relay zone geometry and displacement transfer between normal faults recorded in coal mine plans abstract overlap lengths separations and throw gradients were measured on 132 relay zones recorded on coal mine plans throws on the relay bounding fault traces are usually 2 m and individual structures are recorded on only one seam throw gradients associated with relay zones are not always higher than on single faults but asymmetry of throw profiles is diagnostic of relay zones bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault parallel plane three dimensional structure is defined for two relay zones each recorded on five seam plans these relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces lobes initially terminated at tip lines but as the faults grew gradually rejoined the main fault surfaces along branch lines this type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip line and is an almost inevitable result of a tip line irregularity

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

effect of mechanical interaction on the development of strike slip faults with echelon patterns abstract the overlapping geometry of echelon strike slip faults is well known we have quantified this observation by measuring the amount of overlap and separation and plotting them against each other for over 120 examples although there is large scatter the data show a linear trend suggesting that the overlap increases proportionally with separation up to a limiting value we have analyzed this conspicuous relationship in terms of fault interaction by using a numerical model based on displacement discontinuity the results show that fault interaction is in fact an important factor in contributing to the overlapping geometry of echelon strike slip faults

a comparison of seismic and structural measurements of scaling exponents during tensile subcritical crack growth abstract the observed fractal nature of both fault length distributions and earthquake magnitude frequency distributions suggests that there may be a relationship between the structure of active fault systems and the resulting seismicity in previous theoretical work a positive correlation between the exponent d from the fracture length distribution and the seismic or acoustic emission ae b value has been inferred from a simple dislocation model of the seismic source here we present the first experimental evidence for a correlation between d and b from a series of tensile fracture mechanics tests on crystalline rock carried out in different environmental conditions both air dry and water saturated and at ambient temperature and pressure the microseismic acoustic emissions were monitored during subcritical crack growth under controlled conditions of constant stress intensity k i and quantitative analyses of the resulting fracture patterns were carried out on the same specimens it is found that ae b values ranging from 1 0 to 2 3 correlate negatively with the normalized stress intensity k i k ic where k ic is the fracture toughness of the specimen the microcrack length distribution exponent d ranges from 1 0 to 1 7 fluid presence has a first order influence on both the ae and structure produced in these experiments for experiments at low stress intensity or high fluid content the activation of the stress corrosion mechanism for k i k ic leads to a greater relative proportion both of small cracks and of low amplitude acoustic emissions reflected in higher values of d and b the exponent d is found to correlate positively with the ae b value

practicalities of extrapolating one dimensional fault and fracture size frequency distributions to higher dimensional samples previously published theory which extrapolates fault and fracture population statistics observed in a one dimensional sample to two and three dimensional populations is found to be of limited value in practical applications we demonstrate how significant the discrepancies may be and how they arise there are two main sources for the discrepancies 1 deviations from ideal spatial uniformity spatial poisson process of a fault or fracture pattern and 2 non power law scaling of the size frequency distributions of the population we show that even small fluctuations in spatial density combined with variance in the estimator of population statistics can lead to considerable deviations from the theoretical predictions ambiguity about power law scaling or otherwise of the underlying population is a typical characteristic of natural data sets and we demonstrate how this can affect the extrapolation of one dimensional data to higher dimensions in addition we present new theoretical approaches to the problem of extrapolation when clustering of faults and fractures is explicitly considered clustering is commonly observed in the field as en echelon arrays of fault or fracture segments and we show how this property of natural patterns can be quantified and included in the theory these results are relevant to building more realistic three dimensional models of the physical properties of fractured rocks such as fracture permeability and seismic anisotropy

correlation analysis of fracture arrangement in space abstract we present new techniques that overcome limitations of standard approaches to documenting spatial arrangement the new techniques directly quantify spatial arrangement by normalizing to expected values for randomly arranged fractures the techniques differ in terms of computational intensity robustness of results ability to detect anti correlation and use of fracture size data variation of spatial arrangement across a broad range of length scales facilitates distinguishing clustered and periodic arrangements opposite forms of organization from random arrangements moreover self organized arrangements can be distinguished from arrangements due to extrinsic organization traditional techniques for analysis of fracture spacing are hamstrung because they account neither for the sequence of fracture spacings nor for possible coordination between fracture size and position attributes accounted for by our methods all of the new techniques reveal fractal clustering in a test case of veins or cement filled opening mode fractures in pennsylvanian marble falls limestone the observed arrangement is readily distinguishable from random and periodic arrangements comparison of results that account for fracture size with results that ignore fracture size demonstrates that spatial arrangement is dominated by the sequence of fracture spacings rather than coordination of fracture size with position fracture size and position are not completely independent in this example however because large fractures are more clustered than small fractures both spatial and size organization of veins here probably emerged from fracture interaction during growth the new approaches described here along with freely available software to implement the techniques can be applied with effect to a wide range of structures or indeed many other phenomena such as drilling response where spatial heterogeneity is an issue

