low temperature metamorphism and secondary components in the portage lake volcanics a reassessment we studied the effects of hydrothermal alteration on magnetic properties of the magnetite and hematite components in the 1 1 ga portage lake volcanics upper peninsula michigan results show the importance of the intensity and direction of the hematite magnetization in the detection of secondary magnetite overprints in basalts we collected samples from 20 flows in two stratigraphic sections one from outcrops and the other from drill core the remanence unblocked below 590 c during thermal demagnetization is considered to be due to magnetite and that unblocked between 610 690 c is considered to be due to hematite hematite remanent intensities in the flow interiors are an order of magnitude less intense than those of magnetite and correlate with magnetite intensities from the top of the volcanics to the lower prehnite pumpellyite zone 5 5 km depth in the outcrop section and to the epidote pumpellyite zone 7 0 km depth in the core section these magnetic properties are very similar to those of re

refinements for footwall red bed diagenesis in the sediment hosted stratiform copper deposits model the emplacement of copper in sediment hosted stratiform copper deposits requires the circulation of a low temperature chloride rich brine in rift filling footwall red beds probably driven by meteoric recharge in adjacent highlands the descending oxygen rich meteoric water may become saline probably by leaching of footwall evaporites or possibly by mixing with brines draining down from contemporaneous evaporite pans and evolve toward a moderately oxidized brine as oxygen is consumed during the diagenetic reddening of the initially nonred coarse grained clastic rift sediments under these conditions the pore solution attains its maximum ability to leach and transport trace amounts of copper from the red beds sediment hosted stratiform copper deposits may form where the brine crosses a redox boundary into reduced sulfide rich gray beds with further consumption of oxygen during reddening the brine may become highly reduced and able to take iron into solution possibly explaining synsediment hosted stratiform copper iron oxide alterations and deposits the timing of copper deposition is clearly linked to the diagenetic reddening of immature footwall sediments in rift basins a process considered to extend over millions of years

constraining the north atlantic circulation with tritium data the circulation of the north atlantic as deduced from an inverse calculation is tested against the historical record of tritium the circulation model is first used to do a forward calculation of the tritium transient using published estimates of atmospheric injection rates and of plausible estimates of the tracer history at the open boundaries of the model major discrepancies are found between this calculation and the observations of interior distributions of tritium an attempt is then made to remove these discrepancies by modifying the atmospheric injection rates and the initial estimates of open boundary time histories treating these boundary conditions as control variables the procedure requires answering difficult questions as to the accuracy with which atmospheric transfers to the ocean and the boundary conditions at the northern limits of the model are known although removal of the model and observational tritium estimate differences is not completely satisfactory it appears that existing tritium observations would only weakly constrain a more realistic circulation inversion the computation however does suggest a strategy for future use of transient tracers for constraining circulation models

paleomagnetism of 1 09 ga lake shore traps keweenaw peninsula michigan new results and implications we report paleomagnetic data from a new section of the 1 09 ga lake shore traps exposed on silver island 10 flows and on the adjacent mainland two flows along the northwestern coastline of the

rubidium strontium dating of ore deposits hosted by rb rich rocks using calcite and other common sr bearing minerals the authors have tested a rb sr technique that permits ore deposits to be dated using common gangue minerals such as calcite and fluorite the only conditions the deposit must meet are that 1 it have minerals with a low rb sr ratio and 2 it be enclosed by wall rock with a high rb sr ratio because hydrothermal minerals acquire a strontium isotope composition that is usually similar to that of the wall rock minerals with low rb sr ratio should record and retain the isotopic composition that the wall rock had at the time of mineralization the difference between that ratio and that of the wall rock at present is a function of time and the rb sr composition of the wall rock the technique was tested fusing fluorite and calcite from three deposits ranging in age from tertiary to precambrian in all cases the age determined here closely resembles that obtained by conventional k ar and rb sr dating methods the precision however can be poor and depends chiefly on the strontium isotope heterogeneity of the wall rock and its rb sr enrichment 36 references 1 figure 1 table

