the yavapai mazatzal boundary a long lived tectonic element in the lithosphere of southwestern north america a seismic reflection profile crossing the jemez lineament in north central new mexico images oppositely dipping zones of reflections that converge in the deep crust we interpret these data as a paleoproterozoic bivergent orogen centered on the jemez lineament that formed during original proterozoic crustal assembly by collision of mazatzal island arcs with yavapai proto north american continent at ca 1 68 1 65 ga the two major sets of reflections within the yavapai mazatzal transition boundary dip at 15 20 and we interpret them as part of a south dipping thrust system and as a north dipping crustal scale duplex that formed synchronously with the thrust system the upper crust shows structures recording a succession of tectonic and magmatic events from the paleoproterozoic to the holocene notable among these structures is a system of nappes that formed during development of the bivergent orogen elements of the nappe system are exposed in rocky mountain uplifts and have been dated as having formed at 1 68 ga at depths of 10 km and at temperatures of 500 c we also see continuous bright reflections in the upper part of the middle crust that we associate with basaltic sills that postdate accretion the data show that the yavapai mazatzal suture is low angle 20 an observation that explains why the boundary between the provinces has previously been so hard to define in the surface geology the jemez lineament overlies the root of this bivergent orogen that we also suggest is a paleoproterozoic zone of weakness that has subsequently acted as a conduit for magmas and a locus of tectonism

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

evidence for late middle proterozoic extension in the precambrian basement beneath the permian basin through an integrated interpretation of gravity magnetic seismic reflection and well data we are able to define the geometry lateral extent and tectonic setting of late middle proterozoic mafic rocks the pecos mafic intrusive suite in the basement of the permian basin of west texas and eastern new mexico in this region seismic reflection data are characterized by subhorizontal layered reflectivity within basement that can be tied to compositional variation in a layered mafic intrusion through well cuttings and a synthetic seismogram in places the intrusion forms a sill and the base of the sill is imaged in the reflection data integration of the seismic interpretation with gravity models shows that the pecos mafic intrusive suite is 3 10 km thick and 50 100 km wide the overall structure of the suite is that of a sill with a locally dikelike keel north south trending gravity and magnetic anomalies associated with the pecos mafic intrusive suite suggest that it is composed of four large intrusive centers which cover a minimum area of 9200 km2 making it one seventh the size of the bushveld complex and one of the largest layered mafic intrusions in the world we interpret this massive intrusion to be associated with collisional rifting in the foreland of the grenville orogeny

cocorp profiling across the southern oklahoma aulacogen overthrusting of the wichita mountains and compression within the anadarko basin cocorp consortium for continental reflection profiling deep reflection profiles recorded across the wichita mountains and anadarko basin suggest that significant crustal shortening occurred in the final stages of the evolution of the southern oklahoma aulacogen the crystalline rocks of the wichita mountains were thrust in pennsylvanian time northeastward over sedimentary rocks of the anadarko basin along a series of faults with moderate average 30 to 40 and southwesterly dips these faults can be traced possibly as deep as 20 to 24 km listric thrust faults and hanging wall anticlines developed in the sedimentary rocks of the basin these features contrast with conventional interpretations of pennsylvanian structures as the result of predominantly vertical movements along high angle faults and they suggest that pennsylvanian downwarping of the anadarko basin was at least partially due to thrust loading truncations of reflections from cambrian ordovician rocks in the deepest part of the basin suggest normal faulting which would support ideas of an early extensional stage in the aulacogen cycle the distinctive precambrian layering seen on earlier cocorp data recorded south of the wichita mountains cannot be recognized under the anadarko basin and the proterozoic basin containing that layering may have been bounded on its north side by a precambrian fault this inferred fault was probably twice reactivated during formation of the southern oklahoma aulacogen once during late precambrian early cambrian extension and again during pennsylvanian compression the popular view that aulacogens originated from radial rifting of updomed homogeneous continental crust is probably too simplified and a more important constraint on their location and development may be the nature of pre existing lines of weakness