experimental evidence for different strain regimes of crack populations in a clay model we report results from clay extension experiments used as a model for the evolution of fault populations due to stress interactions at yielding cracks begin to appear and the brittle strain due to them quickly reaches a rate matching the applied stretching rate the crack density number of cracks per unit area initially increases apace then reaches a maximum at a critical strain decreasing thereafter at low strains where the crack population is dilute a power law length distribution is observed which at high strain gradually transitions to an exponential this agrees with fault populations data observed in low and high strain settings these results indicate that fault populations ranging from power law to exponential size frequency distributions reflect the population evolution with increased strain

mechanisms and patterns of o and h isotopic exchange during hydrothermal alteration of the north shore volcanic group and related hypabyssal sills midcontinent rift system minnesota abstract hydrothermal flow systems developed in volcanic and interflow sedimentary rocks in response to the emplacement of 1 1ga mafic plutonic rocks during intracontinental rifting in the midcontinent region oxygen and hydrogen isotopic studies of the north shore volcanic group nsvg northeastern minnesota and related hypabyssal sills demonstrate the importance of both permeability and temperature in controlling isotopic exchange in the rift related hydrothermal system oxygen isotopic values of mineral separates and volcanic whole rocks range from 5 5 to 17 7 and show a strong correlation with whole rock water content and intensity of hydrothermal alteration in the shallow portion of the stratigraphic sequence elevated 18 o values and mineralogical alteration are confined principally to highly permeable amygdaloidal flow tops and bottoms massive flow interiors show little or no perturbation from 18 o values considered normal for fresh basaltic rocks with increased depth and temperature isotopic exchange becomes more pervasive 18 o values remain elevated and oxygen isotopic profiles show no difference between flow margins and interiors at depths where greenschist facies mineral assemblages are found flow margins show a reversal to 18 o values as low as 6 6 and d values decrease to 18 o and d values enhanced rates for hydrogen isotopic exchange relative to oxygen are suggested by whole rock d values that tend to be uniform throughout flows that are less than 7 m thick d profiles that parallel those of 18 o values are found in thicker flows the correlation between degree of hydrothermal alteration and 18 o values suggests that solution recrystallization was the primary mechanism of isotopic exchange in the hydrothermal system individual feldspar phenocrysts from a single lava flow may show a range of 18 o values from 10 5 to 17 7 and ranges in individual altered phenocrysts may be as large as 4 8 feldspar isotopic heterogeneity suggests that permeability varied on a grain to grain scale and that flow through microfractures was important in controlling isotopic variations coexisting plagioclase and pyroxene from hypabyssal sills show a very steep slope on a 18 o plagioclase 18 o pyroxene plot also suggestive of kinetically dominated exchange assuming a fluid buffered system in the permeable flow margins a duration of isotopic exchange on the order of 40 000 years is calculated this time period is short compared to the lifetime of the intrusive activity in the rift and suggests that fluid flow was either episodic in nature or confined to the late stages of magmatic evolution

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

the igneous petrology and magmatic evolution of the midcontinent rift system klewin k w and shirey s b 1992 the igneous petrology and magmatic evolution of the midcontinent rift system in p a ziegler editor geodynamics of rifting volume ii case history studies on rifts north and south america and africa tectonophysics 213 33 40 the 1100 ma midcontinent rift system of north america is one of the world s great igneous provinces containing over one million km3 of mafic volcanic and plutonic rocks volcanic rocks range from picrite to rhyolite although tholeiitic basalt predominates plutonic rocks range from peridotite to granite with plagioclase rich gabbros and troctolites being most abundant the igneous history of the rift can be divided into early and late stages flows of the early stage form a trend having low alumina high magnesia picrite as the primitive endmember whereas flows from the late stage form a trend having high alumina olivine tholeiite as the primitive endmember the early basalts have enriched nd isotopic compositions and were derived largely from old heterogeneous subcontinental lithosphere the late basalts have chondritic to depleted nd isotopic compositions and were derived from large degree partial melts of a mantle plume that had mixed with isotopically depleted asthenospheric mantle apparently melting began initially in the subcontinental lithosphere as it was stretched subsequently a lower mantle plume which had entrained asthenospheric mantle during its ascent rose beneath the rift and melted at large degrees to yield the voluminous later basalts

paleomagnetism of the powder mill group michigan and wisconsin a reassessment of the logan loop the apex and the western arm of the 1 1 ga logan loop a prominent feature of the north american precambrian apparent polar wander path are defined by a large body of data from keweenawan rocks of the lake superior basin until now the eastern arm of this paleomagnetic feature has been largely defined by two data points from the powder mill group of michigan a reexamination of this sequence reveals a primary reversed magnetization which plots close to the apex of the logan loop data from the mellen complex in wisconsin together with our data from the powder mill group suggest that for steeply dipping volcanic units the true direction of a prefolding reversed magnetization is not obtained by simple structural rotation about presently observed strike reversely magnetized sites from localities where the powder mill volcanics have shallow dips yield a pole 146 w 46 n a95 9 2 on the apex of the loop a well defined stratigraphically continuous normal to reversed polarity change near the base of the powder mill group is not confirmed normally magnetized flows have been found at two localities only one of which is at the base of the lava sequence the higher metamorphic grade of normally magnetized sites suggests that normal magnetizations within the powder mill group are secondary and should not be used in definition of the eastern arm of the logan loop

petrology of the keweenawan mamainse point lavas ontario petrogenesis and continental rift evolution the mamainse point formation forms a 5250 m thick sequence of flood basalts ranging from high mgo picrites and basalts to basaltic andesites that formed in the keweenawan 1100 ma midcontinent rift of north america this section is the most continuous and complete sequence of keweenawan lavas and records almost the entire igneous history of the rift the section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics a variety of petrogenetic processes including partial melting of different sources at decreasing depth periodically replenished tapped and fractionated magma chambers crustal contamination and polybaric fractionation were responsible for the evolution of the magmas the dominant petrogenetic process changes with stratigraphy which suggests that the physical evolution of the rift controlled which process operated at any time the initial fracturing of the crust presumably allowed high mgo liquids generated at depth to be erupted the early stage of rifting was apparently slow and allowed the formation of long lived magma chambers in which complex processes such replenishment mixing and assimilation contamination could occur the late stage of rifting however was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted these conclusions from the chemostratigraphy agree with the physical stratigraphy which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

distinct crustal structure of the north american midcontinent rift from p wave receiver functions eighty two broadband seismic stations of the superior province rifting earthscope experiment spree collected 2 5 years of continuous seismic data in the area of the high gravity anomaly associated with the midcontinent rift mcr over 100 high quality teleseismic earthquakes were used for crustal p wave receiver function analysis our analysis reveals that the base of the sedimentary layer is shallow outside the mcr thickens near the flanks where gravity anomalies are low and shallows again in the mcr s center where the gravity anomalies peak this pattern is similar to that found from local geophysical studies and is consistent with reverse faulting having accompanied the cessation of rifting at 1 1 ga intermittent intracrustal boundaries imaged by our analysis might represent the bottom of the mcr s mostly buried dense volcanic layers outside the mcr the moho is strong sharp and relatively flat both beneath the archean superior province and the proterozoic terranes to its south inside the mcr two weaker candidate mohos are found at depths up to 25 km apart in the rift s center the intermediate layer between these discontinuities tapers toward the edges of the mcr the presence of this transitional layer is remarkably consistent along the strike of the mcr including beneath its jog in southern minnesota near the belle plaine fault we interpret these results as evidence for extensive underplating as a defining characteristic of the rift which remains continuous along the minnesota jog where due to its orientation it is minimally affected by the reverse faulting that characterizes the nne striking parts of the rift