u th pb zircon ages of some keweenawan supergroup rocks from the south shore of lake superior new single crystal zircon u th pb ages for plutonic and rhyolitic keweenawan supergroup rocks from the south shore of lake superior provide geochronological constraints on magmatic evolution associated with the 1 1 ga midcontinent rift analyses of a granophyric phase of the mineral lake intrusion and the mellen granite both parts of the mellen intrusive complex and a laterally extensive rhyolite from the top of the kallander creek volcanics have weighted average 207pb 206pb ages of 1102 0 2 8 ma n 2 1100 9 1 4 ma n 5 and 1098 8 1 9 ma n 4 respectively analyses of a pyroclastic rhyolite flow at the top of the porcupine volcanics result in variable 207pb 206pb ages that range from 1080 to 1137 ma this rhyolite exhibits a continuum between morphologically complex and simpler prismatic zircon crystals the latter yielding concordant analyses having a weighted average 207pb 206pb age of 1093 6 1 8 ma n 2 four prismatic zircons from an aphyric rhyolite of the chengwatana volcani

genesis of native copper lodes in the keweenaw district northern michigan a hybrid evolved meteoric and metamophogenic model the keweenaw peninsula of northern michigan is well known for early diagenetic sulfide dominant sediment hosted stratiform copper mineralization hosted mainly by basal siltstone shale units of the nonesuch formation in the white pine district and later native copper lodes hosted mainly by flow top breccias and amygdaloidal portions of flood basalts and interbedded conglomerates of the stratigraphically lower portage lake volcanics the native copper mineralization is generally attributed to downdip burial metamorphism of the host strata an alternative model is proposed here involving a complementary influx of evolved meteoric water elevated in salinity and progressively equilibrating with its aquifers similar to the fluid which formed the white pine mineralization this model has several advantages over the metamorphogenic model alone it would provide the salinity by dissolution of evaporites and moderately oxidizing conditions as a result of hematitization i e reddening of coarse continental rift sediments necessary for copper transport and the evolved meteoric water would have been an important cupriferous brine owing to the leaching of trace amounts of copper from the voluminous aquifers further reduction of the evolved fluid would also explain the updip deposition of native copper in association with dehematitized pumpellyite and chlorite dominant i e reduced hydrothermal mineral assemblages at the same time this influx of meteoric water from highlands adjacent to the keweenaw rift basin would have provided a mechanism to move the deep seated hot dense ore brine upward along the aquifers of the portage lake volcanics on its ascent from deep portions of the rift basin the gravity driven evolved meteoric brine would have mixed with the fluid generated by burial metamorphism to form a hybrid evolved meteoric metamorphogenic oreforming hydrothermal brine

late oligocene climatic changes evidence from calcareous nannofossils at kerguelen plateau site 748 southern ocean abstract palaeoecological changes in nannofossil assemblages in the southern ocean during oligocene times are examined through high resolution quantitative analyses of samples from ocean drilling program odp site 748 kerguelen plateau we quantitatively characterized the palaeoecological preference of groups of species and compared their general trends with those determined at maud rise odp sites 690 and 689 persico d villa g 2004 eocene oligocene calcareous nannofossils from maud rise and kerguelen plateau antarctica palaeoecological and palaeoceanographic implications marine micropaleontology 52 153 179 we then attempt a correlation between the main assemblage variation and sea surface temperature sst changes in the southern ocean at this time relatively stable cool conditions are interpreted to have persisted from earliest to late oligocene times when an increase in abundance of temperate water taxa is recorded both at maud rise and kerguelen plateau before the mi 1 event this reveals a climatic event that probably involved both sites and which is comparable to that indicated by the global oxygen isotope curve miller k g wright j d fairbanks r g 1991 unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion journal of geophysical research 96 6829 6848 zachos j pagani m sloan l thomas e billups k 2001 trends rhythms and aberrations in global climate 65 ma to present science 292 686 693 in the uppermost oligocene at site 689 the temperate water taxa index temperate temperate cool 100 increases from 25 2 ma similarly at site 748 the temperate water taxa index indicates an increase in sst in the late oligocene from about 26 5 ma at the base of chron c8n 2n which is offset by about 1 m y between the two areas the reason for this time difference can probably be found in the location of site 748 with respect to site 689 and in the palaeoceanographic setting taking into account that site 748 lies north of a deep water passage that separates kerguelen plateau from antarctica and north of the present day front of the antarctic circumpolar current the presence of warm water taxa exclusively at site 748 corroborates this hypothesis