seismic reflections from the basal contacts of batholiths although seismic reflection data in batholithic terranes are scarce datasets available to us from three different regions show surprising similarities all three profiles one across precambrian rocks in texas and two across mesozoic batholiths in california and nevada show strong subhorizontal reflections at depths of 6 10 km these reflections are interpreted to originate from the base of the batholiths and indicate that these batholiths are tabular in shape the large amplitudes of the reflections require a marked contrast in acoustic impedance product of velocity and density between the batholith and the underlying rocks two of the profiles show reflective zones having appreciable thicknesses 0 5 2 5 km while the third profile shows a single continuous reflection the four most favored candidates for sub batholithic reflections are 1 underplated gabbro 2 cumulate layers 3 migmatites and 4 thrust faults cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance

formation of alunite jarosite and hydrous iron oxides in a hypersaline system lake tyrrell victoria australia abstract alunite kal 3 so 4 2 oh 6 and jarosite kfe 3 so 4 2 oh 6 are common weathering products of aluminosilicates and pyrite long range transport of the constituents forming these minerals and the subsequent deposition of alunite and jarosite in an evaporite setting have not been previously documented such conditions for the occurrence of alunite and jarosite were investigated in a hypersaline system where acid groundwater enriched in k al fe iii and so 4 enter a saltplaya lake sediment cores were studied by thin section xrd and sem eds groundwater and pore water chemistries were also analyzed the results show that alunite and jarosite occur together or as individual layers and cements within the top 20 cm of the sediments where the groundwater is most concentrated due to evaporation hydrous fe oxides also occur as cements or vein fillings with the alunite and jarosite but are distributed throughout the 70 cm sediment column studied the results are consistent with a model in which alunite and jarosite precipitate as a result of the evaporation of water thermodynamic modeling indicates that the pore water in the playa sediments maybe in equilibrium with alunite jarosite and hydrous fe oxides

self sealing isolation and immobilization a geochemical approach to solve the environmental problem of waste acidic jarosite abstract mimicking geochemical processes to solve environmental problems was implemented in dealing with waste acidic jarosite and alkaline coal fly ash by placing these two chemically different materials adjacent to one another a self sealing layer was formed at the interface between both wastes isolating and immobilizing chemical constituents in the process a series of leaching experiments were performed on each material separately to study the release behavior of the principal constituents radiotracer experiments were conducted to explore diffusion and reaction of constituents such as fe 3 in a combined jarosite fly ash system a model has been developed to simulate the coupled processes of diffusion and precipitation taking into account porosity change due to pore filling by precipitates the formation of a self sealing isolation layer in a hypothetical jarosite fly ash disposal site was modelled leaching results indicate that the release of elements from jarosite is much larger than that from fly ash and that the highly ph dependent release of fe al and zn was controlled by the solubility of their hydroxides leaching results also suggest that precipitation reactions can be expected to occur at the interface between jarosite and alkaline coal fly ash where a large ph gradient exists radiotracer experiments showed that accumulation of constituents occurred at the interface modeled fe 3 profiles in layered jarosite fly ash were well validated by experiments modeling results also showed that with the accumulation of constituents at the interface a new layer with low porosity was formed application of this model suggests that there is a potential use to form a self sealing layer in jarosite fly ash co disposal sites

the limpopo mobile belt a result of continental collision the 600 km long limpopo mobile belt is discussed within the frame of a proterozoic supercontinent model piper 1976 evidence is presented that the rhodesian and kaapvaal cratons may have been separated by distances of more than 1000 km of oceanic crust from about 3350 ma ago the kaapvaal craton appears to have been driven intermittently n nw and then wnw against the rhodesian craton forming the ne sw trending collision zone the limpopo mobile belt and all the major fold and fracture patterns found this movement would be similar to the oblique movement of the pacific plate into the aleutian trench when collision ceased around 2500 ma ago it is likely that the great dyke and other complexes intruded along release fractures formed at right angles to the compression

extraction of deep crustal reflections from shallow vibroseis data using extended correlation lower crustal seismic reflections can be extracted from shallow crustal seismic profiles through the application of extended correlation to uncorrelated vibroseis seismic data fixed bandwidth extended correlation shortens the correlation operator before crosscorrelation producing reflections over an increased correlation time range all with lowered bandwidth self truncating extended correlation preserves the full bandwidth in the original seismic reflection times but loses bandwidth in a predictable manner at the additional later arrival times correlation wavelet shape and extra correlation time are directly related and can be calculated for specific acquisition parameters pre correlation tapering is necessary to avoid undue wavelet distortion at extended correlation times seismic data collected in the basin and range province illustrate the application of the method the results are verified with conventional correlations of long sweep records and with impulsive source data

the sudbury structure controversial or misunderstood the origins of the sudbury structure and associated igneous complex have been controversial most models call for a major impact event followed by impact induced igneous activity although totally igneous models are still being proposed much of the controversy is due in our opinion to a misunderstanding of the size of the original sudbury structure by analogy with other terrestrial impact structures the spatial distribution of shock features and huronian cover rocks at the sudbury structure suggest that the transient cavity was 100 km in diameter which places the original final structural rim diameter in the range of 150 200 km theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in 104 km3 of impact melt more than sufficient to produce a melt body the size of the igneous complex present volume 4 8 103 km3 for the igneous complex to be an impact melt sheet it must have a composition similar to that of the target rocks evidence for this has been presented previously for sr and nd isotopic data which suggest a crustal origin here we also present new evidence from least squares mixing models that the average composition of the igneous complex corresponds to a mix of archean granite greenstone terrain with possibly a small component of huronian cover rocks this is a geologically reasonable mix based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size the igneous complex is differentiated which is not a characteristic of previously studied terrestrial impact melt sheets this can be ascribed however to its great thickness and slower cooling that large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted if this working hypothesis is accepted namely that both the sudbury structure and the igneous complex are impact in origin then previous hybrid impact igneous hypotheses can be discarded and the sudbury structure can be studied specifically for the constraints it provides to large scale cratering and the formation of basin sized multiring impact structures

late middle proterozoic hazel formation near van horn trans pecos texas evidence for transpressive deformation in grenvillian basement the latest middle proterozoic hazel formation near van horn in the trans pecos region of west texas is an orogenic elastic wedge that provides insight into paleotectonic evolution of the southern margin of the laurentian craton the hazel formation is 2 5 km thick adjacent to the contemporaneous streeruwitz thrust fault and consists exclusively of terrestrial sedimentary rocks boulder sized clasts in conglomerate were deposited within alluvial fans north of the streeruwitz thrust sheet 0along the southern margin of a basin of unknown geometry and extent the alluvial fan conglomerate interfingers with eolian sedimentary rocks in the northern part of the basin alluvial fan conglomerate megasequences indicate that at least two episodes of faulting accompanied hazel sedimentation alluvial megasequences are overall aggradational within the hazel succession as revealed by clast size variations within the conglomerate systematic variation of pebble composition up through the hazel conglomerate provides an erosional history within the source terrain carbonate and subordinate mafic volcanic rocks derived from the pre orogenic allamoore and tumbledown formations were the principal source off the hazel formation metavolcanic and metasedimentary rocks from the carrizo mountain group in the present tectonic hinterland are notably absent in the hazel conglomerate moreover a rhyolite terrain presently not exposed in the van horn area was a significant component of the hazel source juxtaposition between source terrain and sedimentary basin coupled with aggradation of alluvial fan megasequences along the southern fault bounded basin margin supports a transpressive regime for this orogenic belt rather than a purely convergent margin deformation associated with the hazel orogenic sequence post dated a 1 126 to 1 070 m y old regional felsic igneous event grenville deformation in the llano uplift of central texas predated the same igneous event this implies that the hazel tectonic episode was a second later grenvillian deformation the terms llano orogeny and hazel orogeny are introduced here for the grenville province in texas and may be comparable with the elzevirian and ottawan orogenies within the grenville province of canada

reconciling physical properties with surface seismic data from a layered mafic intrusion abstract compositional layering of intrusive rocks is often cited as a source of seismic reflectivity in the crystalline crust direct evidence for this is based primarily on synthetic seismograms calculated from laboratory measurements of rock velocity and density or estimates of aggregate rock properties from measurements of mineral velocities and densities little is known about how in situ effects such as fracturing chemical alteration or anisotropy may change the reflectivity character of the surface seismic data an ideal dataset for examining the hazards of using physical properties data for estimating crustal reflectivity occurs at the nellie intrusion a middle proterozoic age layered mafic intrusion that lies beneath the permian basin of west texas due to a well that penetrated 4 5 km of the intrusion we have a dataset that spans the scale from compositional data on the intrusion derived from well cuttings to log data to surface reflection data sonic and density log data from the well measure velocities and densities 10 to 15 below the values calculated from modal mineralogy we attribute this difference primarily to pervasive post emplacement alteration and fracturing of the rock mass in situ thus this unique dataset provides a quantitative estimate of the magnitude of difference between observed and theoretical velocities that can be expected for crystalline rocks in the upper crust and its cause despite the difference analysis of synthetic seismograms shows that primary compositional variation of the intrusion is still responsible for sub horizontal layered reflectivity in both the log based and petrology based synthetic seismograms boundaries of intrusive cycles are characterized by large negative reflection coefficients due to the juxtaposition of mafic rich basal layers against plagioclase rich layer tops in detail only the log based synthetic provides a good match to the surface seismic reflection data this suggests that physical properties from modal mineralogy are useful in obtaining a generalized model for reflectivity but that differences in bulk physical properties of the rock mass preclude the possiblity of obtaining a one to one match with surface seismic data

geophysical and geochronological constraints on the extent and age of mafic intrusions in the basement of west texas and eastern new mexico a deep drill hole recently completed on the central basin platform of west texas provides a rare look at almost 5 km of basement rocks most of the crystalline rocks encountered were basic and ultrabasic in composition a variety of geophysical data is available from the bore hole and vicinity the petrographic and geophysical data suggest that a large layered basic intrusion was penetrated by the well geochronologic u pb data indicate an age of about 1 1 ga for this intrusion this date has significant tectonic implications because it indicates that commonly accepted analogies with the southern oklahoma aulacogen 550 ma rifting are not valid however it is interesting to note that this feature formed during another widespread period of rifting keweenawan

source regions of granites and their links to tectonic environment examples from the western united states this review in honor of ilmari haapala s retirement reflects on lessons learned from studies of three granitic systems in western north america 1 mesoproterozoic samples from west texas and east new mexico 2 laramide granitic systems associated with porphyry copper deposits in arizona and 3 granites of the colorado mineral belt the studies elucidate relationships amongst tectonic setting source material and magma chemistry mesoproterozoic basement samples are from two different felsic suites with distinct elemental and isotopic compositions the first suite the plutonic province is dominantly magnesian calc alkalic to alkali calcic and metaluminous it has low k2o na2o and rb sr and nd model ages of 1 56 to 1 40 ga the second suite the panhandle igneous complex is magnesian metaluminous alkalic and is part of the mesoproterozoic belt of magmatism that extends from finland to southwestern united states samples from the panhandle igneous complex demonstrate three episodes of magmatism the first pulse was intrusion of quartz monzonite at 1380 to 1370 ma the second was comagmatic epizonal granite and rhyolite at 1360 to 1350 ma both of these rock types are high k to slightly ultra high k the third pulse at 1338 to 1330 ma was intrusion of ultra high k quartz syenite nd model ages 1 94 to 1 52 ga are distinct from those of the plutonic province and systematically older than crystallization ages implying a substantial crustal input to the magmas at the sierrita porphyry copper deposit in the mazatzal province of southeastern arizona trace element sr and nd isotopic compositions were determined for a suite of andesitic and rhyolitic rocks 67 ma intruded by granodiorite and granite isotopic composition and chemical evolution are well correlated throughout the suite andesite has the least negative initial end 4 3 and lowest 87sr 86sri 0 7069 it is also the oldest and chemically most primitive having low concentrations of rb sio2 and high concentrations of transition elements these parameters change through the system to the youngest unit granite which has the most negative end 8 5 the highest 87sr 86sri 0 7092 and is chemically most evolved correlation between chemical and nd isotopic evolution probably resulted from a continuous process of progressive assimilation in which mafic magmas invade and incorporate continental crust deposits in arizona with end values more negative than the 8 5 of sierrita lie in the older yavapai province in the northwestern part of the state the difference in the most negative epsilon nd implies that nd isotopic signature is sensitive to the age of the precambrian domain the granites from the colorado mineral belt were emplaced during the transition from laramide convergence to mid tertiary extension three different groups of granites are recognized the first is laramide and was formed during assimilation fractional crystallization involving lower crustal mafic source materials the second and third groups are mid tertiary and represent intracrustal melting of heterogeneous sources this change in source regions and melt regimes in transition from convergence to extension is fundamental to the mesozoic and cenozoic evolution of western north america