scaling of fault displacements and implications for the estimation of sub seismic strain fault displacement populations have been shown to follow a power law scaling relationship characterized by an exponent d this relationship can be used to make predictions of the sub seismic fault population from data derived from seismic surveys although fault populations exist in three dimensions the use of section data is recommended d values derived from sections can be applied directly to several problems and are also related to the d value for the fault set in higher dimensions accurate determination of d requires proper consideration of the scale range and sample size limitations of available data the most common technique of using a cumulative frequency graph often leads to an upwards bias an iterative correction procedure is proposed discrete frequency methods avoid this bias but as a standard linear interval graph has other associated problems a log interval graph method is preferred simulations of these methods applied to random computer generated samples from power law distributions have been made to examine the accuracy of d values derived from typical data equations to estimate the confidence intervals for these d values have been derived from a synthesis of the results the application of the techniques is shown using fault data measured on seismic sections from the southern north sea and the inner moray firth where local differences in d are shown to be significant there is usually a marked change in structural style fault data are used to make improved estimates of crustal extension b by extrapolating the derived powerlaw relationship a value of 13 1 20 is calculated for the inner moray firth applications predicting the intersection of horizontal wells with large sub seismic faults and quality control of fault interpretation on seismic sections are also described

the aeromagnetic method as a tool to identify cenozoic magmatism in the west antarctic rift system beneath the west antarctic ice sheet a review thiel subglacial volcano as possible source of the ash layer in the waiscore abstract the west antarctic ice sheet wais flows through the volcanically active west antarctic rift system wars the aeromagnetic method has been the most useful geophysical tool for identification of subglacial volcanic rocks since 1959 64 surveys particularly combined with 1978 radar ice sounding the unique 1991 97 central west antarctica cwa aerogeophysical survey covering 354 000 km 2 over the wais 5 km line spaced orthogonal lines of aeromagnetic radar ice sounding and aerogravity measurements still provides invaluable information on subglacial volcanic rocks particularly combined with the older aeromagnetic profiles these data indicate numerous 100 1000 nt 5 50 km width shallow source magnetic anomalies over an area greater than 1 2 10 6 km 2 mostly from subglacial volcanic sources i interpreted the cwa anomalies as defining about 1000 volcanic centers requiring high remanent normal magnetizations in the present field direction about 400 anomaly sources correlate with bed topography at least 80 of these sources have less than 200 m relief at the wais bed they appear modified by moving ice requiring a younger age than the wais about 25 ma exposed volcanoes in the wars are the present rapid changes resulting from global warming could be accelerated by subglacial volcanism

amount of extension on small faults an example from the viking graben extension estimates based on palinspastic restoration of only the largest faults in a deformed terrane may be significantly deficient due to neglect of small faults i e unsampled faults most of which have smaller displacements than do sampled faults small faults individually account for little extension but there may be large numbers of them the cumulative extension on small faults can be calculated from the fractal size distributions of fault displacements independent extension estimates support the inference from fault population statistics that small faults account for significant amounts of extension for example restorations of mesozoic faults interpreted from seismic reflection profiles across the viking graben in the north sea yield extension estimates that are only 40 75 of independent estimates the fractal size distribution of fault displacements in the viking graben is consistent with the hypothesis that the remaining 25 60 of extension resulted from displacements on small faults

fractal relations for the diameter and trace length of disc shaped fractures analytic relations are developed between the fractal parameters of a random isotropic population of disc shaped fractures and the parameters of the corresponding population of fracture traces expressed in outcrop these relations indicate that a fractal distribution of fracture diameters translates to a fractal distribution of trace lengths and that the parameters for diameter may be uniquely determined from the parameters for trace length probabilistic results demonstrate the accuracy of the analytic relations and identify nonideal behaviors at the limits of the computed trace length data it is expected that these relations will be useful in the three dimensional characterization of fracture systems in rock from in situ trace length data

changes in fault displacement populations correlated to linkage between faults in carboniferous strata exposed at the western margin of the southern appalachian u s a fold thrust belt mesoscopic faults accommodated strains as the cumberland plateau thrust sheet moved 1 km wnw on a sub horizontal detachment these faults are confined to a layer 75 m thick above the detachment in the upper part of this 75 m thick layer a laterally persistent coal seam separates two stratigraphic units subjected to different amounts of sub horizontal elongation distinct sets of mesoscopic faults that cut bedding at high angles and have normal offsets accommodate the strains in each unit below the coal seam a sample line 500 m long intersects 504 faults a plot of log n size rank vs log d dip separation in mm for these faults is linear suggesting that a scaling law n ad c holds for this population least squares regression gives c 0 8 and log10 a 3 1 above the coal seam a sample line 250 m long intersects 745 faults a plot of log n vs log d for these faults has two distinct linear trends faults with separations 1 25 m yield a scaling law n ad c with c 0 5 and log10 a 2 9 seventeen faults with separations d 1 25 m define a second linear segment n a d c with c 1 8 and log10 a 6 9 the break in slope at d 1 25 m corresponds to faults with heights of 30 m roughly equal to the thickness of the faulted strata faults here with d 1 25 m and d 1 25 m which exhibit different scaling characteristics are respectively analogous to faults within and those that extend across the brittle crust in regional studies comparing the two displacement populations described here small offset faults make a proportionally smaller contribution to the total strain in the larger strain setting this pattern which is observed elsewhere suggests that fault systems within individual layers exhibit different scaling characteristics at different stages in their history this contribution argues that fault linkage is the key to such changes in fault scaling behavior it is a mesoscopic to macroscopic structural change that alters the subsequent development of faults fault linkage like fault size or sampling dimension should be considered in examinations of fault scaling laws

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

aggregate properties of fracture populations abstract empirical studies indicate that the individual attributes of both faults and extension fractures follow power law scaling aggregate properties of fracture populations are important in a variety of problems and can be specified in terms of the scaling parameters of individual fracture attributes development of an expression for an aggregate property requires consideration of a number of independent factors including the topologic dimension of the aggregate property the topologic dimension of sampling and the possibility of scaling changes for fractures that span some mechanically significant layer the riemann zeta function provides an alternative to integration for the analytical and numerical solution of aggregate problems previous work regarding aggregate properties of fracture populations has focused on the strain due to faulting new expressions are developed here for other aggregate properties of interest fracture surface area fracture porosity fracture permeability and shear wave anisotropy a general characteristic of these aggregate properties is that for most values of scaling exponents the aggregate properties are dependent on the size of the sampling domain this implies that the aggregate properties are scale dependent additionally it appears that fracture surface area is concentrated in the smallest fractures of many populations fracture porosity is concentrated in the largest fractures of most populations but not as strongly as fracture permeability which probably derives almost entirely from the largest fractures in populations

consortium for continental reflection profiling michigan surveys reprocessing and results reprocessing of consortium for continental reflection profiling data from central michigan with emphasis on deconvolution and velocity filtering before stack and more detailed velocity analysis has resulted in greatly improved imaging of a previously reported keweenawan rift buried beneath the paleozoic of the michigan basin the seismic stratigraphy of the sub paleozoic units can now be related to the basin filling middle keweenawan volcanic sequence and the upper keweenawan clastic assemblage in greater detail important structural and stratigraphic features can be seen for the first time for example reprocessing has imaged an unconformity within the upper keweenawan it is possible some of these newly imaged features are associated with a late keweenawan compressive episode furthermore reprocessing also reveals previously unrecognized sub keweenawan structures including a deeply penetrating reflection which may represent a major crustal fault and possible weak reflections from midcrustal 25 km and moho 43 km depths

quantitative fault studies on the east pacific rise a comparison of sonar imaging techniques a synthesis is presented of published statistics on the lengths scarp heights and spacing of normal faults on the flanks of the east pacific rise epr between 19 s and 21 n nine data sets are included which were collected at different times employing one of four different sonar mapping systems the scripps deep tow instrument seamarc i seamarc ii and gloria with widely ranging data collection capabilities the epr in this region is morphologically quite similar and in some cases the data sets were obtained from the same area of the seafloor it is assumed therefore that data collection procedure and system resolution have had a greater influence on the fault statistics observed than varying tectonic processes we try to account for these sampling effects such that quantitative comparisons of fault development at midocean ridges may then be made we discuss the problem of identifying faults in sonar images and the errors associated with measurements of the relevant parameters we also investigate the statistical properties of the fault populations and derive expressions for correcting observed values of the population means at finite resolution to obtain the corresponding values at infinite resolution the theoretical predictions compare well with the results of a decimation procedure applied to real data sets to simulate diminishing resolution we also discuss the effects that surveying geometry can have on the interpretation of fault lengths the data synthesis is augmented by an analysis of the correlation between scarp height and fault length for areas near 3 5 s and 12 n on the epr the correlation which we find to be linear is used to estimate 1 the minimum operational fault target size for each of the four mapping systems and 2 the topography due to faulting in an area where only fault lengths are known

estimates of strain due to brittle faulting sampling of fault populations abstract the geometry of sampling domains is a first order consideration in the characterization of brittle fault populations in most cases descriptions of fault size distributions based on map cross section traverse or borehole data systematically underestimate the number of small faults present in a volume the geometry of sampling domains may be accounted for using an empirical proportionality between fault displacement and trace length estimates of strain in which the sampling geometry is considered suggest that small faults accommodate a significant portion of the total strain due to the brittle faulting process

when do fractured media become seismically anisotropic some implications on quantifying fracture properties fractures are pervasive features within the earth s crust and they have a significant influence on the multi physical response of the subsurface the presence of coherent fracture sets often leads to observable seismic anisotropy enabling seismic techniques to remotely locate and characterise fracture systems in this study we confirm the general scale dependence of seismic anisotropy and provide new results specific to shear wave splitting sws we find that sws develops under conditions when the ratio of wavelength to fracture size s d is greater than 3 where rayleigh scattering from coherent fractures leads to an effective anisotropy such that effective medium model emm theory is qualitatively valid when 1 s d 3 there is a transition from rayleigh to mie scattering where no effective anisotropy develops and hence the sws measurements are unstable when s d 1 we observe geometric scattering and begin to see behaviour similar to transverse isotropy we find that seismic anisotropy is more sensitive to fracture density than fracture compliance ratio more importantly we observe that the transition from scattering to an effective anisotropic regime occurs over a propagation distance between 1 and 2 wavelengths depending on the fracture density and compliance ratio the existence of a transition zone means that inversion of seismic anisotropy parameters based on emm will be fundamentally biased more importantly we observe that linear slip emm commonly used in inverting fracture properties is inconsistent with our results and leads to errors of approximately 400 in fracture spacing equivalent to fracture density and 60 in fracture compliance although emm representations can yield reliable estimates of fracture orientation and spatial location our results show that emm representations will systematically fail in providing quantitatively accurate estimates of other physical fracture properties such as fracture density and compliance thus more robust and accurate quantitative estimates of in situ fracture properties will require improvements to effective medium models as well as the incorporation of full waveform inversion techniques

geochemical stratigraphy of the keweenawan midcontinent rift volcanic rocks with regional implications for the genesis of associated ni cu co and platinum group element sulfide mineralization the development of the north american midcontinent rift can be understood in the context of a tectonic and magmatic event in which the far field effects of continental drift influenced magmatism produced by a mantle plume we report and interpret new data for samples collected from stratigraphically controlled sequences of basaltic rocks in the osler volcanic group and the mamainse point volcanic group in ontario canada we confirm that the earliest phase of rift development involved primitive melts that produced basalts in the lower formation of the osler volcanic group with fo74 80 olivine 1 490 2 280 ppm ni the earliest basalts are primitive in both the mamainse point volcanic group and osler volcanic group sections with elevated mgo cr and ni these rocks also have high tio2 and high gd yb which are features that indicate low degrees of melting of a garnet bearing lherzolite mantle the central formation a and b basalts of the osler volcanic group that overlie the lower formation lavas record an upward increase and then decrease in the degree of contamination of basaltic rocks with normal ni and platinum group element pge abundance levels that require no sulfide saturation event to accompany contamination the upper formation of the osler volcanic group is characterized by a limited number of pge depleted flows although these basalts are highly fractionated and typically develop textures with stellate to laminated plagioclase feldspar they are usually not significantly contaminated the lower and upper formations of the mamainse point volcanic group show a diversity in geochemistry consistent with contributions from different mantle sources but contamination is restricted to a small number of flows with normal ni and pge abundance levels there is no indication of a regional metal depletion signature on the scale of the nadezhdinsky formation at noril sk but there is evidence that local s saturation events were triggered in the unexposed conduits feeding some of the upper sequence flows in response to fractionation of the magma massive to heavy disseminated ni cu pge sulfide mineralization in the midcontinent rift is hosted by small primitive intrusions that were formed early in the evolution of the rift and are typically associated with transtensional structures in the cratonic blocks adjacent to the main rift examples include the eagle deposit in michigan the tamarack mineralization in minnesota and the current lake complex in ontario the american examples are associated with splays of the great lakes tectonic zone and the canadian example is associated with the quetico fault zone the intrusions share the geochemical fingerprints of the more primitive lowermost volcanic rocks of the midcontinent rift and they are broadly contemporaneous with the development of the lower stratigraphy of the osler volcanic group and mamainse point volcanic group although the low grade but large tonnage cu ni pge deposits of the duluth were formed significantly later than the small primitive intrusions their host rocks share the same lithophile trace element signature as these early intrusions the parental magmas that produced all of these intrusions were derived from deeper mantle depths where garnet was a stable phase than the magmas that produced the central formation basalts and some of the upper formation basalts investigated in this study

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

stereological analysis of fracture network structure in geological formations relationships between three dimensional fracture networks consisting of poly disperse disks and the corresponding two dimensional trace maps are systematically analyzed bulk densities of disks and disk intersections are related to surface densities of traces and trace intersections this results in a new relation for the average length of disk intersections the probability densities of trace lengths and disk intersections are studied for several disk diameter distributions the inverse problem of deriving the disk distribution from the trace distribution is then solved assuming only that the disks have uniformly random locations and orientations these results are applied to a variety of synthetically generated data as well as to several sets of field data analysis of the field data suggests that fracture diameter distributions follow a power law in agreement with previous conclusions based on trace length histograms with exponents varying from 1 3 to 2 1

steady seepage in trenches and dams effect of capillary flow steady seepage from two dimensional domains is investigated using a dimensionless formulation for variably saturated media that depends on three dimensionless parameters m n and the parameter m is the product of the anisotropy ratio and the squared ratio of the vertical length scale to the horizontal length scale the parameter n increases with the uniformity of the pore sizes and represents the ratio of the domain height to the height of the capillary fringe our modeling results show that the seepage face height in rectangular domains is always larger than the seepage face height computed from saturated flow models the results also show that the seepage face height increases with increasing m increasing n and or decreasing the outflows computed from the present model are always larger than the outflows computed by the dupuit assumption nomographs for rectangular and trapezoidal domains simulating trenches and dams are presented

prediction of macrofracture properties using microfracture information mesaverde group sandstones san juan basin new mexico abstract a better understanding of relationships between microfractures and macrofractures is prerequisite to the extrapolation of observed microfracture properties as a basis for inferring properties of associated macrofractures the hypothesis that micro and macrofractures represent different size fractions of the same fracture sets and consequently have linked attributes was tested on sandstones of the mesaverde group san juan basin new mexico the use of a cathodoluminescence detector attached to a scanning electron microscope sem allowed observation of microfractures not readily visible using conventional microscopic techniques the orientations of microfractures reproduce the orientations recorded at the macroscopic scale from cores and outcrops fracture size distributions are best modeled using power laws and extrapolation of microfracture frequencies to the scale of macrofractures successfully predicts the macrofracture frequencies fracture size distributions change at the length scale of the thickness of the mechanical layer typically by increasing the exponent of their power law distribution this change is independent of censoring biases and reflects changes in sampling topology and or fracture growth mechanisms

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

flexural modeling of the midcontinent rift a basement profile obtained from seismic reflection data has been used to constrain a two dimensional flexural model of basin formation for the midcontinent rift at a latitude of 45 sup 0 25 n model parameters included the thickness of the elastic plate the basin width and the maximum basin thickness modeling suggests that flexure produced a deep narrow basin along the rift axis and that the crust was thinned at the time of basin formation to an elastic thickness of 9 6 km for a plate ruptured by rifting or 5 7 km for an unbroken plate with corresponding maximum basin thicknesses of 14 km and 16 km respectively the plate thickness depends most strongly on the basin width and is well constrained by the seismic data although erosion may have narrowed the basin the maximum basin thickness is poorly constrained because of the lack of seismic data for depths greater than about 10 km and because the strata at the center of the rift have been disturbed by a postrift compressional event which produced the st croix horst despite uncertainty about the basin thickness the load required to flex the crust to produce the midcontinent rift basin is too large to be attributed tomore the weight of the central flood basalts unless the basin subsided into a fluid less dense than the solidified basalts on the basis of seismic refraction data and by analogy with other rifts we hypothesize that a magnetic rift pillow intruded in the lower crust the basaltic pillow subsequently solidified to produce a large high velocity region in the lower crust centered under the rift axis as determined from deep seismic refraction this crystallization and cooling may be responsible for the sag phase of rift evolution as evidenced by laterally widespread occurrence of postvolcanic sediments copyright american geophysical union 1988 less

ground surface temperature history in central canada inferred from 10 selected borehole temperature profiles among the 57 temperature depth profiles recently measured in manitoba and saskatchewan canada only 10 are suitable for inferring recent changes in ground surface temperature many of the rejected temperature profiles show an apparent climatic signal but are affected by topography changes in vegetation or the proximity of lakes at the northernmost site near lynn lake manitoba shallow horizontal variations in temperature have been identified that are related to intermittent permafrost such variations can induce an apparent climatic perturbation in areas where lateral thermal conductivity contrasts have been measured heat refraction effects can also incorrectly be interpreted as ground surface temperature histories the analysis suggests that forest fires that occurred at some of the sites have had little influence on the temperature profiles the 10 selected temperature profiles cover a wide area in northern manitoba and saskatchewan they have been independently and jointly inverted by a singular value decomposition method the ground surface temperature history shows two main episodes a cold period tentatively identified as little ice age with a minimum around 1820 a d was followed by marked warming after 1920 these trends are similar to those recognized in eastern canada

a geophysical profile of the southern margin of the midcontinent rift system in western lake superior a 70 km long seismic reflection profile in western michigan provides new insight into the nature distribution and structure of the keweenawan supergroup volcanic and overlying sedimentary rocks and the controversial keweenaw fault along the southern boundary of the midcontinent rift system in the lake superior basin interpretation of the 5 s reflection data constrained by surface geology magnetics and gravity modeling shows that volcanic rocks which cropout north of the keweenaw fault dip northerly to depths of the order of 17 km south of the fault volcanic rocks overlain by 2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the south range the volcanic pile within the basin thickens rapidly to the north of the keweenaw fault suggesting that the volcanics were deposted in an extensional fault bounded basin clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event the thickness of the volcanic filled basin implies that the upper crust was almost completely broken during the rifting event the available evidence is interpreted to show the keweenaw fault as a moderate to high angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence there is no evidence from the seismic profiling for major faulting except for the keweenaw fault intrusions or folding of the keweenawan supergroup in this region

deformation rates from faulting at the tempe terra extensional province mars 1 rates of deformation are estimated from exposed normal faults within the tempe terra extensional province mars through an analysis that incorporates fault segment linkage and utilizes regional observations of displacement length relationships measured from mars orbiter laser altimeter topographic data moment rates from 1014 1016 n m yr strain rates 10 12 10 11 yr 1 and rifting velocities 0 003 mm yr are comparable to rates of deformation on stable plate interiors of earth the calculated low rates of rifting may result from the poorly constrained timing of deformation on mars cumulative moment release estimates decrease linearly with time from noachian 1 1025 n m to early hesperian 6 1024 n m and late hesperian early amazonian 1 1024 these results indicate that a portion of tharsis related deformation remained localized within the tempe rift throughout much of martian history and provide important constraints for models of martian mantle convection

regional and local patterns of low grade metamorphism in the north shore volcanic group minnesota usa the north shore volcanic group in northern minnesota is part of the middle proterozoic keweenawan sequence one of the largest plateau lava provinces in the world the primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment the subaerial lava flows mainly amygdaloidal olivine tholeiites and tholeiites have undergone low grade metamorphism from zeolite to lower greenschist facies on the basis of alteration phases replacing the primary magmatic minerals infilling amygdales and veins and replacing secondary minerals the following zones have been distinguished 1 thomsonite scolecite smectite 2 heulandite stilbite smectite 3 laumontitechlorite albite 4 laumontite chlorite albite prehnite pumpellyite and 5 epidote chlorite albite actinolite zone in addition to the overall zonation based on mineral parageneses zonations in the composition of the ab content of the newly formed albite replacing primary ca rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows mafic phyllosilicates in the upper part of the sequence mainly smectites and mixed layer smectite chlorites display high si and ca na k contents whereas in the lower part of the sequence the amounts of si and ca na k are markedly lower mainly chlorites and mixed layer chlorite smectites similar zonations are observed within the individual flows the albite content of the newly formed plagioclase is highest and the si and ca na k content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior the above features suggest that the overall pattern is one of burial type metamorphism associated with extension in the rift setting in detail the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts this suggests that at the first stages of alteration lowest grade the patterns of fluid flow were important effects in controlling the assemblages at greater burial depth assemblages are more homogeneous perhaps representative of a more even and pervasive flow pattern using the observed assemblages at face value to define grade and or facies different conditions would be assigned within the different morphological flow portions thus at low grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions