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

precambrian basement geology of the permian basin region of west texas and eastern new mexico a geophysical perspective because most of the permian basin region of west texas and southern new mexico is covered by phanerozoic rocks other means must be found to examine the precambrian upper crustal geology of the region we have combined geologic information on the precambrian from outcrops and wells with geophysical information from gravity and magnetic surveys in an integrated analysis of the history and structure of basement rocks in the region geophysical anomalies can be related to six precambrian events formation of the early proterozoic outer tectonic belt igneous activity in the southern granite rhyolite province an episode of pre grenville extension the grenville orogeny rifting to form the delaware aulacogen and eocambrian rifting to form the early paleozoic continental mar in two geophysical features were studied in detail the abilene gravity minimum and the central basin platform gravity high the abilene gravity minimum is shown to extend from the delaware basin across north central texas and is interpreted to be caused by a granitic batholith similar in size to the sierra nevada batholith in california and nevada this batholith appears to be related to formation of the southern granite rhyolite province possibly as a continental margin arc batholith because of this interpretation we have located the grenville tectonic front southward from its commonly quoted position closer to the llano uplift middle proterozoic mafic intrusions are found to core the central basin platform and the roosevelt uplift these intrusions formed at about 1 1 ga and are related in time to both the mid continent rift system and the grenville orogeny in texas because these features are likely to be rift related they suggest that the concept of a delaware aulacogen needs to be revised only to the extent that the rifting is proterozoic in age not eocambrian precambrian basement structures and changes in lithology have influenced the structure and stratigraphy in the overlying permian basin and thus have potential exploration significance interpretation of the gravity and magnetic data with geologic information also leads us to suggest the existence of pre ellenburger basins which may be extensive and of potential exploration interest

fluid inclusion evidence for impact heating at the siljan ring sweden the siljan ring in central sweden is a devonian impact crater of the complex type the 52 km wide crater occurs in precambrian dala series granites deep drilling in the crater provides samples from depths down to 6000 m these are supplemented by surface samples from the crater and surrounding regions secondary fluid inclusions occur in matrix quartz of nearly all granites examined most inclusions contain low salinity water and water vapor no ch sub 4 or h sub 2 has been detected in surface samples peak trapping temperatures are 327 342 sup 0 c on the crater s inner plateau and minimum trapping temperatures are 135 225 sup 0 c near the annular moat in the subsurface trapping temperatures distinguish two sample populations separated by a flat lying 60 m thick dolerite at 1200 m below sea level in the deep sample group below 1200 m estimated trapping temperatures increase with depth along gradients between 17 and 32 sup 0 c km from 250 300 sup 0 c just below the dolerite to 320 440 sup 0 c at 5711 m trapping temperatures in samples above 1200 m are hotter than would be predicted by extrapolation of the deep trends and are believed to reflect hydrothermal activity generated by impact heating this interpretation is supported by the presence of hotter temperatures of fluid inclusionmore trapping in samples from inside the crater compared to outside and by crosscutting relations which show that some fluid inclusion trails postdate impact related features such as planar elements in quartz impact heated fluids circulated through a circular domain of rock approximately 52 km wide and at least 1 2 km thick the large scale of this circulation system indicates that impact generated hydrothermal activity can be a significant mechanism for heat dissipation following bolide impacts on water bearing planets less

age inheritance and uplift history of the old woman piute batholith california and implications for k feldspar age spectra ion probe u pb zircon analyses yield a crystallization age of 74 pm 3 ma for both peraluminous and metaluminous plutons within the old woman piute batholith these analyses reveal that about 50 of the zircons from the peraluminous sweetwater wash granite are restitic inherited zircons from this batholith yield ages ranging from older than 1800 ma to 1100 ma and most are similar in age to proterozoic igneous rocks in the eastern mojave desert 40 ar 39 ar hornblende ages from the metaluminous granodiorite are 73 pm 2 ma and mica ages from both plutonic series are 70 pm 2 ma k feldspar age spectra yield age gradients with initial steps between 60 and 67 ma and rise to plateau ages of 70 pm 2 ma several k feldspars from the granodiorite yield elevated plateau ages after 50 39 ar released that result from excess argon uplift rates as high as 4 mm yr between 74 and 70 ma are required by the narrow age range for all minerals

characteristics of atmospheric circulation over east asia associated with summer blocking the boreal summer blocking regions were defined using the reanalysis data over the three decades of 1981 2010 and the influence of the blocking on atmospheric circulation in east asia was examined the summer blocking occurred mostly in north europe ural region sea of okhotsk ok and northeastern pacific the summer blocking was the major mode in these four regions according to principal component analysis using 500 hpa geopotential heights among the four blocking regions ok blocking frequencies ok bfs showed negative and positive correlations with summer temperature and precipitation of northeast asia centered around the east sea sea of japan respectively in particular the ok bf had a statistically significant correlation coefficient of 0 54 with summer temperatures in the korean peninsula this indicates that the summer temperature and precipitation in this region were closely related to the ok blocking according to the composite analysis for the years of higher than average bf positive bf years the ok high became stronger and expanded while the north pacific high was weakened over the korean peninsula and japan and an anomalously deep trough was developed in the upper layer 200 hpa as the cool ok high expanded the temperature decreased over northeast asia centered around the east sea sea of japan and the lower level 850 hpa air converged cyclonically resulting in the increased precipitation which induced the divergence in the upper layer and thereby strengthened the jet stream thus the boreal summer ok blocking systematically influencing the area as the most dominant mode

nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america the mountain ranges of arizona and adjacent california and nevada contain large areas underlain by proterozoic anorogenic granites comprising the southwesternmost portion of a transcontinental belt of 1 4 1 5 ga old anorogenic complexes that extends across north america northeast into labrador of these a two mica monazite bearing granitic suite resides in central and southeastern arizona as part of a peraluminous subprovince that is bordered on the south southern arizona to sonora and west western arizona and adjacent portions of california and nevada by marginally metaluminous granites bearing biotite sphene hornblende and fluorite all of these 1 4 ga granites are distinctly more potassic iron enriched relative to mg and depleted in ca mg and sr in contrast to typical orogenic granitoids in general the large ion lithophile element enriched composition is a consequence of limited melting of a water deficient crustal source at depths greater than 25 37 km for the peraluminous granites this contrast is less extreme perhaps resulting from a larger degree of melting as a consequence of a greater metasedimentary component and water in its crustal source the anorogenic granitic magmas intruded into the upper crust at depths of 8 17 km or shallower at temperatures up to 790 c the most dramatic variation in the crystallization intensive parameters resides in the oxygen fugacity which spans three orders of magnitude relative to other anorogenic suites all of the magmas crystallized at elevated levels of o2 as reflected in their assignment to the anorogenic magnetite series yet a regionally significant rise in primary o2 levels unmatched elsewhere in the transcontinental belt occurs for plutons in western arizona including the holy moses and hualapai granites the most extreme case is the hualapai granite whose biotite fe fe mg ratios drop due to high o2 to a low of 0 27 down from more typical levels of 0 54 to 0 75 such extreme variations in primary levels of oxygen fugacity must be an indirect imprint of regional changes of the level of oxidation of the lower crust the high fo2 holy moses and hualapai plutons have intruded near the regional boundary between the metaluminous and peraluminous granites and appear to be imaging a major change in the level of oxidation of the lower crust this boundary is also approximately equivalent to significant changes in the nd and pb isotopic compositions of these granites and the metamorphic and magmatic character of the older orogenic terrane on a global scale the crust forming orogenies ended by 1 6 ga ago and the continents entered a long lived era dominated by localized extension and transcontinental intrusion of anorogenic potassic rapakivi granite mafic dike swarms charnockite and anorthosite the absence of orogenic deformation implies that plate consumption became intraoceanic during this time the profuse and widespread nature of the igneous activity has no phanerozoic analogue and is considered to be unique to the proterozoic a crustal overturn model ties the magmatism to heating within a largely undepleted subcontinental mantle the eventual rise of mantle plumes and the transfer of heat into the youthful undifferentiated proterozoic crust subsequent melting and rise of potassic granitic magmas from the lower crust leads to considerable crustal reorganization a process that would continue until both the mantle and crust reached a stable configuration

thermochronology of isotopically zoned micas insights from the southwestern usa proterozoic orogen abstract we have used three different 40 ar 39 ar laser microprobe methods to explore the distribution of radiogenic 40 ar in 1 0 1 5 mm biotite crystals from the ca 1680 ma horse mountain monzogranite of central arizona incremental heating of two single crystals with a defocused laser beam produced flat age spectra with near plateau ages of 1190 ma showing no indication of intracrystalline 40 ar inhomogeneity in contrast total fusion of twenty five biotite fragments 100 pm yielded apparent ages ranging from 1006 7 to 1212 0 ma detailed age mapping in the 001 plane of two crystals with the laser focused to a minimum spot size confirms that the age dispersion in the fragment data reflects the existence of 200 m y age gradients in single crystals the two mapped crystals display very different age distribution patterns that suggest radiogenic 40 ar loss through two mechanisms volume diffusion on a scale comparable to that of the grain radius and more rapid diffusion along discrete zones of high crystal defect density simple inverse modeling of the overall age dispersion in the two mapped crystals and the fragment population is consistent with the development of the observed age gradients by slow cooling at an average rate of 0 5 k m y the horse mountain results as well as previously published studies indicate that conventional incremental heating of hydrous phases can homogenize intracrystalline gradients in 40 ar thereby masking important details of the thermal history of analyzed samples in contrast detailed isotopic mapping studies offer a wealth of information and will become more powerful with continued improvement in the spatial resolution of 40 ar 39 ar laser microprobes total fusion studies of crystal fragment populations can be readily automated making them less labor intensive than mapping studies our preliminary experiment on a limited horse mountain fragment population suggests that this procedure has great potential as a reconnaissance tool for thermal history research

tightening the siberian connection to western laurentia newly available geological and geophysical data tighten the proterozoic connection between the rifted margins of the northern siberian craton and western laurentia and permit a siberia laurentia australia troika with northern australia connected to the southern margin of the siberian craton the continental assembly is linked by a prominent 2 0 1 8 ga collisional belt and the ca 1 3 1 0 ga grenville orogen the reconstruction also aligns a 1 5 1 45 ga dike sill swarm that extends from the wyoming province through the belt purcell basin into the northern siberian craton separation of australia and siberia may have occurred by the early neoproterozoic but separation of siberia and laurentia may not have been completed until the early cambrian continental extension began along the zigzag siberia laurentia rift zone in the early mesoproterozoic rift related igneous and sedimentary assemblages dated at ca 1 5 1 38 and 1 2 to 1 0 ga correlate across the reconstructed siberia laurentia rift zone renewed rifting in the neoproterozoic and vendian culminated with seafloor spreading and thermal subsidence of the conjugate rift shoulders correlative archeocyathan reefs endemic olenellid trilobite fauna and exchange of detritus between the cratons imply that the rift may have remained relatively narrow until the atdabanian stage black sulfidic shales buried the archeocyathan reefs on the rapidly subsiding rift margins during the botomian sinsk event

southern abitibi greenstone belt archean crustal structure from seismic reflection profiles lithoprobe seismic reflection profiles from the 2 75 2 68 ga southern abitibi greenstone belt reveal subhorizontal reflection zones of regional extent in the upper 15 km of crust that contrast with the steep dips of rock units and structures at the surface the shallowest 6 12 km regional subhorizontal reflection zone is interpreted to be the base of the abitibi greenstone belt supracrustal assemblage which may be an intrusive and or tectonic contact shallowly dipping subparallel reflectors in the upper crust are interpreted as low angle faults consistent with out of sequence stratigraphy revealed by u pb geochronology truncations of reflectors suggest that regional steeply dipping gold mineralized fault zones penetrate to depths of 15 km the profiles are consistent with the kapuskasing crustal model of the central superior province wherein greenstone belts are underlain by middle crustal felsic intrusions and gneiss and a lower crustal zone of mafic gneiss and intrusions the profiles support tectonic models of deformation of the abitibi greenstone belt that invoke either folding and thrusting followed by regional steep faulting or folding and thrusting linked to the steep faults

proterozoic multistage ca 1 1 and 0 8 ga extension recorded in the grand canyon supergroup and establishment of northwest and north trending tectonic grains in the southwestern united states the grand canyon supergroup records at least two distinct periods of intracratonic extension and sedimentation in the late mesoproterozoic and neoproterozoic new 40 ar 39 ar age determinations indicate that the mesoproterozoic unkar group was deposited between ca 1 2 and 1 1 ga basins in which the unkar group was deposited and the related northwest striking faults were created by northeast southwest extension which was contemporaneous with regional northwest southeast grenville contraction new u pb data indicate that the neoproterozoic chuar group was deposited between 800 and 742 ma sedimentary and tectonic studies show that chuar deposition took place during east west extension and resulting normal slip across the butte fault this event is interpreted to be an intracratonic response to the breakup of rodinia and initiation of the cordilleran rift margin laramide monoclines of the grand canyon region have north and northwest trends reactivate faults that originated at the time of unkar and chuar deposition and can be traced for great distances hundreds of kilometers from the grand canyon we use the distribution of monoclines in the southwest to infer the extent of proterozoic extensional fault systems the 1 1 ga northwest trending structures and ca 800 700 ma north trending extensional structures created regional fault networks that were tectonically inverted during formation of the ancestral rocky mountains and laramide contraction and reactivated during tertiary extension

crustal structure and evolution beneath the colorado plateau and the southern basin and range province results from receiver function and gravity studies over the past several decades contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long term stability of the colorado plateau cp and crustal thinning beneath the basin and range province brp in the southwestern united states here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15 500 p to s receiver functions recorded at 72 usarray and other broadband seismic stations on the southwestern cp and the southern brp our results reveal that the brp is characterized by a thin crust 28 2 0 5 km a mean vp vs of 1 761 0 014 and a mean amplitude r of p to s converted wave relative to that of the direct p wave of 0 181 0 014 that are similar to a typical continental crust consistent with the model that the thin crust was the consequence of lithospheric stretching during the cenozoic the cp is characterized by the thickest crust 42 3 0 8 km largest vp vs 1 825 0 009 and smallest r 0 105 0 007 values in the study area in addition many stations on the cp exhibit a clear arrival before the p to s converted phase from the moho corresponding to a lower crustal layer of about 12 km thick with a mafic composition we hypothesize that the lower crustal layer which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines contributed to the long term stability and preuplift low elevation of the colorado plateau

the 40ar 39ar thermochronology of the eastern mojave desert california and adjacent western arizona with implications for the evolution of metamorphic core complexes mesozoic thickening and cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern mojave desert southeastern california and western arizona the application of 40ar 39ar thermochronology to rocks in this region provides information about the timing and nature of thrusting plutonism metamorphism denudation and detachment faulting the 40ar 39ar ages of 175 to 125 ma from the clipper piute turtle mohave bill williams and hualapai mountains are interpreted to be the result of a middle mesozoic thermal event s caused by crustal thickening the 40ar 39ar data from the clipper and piute mountains suggest that this thermal event was followed by a period of cooling at rates of 1 5 c m y orogenesis culminated during the late cretaceous when rocks exposed in the old woman piute chemehuevi and sacramento mountains attained temperatures 500 c which reset the k ar systems of minerals from proterozoic rocks high grade metamorphism in the old woman mountains area was caused by the intrusion of the old woman piute batholith at 73 1 ma cooling rates following batholith emplacement in the old woman mountains were 100 c m y between 73 and 70 ma and 5 10 c m y from 70 to 30 ma between 65 and 25 ma the entire eastern mojave desert underwent a period of cooling at a rate of 2 10 c m y by 30 ma rocks exposed in the old woman piute marble ship clipper and turtle mountains were below 100 c the 40ar 39ar ages from the sacramento mountains suggest that mylonitization caused by the onset of regional extension occurred at 23 1 ma when extension started in the chemehuevi mountains rocks exposed in the southwestern and northeastern portions of footwall to the chemehuevi detachment fault were at 180 c and 350 c respectively this suggests that the exposed part of the chemehuevi detachment fault initiated at a dip of 5 30 or as a series of higher angle faults that cut to a depth of 10 12 km and were later rotated to their present dip unroofing of the footwalls to detachment faults in the sacramento and chemehuevi mountains resulted in average cooling rates of 10 50 c m y between 22 and 15 ma

chronologic and isotopic framework for early proterozoic crustal evolution in the eastern mojave desert region se california the early proterozoic geologic evolution of the eastern mojave desert region as defined by characteristics of its supracrustal rocks granitoids metamorphism structural history and pb and nd isotopic signature contrasts sharply with other proterozoic provinces of the southwestern united states the oldest supracrustal rocks of the mojave desert region contain zircons over 2 0 ga corroborating nd isotopic evidence for a much older crust here than elsewhere in the southwestern united states granitoids widely emplaced within these supracrustal rocks range from 1 76 to 1 64 ga the earlier plutons and surrounding supracrustal rocks were metamorphosed to granulite and high amphibolite facies throughout the province at about 1705 ma in a migmatite producing event that we term informally the ivanpah orogeny subsequent granitoids emplaced from 1 69 to 1 67 ga were voluminous along a north trending belt in the middle of the mojave province younger plutons were emplaced at about 1 66 ga in several places and at about 1 64 ga along the extreme southern part of the province commonalities between the proterozoic evolutions of the mojave and arizona crustal provinces do not conclusively establish the time that the provinces were juxtaposed the data only suggest that the juxtaposition occurred between about 1 76 and 1 64 ga

denudation of metamorphic core complexes and the reconstruction of the transition zone west central arizona constraints from apatite fission track thermochronology apatite fission track thermochronologic results from transects across the basin and range and transition zone provinces in west central arizona provide constraints on the denudational history and structural framework of the region apatite fission track ages decrease from 21 to 14 ma in the harcuvar mountains and from 16 to 13 ma in the buckskin rawhide mountains in the slip direction sw ne of detachment faults in the lower plates of the metamorphic core complexes mean lengths of confined fission tracks from the core complexes are all 14 m indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone 40 c m y for lower plate rocks apparent apatite ages in the transition zone province generally increase from 25 ma to 100 ma away from the basin and range province this trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the weaver mountains and the colorado plateau gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the phillips kirkland drill hole reveal parts of denuded mesozoic and cenozoic apatite partial annealing zones these paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks the reconstructions indicate that the major faults in the transition zone province have relative displacements of 1 km and that offset on them occurred mostly after 25 ma these data also indicate that most of the rocks now exposed in the transition zone of west central arizona were not exposed until miocene time

a diagnosis of two blocking events that occurred simultaneously in the midlatitude northern hemisphere using the goddard laboratory for atmospheres goddard earth observing system 1 analyses the horizontal distribution of atmospheric forcing processes involved in the growth maintenance and decay of two northern hemisphere midlatitude blocking anticyclones that occurred simultaneously were examined with the goal of determining whether there was a dynamic connection between the two cases the complete form of the zwack okossi equation which is a generalization of the petterssen sutcliffe development equation was used as the primary diagnostic tool the basic analyses were partitioned into their planetary scale and synoptic scale components noninteraction 500 hpa height tendencies as well as scale interaction 500 hpa height tendencies based on the results of this diagnosis there was no dynamic connection between these blocking anticyclones this result agrees with findings of other studies based on a statistical analysis of simultaneous blocking these results also imply that blocking may be a local phenomenon the diagnostic results from these blocking events were in many respects similar to those of previous studies however some key differences were found for example it was found here that for both mode 2 blocking events temperature advection was an important mechanism in block formation earlier results showed vorticity advection as the primary atmospheric forcing process in block formation for a mode 1 block also the scalepartitioned results show that upstream cyclones contributed to block formation and intensification in these events directly through the synoptic scale component often with the support of the interaction component of the total forcing earlier scale partitioned results showed that the interaction component of the total forcing was most important in the formation of a mode 1 block

mechanisms determining the atmospheric response to midlatitude sst anomalies abstract previous gcm experiments demonstrated that a model atmosphere produces two different responses to a midlatitude warm sst anomaly over the pacific under perpetual january and february conditions to elucidate the mechanisms responsible for the different gcm responses and their dependence on the background flow experiments with two idealized models are conducted experiments with a linear baroclinic model reveal that the gcm responses at equilibrium are primarily maintained by the anomalous eddy forcing the anomalous flow induced directly by an idealized initial heat source exhibits little sensitivity to the background flow eddy feedbacks on the heating induced anomalous flow are examined using a linear storm track model the anomalous eddy forcing produced by the storm track model is sensitive to the basic state the eddy forcing in january acts to shift the heating induced upper level ridge toward the northeast of the gulf of alaska while in february it acts to reinforce the ridge this sugge

a grenville age layered intrusion in the subsurface of west texas petrology petrography and possible tectonic setting the nellie intrusion is a thick more than 4420 m mafic to ultramafic layered intrusion with a radiometric age of 1163 ma rock types change abruptly with stratigraphic height and include norite pyroxenite gabbronorite hornblende gabbro gabbro anorthosite harzburgite and lherzolite norite is most abundant but gabbro and hornblende gabbro are locally abundant rare olivine rich layers are also present the general order of crystallization was olivine orthopyroxene plagioclase clinopyroxene and hornblende mg s expressed as 100 mg mg fe range from 76 3 to 85 8 for olivine 56 7 to 84 9 for orthopyroxene 62 5 to 90 3 for clinopyroxene and 52 4 to 82 8 for amphibole mg s vary with height and display abrupt reversals which indicate open system addition of new mafic magma eleven cyclic units were identified on the basis of evidence for injection of basaltic magma these can be grouped into three megacyclic units the abundance of orthopyroxene and mineral compositional evidence for

discovery of major basement cored uplifts in the northern galiuro mountains southeastern arizona implications for regional laramide deformation style and structural evolution lowell institute for mineral resources at the university of arizona arizona geological society j harold courtright scholarship

seismic identification of basement reflectors the bagdad reflection sequence in the basin and range province colorado plateau transition zone arizona although seismic reflection profiling has contributed greatly to our knowledge of the crust and upper mantle it is often difficult to unambiguously identify the geologic origin of reflections this is particularly true of intrabasement reflectors whose geometry and origin we need to know in order to understand tectonic processes operating in the crust we present synthetic seismogram modeling of recently collected p and s wave seismic reflection data which suggests that the intrabasement bagdad reflectors in west central arizona are intrusive mafic sheets the identification is based on the determination of the phase positive polarity and amplitudes of the reflected arrivals if the bagdad reflectors are cenozoic age mafic sheets they may be the product of rapid intrusion into an area of weak extension where the principal stresses are approximately equal

subsurface structure of the eastern arkoma basin analysis of 425 km of seismic reflection data in the eastern arkoma basin reveals a structure and history quite different from those previously reported for the arkoma basin the eastern arkoma basin has three structural styles that formed in the following chronological order deep steep normal faults shallow listric normal faults and thrust faults the origin of the structural styles and the paleozoic history of the eastern arkoma basin may be summarized as follows late proterozoic or early cambrian rifting was followed by deposition of cambrian through upper mississippian strata on a passive plate margin the mississippian pennsylvanian boundary marks a time of major down to the south normal faulting with coincident folding of the footwall blocks and truncation of t e faulted and folded terrain by a pre morrowan mississippian pennsylvanian unconformity the unconformity slopes southward and steepens locally across the erosionally truncated footwall blocks during the pennsylvanian down to the south listric normal faults cut the atokan and morrowan sections and merged with but did not displace the steeper segments of the sub morrowan unconformity surface folds north of the ross creek thrust are rollover anticlines overlying these listric normal faults major deformation in the eastern arkoma basin terminated with emplacement of the ross creek thrust in the late pennsylvanian

physical volcanology of mid proterozoic plateau lavas the keweenawan north shore volcanic group minnesota the north shore volcanic group a thick sequence of plateau lavas of keweenawan proterozoic 1 100 m y age was erupted during the development of the midcontinent rift of north america it contains many well exposed physical features that imply a range of eruptive styles and flow mechanisms these include a high volume and high effusion rate floods of basalt and basaltic andesite which ponded and crystallized largely after flow had ceased b moderate to small volume and effusion rate flows and small floods some probably building low shields and c small flows to very large flows and ignimbrites of icelandite and rhyolite lacking visible vent areas viscosities calculated from 183 chemical analyses correlate well with the physical features the style of volcanism combined characteristics of the columbia plateau and the snake river plain plains volcanism and was very similar to that of the tertiary of eastern iceland

the role of high and low frequency dynamics in blocking formation time evolutions of prominent blocking flow configurations over the north pacific and europe are compared based upon composites for the 30 strongest events observed during 27 recent winter seasons fluctuations associated with synoptic scale migratory eddies have been filtered out before the compositing a quasi stationary wave train across the atlantic is evident during the blocking amplification over europe while no counterpart is found to the west of the amplifying blocking over the north pacific correlation between the tropopause level potential vorticity pv and meridional wind velocity associated with the amplifying blocking is found to be negative over europe in association with the anticyclonic evolution of the low pv center but it is almost zero over the north pacific feedback from the synoptic scale eddies as evaluated in the form of 250 mb geopotential height tendency due to the eddy vorticity flux convergence accounts for more than 75 of the observed amplification for the pacific blocking and less than 45 for the european blocking this difference is highlighted in two types of contour advection with surgery experiments in one of them pv contours observed four days before the peak blocking time were advected by composite time series of the low pass filtered observational wind and in the other experiment they were advected by the low pass filtered wind from which the transient eddy feedback evaluated as above had been removed at every time step hence the latter data should be dominated by low frequency dynamics for the european blocking both experiments can reproduce the anticyclonic evolution of low pv air within a blocking ridge as observed for the pacific blocking in contrast the observed intrusion of low pv air into the higher latitudes cannot be reproduced without the transient feedback furthermore in a barotropic model initialized with the composite 250 mb flow observed three days before the peak time a simulated blocking development over the north pacific is more sensitive to the insertion of the observed transient feedback than that over europe these results suggest that the incoming wave activity flux associated with a quasi stationary rossby wave train is of primary importance in the blocking formation over europe whereas the forcing by the synoptic scale transients is indispensable to that over the north pacific

cocorp arizona transect strong crustal reflections and offset moho beneath the transition zone the consortium for continental reflection profiling cocorp transect across the southwest margin of the colorado plateau in arizona reveals prominent subhorizontal zones of reflections here termed the bagdad reflection sequence brs which dominate the upper and middle crust in the transition zone tz between the colorado plateau and the basin and range province a cenozoic origin for the brs is considered most likely it may represent fragmented or sheared zones related to detachment faulting or a series of intrusions possibly emplaced along preexisting zones of weakness moho reflections observed beneath the tz and the basin and range province contrast with the nonreflective moho beneath the colorado plateau suggesting that extension and its associated igneous processes probably played a key role in the development of reflectors at the moho the moho and a prominent midcrustal reflector are offset by about 3 km in a normal sense in at least one place this is the best example of offset moho yet found by cocorp and contrasts sharply with cocorp observations of a smooth moho elsewhere this offset is probably a late tectonic expression of crustal extension and thinning across the tz

the anomalous structures of atmospheric and oceanic variables associated with the frequency of north pacific winter blocking 1 from empirical orthogonal function eof analysis of the atmospheric and oceanic variables over the north pacific during the northern hemispheric winter 1960 2009 we were able to determine that the second mode of the eof is related to north pacific winter blocking our composite analysis reveals that the structures of the variables during winters with contrasting frequencies of blocking events are systematically different and show quite opposite pattern in the north pacific during winters with frequent blocking occurrence a north south dipole pattern of geopotential height anomaly is formed and the associated westerly winds are weakened strengthen north south of 35 n these factors bring variations of ocean advection and ocean surface heat flux resulting in a dipole pattern of sea surface temperature sst with a warm anomaly north of 35 n and a cold anomaly to the south the effect of sst on blocking is examined using an atmospheric general circulation model agcm experiment in the experiment the sst anomaly related to blocking is applied as a boundary forcing of the agcm to investigate the effect of sst on the formation of blocking from the experiment we also concluded that the winter blocking is not induced by north pacific sst forcing although consistent linkage between oceanic and atmospheric variables is evident

miocene platform margin reefs gulf of suez egypt jebel abu shaar is a completely dolomitized carbonate platform atop a crystalline basement horst on the western side of the gulf of suez margins of the platform where not removed by synsedimentary faulting are formed by well developed coral reefs the massive reef carbonates consistently illustrate two stages of growth a basal paucispecific unit of branching coral bafflestone mostly stylophora and a thicker upper unit of diverse coral framestone dominated by faviids in the upper unit the reef crest is massive columnar porites and less common caulastrea framestone the back reef is a framestone of diverse faviids mainly montastrea favites and tarbellastrea and interbedded reef flat rhodolite rudstones the back reef and reef flat facies grade onshelf into stylophora bafflestone biostromers and faviid bioherms the reef front is a shallow to intermediate depth zone of numerous and diverse faviids dominated by montastrea and acanthastrea framestones bioclastic sands and hardgrounds deeper zones are mostly small acanthastrea mounds or rhodolite bivalve rudstones with scattered faviids and acanthastrea synsedimentary lithification internal sedimentationm and bioerosion prevail throughout a deep water slope parallel biostrome of ahermatypic corals dominated by dendrophyllia and containing numerous balanophyllia and madracis is present 10 km north of abu shaar corals are well cemented by numerous rinds of marinemore cement which is overlain by geopetal internal sediment containing planktonic foraminifers and pteropods less

miocene low angle normal faulting and dike emplacement homer mountain and surrounding areas southeastern california and southernmost nevada homer mountain and surrounding regions are within or adjacent to the western part of a broad region of low angle normal faults exposed within the lower colorado river trough during middle miocene time upper plate rocks in the homer sacramento dead and newberry mountains moved eastward or northeastward relative to the lower plate above single or multiple low angle normal faults deposition of coarse clastic sedimentary rocks occurred during extensional faulting and was accompanied by and closely followed by eruption of basaltic volcanics upper plate fault blocks of miocene volcanic and sedimentary rocks and older underlying crystalline rocks are tilted gently to steeply to the west or southwest low angle normal faults have complex sinuous traces due to the irregular form of fault surfaces antiformal warping and uplift of the lower plate about a north south trending axis due to tectonic denudation and isostatic rebound divided the regionally east thickening extensional allochthon into separate synformal and wedge shaped components the homer mountain area lies within or adjacent to the synformal component lower plate rocks are intruded by numerous middle miocene east west trending dikes which are in turn intruded by subhorizontal dikes both sets of dikes are cut and displaced by low angle normal faults except on the west flank of the newberry mountains where an east west trending dike cuts the basal low angle normal fault k ar geochronologic and field data establish the approximate synchroneity of dike emplacement and low angle faulting the state of stress in the lower plate as inferred from dike orientations differed radically from the state of stress in the upper plate as inferred from fault geometry low angle faulting and east northeast west southwest distension of upper plate rocks reflect regional reduction of compression in the east northeast west southwest direction and associated large scale east northeast west southwest crustal extension in contrast concave upward flexure of the lower plate in response to tectonic denudation and resultant isostatic uplift is inferred to have produced local subhorizontal compression at shallow crustal levels in the lower plate that overwhelmed the regional extensional stress and prevented emplacement of dikes oriented perpendicular to the direction of regional extension north south crustal extension during dike emplacement appears to have resulted from minor divergence of lower plate crustal blocks as they were displaced in a west southwest direction away from the colorado plateau

intrusion of horizontal dikes tectonic significance of middle proterozoic diabase sheets widespread in the upper crust of the southwestern united states initially horizontal sheet intrusions of middle proterozoic diabase are abundant in a region 650 by 300 km across in arizona and california the diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences massive granite is the most common basement host for the sheets probably because it fractured more easily than foliated hosts during sheet emplacement steep feeder dikes are rare compared to the sheets the diabase in many places is exposed in fault blocks that were tilted during tertiary tectonic extension structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust to depths of at least 13 km sheet intrusion implies a vertical orientation of the least compressive stress so i conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1 1 ga magma overpressures water encountered by rising magma vertical changes in the crustal stress regime and flotation of low density granite all may be important factors for sheet intrusion the stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event the diabase province contrasts structurally with similar age provinces of basaltic magmatism elsewhere in north america that show evidence of tectonic extension such as the midcontinent rift the consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks sheet intrusions in the geologic record may be seriously underreported their recognition is important for the interpretation of seismic reflection profiles of continental crust sheets in arizona may be responsible for the bagdad reflection sequence which extends to depths of at least 15 km

high frequency transient eddies and blocking abstract in this paper we investigate the role of high frequency transient eddies in the maintenance of the blocking high which occurred over europe on 16 25 february 1979 time filtering is used to decompose the atmospheric large scale flow into contributions from high frequency and low frequency fluctuations the separation being roughly at the period of six days the net effect on the ten day mean flow of the high frequency cyclone scale eddies is studied using several methods the vorticity flux convergence associated with the high frequency eddies although small in comparison with the dominating terms in the vertically averaged vorticity equation appears to be important for counterbalancing the effects of the time mean surface stress especially in the vicinity of the storm tracks the eddies are thus important for the distribution of the time mean surface pressure also the tendency method of illustrating the eddy effects supports this view the hemispheric average of the barotropic energy conve

the transient atmospheric response to midlatitude sst anomalies abstract to study the transient atmospheric response to midlatitude sst anomalies a three layer quasigeostrophic qg model coupled to a slab oceanic mixed layer in the north atlantic is used as diagnosed from a coupled run in perpetual winter conditions the first two modes of sst variability are linked to the model north atlantic oscillation nao and eastern atlantic pattern eap respectively the dominant atmospheric modes in the atlantic sector the two sst anomaly patterns are then prescribed as fixed anomalous boundary conditions for the model atmosphere and its transient responses are established from a large ensemble of simulations in both cases the tendency of the air sea heat fluxes to damp the sst anomalies results in an anomalous diabatic heating of the atmosphere that in turn forces a baroclinic response as predicted by linear theory this initial response rapidly modifies the transient eddy activity and thus the convergence of eddy momentum and heat fluxes the latter transforms t

a general circulation model study of the atmospheric response to extratropical sst anomalies observed in 1950 79 abstract a 30 year experiment with an atmospheric general simulation model has been performed the lower boundary condition at all oceanic grid points between 38 s and 60 n has been prescribed to follow the observed month to month variation of the sea surface temperature sst field during the 1950 79 period much of the model diagnosis presented here pertains to the midlatitude atmospheric response to recurrent sst patterns in the north pacific and north atlantic in winter the principal modes of variability of the seasonally averaged 515 mb height and sst fields have been identified using rotated principal component rpc analysis the extrema of the first atmospheric mode reside over the north atlantic and eurasia whereas the second mode is associated with height anomalies in the north pacific north american sector cross correlation analysis reveals that these two atmospheric modes am linked to leading patterns of the sst field in the north atlantic and north pacific respectively it is also demonst

footwall rocks to the mid tertiary chemehuevi detachment fault a window into the middle crust in the southern cordillera crystalline rocks unroofed from the middle crust during tertiary crustal extension by the chemehuevi detachment fault record a complex history of comprises both subhorizontal and subvertically foliated mylonitic gneisses sharing a common gently sw plunging stretching lineation which are intruded by members of the voluminous laccolith shaped chemehuevi mountains plutonic suite proterozoic and mesozoic gneisses migmatites and early members of the chemehuevi mountains plutonic suite in the eastern chemehuevi mountains are deformed into a gently southwest dipping zone of protomylonite mylonite and locally ultramylonite in excess of 1 5 km thick a belt of steeply foliated mylonitic gneiss up to 5 km across traceable for 12 km along strike ne sw crops out in the northern part of the range this belt passes abruptly downward into the subhorizontal shear zone exposed in the eastern chemehuevi mountains a preliminary shear sense study indicates ne directed shear within the sub horizontal mylonitic rocks and sinistral shear parallel to the steeply foliated mylonites deformation in these two zones took place under upper greenschist to middle amphibolite facies conditions mylonitic deformation associated with the two shear zones was spanned by intrusion of the chemehuevi mountains plutonic suite cross cutting contacts within the suite indicate that ductile deformation occurred after intrusion of hornblende biotite granodiorite four zircon fractions from this unit are discordant but define a chord with a lower intercept of around 90 ma and an upper intercept of 1900 ma thirteen zircon fractions from two separate undeformed members of the suite biotite granodiorite and porphyritic biotite granodiorite are variably discordant but fractions for each unit define chords that yield latest cretaceous crystallization ages with upper intercepts of 1600 ma together these data constrain the timing of plutonism and ductile deformation with associated thick zones of fabric reorientation and mylonitic transposition to the late cretaceous superimposed on this fabric and involving all rocks types in the footwall are the localized brittle and ductile effects of the mid tertiary extensional deformation movement direction and the timing of mylonitic deformation in the chemehuevi mountains suggest that the effects of the cordilleran fold and thrust belt extends into the chemehuevi mountains region where it in part overlapped with and was heated by plutonism along the eastern margin of the mesozoic magmatic arc during late cretaceous time

geochemistry and petrogenesis of proterozoic diabase in the southern death valley region of california diabase sills and dikes of proterozoic age intrude crystalline basement and the overlying crystal spring formation in the southern death valley region of california despite pervasive deuteric alteration analyses of relict plagioclase an66 45 titaniferous augite and ilmenite permit the calculation of initial crystallization temperatures of 1 165 25 c for plagioclase and 1 110 25 c for augite with an oxygen fugacity of 10 11 atm the early crystallization of plagioclase is consistent with the generally subophitic texture of the diabase

the climatology of blocking anticyclones for the northern and southern hemispheres block intensity as a diagnostic abstract a 30 yr climatology of blocking events was compiled by stratifying the data into seasonal and three regional categories for both the northern and southern hemispheres using the ncep ncar reanalyses several characteristics of blocking anticyclones were included in the study and these were frequency of occurrence preferred formation regions duration blocking days and intensity the block intensity bi calculation was modified successfully from a previous study in order to automate the procedure for use with large datasets and it is applied for the first time to derive a long term observational record of this quantity this modification also makes bi suitable for use as a diagnostic tool blocking events in the northern southern hemisphere were the most persistent and strongest during the cold season and over the atlantic pacific region as found using bi to measure intensity the characteristics of blocking events derived in this study were compared to previous long term climatological s

major proterozoic basement features of the eastern midcontinent of north america revealed by recent cocorp profiling cocorp profiling in the eastern midcontinent of north america has 1 traced an extensive sequence of precambrian layered rocks beneath southern illinois indiana and western ohio 2 detected a broad zone of east dipping basement reflectors associated with the grenville front beneath western ohio and 3 discovered a wide region of west dipping reflectors penetrating most of the crust beneath eastern ohio the precambrian layered assemblage may be as much as 11 km thick beneath southern illinois extends at least 170 km in an east west direction and contains several strong reflectors that have a lateral continuity of 80 km or more industry seismic data indicate that the layering is extensive in a north south direction as well possible explanations for the layering include the silicic igneous rocks of the ca 1 48 ga eastern granite rhyolite province which are penetrated by basement drill holes throughout the region perhaps intermixed or underlain by mafic igneous or sedimentary rocks the 40 50 km wide zone of strong east dipping 25 30 reflectors beneath west central ohio corresponds to the position of the grenville front as determined from potential field and drill hole data these dipping reflectors in the upper and middle crust are interpreted to result from ductile deformation zones mylonites like those exposed at the grenville front in canada and imaged on the glimpce seismic reflection lines in lake huron both the cocorp and glimpce lines show a remarkably similar reflection geometry despite the more than 500 km separating the two profiles easternmost ohio appears to be underlain by pronounced west dipping 40 reflectors in the middle and lower crust which are also interpreted as marking a region of pervasive ductile deformation 80 km or more in width analogy with similar reflection packages elsewhere suggests that these reflections may mark a major collision zone the west dipping reflectors may be correlative with similar reflectors imaged on another cocorp survey in northern alabama the correlations suggested by these new results though tentative imply that the eastern midcontinent is composed of a relatively simple assemblage of crustal blocks

the laccolith stock controversy new results from the southern henry mountains utah domes of sedimentary strata at mount holmes mount ellsworth and mount hillers in the southern henry mountains record successive stages in the growth of shallow 3 to 4 km deep magma chambers whether the intrusions under these domes are laccoliths or stocks has been the subject of controversy according to g k gilbert the central intrusions are direct analogues of much smaller floored intrusions exposed on the flanks of the domes that grew from sills by lifting and bending of a largely concordant overburden according to c b hunt the central intrusions are cylindrical stocks sheathed with a zone of shattered sedimentary rocks and the small flanking sills and laccoliths grew laterally as tongue shaped masses from the discordant sides of these stocks new geologic mapping demonstrates that the sedimentary overburden now partially eroded from the domes was uplifted about 1 2 km at mount holmes 1 8 km at mount ellsworth and at least 2 5 km at mount hillers the radii of the domes are similar between 5 and 7 km the strata over the domes have a doubly hinged shape consisting of a concave upward lower hinge and a concave downward upper hinge a limb of approximately constant dip joins these two hinges and dips 20 at mount holmes 50 to 55 at mount ellsworth and 75 to 85 at mount hillers the distal portion of each dome is composed of a gently dipping peripheral limb 3 to 4 km long presumably underlain by sills and minor laccoliths although geologic cross sections and regional aeromagnetic data for the three domes are consistent with floored laccolithic central intrusions these data alone do not rule out the possibility of a stock at depth at mount hillers paleomagnetic vectors indicate that tongue shaped sills and thin laccoliths overlying the central intrusion were emplaced horizontally and were rotated during doming through about 80 of dip this sequence of events is not consistent with the emplacement of a stock and subsequent or contemporaneous lateral growth of sills and minor laccoliths growth in diameter of a stock from about 300 m at mount holmes to nearly 3 km at mount hillers as hunt suggested should have been accompanied by considerable radial shortening of the sedimentary strata and a style of folding which is not observed geologic and geophysical data and mechanical models support a laccolithic origin for the central magma chambers underlying the domes

facies and geologic history of an exposed miocene rift margin carbonate platform gulf of suez egypt abu shaar is a small exhumed carbonate platform of lower to middle miocene age on a horst of precambrian crystalline rock along the western side of the gulf of suez the reef rimmed complex is entirely dolomite but with exquisitely preserved primary and diagenetic microfabrics similar platforms some of which contain hydrocarbons are buried beneath evaporites in the gulf the platform developed in three stages defined as local members of the rudeis formation the first stage records deposition during progressive marine onlap as aprons of mixed terrigenous carbonate sediments evolved into a wide reef rimmed platform synsedimentary platform margin collapse subsequently removed the most seaward portions of the reefs along the eastern gulf facing margin these truncation surfaces were then covered by a second phase of reef growth and deposition of fore reef carbonates the third and final stage deposited following a sea level drop and subaerial exposure is a series of well bedded peritidal carbonates that become progressively more evaporitic upward fore reef facies are again truncated by synsedimentary slope failure former evaporites are now recorded as collapse breccias contorted bedding black chert nodules and evaporite molds porosity is mainly secondary as vugs and molds and is highest in platform interior facies these more porous sediments contained more aragonitic components originally and were less affected by submarine cementation in contrast the higher degree of synsedimentary lithification of platform margin reef and fore reef sediments is comparable with what is commonly observed in modern reef dominated platform margins

eustatic control of turbidites and winnowed turbidites global changes in sea level primarily the results of tectonism and glaciation control deep sea sedimentation during periods of low sea level the frequency of turbidity currents is greatly increased episodes of low sea level also cause vigorous contour currents which winnow away the fines of turbidites in the rock record the occurrence of most turbidites and winnowed turbidites closely corresponds to global lowstands of paleo sea level this observation may be useful in predicting the occurrence of deep sea reservoir facies in the geologic record

keweenaw hot spot geophysical evidence for a 1 1 ga mantle plume beneath the midcontinent rift system the proterozoic midcontinent rift system of north america is remarkably similar to phanerozoic rifted continental margins and flood basalt provinces like the younger analogues the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad asthenospheric thermal anomaly which the authors call the keweenaw hot spot great lakes international multidisciplinary program on crustal evolution seismic reflection profiles constrain end member models of melt thickness and stretching factors which yield an inferred mantle potential temperature of 1 500 1 570c during rifting combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material the isotopic signature of keweenawan volcanic rocks presented in a companion paper by nicholson and shirey this issue is consistent with the model of passive rifting above an asthenospheric mantle plume

correlation of metamorphosed paleozoic strata of the southeastern mojave desert region california and arizona isolated outcrops of deformed regionally metamorphosed paleozoic strata are scattered within the southeastern mojave desert region of california and western arizona these strata unconformably overlie a basement of proterozoic crystalline rocks and are overlain in turn by metamorphosed mesozoic sedimentary rocks the strata can be correlated lithostratigraphically with the classic cratonal paleozoic section of the western grand canyon arizona and with nonmetamorphosed paleozoic sections transitional between cratonal and miogeoclinal in the ship marble and providence mountains california the strata evidently were once continuous with paleozoic epicontinental strata exposed throughout the southern great basin and colorado plateau outcrops of paleozoic strata and of the underlying proterozoic basement in the southeastern mojave desert region define a terrane that has been disrupted by mesozoic thrust faults and by tertiary detachment faults but that nevertheless retain a gross paleogeographic coherence this coherent terrane extends at least as far west and southwest as the big maria palen and calumet mountains and possibly beyond to include paleozoic exposures in the san bernardino mountains and near victorville poorly understood tectonic boundaries separate the area of paleogeographic coherence from known or suspected allochthonous terranes in the western mojave desert and the eastern transverse ranges

interpretation of seismic reflection gravity and magnetic data across middle proterozoic mid continent rift system northwestern wisconsin eastern minnesota and central iowa seismic reflection gravity and magnetic data were used along four profiles to study the mid continent rift system between northwestern wisconsin and central iowa mafic lavas in medial horsts are characterized by strong laterally continuous reflections that indicate a stratigraphic package as much as 10 km thick mafic roots extending beneath the reflective sequence to mid crustal depths were defined from gravity modeling in northwestern wisconsin southeastern minnesota and central iowa a strong normally directed remanent magnetization was modeled for most lavas although some lavas deep in the section of northwestern wisconsin and central iowa were modeled as magnetically reversed the horsts are apparently bounded by faults that are vertical or dip moderately inwar several of these faults originated as growth faults in fault bounded grabens but these faults were transposed into reverse faults by a late stage compressional event the medial horsts are flanked by several half graben basins that contain as much as 5 km of sedimentary material inferred to be lithologically akin to the oronto and bayfield groups of michigan and wisconsin several synclinal basins characterized by 2 4 km of sedimentary rocks are developed atop the medial horsts when viewed regionally the four interpretive profiles show a marked asymmetry with regard to the off axis placement of the deep mafic roots and the positions of postulated growth faults this asymmetry is consistent with the rift propagating as a series of northeast striking en echelon half grabens separated by northwest striking scissor faults or accommodation zones

characteristics depositional environment and tectonic interpretations of the proterozoic cardenas lavas eastern grand canyon arizona the 1 1 b y old cardenas lavas were deposited in shallow hypersaline water probably under tidal flat conditions and in an epicratonic basin that subsided without tilting at about the same rate as the lavas built up late in cardenas time the lavas were deposited subaerially in at least some places subsequently the lavas were probably tilted gently to the northeast then eroded and buried by the nankoweap formation which was deposited in shallow sea water most of the lavas exposed today issued from nearby vents the pre nankoweap tilting is in the same direction as a later and more intense pre tapeats tilting suggesting reactivation of existing structures if the earlier tilting occurred shortly after deposition of the lavas the reactivation occurred following a very long period of relative quiescence 300 m y if the pre nankoweap erosion interval was long the tilting events would have been relatively close in time the erosion interval would have been a major hiatus in the grand canyon sequence and the nankoweap formation and chuar group would be much younger than the unkar group of which the cardenas lavas are part

proterozoic geochronologic and isotopic boundary in nw arizona u pb ages determined from zircon sphene and apatite in conjunction with pb isotopic analyses of alkali feldspar establish a regional geochronological framework and constrain the location of a major north trending proterozoic crustal boundary in northwestern arizona two regions west of the boundary hualapai mountains and lost basin range garnet mountain are characterized by complex u pb zircon systematics evidence for inheritance of an older zircon component 1 8 2 3 ga and elevated sup 207 pb sup 204 pb from feldspars compared to the east although the discordia patterns are complex supracrustal rocks are interpreted to be ca 1 73 ga and are intruded by plutonic rocks ca 1 70 ga deformation is younger than ca 1 70 ga foliated granites and older than the 1 682 plus minus 4 ma garnet mt monzogranite the rocks in one area east of the boundary cottonwood cliffs are characterized by relatively simple u pb zircon systematics no evidence for inheritance of any older component and feldspar sup 207 sup 204 pb near model mantle values supracrustal rocks are older than 1 73 ga as they are intruded by a 1 730 plus minus 9 ma foliated granodiorite timing of deformation is constrained by the late syn kinematic valentine granite dated at 1 713 plus minus 12 ma mineral ages indicate thatmore the rocks on either side of the boundary had different cooling histories and inferentially different uplift histories west of the boundary the cooling history is inferred from minerals separated from an amphibolite metamorphic zircon is 1 687 13 minus 8 ma sphene is 1 660 plus minus 5 ma hornblende has a sup 40 ar sup 39 ar age of 1 552 plus minus 5 ma and apatite has a u pb age of 1 520 plus minus 45 ma east of the boundary an amphibolite has sphene with an age of 1 670 plus minus 11 ma and apatite with an age of 1 630 plus minus 8 ma less

seismic constraints on the nature of lower crustal reflectors beneath the extending southern transition zone of the colorado plateau arizona events maintain a constant amplitude with offset which is most consistent with a mafic layering model one exception is a high amplitude 10 db above background event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets the event has a pronounced s wave response which along with its negative avo trend is a possible indication of the presence of fluids in the lower crust the arizona transition zone is an active but weakly extended province which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers instead we favor horizontal basaltic intrusions as the primary origin of high impedance reflectors based on 1 the fact that most xenoliths in eruptive basalts of the transition zone are of mafic igneous composition 2 indications that a pulse of magmatic activity crossed the transition zone in the late tertiary period and 3 the high regional heat flow observed in the transition zone the apparent presence of fluids near the base of the reflective zone may indicate a partially molten intrusion we present a mechanism by which magma can be trapped and be induced to intrude horizontally at rheologic contrasts in extending crust

origin of gneisses in the aureole of the san gabriel anorthosite complex and implications for the proterozoic crustal evolution of southern california granulite facies gneisses along the southwestern margin of the san gabriel anorthosite complex include compositionally banded locally migmatitic felsic gneisses and mafic granulites with rare interlayered augen gneiss and garnet hypersthene spinel aluminous gneiss previous workers accepted a 1 4 ga age for these granulites and a 1 2 ga age for the adjacent anorthosite complex and hence that this proterozoic basement is allochthonous with respect to the southwestern united states part of the north american craton solvus geothermometry in felsic and mafic granulites yields minimum metamorphic temperatures of 900 950 c and new u pb zircon data yield a minimum metamorphic age of 1186 ma coincident with that of the adjacent anorthosite we reinterpret these rocks as a 1190 ma anorthosite complex and its granulite grade thermal aureole geochemical data suggest that the anorthosite complex intruded early proterozoic rocks correlative with the mojave crustal province representing the north american craton exposed to the northeast we conclude that these proterozoic basement rocks represent the southwestern margin of the north american craton and not an exotic terrane accreted in mesozoic or tertiary time

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

seismic imaging of extended crust with emphasis on the western united states understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust this is particularly true in areas where the last tectonic event was extensional such as the basin and range province of the western united states and much of western europe in these regions a characteristic reflective pattern has emerged whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective in the metamorphic core complex belt in the western united states where extension can be as much as an order of magnitude greater than in the more classic continental rift zones the lower crustal reflectivity thickens and rises yielding a picture of a crust that is reflective throughout synthetic seismic studies have documented that the reflectivity in these regions can be modeled by numerous laminae tens of meters thick and hundreds of meters across characterized by inter layered high and low velocities two geologic factors are interpreted as contributing to this layered character ductile strain responding to stress in the thermally weakened middle and lower crust and intrusive layering corresponding to injection of subhorizontal sheets of mantle derived magmas these two processes yield a variety of geologic structures including transposed compositional layering mylonitic ductile shear zones and intrusive mafic sheets all of which occur at the proper scales to cause the prominent reflectivity observed if metamorphic core complexes are representative of extended continental crust world wide then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world

thermal and unroofing history of a thick tilted basin and range crustal section in the tortilla mountains arizona we estimate here a geothermal gradient of only 17 5 c km 1 for the tilted grayback fault block in southeastern arizona when extension began 25 ma this gradient is lower than preextension gradients estimated elsewhere in the basin and range is only about 50 of typical gradients in the basin and range today and needs to be accounted for in models of continental extension the grayback block exposes a 12 km thick crustal section of proterozoic and cretaceous granitoids which was tilted 90 during extension between 25 and 15 ma zircon fission track ages decrease structurally downward westward across the block and were all within a zone of partial track annealing prior to tilting and quenching the zircon age gradient suggests that the 220 240 c isotherm migrated downward 5 6 km during paleogene erosion and regional cooling apatite fission track ages decrease westward from 83 ma in the structurally highest crystalline rocks to 24 ma at 6 km paleodepth and then to 15 ma another 6 km farther west track length analysis confirms that apatites above the break in slope in age at 5 7 km paleodepth resided in a zone of partial annealing prior to tilting and deeper apatites record rapid cooling upon tilting and unroofing beginning 25 ma at that time the 110 10 c isotherm determined by the depth at which tracks in apatite were fully erased was at a basement paleodepth of 5 7 km and the 220 30 c isotherm as estimated from zircon data resided at a pretilting basement depth of 12 15 km from consistent values of paleogeothermal gradient for two depth intervals we estimate the pretilt gradient was 17 5 c km 1 from 25 to 15 ma the rotating grayback block cooled rapidly as higher westward moving blocks unroofed it tectonically at a rate of 1 km m y 1

paleomagnetism of middle proterozoic diabase sheets from central arizona paleomagnetic results from 1090 ma diabase sheets of the southwestern u s a diabase province exposed in central arizona yield two distinct remanent magnetizations herein termed adn and adr in accordance with the findings of previous investigations magnetization adn is well defined and has an in situ mean direction of d 283 3 i 45 1 k 17 7 95 8 7 n 17 independent observations a mean pole after correction of paleomagnetic site means for a net 5 clockwise rotation of the colorado plateau and transition zone is located at 22 7 n 179 3 e k 21 9 a95 7 8 the second magnetization adr gives an in situ mean direction of d 161 1 i 87 5 k 22 2 95 19 9 n 4 independent observations with a poorly defined pole at 37 6 n 247 6 e k 6 5 a95 38 9 rock magnetic and alternating field and thermal demagnetization characteristics indicate the adn and adr magnetizations are both carried by low ti titanomagnetite both magnetizations are interpreted to be primary

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

the effect of boreal late autumn snow cover over western and central china on the northern hemisphere wintertime blocking frequency abstractthe impact of snow cover in western and central china during late autumn on wintertime blocking occurrence is investigated using reanalysis data the study results show that wintertime atmospheric circulations affected by late autumn snow cover anomalies form favorable conditions for increased blocking frequency bf especially in the north pacific and north atlantic evidence is also presented that the stratosphere troposphere interactions are the key mechanism of the lag response of wintertime north pacific and north atlantic bfs to the late autumn snow cover that is positive anomalous snow cover can induce a dipole anomaly in the geopotential height field over the lower stratosphere due to the decrease of the 300 1000 hpa thickness and the concurrent variation between the east asian plateau jet and the polar front jet the associated positive geopotential height anomalies are located over northwestern eurasia meanwhile western and central china features remarkably negative geopotential he

late cretaceous juxtaposition of metamorphic terranes in the southeastern san gabriel mountains california new structural petrological and geochronological data from the southeastern san gabriel mountains define the amalgamation history of suspect terranes in this region four metamorphic terranes constitute the area the cucamonga san antonio san gabriel and baldy terranes and were juxtaposed in two discrete episodes during the late cretaceous the cucamonga san antonio and san gabriel terranes were juxtaposed along subparallel east west trending left lateral mylonitic fault zones during the first episode the cucamonga terrane consists of lower cretaceous granulite facies gneissic rocks these rocks were intruded in the late cretaceous by granitic rocks as the cucamonga terrane was faulted ductilely against the southern margin of the san antonio terrane mylonitized units were subjected concurrently to retrograde amphibolite facies metamorphism metasedimentary pendants in the san antonio terrane were metamorphosed under upper amphibolite facies conditions as plutonic rocks were emplaced syntectonically along its northern boundary the san antonio terrane was juxtaposed concurrently against the san gabriel terrane a terrane consisting of precambrian gneiss and precambrian and mesozoic intrusive rocks the boundary is represented by middle fork complex a ductile zone intermingling rocks of the two terranes after palinspastic restoration of local cenozoic fault displacements the left lateral mylonitic fault zones separating the terranes appear as parts of a major sinistral transform system along the southern margin of the san gabriel mountains displacement for each of the mylonitic fault zones was approximately several tens of kilometers and was part of a broad left lateral shear system this system is modeled as a lateral ramp or tear fault to a west directed synplutonic ductile thrust system underlying the san gabriel terrane crosscutting syn and post tectonic intrusives indicate that most of the displacement occurred between 88 and 78 m y ago no lithologic isotopic or structural data support previous assertions that these terranes were accreted to north america during the tertiary the late cretaceous synplutonic juxtaposition of the cucamonga san antonio and san gabriel terranes was followed by underthrusting of oceanic rocks of the baldy terrane that is pelona schist along the vincent thrust precise timing of this episode is as yet uncertain

petrology of middle proterozoic diabases and picrites from lake nipigon canada mafic rocks at lake nipigon provide a record of rift related continental basaltic magmatism during the keweenawan event at 1109 ma the mafic rocks consist of an early volumetrically minor suite of picritic intrusions varying in composition from olivine gabbro to peridotite and a later suite of tholeiitic diabase dikes sheets and sills the diabase occurs primarily as two 150 to 200 m thick sills with a textural stratigraphy indicating that the sills represent single cooling units compositional variation in the sills indicates that they crystallized from several magma pulses

possible extension of the midcontinent rift in west texas and eastern new mexico the midcontinent rift system forms one of the most prominent gravity features in north america the recognized geophysical anomaly extends in an arc from southern oklahoma to lake superior and then into southern michigan the midcontinent rift system was active between 1185 1010 ma as indicated in the lake superior region by age determinations on intrusive igneous rocks we suggest that the period of formation of the midcontinent rift was also a time of extensive igneous activity in texas and new mexico this activity is represented by intrusions beneath the central basin platform texas and new mexico intrusions which crop out at the pajarito mountain in the sacramento mountains new mexico a basaltic debris flow in the franklin mountains texas basalt flows at van horn texas and the crosbyton geophysical anomaly east of lubbock texas these bodies and other bodies located by geophysical anomalies and wells drilled into mafic precambrian rocks may be related to the midcontinent rift system

the abitibi dyke swarm a consequence of superior grenville interaction both the age 1150 ma and the strike of the abitibi dykes east of the kapuskasing structural zone in the superior province of the canadian shield can be explained as a consequence of the grenvillian orogeny in the adjacent grenville province modelling the superior province as an elastic brittle plate acted on its southeastern boundary by tectonic stresses consequent upon the grenvillian orogeny we obtain families of maximum and minimum horizontal stress trajectories that match very well the orientation of the dykes

petrogenesis of mid proterozoic granitic magmas examples from central and west texas abstract circa 1 1 ga granitic magmatism in texas was manifested as two compositional groups 1 the 1 12 ga red bluff granitic suite in west texas and 2 1 12 1 07 ga granites of the llano uplift of central texas both suites share some characteristics typical of anorogenic granites e g potassium and iron rich bulk compositions fe rich hydrous silicates emplacement conditions involving low oxygen fugacities and water contents and exhibit similar isotopic characteristics however rock associations mineral chemistries and trace element compositions of the two suites are distinct and no single petrogenetic model for the two suites is possible the red bluff granitic suite includes cogenetic syenites quartz syenites and granites transitional ferrobasaltic dikes are also present in contrast syenitic and mafic rocks are not associated with the llano granites the llano granites contain biotite and calcic amphibole with lower fe fe mg ratios compared to those occurring in the red bluff rocks alkali amphiboles e g arfvedsonite occur in the red bluff granites but not in the llano granites the red bluff granitoids are characterized by high feo t mgo ratios high na2o k2o high concentrations of hfse and rare earth elements ree and other features typical of a type within plate granites e g the pikes peak batholith ppb the llano granites are geochemically distinct with generally higher p2o5 and sr lower na2o feo t mgo zr y and ree and much lower ta and nb nd isotopic data overlap between the two granite suites and have juvenile signatures however trace element data suggest different petrogeneses for the two suites the red bluff suite is interpreted as having a direct derivation from mantle sources via extended fractional crystallization of basaltic parental magmas with minor crustal assimilation the llano granites appear to represent anatectic melts derived from slightly older juvenile crustal sources some melts underwent fractional crystallization controlled by feldspar and accessory minerals the petrology and geochemistry of 1 1 ga granites in texas indicate that they should not be considered as part of a single anorogenic magmatic event the red bluff granitic suite was emplaced into a shelf sequence north of the grenville front within a broad zone characterized by mild extension in contrast llano granites are late stage intrusions emplaced into multiply deformed and metamorphosed crust south of the grenville front during or after waning stages of grenville orogenesis

sea surface temperature anomalies planetary waves and air sea feedback in the middle latitudes the mechanisms that contribute to the generation and damping of large scale mid latitude sea surface temperature sst anomalies are discussed the sst anomalies reflect primarily the response of the upper ocean to the changes in air sea fluxes that are associated with daily weather fluctuations heat flux forcing is dominant in the lower middle latitudes while wind driven entrainment may be most effective in the high latitudes advection by anomalous ekman current is generally less important and ekman pumping is negligible the sst anomalies decay in part because of entrainment effects associated with mixed layer deepening and oceanic mixing and in part because of heat exchanges with the atmosphere the three approaches commonly used to model the evolution of sst anomalies are reviewed case studies based on monthly or seasonal anomaly maps of the large scale sst and atmospheric anomalies numerical simulations with one dimensional mixed layer models and stochastic forcing models we stress the similarities in the different approaches and discuss their main advantages and limitations the response of the atmosphere to mid latitude sst anomalies is considered first we discuss the poorly known relationship between sst anomalies and diabatic heating using a crude assumption for the air sea coupling we consider a two layer quasi geostrophic channel model and discuss the stationary wave response to sst anomaly forcing and the resulting air sea feedback it is found that the back interaction of the sst anomalies onto the atmosphere causes a weak sst anomaly damping at large scales and a strong one at small scales the air sea coupling should also act as an eastward propagator for the sst anomalies the response of more realistic linear wave models to prescribed diabatic heating is then reviewed and it is suggested that realistic mid latitude sst anomalies have a weak influence on the atmospheric circulation corresponding to changes in the geopotential height of 10 30 m at most this order of magnitude is consistent with the results of general circulation model experiments and with the limited climate predictability associated with mid latitude sst anomalies

tectonic control on carbonate platform facies distribution and sequence development miocene gulf of suez abstract mid miocene deposition in the southwestern gulf of suez was governed by an extensional fault block topography shallow marine carbonates and clastics were deposited around the exposed block crests and in marginal half grabens of the rift while central troughs were filled by slope aprons and turbiditic and pelagic mudstones fan deltas developed at points along the fault block escarpments syn rift platform carbonates from one block crest esh el mellaha are described here the platform evolved in three stages generating three sedimentary sequences sequences 1 and 2 were separated by boundary fault reactivation sequences 2 and 3 by sea level drawdown exposure and karstification the sediments comprise six lithofacies associations la 1 6 reef la 1 fore reef la 2 and back reef embayment la 3 sediments form a small reef rimmed platform in sequence 1 at the southern end of the block fringing reefs and reef lagoon systems also developed along the footwall escarpment and on fan deltas which bridged the block bounding fault fringing reefs in sequence 2 generated a carbonate talus veneer la 4 along the whole 80 km length of the esh el mellaha fault scarp carbonate slope and reefal sediments preserved on the footwall exhibit abundant evidence of syndepositional fault movements with truncation of escarpment fringing sequences and repeated generation of reefal wedges minor drowning at the sequence 1 2 boundary caused deepening of lagoons progradation of leeward reefal margins with lagoonal patch reef development and back stepping and vertical accretion of the reefal rim platform development was followed by algal laminated dolomitic sediments la 5 in the area of the old platform interior sequence 3 much of the esh el mellaha ridge remained exposed during platform growth and shed coarse alluvial clastic sediments la 6 into the peripheral marine environments the relationships demonstrated between miocene sedimentation and structural setting have parallels in the modern environments of the zeit bay area

a modelling and observational study of the relationship between sea surface temperature in the north west atlantic and the atmospheric general circulation results from four pairs of 50 day wintertime integrations of the meteorological office s 5 level general circulation model with warm and cold sea surface temperature s s t anomalies of about 3 k in the northwest atlantic are described difference fields between the warm and cold integrations are statistically significant at the 1 level with positive geopotential height over the central north atlantic and weaker negative height over europe the storm track over the atlantic is displaced from its normal position results from four further pairs of integrations with halved s s t anomalies are also described the response is approximately linear with systematic differences in 500 mb geopotential height over the atlantic parts of which are just significant at the 10 level with half the full s s t anomaly overall however the model s response is weaker than could be obtained with tropical s s t anomalies of the same magnitude results from the model integrations are compared with results from an observational study of the relationship between wintertime s s t in the north west atlantic and mean sea level pressure and 500mb height two independent 30 year periods were chosen for study thus minimizing the influence of long term trends in s s t over the atlantic and europe the model results compared well with the observations with s s t data lagging the atmosphere by one month the observational study appears to show that the s s t anomalies are initially forced by perturbations in the atmospheric circulation with s s t data leading the atmosphere by one month results show that atmospheric and s s t anomalies are most persistent in the period october to december throughout the winter these lagged relationships are much weaker and not statistically significant diagnostics of e vector divergence from the gcm experiments are used to suggest that anomalous baroclinic wave activity over the atlantic is important as a momentum forcing for the anomalous time mean flow pattern on the other hand the role of thermal forcing provided by anomalous diabatic heating and transient eddy heat flux convergence may be important to substantiate this statement a simple linear steadystate two layer model of the response to extratropical thermal forcing is described with a suitable basic state flow and a mid tropospheric heat source given mainly by the transient eddy heat flux convergence the response is shown to be equivalent barotropic with a downstream ridge and ascent over the thermal source conversely results from an ocean mixed layer model are discussed which suggest that warm s s t anomalies could be maintained by a positive surface pressure perturbation positioned downstream of the anomaly through anomalous northward advection of warm ocean water by ekman drift currents this northward advection would balance the sensible and latent heat loss into the atmosphere over the s s t anomaly hence it is possible that some positive ocean atmosphere feedback may account for the persistence of such atmospheric and oceanic anomalies