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

remagnetization in keweenawan rocks part i conglomerates conglomerate tests are used to show that late precambrian keweenawan conglomerates on the keweenaw peninsula michigan and at mamainse point ontario have suffered a partial chemical remagnetization that predated tectonic tilting at mamainse point the overprint was acquired during the final normal polarity stage of keweenawan volcanism whereas on the keweenaw peninsula it occurred after cessation of volcanism and has a maximum late freda sandstone age the extent of overprinting on the keweenaw peninsula increases westwards along a 40 km strike length in the copper harbor conglomerate and may at least in part be related to secondary mineral zonation including native copper in the underlying portage lake volcanics it is shown that neither of the two overprinting episodes can be responsible for the reversal asymmetry observed in keweenawan igneous rocks

paleomagnetism of jurassic to miocene sediments from the apenninic carbonate platform southern apennines italy evidence for a 60 counterclockwise miocene rotation abstract the tectonic evolution of the apennine belt southern tyrrhenian sea system is addressed through a paleomagnetic study of lias to langhian sediments from the apenninic carbonate platform southern apennines italy reliable paleomagnetic data gathered from 21 sites document a regional scale post langhian 80 counterclockwise ccw rotation since previous studies of the plio pleistocene clays spread over the orogen had shown a 20 ccw rotation we conclude that the southern apennines rotated by 60 during middle late miocene our data provide evidence that the southeastward drift of calabrian block and synchronous spreading of the southern tyrrhenian sea induced saloon door like deformation of the southern apennines and sicily which underwent similar magnitude although opposite in sign orogenic rotations a paleomagnetically derived paleogeographic reconstruction shows that at 15 ma late langhian the alpine apennine belt collided with a nne oriented carbonate platform corridor surrounded by oceanic basins we speculate that both the end of the corsica sardinia rotation and the eastward jump of the locus of back arc extension from the liguro provencal to the tyrrhenian sea may have been consequences of this event

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

paleomagnetism and the orocline hypothesis eldredge s bachtadse v and van der voo r 1985 paleomagnetism and the orocline hypothesis in n l carter and s uyeda editors collision tectonics deformation of continental lithosphere tectonophysics 119 153 179 oroclines were originally defined by carey as curved mountain belts which initially were straight or at feast straighter than they are today in the last few years the definition has been broadened to include any curved mountain belt regardless of its original shape since the occurrence of oroclinal bending is best recorded in the change of declination as a function of tectonic setting paleomagnetic and structural data from six potential oroclines have been compiled and analyzed to determine the amount of rotation displayed by the change of paleomagnetic declination relative to the change in strike of the fold belt the arcuate belts investigated are the sicilian calabr n arc and the umbrian arc of italy the swiss portion of the jura mountains the central portion of the appalachian mountains from pennsylvania to virginia u s a the wyoming idaho overthrust belt of western north america and the hercynides of western and central europe the jura mountains and the pennsylvania virginia portion of the appalachians fail to show significant oroclinal bending the wyoming idaho belt shows a combination of rotated possibly oroclinai and unrotated thrust sheets in the sicilian calabrian arc significant oroclinat bending caused by the impingement of the calabria peloritani nappes in the late tertiary can be demonstrated while the umbrian arc of similar age in the northern apennines also shows oroclinal bending on a smaller scale hercynian europe the only belt included in which deformation of basement rocks can be demonstrated shows oroclinal bending at least so as well as a marked original curvature 70 in its western part common to all the orociines studied in this paper is the probable impingement of a rigid block or continental margin during the orogeny causing subsequent deformation and bending of the fold belt

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

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

paleomagnetism and u pb geochronology of the 2 17 ga biscotasing dyke swarm ontario canada evidence for vertical axis crustal rotation across the kapuskasing zone u pb dates on baddeleyite yield ages of 2167 8 2 2 and 2171 6 1 2 ma on two northeast trending dykes west of the kapuskasing zone in ontario canada these ages identify the dykes as belonging to the biscotasing dyke swarm east of the kapuskasing zone which was previously dated at 2166 7 1 4 ma by u pb on baddeleyite and zircon the new dates show that the biscotasing swarm was emplaced over an area of at least 300 000 km2 much larger than hitherto suspected and in a geologically short period of time of about 5 million years a comparison of paleomagnetic data from biscotasing and 2 45 ga matachewan dykes on either side of the kapuskasing zone suggests that the western half of the superior province has rotated about 10 20 counterclockwise relative to the eastern half across the kapuskasing zone this movement may have been accompanied by rifting farther north which ultimately led to the paleoproterozoic embayment underlying hudson bay that gives the superior province its characteristic butter

paleosecular variation during sequential geomagnetic reversals from hawaii abstract paleomagnetic directions measured from 175 flows in four volcanic sequences on the island of o ahu hawaii cover about 0 3 m y of geomagnetic field changes and include detailed records of the successive gilbert gauss 3 57 ma and lower mammoth reversals 3 33 ma the dominant pattern of the directional changes is the presence of large inclination variations with increasing amplitude and steep values during the transitional periods the morphology of these recurring features during different transitions is reminiscent of the characteristics of the non dipole field inferred from studies of paleosecular variation in the pacific these observations are consistent with transitional mechanisms yielding the emergence of non dipole components with characteristics similar to those during periods of stable polarity the three different records of the lower mammoth reversal show that clusters of transitional pole positions are not reproducible between parallel sections and are thus linked to local eruptions and not to long lived states of the transitional field

mid to late permian microfloristic evidence in the metamorphic successions of the northern apennines insights for age constraining and palaeogeographical correlations recent studies on the stratigraphy of paleozoic successions belonging to key sectors of the inner northern apennines provide data that can be used to propose new hypotheses about the palaeogeography of the western mediterranean domain during post hercynian times we report here the first evidence of mid to late permian guadalupian lopingian palynomorphs in the chronostratigraphically highly debated metamorphic units of the northern apennines the rio marina formation the mt calamita formation and the le cetine formation and consider the stratigraphic implications for the tuscan basement to which the study rocks belong and their palaeogeographical attribution considering their gondwana affinity these results agree with the interpretation that the alpine tethyan sedimentary cycle was already active during the mid to late permian thematic collection this article is part of the apennines and tyrrhenian system collection available at https www lyellcollection org cc the apennines tyrrhenian system

remagnetization in keweenawan rocks part ii lava flows within the copper harbor conglomerate michigan alternating field and thermal demagnetization studies of a thin lava flow unit within the copper harbor conglomerate michigan native copper district show a westerly increase in chemical remanent magnetization crm overprinting along a 50 km strike length the crm may have formed while secondary minerals including native copper were being generated in the underlying portage lake volcanics and when a rhyolite plug was injected into nearby siltstones of the overlying freda formation the direction of magnetization during copper harbor time as given by the flows is virtually indistinguishable from that found from the portage lake volcanics thus supporting previous geological observations that the two formations are closely associated in time

hydrogeochemical and isotopic insights into mineralization processes and groundwater recharge from an intermittent monsoon channel to an overexploited aquifer in eastern haryana india this study aimed at characterizing the groundwater flow pattern in a semi arid agricultural area in northern india crossed by an intermittent monsoon controlled watercourse the najafgarh drain more specifically it focused on studying the impact of groundwater recharge from the riverbed to the regional aquifer using hydrogeochemical and isotopic data significant hydrogeochemical zonation was observed between the northern central and southern sides of the drain linked to different mineralization processes and mixings northward from the drain groundwater was mainly brackish 4 1 23 4 ms cm due to dissolution of evaporites halite and anhydrite southward from the drain mostly fresh groundwater was found from 0 5 to 2 3 ms cm revealing notable cation exchange processes in the vicinity of the drain central area mineralization was intermediate 0 7 4 ms cm and groundwater showed low geochemical evolution supposing a distinct origin stable isotopes of water 18o 2h confirmed that central groundwater was not a simple mixing between northern and southern groundwater masses but had a significant component of infiltrated surface water from the drain potentiometric data supported these findings and confirmed the contribution of the drain to the recharge of the aquifer setting up a hydraulic barrier between north and south despite surface water availability limited to the monsoon season and low hydraulic conductivity of the riverbed this study demonstrates the value of the geochemical and isotopic analysis of groundwater to characterize groundwater flow pattern in peri urban agricultural areas especially surface water groundwater interactions

paleomagnetic study of thrust sheet rotation during foreland impingement in the wyoming idaho overthrust belt upper jurassic and lower cretaceous strata in the darby and absaroka thrust sheets were sampled in order to paleomagnetically evaluate thrust sheet rotation in the wyoming idaho overthrust belt no significant rotations were found lack of rotation in these thrust sheets suggests that previously reported rotation of the prospect thrust sheet was not transmitted to the older darby and absaroka thrust sheets indicating that the prospect thrust sheet only experienced local rotations in response to collision with the teton gros ventre uplift the upper jurassic stump formation possesses a magnetization that passes the fold test and gives a paleomagnetic pole at 64 n 170 e k 18 a95 15 in good agreement with other published upper jurassic poles for cratonic north america samples from all lower cretaceous sites are believed to possess a synfolding remagnetization of chemical origin

paleomagnetism of the fond du lac formation and the eileen and middle river sections with implications for keweenawan tectonics and the grenville problem overlying the keweenawan lavas of northern wisconsin and michigan is a thick sequence of terrestrial sandstone shale and siltstone that has undergone some tectonic deformation associated with movement along thrust faults and the development of the lake superior syncline thermal and alternating field demagnetization chemical leaching and multivector analysis using orthogonal projections reveal a trivector structure of the natural remanent magnetization nrm of the fond du lac formation and middle river section amnicon and orienta formations and a bivector structure of the nrm of the eileen section eileen formation the components may be classified by their physical properties as revealed by demagnetization a population of high blocking temperature components k1 is found in all three sections and gives poles as follows fond du lac 16 n 160 e middle river 25 n 148 e eileen 20 n 156 e after structural correction is applied a population of intermediate blocking temperature components k

middle paleozoic paleomagnetism of east kazakhstan post middle devonian rotations in a large scale orocline in the central ural mongol belt abstract in order to test different hypotheses concerning the paleozoic evolution of the ural mongol belt umb and the amalgamation of eurasia we studied middle devonian basalts from two localities 11 sites and lower silurian volcanics redbeds and intra formational conglomerates from three localities 20 sites in the chingiz range of east kazakhstan the devonian rocks prove to be heavily overprinted in the late paleozoic and a high temperature presumably primary southerly and down component is isolated at only four sites from a homoclinal section most silurian redbeds are found to be remagnetized in the late paleozoic in contrast a bipolar near horizontal remanence isolated from silurian volcanics is most probably primary as indicated by positive tilt and conglomerate tests analysis of paleomagnetic data from the chingiz range shows that southward pointing directions in ordovician silurian and devonian rocks are of normal polarity and hence indicate large scale rotations after the middle devonian the chingiz paleomagnetic directions can be compared with paleozoic data from the north tien shan and with the horseshoe shaped distribution of subduction related volcanic complexes in kazakhstan both paleomagnetic and geological data support the idea that today s strongly curved volcanic belts of kazakhstan are an orocline deformed mostly before mid permian time despite the determination of nearly a dozen new paleozoic paleopoles in this study and other recent publications by our team significant temporal and spatial gaps remain in our knowledge of the paleomagnetic directions during the middle and late paleozoic however the paleomagnetic results from the chingiz range and the north tien shan indicate that these areas show generally coherent motions with siberia and baltica respectively

paleomagnetic considerations of the development of the pennsylvania salient in the central appalachians abstract late paleozoic remagnetizations of central appalachian clastic and carbonate rocks of ordovician to devonian age constrain the structural evolution of the pennsylvania salient new paleomagnetic results from 26 sites located on nine folds combined with previously published results from 60 sites yield a mean direction for the northeastern limb of the salient at 172 1 k 49 and a mean direction for sites southwest of the salient at 173 1 k 70 these two directions are not statistically different from each other and are not significantly rotated from the expected permian paleomagnetic direction for north america individual fold test results show that this secondary magnetization is synchronous with or even older than folding across the region these paleomagnetic results suggest that curvature of fold axes around the salient is an original feature of fold growth and not the result of post orogenic oroclinal bending in contrast analysis of the primary magnetization in the silurian bloomsburg formation shows that directions from the two limbs of the salient are significantly different new results from twenty sites yield a mean direction for the northeastern limb at 4 40 k 31 and for the southwestern limb at 338 8 k 23 combining these results with previously published directions from the bloomsburg formation yields a mean direction for the northeastern limb at 1 34 k 32 and for the southeastern limb at 343 34 k 30 the difference in declination does not appear to be the result of internal deformation of the rocks or acquisition of magnetization during a period of rapid apparent polar wander rather these directions indicate that the northeastern limb has been rotated clockwise 18 9 relative to the southwestern limb the sense and amount of this rotation is consistent with the curvature of the orogen around the salient however within each limb of the salient the primary component declinations do not directly correlate with the orientation of the first order fold axes collectively therefore these paleomagnetic results suggest that the structural evolution of the salient included pre orogenic rotations or rotations that occurred early in the alleghanian orogeny followed by later remagnetization and folding of the central appalachian foreland

speculations on the origin of the north american midcontinent rift cannon w f and hinze w j 1992 speculations on the origin of the north american midcontinent rift in p a ziegler editor geodynamics of rifting volume ii case history studies on rifts north and south america and africa tectonophysics 213 49 55 the midcontinent rift is an example of lithospheric extension and flood basalt volcanism induced when a new mantle plume arrived near the base of the lithosphere very large volumes of basaltic magma were generated and partly erupted before substantial lithospheric extension began volcanism continued along with extension and deep rift subsidence for the ensuing 15 m y much of the basaltic magma including some of the earliest flows was formed by partial melting of isotopically primitive asthenosphere contained in the plume head the intense but relatively short duration of rifting and magmatism is a result of the dissipation of thermal and mechanical energy in the plume head as the plume head spread beneath the lithosphere it stretched the overlying lithosphere radially away from the lake superior region the triple junction of the rift system and partially melted to form the great volume of basalt and related intrusive rocks of the region the plume arrived beneath a continent that was under compression as a result of the ongoing grenville orogeny that affected a large region east of the rift that compression prevented full continental separation and eventually returned the region to compressional tectonics as the energy of the plume head waned

transpressive tectonism in the eastern penokean orogen canada consequences for proterozoic crustal kinematics and continental fragmentation abstract results of a structural analysis of archean basement huronian cover and various igneous rocks in the sudbury area ontario and regional tectonometamorphic correlations suggest that structures in the eastern penokean orogen formed dominantly by dextral transpression bulk horizontal shortening started with the ca 2 4 to 2 2 ga blezardian orogeny and was accompanied by peak metamorphism and local granitoid magmatism during the 1 89 to 1 83 ga penokean orogeny horizontal dextral shear parallel to the curved axis of the eastern penokean orogen gained in importance and is most likely related to docking of proterozoic magmatic arc terranes to the superior province margin this tectonic activity led to distributed heterogeneous deformation of archean basement rocks and exhumation of deep crustal rocks in the sudbury area and the kapuskasing structural zone but may have failed to accrete paleoproterozoic terranes in the lake huron area the lack of evidence for horizontal extension following blezardian and penokean contraction suggests that crustal fragmentation of the archean supercontinent started at 2 45 ga finally the great lakes tectonic zone and the murray fault master dislocations in the penokean orogen may have acted as crustal weakness zones which influenced the orientation and development of 1 1 ga midcontinent rift segments

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