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

mineral deposits of the mesoproterozoic midcontinent rift system in the lake superior region a space and time classification abstract the mesoproterozoic midcontinent rift system mrs of north america hosts a diverse suite of magmatic and hydrothermal mineral deposits in the lake superior region where rift rocks are exposed at or near the surface historically hydrothermal deposits such as michigan s native copper deposits and the white pine sediment hosted stratiform copper deposit were major mrs metal producers on going exploration for and potential development of copper nickel sulfide deposits hosted by the duluth complex of minnesota and the opening of the eagle nickel mine in michigan indicate an expanding interest in mrs magmatic deposits mrs hydrothermal and magmatic mineral deposits many of which are significant past present and likely future providers of critical minerals here are placed into a space and time metallogenic framework to construct this framework regional mrs mineral deposits extracted from the u s geological survey mineral resources data system mrds and the ontario ministry of energy northern development and mines mineral deposit inventory mdi were supplemented by other known and recently recognized mineral deposits described in the literature all mineral deposits were classified by deposit type host rock age and type and estimated timing of mineralization deposits were then put into a tectonic evolutionary framework stages for the mrs which shows that deposits formed within discrete spatial and temporal stages of rift evolution each stage of rift evolution is characterized by specific mineral deposit types that are largely confined both by their physical location in rift rocks and type and timing of mineralization examples include mrs nickel rich conduit type magmatic sulfide deposits which are restricted to an early magmatic stage mrs history when magma compositions were characterized by ni rich picrites and high mg basalts in contrast contact type magmatic sulfide deposits with cu ni were derived from more evolved al rich tholeiitic magmas that dominated a later time of voluminous magmatic activity hydrothermal sediment hosted stratiform chalcocite mineralization along the margins of western lake superior in michigan is economic only where fluid flow through red beds was concentrated by structures along the margins of a post volcanic mrs sedimentary basin widespread native cu and native ag mineralization occurred about 40 million years after formation of host basalt lava flows and interflow sedimentary rocks these descriptions of the diverse mineral deposits in the lake superior region cataloged within the space of the local and regional geology and over the more than 60 million year mineralizing history of the mrs are indicative of the complex and at times overlapping magmatic and hydrothermal mineral systems that operated within this major large igneous province

paleomagnetism of the native copper mineralization keweenaw peninsula michigan the age and genesis of michigan s world class native copper deposits are poorly constrained the copper is hosted by basaltic flow tops and conglomeratic interbeds of the 1095 2 ma keweenawan por

district scale concentration of native copper lodes from a tectonically induced thermal plume of ore fluids on the keweenaw peninsula northern michigan economic native copper lodes in the lake superior district are confined to a 45 km long portion of the portage lake volcanics on the keweenaw peninsula despite similar host rocks extending for more than 1 000 km around lake superior the focus of ascending ore forming brine into a narrow segment of portage lake strata may be attributed to upward thrusting on the keweenaw fault during closure of the midcontinent rift forming anomalously hot steeply dipping aquifers on the keweenaw promontory and to the development of a single dominant thermal plume within the incipiently buoyant brine at the scale of the mine district on that promontory