durable terrestrial bedrock predicts submarine canyon formation though submarine canyons are first order topographic features of earth the processes responsible for their occurrence remains poorly understood potentially analogous studies of terrestrial rivers show that the flux and caliber of transported bedload are significant controls on bedrock incision here we hypothesize that coarse sediment load could exert a similar role in the formation of submarine canyons we conducted a comprehensive empirical analysis of canyon occurrence along the west coast of the contiguous united states which indicates that submarine canyon occurrence is best predicted by the occurrence of durable crystalline bedrock in adjacent terrestrial catchments canyon occurrence is also predicted by the flux of bed sediment to shore from terrestrial streams surprisingly no significant correlation was observed between canyon occurrence and the slope or width of the continental shelf these findings suggest that canyon incision is promoted by a greater yields of durable terrestrial clasts to the shore

zircon and rutile upb geochronology of the niquelândia layered mafic and ultramafic intrusion brazil constraints for the timing of magmatism and high grade metamorphism abstract the niquel ndia intrusion is among the largest layered intrusions in the world it is part of a 350 km long belt of large mafic ultramafic complexes in central brazil both intrusion and country rocks were subjected to highgrade metamorphism ranging from amphibolite to granulite facies and associated ductile deformation the first u pb zircon and rutile ages for this region are presented zircon fractions from a meta anorthosite from the upper gabbro anorthositic unit are variably discordant 0 28 with regard to the lower intercept along a mixing line from 777 10 ma lower intercept and 1583 25 ma upper intercept zircon fractions from a later quartz diorite within the central mafic unit are variably discordant 0 14 with regard to the lower intercept along a mixing line from 794 6 4 lower intercept and 1565 22 upper intercept the upper intercepts for both rocks interpreted to be the magmatic age overlap and suggest a mesoproterozoic age for the intrusion the lower intercept is interpreted to be the high grade metamorphic event zircons from two quartz rich blastomylonites representing different metamorphic domains and cutting through the layered intrusions were analyzed a single zircon from the amphibolite facies domain blastomylonite gives a concordant 0 3 207 pb 206 pb age of 777 3 ma and a single zircon from the granulite facies blastomylonite gives a 207 pb 206 pb age of 786 5 ma 0 8 discordance both results are consistent with the lower intercept ages and set the 770 795 ma interval for the high grade metamorphic event rutile ages 650 ma are considerably younger than zircon metamorphic ages and suggest a milder regional tectonomagmatic episode or a complex and long lasting cooling history for the region the u pb data provide evidence for an important mesoproterozoic extensional tectonic event in central brazil it also sets the 770 795 ma interval for the high grade amphibolite to granulite facies prograde metamorphism interpreted to represent the timing of a continental collision event

the reflectivity of magmatic underplating using the layered mafic intrusion analog abstract the reflection characteristics of layered mafic intrusions provide an analog for characterizing the reflectivity of magmatic underplates one dimensional seismic models of a variety of mafic intrusions with different parent magmas demonstrate that cumulate layering may typically be strongly reflective with reflection coefficients as high as 0 18 a seismic profile across the bjerkreim intrusion in southern norway shows that even a moderately reflective cumulate sequence with reflection coefficients on the order of 0 05 produces detectable reflections two dimensional models demonstrate that the subhorizontal cumulate layering in typical mid to upper crustal intrusions is conducive to seismic imaging but more complicated geometries such as concentric layering created during synmagmatic deformation complicate the seismic image reflection coefficients predicted for granulite facies mineral assemblages are similar to those predicted for the igneous assemblages reflection coefficients predicted for cumulates intruded or re equilibrated at eclogite facies conditions may be either decreased or enhanced with respect to the igneous assemblages depending on the abundance of plagioclase in the cumulate layers and the pressure results suggest that magmatic underplating should not be invoked as an explanation for non reflective crust except in cases where eclogitization has occurred

the oslo rift a review the tectonomagmatic history of the oslo rift may be subdivided into 5 main periods stage 1 300 ma development of a shallow depression stage 2 300 295 ma roughly simultaneous onset of widespread basalt volcanism b1 and vertical movement along nnw sse to n s trending faults stage 3 295 275 ma main rifting period accompanied by volcanism dominated by rhomb porphyry lavas latites stage 4 275 240 ma change of magmatic style from basaltic shield volcanism to central volcanoes of mixed compositions many of which went through caldera collapses intrusion of composite batholiths of intermediate to granitic compositions stage 5 240 ma dike intrusions may have continued into triassic time both the crust and total lithosphere under the rift are thinned relative to the precambrian basement on both sides a 12 km thick high density and high velocity body occupies the lower crust along the rift axis and extends under the eastern and western rift shoulder petrological and geochemical data imply that this massive layer represents dense cumulates and gabbroic rocks formed from mantle derived magmas in deep crustal magma chambers two mantle sources one nearly undepleted age corrected nd of about 1 and sr of 10 to 15 the other mildly depleted nd of about 4 sr of 10 contributed melts to the oslo rift magmatism the two sources are interpreted as different parts of a heterogeneous upper mantle which before the rifting event belonged to the subcontinental lithosphere on their way to the surface the mantle derived magmas were retained in deep crustal magma chambers where they underwent extensive fractional crystallization some also suffered moderate contamination anatectic melts and melts formed by mixing between mantle derived and anatectic melts gave rise to syenitic and granitic intrusions

the magnetic fabric of the wolfcamp shale midland basin west texas understanding petrofabric variability hydrocarbon associations and iron enrichment rock fabrics have long been recognized as a key variable influencing unconventional reservoir properties and yet petrofabric analysis is rarely incorporated in reservoir characterization in a quantitative and scalable manner this work introduces anisotropy of magnetic susceptibility ams coupled with handheld x ray fluorescence to define the origin of petrofabrics in the wolfcamp shale and examine their relationship with total organic carbon toc and oil saturation indices osi standard paleomagnetic plugs were drilled from three vertical cores spanning the entire wolfcamp shale in the midland basin west texas low temperature magnetic experiments suggest paramagnetic minerals contribute between 93 4 and 98 6 to the low field bulk magnetic susceptibility klf across all lithotypes scanning electron microscopy revealed frequent occurrences of paramagnetic minerals such as chlorite illite ferroan dolomite marcasite and pyrite the fe rich clays such as chlorite carry the ams signal in mudrocks with a minor contribution from ferroan dolomite conversely ferroan dolomite carries the ams signal in most carbonate facies with a minor influence from fe rich clays the degree of magnetic anisotropy pj and shape factor t in mudrocks are compositionally dependent and respond to changes in both clay and framework grain concentration for carbonate facies pj and t vary widely because of changes in depositional facies and the concentration of ferroan dolomite the toc correlates well with pj and strongly oblate magnetic fabrics this relationship likely stems from the adsorption of organic matter om to clays during deposition and may indicate that organoclay composites were responsible for delivering om to the midland basin elevated osi are fabric selective favoring low pj and t fabrics suggesting that such fabrics facilitate hydrocarbon migration more so than high pj and t fabrics fuzzy c means clustering provides a framework to describe ams and geochemical transitions at the core scale and could serve as a tool to map barriers and pathways to hydrocarbon migration finally ti nb and klf data point to a mafic provenance lithotype and suggest layered intrusive complexes in the central basin uplift were exposed and eroded during deposition of the wolfcamp shale mafic source rocks probably increased supply of micronutrients such as dissolved iron and may have enhanced productivity in the midland basin

seismic response and construction of seaward dipping wedges of flood basalts voring volcanic margin geological and geophysical data from the 900 m of volcanic basement drilled at ocean drilling program site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins the main part of the drilled section corresponding to a prominent seaward dipping reflector sequence consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments the basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity pore aspect ratio distribution and alteration stacks of laterally continuous basalt flows appear to have thin layer transverse isotropic properties for typical wavelengths in multichannel seismic data vertical seismic profiling and average sonic log velocities are similar 3 77 km s and 3 88 km s respectively while comparable refraction velocities are 10 20 higher synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces though the most continuous reflectors are related to thick flows seismic models based on the characteristic velocity and density basalt flow distributions and site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows the base of the dipping reflector sequence reflector k correlates with flows in the lower part of upper series basalts while the transition from basaltic to underlying dacitic andesitic lavas correlates with a locally defined reflector a model for the breakup related volcanism includes 1 prebreakup dacitic andesitic volcanism 2 early breakup basaltic volcanism infilling the prebreakup relief 3 main breakup stage with intense focused volcanism large subsidence and lava pounding and 4 late breakup volcanism during a period of decreased subsidence and local off axis activity

from trash to treasure three dimensional basement imaging with excess data from oil and gas explorations modern oil and gas seismic surveys commonly use areal arrays that record continuously and thus routinely collect excess data that are not needed for the conventional common reflection point imaging that is the primary goal of exploration these excess data have recently been recognized to have utility not only in resource exploration but also for addressing a diverse range of scientific issues here we report processing of such discarded data from recent exploration surveys carried out in southeastern new mexico these have been used to produce new three dimensional 3 d seismic reflection imagery of a layered complex within the crystalline basement as well as elements of the underlying crust this enigmatic basement layering is similar to that found on industry and academic seismic reflection surveys at many sites in the central united states correlation of these reflectors with similar features encountered by drilling in northwestern texas suggest that they may be part of an extensive continental scale network of tabular mafic intrusions linked to keweenawan rifting of the igneous eastcentral unites states during the late proterozoic more importantly this analysis clearly demonstrates that the new generation of continuously recorded 3 d exploration datasets represent a valuable source of fresh information on basement structure that should be examined rather than discarded such basement information is not only important to understanding crustal evolution it is directly relevant to assessing risks associated with fossil fuel extractions such as induced seismicity related to waste water injection

precambrian basement structure and laramide deformation revealed by seismic reflection profiling in the laramie mountains wyoming the university of wyoming conducted seismic reflection profiling in the laramie mountains of wyoming to investigate the nature of precambrian crustal structure and laramide deformation in the region here the imaging of layering in the laramie anorthosite complex confirms that layered intrusions can be reflective and establishes an analog for interpreting events in other seismic reflection data sets where layered intrusions could be the cause of multicyclic events moreover the presence of prominent primary layering in the complex supports a conclusion that it formed in situ and not as the result of the emplacement of a diapiric crystal mush imaging of laramide thrusts in the laramie mountains adds credence to the idea that brittle faults can be reflective in some situations migrated apparent dips on these faults are 30 westward basement involved compressive structures consisting of monoclinal folding and a fault triple junction demonstrate that compression controlled the style of deformation during the laramide orogen in wyoming west dipping precambrian structures imaged in these data have a similar attitude to the laramide structures suggesting that laramide deformation was influenced by precambrian features these precambrian structures may be subthrust slices of exotic proterozoic terranes

widespread buried precambrian layered sequences in the u s mid continent evidence for large proterozoic depositional basins large regions of the north american mid continent are underlain by precambrian layered rocks buried beneath phanerozoic sedimentary strata south of the wichita mountains published seismic reflection profiles show a precambrian layered assemblage extending for at least 40 km in both the north south and east west directions and industry data show that it may continue 150 km to the southeast seismic reflection data in the illinois region show a precambrian layered assemblage extending 320 km in an east west direction and 200 km in a north south direction in both cases the layered rocks are as much as 12 km thick apparent sequence boundaries onlap downlap within these assemblages suggest they are parts of large depositional basins with diffractions and dipping strat due to faulting the layered sequences correlate with regions of relatively long wavelength and low amplitude magnetic anomalies the extent of this magnetic signature suggests that about 200 000 km 2 of illinois indiana and western ohio about 50 000 km 2 of southernmost oklahoma and north central texas and about 32 000 km 2 of southern missouri and northern arkansas may be underlain by similar precambrian strata drill holes indicate that the top of the mid continent precambrian basement is composed largely of silicic igneous rocks such material may comprise a large part of the layered sequences alternatively these igneous rocks could be intermixed with or underlain by nonvolcanic meta sedimentary strata the strong reflectivity of some layers suggest that minor mafic flows and or sills may also be present analysis of u pb and nd sm isotopes within the granites and rhyolites imply that the layered sequences postdate crustal formation at 1 7 2 0 ga and predate or are contemporaneous with the 1 3 1 5 ga crystallization ages of the granites and rhyolites though these layered rocks have a spatial association with igneous rocks and thus have likely been metamorphosed the possibility tha they contain precambrian hydrocarbons that escaped heating is as yet untested

architecture of a 1 38 1 34 ga granite rhyolite complex as revealed by geochronology and isotopic and elemental geochemistry of subsurface samples from west texas usa abstract during the evolution of laurentia a mesoproterozoic felsic igneous belt extended from fennoscandia through canada to the southwestern united states this belt referred to as the granite rhyolite province in north america forms much of the west texas and eastern new mexico basement we present data from 41 closely spaced wells in west texas that penetrated several hundred meters into the southern granite rhyolite province and provide the first opportunity to develop a three dimensional view of the basement from subsurface samples the felsic rock types include hornblende bearing quartz monzonite ignimbritic rhyolite and comagmatic granite with eutectic textures these rocks plot in the high k to ultra high k field quartz syenite which plots exclusively in the ultra high k field is the final felsic phase of magmatism all of the felsic magmas were variably oxidizing based on biotite compositions u pb chronology supports the intrusive relations established by petrography the quartz monzonite is oldest 1380 ma and is followed by the granite rhyolite sequence 1360 ma and the quartz syenite 1340 ma nd model ages range from 1520 to 1740 ma indicating involvement of older proterozoic crust in many wells thick mafic sills intrude the granite rhyolite sequence nd model ages for the mafic rocks range from 1560 to 1440 ma implying that they are mesoproterozoic in age the mafic rocks include an alkaline oib like suite which is not supportive of a subduction origin u pb ages are also reported for the mescalero well 1 in eastern new mexico in this well metasedimentary and metavolcanic rocks of the debaca sequence unconformably overlie a quartz syenite similar to the quartz syenite from the west texas wells detrital zircons from the basal meta arkose of the sequence are 1690 and 1320 ma indicating a maximum age of 1300 ma for this sequence

deep geometry of the sudbury structure from seismic reflection profiling seismic reflection data show the deep geometry of the sudbury structure to be markedly asymmetric the sudbury north range comprises shallowly south dipping sedimentary strata overlying a thick unit of heterolithic breccia and melt bodies layered rocks of the sudbury igneous complex and footwall gneiss reflections from the upper layers are interrupted by faults near the center of the sudbury basin whereas the basal mafic units and footwall gneiss continue dipping southward and appear to be tightly folded or truncated near the creighton fault in contrast the seismic image of the south range is dominated by a distinctive series of moderately south dipping reflections interpreted as thrust faults or shear zones on which severe telescoping and imbrication of lithological units and considerable northwest southeast shortening of the sudbury structure have occurred such shortening explains the noncircular shape of the structure and thus removes a critical objection to the sudbury impact crater hypothesis the new structural model is shown to be consistent with regional magnetic and gravity fields a large hidden ultramfic mafic mass is no longer required to explain the potential field data

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

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

proterozoic basin in the southern midcontinent of the united states revealed by cocorp deep seismic reflection profiling cocorp deep crustal seismic profiles in southwestern oklahoma show strong persistent continuous and undeformed layering in the basement over an area probably very much greater than 2 500 km 2 such layering is very unusual judging by cocorp experience with basement rocks elsewhere in the united states the data can be interpreted as representing a proterozoic basin filled with clastic sedimentary and felsic volcanic rocks 7 to 10 km thick whose base lies 10 to 13 km deep these rocks are believed on the basis of sparse evidence from regional geology to have been deposited or extruded about 1 200 to 1 400 m y ago and some of them may now be metamorphosed this basin lies on the south side of the wichita mountains under the paleozoic hardeman basin and is similar in depth to the paleozoic anadarko basin north of the mountains the deep basement layering is truncated on the south side of the wichita mountains probably by precambrian faults in conjunction with granitic intrusions pennsylvanian compression probably reactivated these precambrian trends extensive precambrian basin deposits in this area were unexpected on the basis of evidence from sparse well control and reports of other layered basement reflections elsewhere in the southern midcontinent suggest that precambrian basins may be an important feature of this region simple models for the evolution of southwestern oklahoma as an aulacogen must be reformulated in the light of these new data

high velocity breakup related sills in the voring basin off norway multichannel seismic reflection profiles in the hel graben v0ring basin reveal a sill complex at approximately 5 km depth it is associated with exceptionally high 7 4 km s 1 seismic wide angle velocities the existence of observable wide angle arrivals shows that the sills act as efficient waveguides seismic reflection data and amplitude modeling constrain the thickness of individual sills to approximately 100 m sonic logs from sills of similar thickness on the nearby utgard high show an average velocity of 7 0 km s 1 such high velocities require an olivine gabbroic sill composition and emplacement under conditions which allowed growth of relatively large crystal sizes a possible reason for such an emplacement environment is the hei graben s role as an intrusion center during breakup volcanism this would provide the necessary duration of the magmatic activity as well as locally increased melt volumes and cooling times sill complexes of this kind decrease the accuracy of determined velocity fields and crustal geometries below the top of the sill complex affecting depth conversion and gravity modeling furthermore the results question the concept of lower crustal bodies as large scale homogeneous accumulations of mafic melt