oaxaca event indicate an extremely simple source at the period range of the wwssn long period seismographs this simplicity suggests that the p waves were generated by a limited portion of the rupture surface perhaps by the breaking of a fault zone asperity this result may be further supported by the discrepancy between the larger surface wave moment compared with that determined from body waves such simplicity also appears to be the case for the 1965 and 1968 mexican earthquakes on the adjacent eastern and western ends of the oaxaca aftershock zone respectively chael et al 1980 this type of body wave simplicity for a large subduction zone earthquake is an important characteristic of the mode of strain release along some subducting plate boundaries

the 1982 enola arkansas swarm and scaling of ground motion in the eastern united states abstract the north central arkansas swarm which began in january of 1982 and was still active as of december 1983 has produced over 30 000 events three of these events had duration magnitudes greater than 4 0 fifty three locatable events in the magnitude range 3 8 and below were recorded during the 2 1 2 weeks our 10 digital seismographs were in place the hypocentral locations are confined to a tight cluster with a radius of approximately 1 km depths range between 4 and 7 km focal mechanisms determined for 10 of the events show a combination of strike slip and normal faulting source parameters seismic moment source radius and stress drop were calculated for 48 of the events the seismic moments range from 10 18 to 10 21 dyne cm while the stress drops range from 0 2 to 60 bars acceleration spectra are band limited at the high end at 30 hz the stress drops increase with seismic moment up to 10 20 dyne cm but appear to be constant at greater moment values the arkansas source parameters are similar to other body wave source parameters determined from near source investigations of earthquakes in eastern and central north america e g new brunswick and south carolina and in california e g mammoth lakes oroville and anza but differ substantially from lg wave source parameters estimated for earthquakes in the eastern and central united states

strong ground motion from the michoacan mexico earthquake the network of strong motion accelerographs in mexico includes instruments that were installed under an international cooperative research program in sites selected for the high potenial of a large earthquake the 19 september 1985 earthquake magnitude 8 1 occurred in a seismic gap where an earthquake was expected as a result there is an excellent descripton of the ground motions that caused the disaster

crust mantle interaction in continental arcs inferences from the mesozoic arc in the southwestern united states arc magmas ranging in composition from basaltic andesites to rhyolites and intrusive equivalents were emplaced into the western margin of the north american craton starting in late triassic time giving way to rift0related sedimentation in the late jurassic the region of this study cuts across proterozoic basements of contrasting nd model ages 1 7 1 8 ga average nd 11 in eastern arizona and 2 0 to 2 3 ga average nd 18 in western arizona and eastern california bennett and depaolo 1987 the mesozoic rocks have initial nd of 3 4 to 6 4 in the eastern part of the study area and 7 1 to 9 2 in the western part all of the rocks have elevated 87sr 87sr initial ratios 0 706 trends in initial nd values of mesozoic arc rocks are directly correlated with the nd model ages of the basement through which they passed simple two component mixing calculations indicate that recycled continental crust in the arc magmas represents on average about 65 a minimum of 35 mantle input into continental arc magmas as recent as the mesozoic represents a significant contribution to the growth of the continental crust in the absence of a return flow of continental material into the mantle of similar magnitude in a detailed study in the santa rita mountains arizona there is a pattern of increase of nd with time early basaltic andesites have more negative nd than later felsic rocks a correlated pattern of depletion with time is also observed with trace element and major element data we attribute this either to progressive hybridization of the lower crust by repeated injection of mantle magmas or the progressive thinning of the continental crust during prolonged arc magmatism the present data do not allow distinction between the two models progressive decrease in crustal contribution to arc magmas with time may be an important feature of continental arc evolution hybridization of the lower crust due to repeated injection of mantle melts during arc magmatism may help contribute to small scale heterogeneities in lower crust inferred from seismic and xenolith data similarly whether there is a well defined moho or sharp crust mantle boundary in any given segment of the continental crust may in part depend on the extent of crust modification as a result of continental arc magmatism

crustal structure of oaxaca mexico from seismic refraction measurements seismic refraction and gravity data have been analyzed to obtain a model of the compressional wave structure of the ocean to continent transition in the state of oaxaca in southwestern mexico crustal thickness on the continent at the latitude 18 n is 45 4 km based on reflected phases from the moho discontinuity the crust has been modeled with three layers with velocities of 4 3 to 4 6 5 0 to 5 7 and 6 85 to 7 0 km sec each with positive velocity gradient the crust thins to 10 km at the coast near pinotepa nacional where precambrian metamorphic rocks are exposed 45 km from the mid america trench offshore the oceanic structure consists of an 8 km thick crust with a normal crustal velocity structure spudich and orcutt 1980 the apparent dip of the subducting plate beneath western mexico is 10 on the oceanic side strong reflections suggest a minimum depth of 35 km for the lithosphere asthenosphere boundary the asthenosphere has a seismic velocity of 7 6 km sec and a thin lid in which the velocity is 8 6 km sec

source parameters of the 1980 mammoth lakes california earthquake sequence from the more than 1500 mammoth lakes earthquakes recorded on three component digital seismographs spudich et al 1981 150 were used in an analysis of the locations mechanism and source parameters a composite fault plane solution of nine earthquakes 3 9 m 5 1 defines a right lateral strike slip mechanism on a steeply dipping nearly east west plane striking s75 e or left lateral strike slip on a nearly north south plane striking n10 e vertical cross sections of well located aftershocks indicate possibly three east west planes that coincide with the locations of the four largest earthquakes with ml 6 0 using the spectral analysis of s waves brune 1970 source parameters for 67 earthquakes were determined forty eight had magnitudes greater than or equal to 3 0 seismic moments ranged from 9 20 1018 dyn cm to 2 33 1024 dyn cm earthquakes with seismic moment greater than about 1 0 1021 dyn cm had nearly constant stress drops 50 bars earthquakes with seismic moment less than about 1 0 1021 dyn cm had stress drops that apparently decrease as seismic moment decreases

latest cretaceous and early tertiary orogenesis in south central arizona thrust faulting regional metamorphism and granitic plutonism rocks in a number of mountain ranges in southernmost central arizona are juxtaposed by thrust faults regionally metamorphosed and intruded by garnet two mica granites field relations and k ar and u pb isotopic geochronology indicate that thrust faulting metamorphism and granitic plutonism were closely related aspects of a latest cretaceous and early tertiary orogenic episode the regionally metamorphosed rocks chiefly greenschist facies quartzofeldspathic schists were derived from jurassic and cretaceous sedimentary volcanic and plutonic rocks in five ranges these metamorphic rocks are overlain along synmetamorphic thrust faults by precambrian gneiss or late jurassic or late cretaceous plutonic rocks in three additional ranges upward increase of textural grade within the metamorphic rocks and analogy with the areas of exposed thrust faults indicate the likelihood of a concealed thrust fault flanking the range seven of these eight thrust faults have single lower plates but one thrust is underlain by a duplex consisting of several imbricate structural sheets separated by mylonitic tectonic slides areal structural relations and the regional arrangement of distinctive jurassic igneous rock units strongly suggest that one large range the baboquivari mountains and several smaller ranges are fensters in a thrust fault system of regional extent and that several adjacent ranges are allochthonous the regionally metamorphosed rocks are intruded by synmetamorphic to postmetamorphic early tertiary leucocratic granites characterized by various combinations of accessory biotite muscovite and garnet at least two of these granites contain inherited precambrian zircon indicative of generation by crustal anatexis k ar and or u pb isotopic ages of premetamorphic granites metamorphic rocks thrust zone mylonites synmetamorphic granites and metamorphogenic fissure veins indicate that the orogenic episode commenced in late cretaceous time and culminated in early tertiary time 58 to 60 m y ago the observed and inferred close spatial and temporal relations between thrust faulting regional metamorphism and granitic plutonism lead to the following hypothesis for orogenesis in south central arizona crustal compression caused overthrusting of crystalline rocks resulting in crustal thickening and crustal thickening and conductive and magmatic heat flux from the mantle together set up a thermal regime within which regional metamorphism and the generation and emplacement of the anatectic granites took place

geochemical and nd isotopic systematics of granites from the arunta inlier central australia implications for proterozoic crustal evolution geochemical and sm nd isotopic results are reported for granites from the proterozoic arunta inlier of central australia these combined with new geochronological data for the granites zhao and bennett 1995 allow important constraints to be placed on proterozoic tectonic and crustal evolution in central australia granites from the arunta inlier can be divided into three major geochemical groups a calcalkaline trondhjemitic group cat a high heat production group hhp and a volumetrically significant main group the cat group which occurs only in the southern margin of the inlier is characterised by high na2o nak sr krb and sry and relatively low k2o rb rbsr th u ree nb and y analogous to calc alkaline suites occurring in modern convergent plate margins the hhp group which occurs mainly in the interior of the arunta inlier and is spatially associated with the main group is characterised by high k rb th u rbsr and rbzr and relatively low sr ba nak krb barb mgo cr and ni the main group which occurs throughout the arunta inlier is geochemically intermediate between the cat and hhp groups it is geochemically analogous to the 1880 1850 ma old barramundi igneous association recognised in other proterozoic terrains of central australia and can be further subdivided into four age subgroups 1820 1780 1750 1650 and 1615 1600 ma respectively all three groups show similar negative nb anomalies on the trace element normalized diagrams despite the geochemical diversity of the arunta granites no correlations between the nd isotopic and geochemical signatures are observed the ranges of initial nd values and nd depleted mantle model ages tnddm for the three groups of granites overlap with each other overall there are two groups of tnddm ages the most common ranging from 2 33 to 1 96 ga and the other from 1 83 to 1 72 ga although initial nd values of the granites show large variations there is a general trend for the nd values to increase i e become less negative with decreasing crystallization ages the large geochemical and isotopic variations of the arunta granites reflect considerable heterogeneity in the sources of the granites and do not support a uniform 2 3 2 1 ga old barramundi type underplate source for the origin of the granites the combined geochemical and nd isotopic data suggest the sources of the granites contain at least two main components an palaeoproterozoic mantle derived component and an older crustal component the older crustal component was incorporated into the source region of the granites either through magma assimilation of crustal materials at lower crustal levels or more probably via sediment subduction and mantle wedge metasomatism the increasing nd values with decreasing crystallization ages may be a result of progressively increasing proportions of newly accreted island arc materials being incorporated in the subducted sediments alternatively a juvenile mantle derived component may have been added to the source region of the younger granites it is apparent that the nd isotopic signatures in the granites has resulted from complex processes and neither simple mixing nor simple two stage protolith models can as yet satisfactorily explain the observations the petrogenesis of the three geochemical groups of granites can be interpreted in a plate tectonics scenario it is considered that the cat group was formed by fractionation of arc type magmas and or partial melting of arc related intrusions or underplates in a subduction related continental margin setting whilst the formation of the main group may involve partial melting of fractionated and or modified arc related underplates but not necessarily in a subduction environment the younger hhp group was probably generated by remelting of the coexisting main group during subsequent tectonothermal events

analysis of the petatlan aftershocks numbers energy release and asperities the petatlan earthquake of march 14 1979 ms 7 6 occurred between the middle america trench and the mexican coast 15 km southwest of petatlan guerrero mexico from seismograms recorded on smoked paper fm and digital tapes we have identified 255 aftershocks with coda lengths greater than 60 s that occurred 11 hours to 36 days after the main shock based on these events the aftershock epicentral area defined during the period between 11 and 60 hours was about 2000 km2 between 11 hours and 6 days it was about 2400 km2 although the area grew to 6060 km2 in 36 days most of the activity was still confined within the area defined after 6 days this suggests that the smaller aftershock area might represent an asperity the distribution of events and energy release per unit area confirm the existence of heterogeneity in the aftershock area thus our data support the concept of an inhomogeneous rupture area that includes an asperity as suggested by chael and stewart 1982 to account for the differences they computed for the body and surface wave moments from wwssn data however the combination of the moments reichle et al 1982 report for body and surface waves from ida data and the rupture areas reported in this paper results in a solution that is most physically realizable in terms of stress drop and slip we calculate stress drops of 5 and 15 bars the former for the average over the entire area the latter for the asperity and an average slip of 60 cm for the entire area and 120 cm for the asperity these values for slip are 30 and 60 respectively of the convergence of the cocos plate relative to the north america plate during the 36 year period between the last two major earthquakes in the petatlan area hypocenters of the aftershocks define a zone about 25 km thick dipping 15 with an azimuth of n20 e which is perpendicular to the middle america trench most aftershocks are below the main shock the b value estimated for aftershocks in the range 3 0 mcl 4 5 is 60 higher than one determined similarly by hsu 1981 for the foreshocks

local seismicity preceding the march 14 1979 petatlan mexico earthquake ms 7 6 local seismicity surrounding the epicenter of the march 14 1979 petatlan mexico earthquake was monitored by a network of portable seismographs of the hawaii institute of geophysics from 6 weeks before to 4 weeks after the main shock prior to the main shock the recorded local seismic activity was shallow and restricted within the continental plate above the benioff zone the relocated main shock hypocenter also lay above the benioff zone suggesting an initial failure within the continental lithosphere four zones can be recognized that showed relatively higher seismic activity than the background activity within these zones has followed a number of moderate earthquakes that occurred before or after the initial deployment of the network three of these moderate earthquakes were near the mexican coastline and occurred sequentially from southeast to northwest during the three months before the petatlan earthquake the petatlan event occurred along the northwestern extension of this trend we infer a possible connection between this observed earthquake migration pattern and the subduction of a fracture zone because the 200 km segment that includes the aftershock zones of the petatlan earthquake and the three preceding moderate earthquakes matches the intersection of the southeastern limb of the orozco fracture zone and the middle america trench the petatlan earthquake source region includes the region of the last of the three near coast seismic activities zone a earthquakes of zone a migrated toward the petatlan main shock epicenter and were separated from it by an aseismic zone about 10 km wide we designate this group of earthquakes as the foreshocks of the petatlan earthquake these foreshocks occurred within the continental lithosphere and their observed characteristics are interpreted as due to the high stress environment before the main shock pre main shock seismicity of the petatlan earthquake source region shows a good correlation with the aftershocks in their spatial distribution this suggests that an asperity existing along the benioff zone may have affected both the pre main shock activity in the continental lithosphere and the aftershocks along the benioff zone although major thrust earthquakes at trenches occur along benioff zones in the present study we find little activity on this interplate boundary before the petatlan earthquake the overlying continental block on the contrary is very active seismically our data suggest that the activity is probably governed by the stress transmitted from below due to coupling between two plates and the heterogeneity within the continental lithosphere the continental material is probably the more likely place for precursors

migration of tertiary volcanism in the southwestern united states and subduction of the mendocino fracture zone abstract a compilation of 417 isotopic dates on mid and late cenozoic igneous rocks from the southwestern united states shows that volcanism migrated northward with time the principal locus of volcanism at any given time was an east west band that corresponded closely with the calculated position of the subducted mendocino fracture zone mfz under the north american plate this correspondence supports the theory that volcanism was triggered by subduction of the mfz which was a major 1 km north facing topographic step in the farallon plate both volcanism and the mfz moved northward at about 3 1 cm year andesites and rhyolites show close correspondence to mfz passage but many basalts were erupted significantly later cooling dates on basement rocks in southern arizona cluster at the time the mfz passed under that area earlier models proposing rapid late tertiary steepening of the subducted farallon plate were based on a westward sweep of volcanism our compilation shows no evidence for such a westward sweep

recent large earthquakes along the middle american trench and their implications for the subduction process several large shallow earthquakes ms 7 0 have occurred along the middle american trench since the installation of the wwssn network included are the 1965 1968 and 1978 oaxaca events the 1970 chiapas event the 1973 colima event and the 1979 petatlan event these earthquakes have been studied in an attempt to identify similarities and differences between them that may lead to a better understanding of fracture and subduction processes the events have seismic moments ranging from 1 0 1027 dyne cm for the 1968 event to 3 2 1027 dyne cm for the 1978 event all events are of predominantly thrust type consistent with subduction to the northeast of the cocos plate body waves for the 1965 1968 1978 and 1979 events along the trench indicate rather simple faulting processes these events all had focal depths of 15 to 20 km and stress drops of the order of 10 bars the 1970 and 1973 events the easternmost and westernmost respectively of the events studied here are located close to triple junctions for the cocos north america caribbean plates 1970 and the cocos north america rivera plates 1973 these two events generated more complex body waves than the oaxaca and petatlan quakes this source complexity may have been related to the more complicated tectonics near the 1970 and 1973 epicenters this may offer a new perspective to viewing source complexity in relation to subduction zones

metallogenesis and ore controls of cenozoic porphyry mo deposits in the gangdese belt of southern tibet abstract the gangdese is a newly explored porphyry copper ore belt in china except for the cu mo au porphyry deposits there are some mo cu w porphyry and skarn deposits in this belt two pulses of molybdenite mineralization are recognized in the central lhasa subterrane with ancient continent crust including the paleocene eocene 65 52 ma porphyry mo and skarn mo w deposits formed during the rollback of neo tethyan oceanic slab in collisional setting and miocene 21 15 ma porphyry mo cu deposits generated in the postcollisional setting the gangdese copper deposits also occur during these two periods but are distributed in the southern lhasa subterrane dominated by juvenile crust through mantle derived magmatism resulting from neo tethyan ocean lithosphere subduction the intrusions related to molybdenite mineralization have relatively lower bulk rock e nd t and higher 87 sr 86 sr i values lower zircon e hf t values and older hf model ages lower molybdenite re contents and more radiogenic pb isotopes than the coeval copper related porphyries in the gangdese belt in addition the miocene mo cu related intrusions generally have lower zircon ce 4 ce 3 ratios than the miocene cu mo related intrusions these data indicate that more contributions from mature continental crustal materials play crucial role in the generation of mo than cu porphyry mineralization in the gangdese belt the eocene mo related intrusions are characterized by high sio 2 68 76 and k 2 o 5 1 5 9 concentrations enrichment of large ion lithophile elements lile depletion of high field strength elements hfse and low sr y 7 5 23 8 ratios their sr nd isotope compositions e nd t 3 4 to 4 6 87 sr 86 sr i 0 70622 0 70679 indicate that they are associated with partial melting of mantle wedge mixing with significant amounts 55 65 of middle upper crustal materials the miocene mo cu related intrusions have relatively high sio 2 64 73 and k 2 o 1 9 4 8 contents and are characterized by enrichment of lile depletion of hfse and high sr y ratios 55 189 their sr nd isotope compositions e nd t 4 6 to 3 4 87 sr 86 sr i 0 70622 0 70679 are indicative of derivation from remelting of previously subduction modified tibetan lithosphere with significant amounts 50 70 of middle upper crustal materials for the gangdese mo cu porphyry mineralization mo metal was suggested to be derived from the middle upper crustal materials particularly the carbonaceous permian marine sedimentary whereas cu metal to be sourced from the gangdese miocene cu fertile magmas the exploration targeting of the gangdese molybdenite deposits should be focused on southern part of the central lhasa subterrane where the carbonaceous permian strata occur the occurrence of mo cu porphyry deposits could provide clue for targeting cu mo porphyry deposits in the gangdese belt the gangdese molybdenite deposits share many similarities with the east qinling molybdenite deposits such as type of ore deposits geodynamic setting of their formation variable mo grades and high f contents

depth dependence of source parameters at monticello south carolina three estimates of stress differences which include brune stress drop stress drop from rms of acceleration arms and the apparent stress have been determined for 13 earthquakes at monticello south carolina a site of reservoir induced seismicity data for nine of the events come from digitally recorded three component seismograms at four or five stations that were deployed around the monticello reservoir in may and early june 1979 the data from the other four events come from a strong motion accelerograph located on the dam abutment at the southwest end of the reservoir estimates of the seismic moment m o range from 4 6 1017 to 3 4 1020 dyne cm s waves and radiated energy from about 1011 to 3 1016 dyne cm s waves brune stress drops ranged from 0 5 bars to about 90 bars and show a strong dependence on depth focal depths range from 0 07 to 1 4 km and a moderate dependence on m o arms stress drops from the direct s wave span a similar range of values and also exhibit a strong dependence on depth apparent stresses are usually lower than the other estimates of stress differences by a factor of 2 to 4 seismic stress differences are highest in the topmost 0 2 to 0 3 km a depth range for which the in situ measurements of stress and pore pressure suggest that the rock is in a state of incipient failure in this depth range where the four largest events occurred the stress drops are of the same order as the ambient shear stress these data suggest that at monticello where pore fluids have a strong influence on the failure process the largest stresses released seismically are in regions most conducive to failure and that the seismic efficiencies for events at monticello are larger than have been reported for other tremors in different tectonic settings

common pb isotopic compositions of the lima arequipa and toquepala segments in the coastal batholith peru implications for magmagenesis abstract common pb isotopic compositions for 65 feldspar samples from the 188 37 ma old gabbroic to monzogranitic plutons of the peruvian coastal batholith show changes along strike that can be related to variable contamination of mantle derived magmas by the local precambrian basement results have the following isotopic ranges 206 pb 204 pb 17 580 20 803 207 pb 204 pb 15 555 15 709 and 208 pb 204 pb 38 104 41 177 with averages of 206 pb 204 pb 18 630 207 pb 204 pb 15 610 and 208 pb 204 pb 38 500 the gabbroic dioritic and tonalitic plutons north and east of lima the lima segment have ratios similar to the isotopically homogeneous reservoir identified by tilton 1979 tilton and barreiro 1980 and barreiro and stern 1982 for rocks in central and southern chile the homogeneous reservoir has been suggested to be enriched subcontinental mantle more siliceous rocks granodiorites and monzogranites in the lima segment have considerably higher ratios 206 pb 204 pb 18 680 20 803 207 pb 204 pb 15 610 15 709 and 208 pb 204 pb 38 500 41 177 nine of ten monzogranites from the ring dike complexes in the lima segment define a linear array with a slope of 0 1143 correlation coefficient 0 9398 and a pb isochron age of 1 9 ga this isochron age has no simple chronological meaning although it may represent a component of sedimentary debris derived from 1 9 ga crustal materials feldspars from plutons in the arequipa and toquepala segments south of lima tend to have low pb isotopic ratios 206 pb 204 pb 17 580 18 603 207 pb 204 pb 15 555 15 633 and 208 pb 204 pb 38 104 38 749 the ratios plot between the isotopic signatures of local precambrian granulites and gneisses and the chile type enriched subcontinental magmatic reservoir indicating that these end members were the principal magma sources the oldest plutonic rocks in the segments early to mid jurassic are closest to the pb isotopic ratios of the precambrian basement contamination of plutons in the arequipa and toquepala segments by unradiogenic pb from the precambrian basement and lack of it in the lima segment supports crustal models by couch et al 1981 and jones 1981 which show a thick precambrian basement layer throughout southern peru and an extremely thin layer beneath the lima segment

source mechanisms of the june 7 1982 ometepec mexico earthquake body wave synthetic seismograms are used to study the source parameters and rupture time history of the doublet of large m s 6 9 7 0 earthquakes which occurred within five hours of one another near the town of ometepec mexico on june 7 1982 the events are thrust events consistent with the subduction of the cocos plate beneath the north america plate our results indicate that these are not the simple single source events previously thought to be typical of the middle america trench in this region while the first event had a simple source time function the second did not this relatively complex behavior can not be explained by any tectonic complications that have been proposed to account for the complexity of the 1970 chiapas earthquake and the 1973 colima earthquake multiple earthquake sequences in 1962 1950 and 1928 have occurred in the general area of the two 1982 ometepec earthquakes suggesting that earthquake doublets are characteristic of this region

nd and sr isotopic compositions of lower crustal xenoliths from north queensland australia implications for nd model ages and crustal growth processes abstract sr and nd isotopic analyses of granulite facies lower crustal xenoliths from northern queensland australia fall on a mixing array on a 87 sr 86 sr vs 143 nd 144 nd diagram at 300 ma this was a time of intense orogeny in northern queensland attended by granitic magmatism in the upper crust and granulite facies metamorphism in the lower crust ion microprobe zircon dating indicates that most of the xenoliths formed at this time the sr and nd isotopic compositions of these rocks are therefore interpreted to have resulted from large scale mixing in the lower crust between mantle derived basaltic melts and isotopically evolved early proterozoic continental crust both end members are represented in the xenolith suite depleted mantle nd model age tdm ages for meta igneous granulite xenoliths having appropriate 147 sm 144 nd i e between 0 12 and 0 15 range from 1 9 to 2 2 ga for mafic melt like xenoliths and 1 2 to 1 4 ga for felsic melt like xenoliths however zircon ages on these same samples show that their protoliths crystallized during the permo triassic orogeny thus the old tdm ages for the mafic xenoliths do not accurately reflect their crustal residence ages possibly due to crustal assimilation or their derivation from an enriched mantle source this is a common feature of mafic granulite xenoliths the world over and highlights the difficulty in determining the age of mafic underplates hence the timing of crustal growth from tdm ages because the felsic melt like xenoliths lie on the mixing array their mid proteozoic tdm ages are likely to have resulted from mixing and therefore represent an average crustal residence age the combined data suggest that significant crustal growth and reworking occurs in the lower crust during phanerozoic crustal orogenies

isotopic and paleomagnetic constraints on the mesozoic tectonic evolution of south china in order to better constrain the paleogeographic evolution of south china we measured sm nd and rb sr isotopic compositions for 23 mesozoic granites that crop out throughout the area tightly grouped neodymium depleted mantle model ages 1 4 0 3 ga suggest the region is underlain by relatively homogeneous proterozoic crust and fail to define crustal provinces neither the isotopic nor geologic data suggest that a mesozoic suture exists however granites possessing anomalously high sm 8 ppm and nd 45 ppm concentrations relatively high initial epsilon neodymium 4 to 8 and high but variable initial 87sr 86sr 0 759 to 0 713 form a northeast trending zone that coincides with two prominent mesozoic basins southeast of the zone lie the majority of mesozoic intrusives and upper triassic to lower cretaceous extensional basins found in south china mesozoic paleomagnetic poles are well clustered northwest of the zone pre cretaceous poles southeast of it are discordant with respect to those from the northwest the only recognized tectonostratigraphic terrane in south china lies southeast of the zone the terrane is bordered by a northeast trending sinistral fault that was active in the mesozoic other faults in south china have similar attitudes ages and sense of shear together the observations suggest that the mesozoic tectonic regime in south china consisted of strike slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults the zone defined by the granites enriched in nd and sm demarcates displaced terranes to the southeast from relatively stable land to the northwest

isotopic evidence for the provenance of turquoise in the southwestern united states the archaeological record shows that turquoise was widely mined and highly valued by pre hispanic societies in the southwestern united states and it has long been assumed that much of the turquoise noted in ancient mesoamerica was traded from this region however little is understood about the acquisition and exchange of turquoise by native american societies because the geological sources of most turquoise artifacts from archaeological sites in the southwestern u s and mexico are not known we evaluate the potential for pb and sr isotopic ratios to indicate the geological provenance of turquoise artifacts recovered from these regions pb and sr isotopic measurements were made on 137 geological samples of turquoise from 19 mining districts across the southwestern united states and northern mexico these data reveal that isotopic signatures of turquoise deposits vary geographically according to regional and local differences in the geologic settings of turquoise mineralization as an archaeological case study we also report pb and sr isotopic data on 10 turquoise artifacts recovered from three late pre hispanic ca a d 1250 1400 ancestral zuni sites located in the el morro valley of western new mexico these have isotopic signatures uniquely consistent with the turquoise deposits of the cerrillos hills a location identified in zuni traditional history as an ancient source of turquoise these data thus establish pb and sr isotopic measurements as powerful tools for determining the sources of turquoise artifacts and provide a new framework for evaluating the role of turquoise in pre hispanic exchange networks across north america

a chemical and isotopic study of the laramide granitic belt of northwestern mexico identification of the southern edge of the north american precambrian basement along the laramide belt of northwestern mexico granitic rocks of similar bulk composition show isotopic and trace element signatures that help to delineate the position of the southern edge of the north american precambrian basement in the northern part the laramide plutons the northern granites intruded proterozoic crystalline rocks and a thick late proterozoic through paleozoic miogeoclinal cover of north american affinity in the central part the granitic bodies the central granites were emplaced into a sequence of paleozoic eugeoclinal rocks overlain by late triassic clastic units the southern part of the belt the southern granites intruded a less known crust characterized by middle to late mesozoic island arc related volcanic and sedimentary rocks of the guerrero terrane data from a suite of metaluminous to slightly peraluminous calc alkalic granitic rocks along the belt display north to south geochemical and isotopic variations which could correlate with the type of intruded basement the northern and central granites are characterized by strongly fractionated light rare earth element ree enriched patterns which display generally pronounced negative europium anomalies whereas the southern granites have lower total ree enrichments and much flatter chondrite normalized slopes displaying almost no europium anomalies isotopic results also suggest regional variations as shown by the following initial sr and end ranges 0 7070 to 0 7089 and24 2 to25 4 respectively for the northern granites 0 7060 to 0 7079 and23 4 to25 1 for the central granites and 0 7026 to 0 7062 and20 9 to 14 2 for the southern granites on the basis of their isotopic similarities the proterozoic mafic to intermediate lower crust revealed by xenoliths from young volcanic flows in southern arizona and northern mexico is interpreted as a reasonable parental source for the northern and central granites however mantle derived melts are not excluded the more primitive southern granites are interpreted to come from a source that lacked proterozoic basement instead they were probably derived by mixing of juvenile mantle melts with partial melts of the lower parts of the guerrero terrane in general the north to south compositional variations of the laramide granitic rocks of northwestern mexico reflect the crustal structure underneath the batholiths the sr and nd data indicate that the edge of the north american precambrian basement extends approximately southeastward from the coastal batholith of central sonora then about 200 km south of hermosillo in southern sonora the edge bends eastward and continues to the east beneath the sierra madre occidental volcanic province

re evaluation of the porosity measurements under different confining pressures a better appraisal of reservoir porosity porosity is one of the most important rock properties in describing hydrocarbon reservoirs tests on core samples provide direct and representative porosity data and the measurement of porosity at high confining pressures is recognized to correlate well with subsurface reservoir porosity whereas theoretical deductions of the changes and relationships of pressures volumes and compressibility suggest that porosity is reduced during the coring and lifting processes the porosity measurement at elevated confining pressure does not evaluate original reservoir porosity this theory is quantitatively validated by repeated laboratory experiments of loading and unloading on sandstone core samples when the in situ confining pressure is approximately 30 35 mpa 4350 5076 psi coring and lifting would cause a porosity reduction of approximately 1 2 1 6 and the porosity test under high confining stress results in further porosity loss a revised approach in calculating reservoir porosity from cored samples is proposed and can have significant implications for reserve calculations recovery factors and geostatistical reservoir models the study is important for both conventional and unconventional reservoirs because it discusses a fundamental mechanism of porosity change

paleoconvergence rates determined from k2o sio2 ratios in magmatic rocks and their application to cretaceous and tertiary tectonic patterns in southwestern north america spatial temporal and chemical data may be used to calculate paleodip angles for paleosubduction zones assuming the position of the paleotrench is reasonably well known in turn paleoconvergence rates may be calculated according to the equation developed by luyendyk 1970 that relates dip angle to convergence rate data from cretaceous tertiary magmatic rocks indicate paleoconvergence rates of 7 to 8 cm yr for the period 130 to 85 m y ago convergence rates markedly increased to an average of about 14 cm yr for the period 85 to 45 m y decreased convergence rates are indicated from 45 to 20 m y paleoconvergence rates determined from magmatic rocks are generally in agreement with those determined independently by other workers from paleomagnetic data on ocean floor rocks the anomalously high rates of convergence determined from the igneous rock data may be due to subduction of younger more buoyant oceanc crust after 65 m y b p although recent observations from independent data suggest that eocene convergence may have been a high 20 to 26 cm yr paleomagnetically the period of constant convergence constant dip subduction from 130 to 85 m y ago correlates almost exactly with the long cretaceous normal polarity interval as suggested on the new lowrie alvarez 1981 geomagnetic time scale in contrast the post 85 m y period of variable convergence and variable dip subduction correlates with polarity events of short duration and frequent reversal tectonically the period of constant convergence rates from 130 to 85 m y ago correlates with sevier orogeny in nevada and utah the period of increased convergence from 85 to 45 m y ago correlates with classic laramide orogeny in wyoming and colorado the period of decreased convergence from 45 to about 20 m y ago correlates with a poorly understood and incompletely defined mid tertiary orogenic event

extensional shearing deformation and kilometer scale a type folds in a cordilleran metamorphic core complex raft river mountains northwestern utah cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations in the raft river mountains a mylonitic zone and a late superimposed detachment fault separate precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous paleozoic rocks which suffered a brittle extensive deformation we suggest that the mylonitic ductile deformation is the result of tertiary extensional tectonics strain analysis shows that 1 strain results from large scale progressive shearing deformation in a non coaxial regime 2 e w regional stretching lineations represent the direction of shearing and transport 3 shear criteria indicate an east directed movement 4 strain magnitude is increasing eastward 5 kilometer scale folds parallel to the lineations are interpreted as a type folds formed by eastward shearing of a previously northward dipping layering measurement of striated faults in the upper plate gives an e w direction of extension parallel to the lineations thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics a model is proposed to explain the observed geometry and kinematics of the structures the mesozoic to paleocene cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes important uplift abnormal heating and granite emplacement occurred later during the collapse of the former belt the combined effects of thermal and gravitational instabilities during tertiary extensional tectonics enabled the development of a ductile shear zone localized at the precambrian basement paleozoic cover interface the brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust

metamorphic terranes isotopic provinces and implications for crustal growth of the western united states simplified maps of the western cordillera indicating distribution of metamorphic facies and times of principal recrystallization have been compiled despite rearrangement by transcurrent shuffling general trends are recognizable narrow belts of phanerozoic high p blue schist and serpentinized peridotite lie outboard from broad penecontemporaneous high t metamorphosed calcalkaline magma intruded belts bordering the pacific ocean and both are disposed approximately parallel to the present day north american margin they represent recrystallized products of mid paleozoic and younger subduction zones andean type margins and exotic island arcs in contrast dominant metamorphic mineral parageneses within the continental interior are spatially associated with chiefly mesozonal granitic plutons glaucophane schists eclogites and large tracts of ophiolitic ultramafics are absent in part due to selective overprinting of old accretionary margin lithotectonic assemblages by subsequent orogenies high t recrystallization and anatexis for example at depth within the california coast ranges high p phases are currently being destroyed farther inland a general paucity of oceanic petrotectonic assemblages is evident possibly reflecting the differential sinking of dense basaltic peridotitic units during thermal softening preserved metamorphic field gradients reflect the magnitude of subsequent uplift and erosion as well as original pt conditions late mesozoic to mid cenozoic recrystallization accompanied earlier crustal thickening ductile deformation succeeded by later extension brittle faulting mylonitization mesozoic cenozoic metamorphism involved cratonal rocks in the mojave sonoran desert but in nevada and to the north eugeoclinal sections were recrystallized metamorphic intensity decreases eastward toward the phanerozoic hinge line basement rocks in the rockies display the effects of shallow level crustal deformation but much less widespread evidence of post proterozoic plutonism and regional metamorphism initial pb nd and sr isotopic compositions and crystallization ages of igneous rocks in comparison with times of intrusion recrystallization and metamorphic facies distributions are compatible with a geologic scenario involving gradual proximal growth of the ancient continent chiefly southward from the archean wyoming craton during early and mid proterozoic time followed by late proterozoic phanerozoic westward development accretion involved the incorporation of some exotic mostly oceanic outboard terranes however continental growth in the western u s cordillera during both proterozoic and phanerozoic time periods appears to have been due chiefly to partial fusion and magma ascent above subducting paleo pacific lithospheric plates combined with metamorphic sedimentary reworking in the forearc trench back arc setting rather than resulting from the amalgamation of preexisting ancient continental fragments

zircon u pb and molybdenite re os dating and geological implications of the shimadong porphyry molybdenum deposit in eastern yanbian northeastern china the shimadong porphyry mo deposit is located in eastern yanbian in the eastern part of the north margin of the north china craton northeastern china here we present the whole rock major and tra

crustal affinities of metallogenesis in the american southwest the source of metals in many lands of ores and ore deposit styles remains uncertain notwithstanding more than a century of field studies scientific advances in concepts and enhancement of analytical capability however numerous geologic research efforts that have addressed many relevant themes in southern arizona have resulted in different kinds of data supporting and consistent with the interpretation of significant association of some ore metals of this region with cratonic crust these themes range in scale from studies of regional geology geophysical interpretations and precious metal production through petrology to trace element and isotope geochemistry the notion of a crustal source for metals contrasts with many long standing ideas of direct mantle sources for this metal province or more recently direct derivation of metals from subducted materials a hundred years of recorded silver and gold production reveal a two fold separation of mining districts in arizona and new mexico on the basis of ranges of produced ag au ratio that correspond in turn with a separation of two distinct geologic domains of the southern cordilleran metal province the produced ag au ratio in districts of different ages ore styles tectonic setting and metal habit are not affected by deposit style or ore control or igneous rock type or vertical zoning and weathering studies of the isotopes and trace element chemistry of related igneous rocks reveal that ore related intrusions have a significant crustal progenitor studies of pb and os isotopes have discovered strong signals of crustal parentage in ore minerals the regional geology metallization history production and chemical and isotopic data strongly support a hypothesis of crustal sources of some of the metal in this large and complex province

stress drop estimates for some aftershocks of the ometepec guerrero mexico earthquake of june 7 1982 brune s stress drop apparent stress and arms stress drop are estimated at a single station for 25 aftershocks of the ometepec earthquakes ms 6 9 and ms 7 0 the arms stress drops and apparent stresses are systematically smaller than brune s stress drops stress drops from the root mean square of acceleration and apparent stress range from 0 01 to 10 2 bars 1 bar 100 kpa except for two values 21 4 and 33 0 bars on the other hand brune s stress drops range from 0 6 to 239 bars seismic moments ranging from 0 5 1019 to 289 1019 dyn cm 1 dyn cm 10 n cm were estimated for events with coda magnitudes between 0 6 and 2 2 values of radiated seismic energy calculated by integration of the displacement spectra range from 2 5 1012 to 2 3 1016 dyn cm the fmax values lie between 16 and 30 hz seismic coda wave attenuation measured on narrow band pass filtered seismograms show a linear dependence of the seismic quality factor of the form in the range of frequencies from 3 to 24 hz

triggers for the formation of porphyry ore deposits in magmatic arcs porphyry ore deposits supply much of the copper molybdenum gold and silver used by humans a review of the main processes that trigger porphyry ore formation suggests that sulphide saturation of the magmas that supply the metals could be the overriding mechanism that helps control the temporal and spatial distribution of the ore deposits

an earthquake doublet in ometepec guerrero mexico on june 7 1982 an earthquake doublet occurred in a gap near ometepec guerrero mexico which had been given a high seismic potential the two earthquakes first event m s 6 9 m b 6 0 16 3 n 98 4 w d 25 km second event m s 7 0 m b 6 3 16 4 n 98 5 w d 8 km of the doublet occurred within five hours of each other we determine the source parameters of these events by inverting surface wave data at a period of 256 s the results are for the first event strike 116 dip 77 slip 88 and seismic moment of 2 8 10 26 dyne cm and for the second event strike 116 dip 78 slip 89 and seismic moment of 2 8 10 26 dyne cm modeling of long period p waves suggests that the first event has a depth of 20 km and is represented by a single trapezoidal source time function with an effective duration of 6 s the second event is best modeled by two sources at depths of 15 and 10 km the combined effective source duration time for the two sources is about 10 s the ratio of the seismic moment obtained from body waves to that from surface waves is 0 5 for the first event and 1 for the second adding the seismic moment of the two events and considering the first week aftershock area 3200 km 2 the stress drop is 4 bars these results suggest that the first event that involved a deeper asperity caused an incremental stress change large enough to trigger the second event if the two events of the doublet broke distinct areas of the subduction zone the coseismic slip is 0 58 m and accounts for about 25 of the total plate motion between the cocos plate and the north america plate accumulated since the last large earthquake in the region other doublets similar to the ometepec doublet have occurred along the middle america trench during the past 70 years a regional distribution of comparable size asperities may be responsible for this relatively frequent occurrence of doublets and for the simplicity of earthquakes in the region the high convergence rate which produces rapid strain accumulation and short recurrence intervals for large earthquakes and the smooth sea floor subducted along the middle america trench may contribute to the homogeneous distribution of comparable size asperities we found a relation log t 1 3 log m 0 t is the average recurrence time and m 0 is the average seismic moment for large earthquakes along the subduction zone in the guerrero oaxaca region where the convergence rate and the properties of the subducted plate are considered relatively uniform a simple asperity model predicts this relation