primary curvature in the mid continent rift paleomagnetism of the portage lake volcanics northern michigan usa abstract rocks of the north american mid continent rift mcr in the keweenaw peninsula of northern michigan display a change in structural trend from east to west varying in strike from 100 in the east to 35 farther west before returning to a more east west trend of 80 near wisconsin in general curvature can be described either as of primary origin meaning that the arc developed in its present curved state or as of secondary origin when the arc formed from an initially straighter geometry a powerful tool in evaluating the origin and degree of curvature in any deformed belt is paleomagnetism where coincident change in declination and strike is evidence for secondary curvature several paleomagnetic studies have been completed on rift related sedimentary rocks as well as lava flows from the mcr in the lake superior region paleomagnetic directions for the sedimentary rocks vary much more in declination than in inclination possibly reflecting a vertical axis rotation if secondary rotation has affected the sedimentary rocks then underlying volcanic rocks should also demonstrate a similar rotation thirty one sites were collected from the portage lake volcanics in the most highly curved part of the keweenaw peninsula in the upper peninsula of michigan sites were chosen to maximize the variation in structural trend thermal demagnetization results showed two components in samples from most sites a lower temperature a component 580 c both components were tested for primary remanence using a conglomerate test the a component carried by magnetite passes the conglomerate test at a 95 confidence level and is therefore considered a primary remanence the b component carried by hematite fails at the 95 level and is considered a secondary magnetization most importantly declination of the primary a magnetization between sites shows no correlation with strike demonstrating that vertical axis rotations cannot explain the curvature of the mid continent rift we conclude that curvature in the lake superior region is a primary feature likely reflecting a pre existing zone of weakness that was exploited during rifting

closing of the midcontinent rift a far field effect of grenvillian compression the midcontinent rift formed in the laurentian supercontinent between 1109 and 1094 ma soon after rifting stresses changed from extensional to compressional and the central graben of the rift was partly inverted by thrusting on original extensional faults thrusting culminated at about 1060 ma but may have begun as early as 1080 ma on the southwest trending arm of the rift the crust was shortened about 30 km on the southeast trending arm strike slip motion was dominant the rift developed adjacent to the tectonically active grenville province and its rapid evolution from an extensional to a compressional feature at ca 1080 ma was coincident with renewal of northwest directed thrusting in the grenvllle probably caused by continent continent collision a zone of weak lithosphere created by rifting became the locus for deformation within the otherwise strong continental lithosphere stresses transmitted from the grenville province utilized this weak zone to close and invert the rift

an adjoint sensitivity study of chlorofluorocarbons in the north atlantic 1 adjoint sensitivities of cfc 11 concentrations and cfc 11 cfc 12 ratio ages in a north atlantic general circulation model are analyzed these sensitivities are compared with those of spiciness t s where are the thermal and haline expansion coefficients respectively high sensitivity fields are candidates for providing the most powerful constraints in the corresponding inverse problems in the dual adjoint solutions all three variables exhibit the major ventilation pathways and define the associated timescales in the model overall however spiciness shows the highest sensitivity to the flow field in the north atlantic deep water sensitivities of cfc properties and spiciness to the isopycnal mixing and thickness diffusion are of the same order of magnitude in the lower subtropical thermocline sensitivities of cfc properties to the isopycnal mixing and thickness diffusion are higher the utility of this sensitivity is undermined by the need to reconstruct their boundary conditions given the influence of t s measurements on the density field they produce the most powerful constraints on the model on the large scale it still remains possible however that transient tracers can provide a larger relative information content concerning the mixing process between the near surface boundary layer and the thermocline but dependent upon the ability to reconstruct accurate initial and boundary conditions

age of native copper mineralization keweenaw peninsula michigan amygdaloidal flood basalts and conglomerates are the host for substantial deposits of native copper within the portage lake volcanics in the keweenaw peninsula of michigan a wide variety of gangue minerals are associated with the regional hydrothermal alteration mineralization event application of the rb sr method to amygdule filling microcline calcite epidote and chlorite suggests an age of mineralization between 1 060 and 1 047 m y or asymp 20 m y these results are supported by a fission track age on epidote of 1 044 or 169 m y the age of native copper mineralization determined in this study is consistent with geologic evidence which suggests that mineralization postdated the deposition of most or all of the overlying freda sandstone variable initial 87 sr 86 sr ratios between the low rb sr phases suggest mixing of isotopically distinct sources of sr during the generation and equilibration of the hydrothermal solutions

the precambrian nonesuch formation of the north american mid continent rift sedimentology and organic geochemical aspects of lacustrine deposition abstract the precambrian keweenawan trough in the lake superior region of north america is part of a failed intractonic rift that is filled with a thick sequence of volcanic and sedimentary rocks one unit in this sequence the none such formation is composed of black to green siltstones and shales and is conformable with the underlying alluvial copper harbor conglomerate and the overlying fluvial freda sandstone the formation is exposed as a lens in northern michigan and wisconsin and attains a maximum thickness of 200 m based on the integration of outcrop and core data nine facies have been recognized in the formation each has been assigned to one of three genetic facies assemblages a marginal lacustrine assemblage characterized by interbedded lithic sandstones siltstones mudstones and sandstone shale couplets is interpreted as a record of deposition on a sandflat mudflat complex a lacustrine assemblage is characterized by massive to well laminated dark shaly siltstones carbonate laminites shales siltstones and mudstones these sediments were probably deposited in a progressively shallowing perennial lake bottom conditions were anoxic and became increasingly more oxic as lake regression progressed a gradual transition from a lacustrine environment to a fluvial environment is represented by the red to brown massive horizontally laminated and rippled fine sandstones siltstones and mudstones of the fluvial lacustrine assemblage interactions among rates of subsidence rates of sedimentation fluctuations in lake level possible changes in climate and differences in tectonic setting controlled sedimentation in the nonesuch lake and resulted in variable vertical facies sequences total organic carbon toc analyses show a strong correlation between organic richness and the shale facies lacustrine assemblage and mudstone facies marginal lacustrine assemblage detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two prevalent organic petrographies which could be distinguished using combined pyrolysis gas chromatography mass spectrometry py gc ms and to a limited extent by stable carbon isotope mass spectrometry the geographic and stratigraphic distribution of rocks containing the two organic petrographies suggests that differential preservation e g microbial degradation accounts for the difference rather than maturity or variable source input

crustal scale thrusting and origin of the montreal river monocline a 35 km thick cross section of the midcontinent rift in northern michigan and wisconsin a structurally simple 35 km thick north facing stratigraphic succession of late archean to middle proterozoic rocks is exposed near the montreal river which forms the border between northern wisconsin and michigan this structure the montreal river monocline is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the keweenawan supergroup middle proterozoic along the south limb of the midcontinent rift and disconformably underlying sedimentary rocks of the marquette range supergroup early proterozoic these rocks lie on an archean granite greenstone complex about 10 km of which is included in the monocline this remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension the faults responsible for the monocline are newly recognized components of a well known regional fault system that partly closed and inverted the midcontinent rift system resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 20 ma and followed very shortly after extension that formed the midcontinent rift system

temporal evolution of the tracer signal in the deep western boundary current tropical atlantic four world ocean circulation experiment woce repeat cruises october 1990 to march 1994 in the tropical atlantic off brazil are used to study the spatial and temporal evolution of the chlorofluorocarbon cfc components cfc 11 and cfc 12 and tritium signal in the upper north atlantic deep water nadw its shallowest part located in the tropical atlantic around 1600 m depth is the shallow upper north atlantic deep water sunadw it is characterized by a distinct tracer maximum which is presumably received through winter time convection in the subpolar north atlantic here we discuss the tracer fields and the temporal evolution of the tracer signal of the sunadw in the tropical atlantic along two meridional sections at 44 degrees and 35 degrees w and two zonal sections at 5 degrees and 10 degrees s off brazil the spatial and temporal development of the tracer field in the tropical atlantic as well as the correlation with hydrographic parameters show that the temporal tracer change being due to the arrival of younger water is disturbed by other processes in particular the impact of variable mixing and spreading pathways on the observed tracer variability in the sunadw is evident in the observations

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

paleomagnetism and upb geochronology of volcanic rocks from michipicoten island lake superior canada precise calibration of the keweenawan polar wander track abstract paleomagnetic poles have been determined for normally magnetized samples of late proterozoic volcanic rocks from 23 sites on michipicoten island lake superior canada the results are in general accord with an earlier less precise study by one of the authors and indicate a small polar shift between rocks of the mamainse point formation 186 e 38 n and the michipicoten island formation 175 e 26 n a quartz feldspar porphyry intruding the lower mamainse point formation but unconformably overlain by the michipicoten island formation was sampled for u pb zircon dating the zircons define an age of emplacement of 1086 5 1 3 3 0 ma this age and the pole positions are consistent with and supplement ages previously determined for the keweenawan paleomagnetic polar wander curve these data define an apparent polar wander rate of 14 cm a 1 this age combined with previously published u pb ages indicates a time span of 22 ma for most keweenawan igneous activity in addition the time of the keweenawan paleomagnetic reversal is constrained to be between 1086 5 1 3 3 0 and 1097 6 3 6 ma the age of the upper part of the reversely magnetized osler group the zircon population contained an inherited component from a source 1403 50 ma old this suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid proterozoic

a process based approach to estimating the copper derived from red beds in the sediment hosted stratiform copper deposit model most estimates of the total tonnage of copper metal leached from footwall red beds to form sediment hosted stratiform copper deposits are based on the amounts of copper at ppm levels that might have been derived from the volume of red beds underlying given sediment hosted stratiform copper deposits that static calculation is replaced here with a process based approach in which the release of copper results from the progressive diagenetic alteration of the footwall aquifers by oxygen rich meteoric water driven by topographic recharge in highlands adjacent to the intracontinental rift basins hosting these deposits the total amount of liberated copper depends on the volume of meteoric water its oxygen content and the duration of the diagenetic leaching of copper estimates suggest that adequate amounts of copper may be derived from footwall red beds within reasonable periods of early and possibly more advanced diagenesis

latent thermal effects from porcupine volcanics calderas underlying the white pine presque isle stratiform copper mineralization northern michigan economic stratiform copper mineralization in the fine grained carbonaceous basal nonesuch formation of the 50 km wide white pine presque isle mining district northern michigan has long been explained by the essentially vertical infiltration of cupriferous brine across the full width of basal nonesuch strata from the underlying copper harbor conglomerate aquifer followed by copper precipitation mainly by reactions with in situ iron sulfide based on that model why do lesser amounts of total copper measured in vertical sections occur in coarser grained presumably more permeable basal nonesuch strata situated laterally to the east and west of the mining district the answer may lie in increased infiltration rates for copper harbor hosted ore forming brine warmed locally by latent volcanic heat from a resurgent caldera or calderas located within the large porcupine volcanics dome underlying the mine district not only would caldera heat have warmed the brine and lowered its density but more importantly it would have produced a remarkable decrease in the brine viscosity an increase in temperature from 15 to 100 c could have increased the brine infiltration rate by more than four times in very early diagenetic time when the sediments were poorly compacted and highly porous because progressive compaction would have opposed the temperature effect a very early diagenetic timing is suggested for main stage copper mineralization over the porcupine volcanics caldera textural evidence has also long supported an early mineralization event at the rift basin scale the exceptionally rapid upward flux of warmed brine into fine grained nonesuch over the caldera would have required equivalent volumes of replacement brine which would in turn have required a lateral convergence of brine within the footwall copper harbor conglomerate aquifer toward the white pine presque isle district this lateral focus of brine toward the warm caldera area would have created a more efficient basin scale ore forming system than that resulting from a linear hydrogeologic flow most concepts described here deserve further evaluation through quantitative hydrogeologic modeling thermal effects on ore brine viscosities and densities developed here for the sediment hosted stratiform copper mineralization of the white pine presque isle district could be considered in genetic models for other deposits and deposit types the effects are especially strong for solutions warmed from low temperatures e g 20 c or lower to 100 c and could also be important to ore fluid flow for ores involving much higher hydrothermal temperatures if solutions with initially low temperatures played a role in their genesis

magnetization age from paleomagnetism of the copper harbor red beds northern michigan usa and its keweenawan geologic consequences the copper harbor formation on lake superior s keweenaw peninsula records the transition from volcanic to sedimentary infilling of north america s 1 1 ga keweenawan rift radiometric dating shows t

the copper harbor conglomerate a late precambrian fining upward alluvial fan sequence in northern michigan the keweenawan copper harbor conglomerate exposed in northern michigan is part of a thick succession of volcanics and terrigenous clastics deposited in the keweenawan trough a failed intracontinental rift it is a fining upward wedge of volcanogenic conglomerate and sandstone deposited as proximal to distal braided stream and sheet flood facies on coalesced alluvial fans and sand flats this unit along with the lower portion of the overlying nonesuch shale consists of an alluvial fan shallow lake sequence that filled the rift during and following volcanic activity the fining upward nature of the sequence reflects a waning sediment supply probably due to lowering of the source area following faulting the copper harbor conglomerate exhibits attributes of both arid fan and humid fan alluvial models coarse grained facies are lithologically similar to alluvial fan units deposited in modern humid regions conversely finer distal copper harbor facies exhibit numerous features indicating flashy discharges and desiccation processes typical of deposition in modern arid regions ambiguity of sedimentary features useful for climatic inference probably reflects the limited applicability of holocene models to more ancient alluvial sequences because precambrian hydraulic regimes developed under various climatic settings may have differed from their holocene counterparts

constraining the north atlantic circulation between 4 5 s and 39 5 n with transient tracer observations 1 a1 horizontal resolution north atlantic general circulation model gcm in the latitude band 4 5 s to 39 5 n is compared to and then combined with chlorofluorocarbon and tritium transient tracer data the method of lagrange multipliers adjoint is used tracer distribution within the model ocean interior is primarily sensitive to the flux through the open northern boundary this flux is determined by the estimation method and is found to deviate from the initial estimate significantly for all tracers an attempt to carry the model domain to 78 5 n showed the gcm inadequacy believed to be primarily a problem of spatial resolution to compute the convective injection of tracer into the deep ocean any such errors persist throughout the tracer integration and corrupt all further tracer concentrations in general and consistent with earlier much simpler computations uncertainties in surface and northern boundary conditions dominate the computed tracer concentrations the final results here are improved estimates of the three dimensional time histories of the tritium and cfcs and their boundary conditions over the model domain with very little information available to constrain the gcm itself in the latitude band index terms 4532 oceanography physical general circulation 4842 oceanography biological and chemical modeling 4255 oceanography general numerical modeling 4271 oceanography general physical and chemical properties of seawater keywords transient tracer state estimation adjoint model

global ocean circulation during 1992 1997 estimated from ocean observations and a general circulation model 1 a three dimensional oceanic state is estimated for the period 1992 1997 as it results from combining large scale ocean data sets with a general circulation model at the cost of increased computational load the estimation assimilation method is chosen specifically so that the resulting state estimate is consistent with the model equations having no artificial sources or sinks to bring the model into close agreement with observations its initial temperature and salinity conditions are permitted to change as are the time dependent surface fluxes of momentum heat and freshwater resulting changes of these control vectors are largely consistent with accepted uncertainties in the hydrographic climatology and meteorological analyses the assimilation procedure is able to correct for many of the traditional shortcomings of the flow field by changing the surface boundary conditions changes in the resulting flow field are predominantly on the gyre scale and affect many features that are often poorly simulated in traditional numerical simulations such as the strengths of the gulf stream and its extension the azores current and the anticyclonic circulation associated with the labrador sea tests of the results and their consistency with prior error assumptions show that the constrained model has moved considerably closer to the observations imposed as constraints but has also moved closer to independent data from the world ocean circulation experiment not used in the assimilation procedure in some regions where the comparisons remain indeterminate not enough ocean observations are available and it is difficult to ascribe the residuals to either the model or the observations although problems remain a useful first solution to the global time dependent ocean state estimation problem has been found the estimates will continue to improve through the evolution of numerical models computer power increases more data and more efficient estimation methods

no asymmetry in geomagnetic reversals recorded by 1 1 billion year old keweenawan basalts 1 1 billion year old volcanic rocks in north america are thought to record asymmetric geomagnetic reversals indicating non axial dipolar behaviour of the magnetic field high resolution data from ontario suggest that the reversals were instead symmetric and that the apparent reversal asymmetry is an aliasing effect of the low resolution of earlier samples combined with the rapid motion of north america

paleomagnetism and the age of copper mineralization in the portage lake volcanics upper peninsula michigan the close association of hematite with native copper in the flow tops of the portage lake volcanics invites the opportunity to establish the relative age of copper mineralization for this purpose

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference