tectonic history of the lunksoos composite terrane in the maine appalachians several ordovician volcanic terranes are preserved between the paleozoic north american margin and the avalon terrane in the northern appalachians of maine they have been interpreted as remnants of an island arc on the basis of their current position in the appalachians or as ocean islands near the margin of avalon on the basis of paleontologic data a study of one of these terranes the lunksoos terrane in northern maine was undertaken through a combination of paleomagnetic and structural analyses in order to understand its tectonic evolution paleomagnetic study of the pre caradoeian pre late ordovician stacyville volcanics revealed three components of magnetization the final magnetization removed d 90 i 36 95 11 8 is interpreted to be primary and yields a paleolatitude of approximately 20 s however a late ordovician remagnetization cannot be excluded the stacyville volcanics now overlie a cambrian and lower ordovician sequence the grand pitch and shin brook formations that is believed to have been deposited at the southern side of iapetus possibly near the margin of avalon during the middle and late ordovieian avalon was at paleolatitude s higher than 40 s while the volcanics were either formed or remagnetized at a paleolatitude of 20 s the pre caradocian stacyville volcanics therefore could not have been deposited directly on the grand pitch shin brook sequence but must have been tectonically emplaced the rockabema diorite which intrudes the entire cambro ordovician sequence constrains the timing of this emplacement to be pre late early silurian these relationships indicate that the lunksoos area is a composite terrane within the maine appalachians the tectonic emplacement scenario is unnecessary if the grand pitch and shin brook formations were not located on the south side of iapetus during the ordovician this alternative however poses conflicts with the interpretation of faunal affinities

pre folding magnetization reconfirmed for the late ordovician early silurian dunn point volcanics nova scotia johnson r j e and van der voo r 1990 pre folding magnetization reconfirmed for the late ordovician early silurian dunn point volcanics nova scotia tectonophysics 178 193 205 paleomagnetic results have been obtained for three new sites in steeply inclined subarea1 volcanic flows of the dunn point formation from the avalon terrane of nova scotia demagnetization analysis reveals a characteristic magnetization carried by hematite which is similar to that reported in previous studies of this unit a new and improved fold test for the characteristic component combining paleomagnetic and structural data from the present study with paleomagnetic data for vertically dipping flows and laterites studied previously by the authors is significantly positive the new fold test reconfirms the prefolding age of the characteristic component a primary age of magnetization is inferred based on field evidence for early oxidation of the flows the overall mean direction for the characteristic component of d 344o i 60 k 68 6 ass 4 1 paleopole at 2on 130 e corresponds to a late ordovician earliest silurian paleolatitude for avalon of 41 s the latter is much higher than that predicted for the area on the basis of the cratonic north america apparent polar wander path implying a substantial post ordovician displacement 1700 km of avalon with respect to the craton the dunn point result supports previous paleogeographic reconstructions that show avalon together with the continental blocks of hercynian europe and northwestern gondwana

aftershocks of the 2010 mw 7 2 el mayor cucapah earthquake reveal complex faulting in the yuha desert california 1 we detect and precisely locate over 9500 aftershocks that occurred in the yuha desert region during a 2 month period following the 4 april 2010 mw 7 2 el mayor cucapah emc earthquake events are relocated using a series of absolute and relative relocation procedures that include hypoinverse velest and hypodd location errors are reduced to 40 m horizontally and 120 m vertically aftershock locations reveal a complex pattern of faulting with en echelon fault segments trending toward the northwest approximately parallel to the north american pacific plate boundary and en echelon conjugate features trending to the northeast the relocated seismicity is highly correlated with published surface mapping of faults that experienced triggered surface slip in response to the emc main shock aftershocks occurred between 2 km and 11 km depths consistent with previous studies of seismogenic thickness in the region three dimensional analysis reveals individual and intersecting fault planes that are limited in their along strike length these fault planes remain distinct structures at depth indicative of conjugate faulting and do not appear to coalesce onto a throughgoing fault segment we observe a complex spatiotemporal migration of aftershocks with seismicity that jumps between individual fault segments that are active for only a few days to weeks aftershock rates are roughly consistent with the expected earthquake production rates of dieterich 1994 the conjugate pattern of faulting and nonuniform aftershock migration patterns suggest that strain in the yuha desert is being accommodated in a complex manner

application of paleomagnetism to accretionary tectonics and structural geology the use of paleomagnetism to determine translations and rotations of geologic terranes has contributed to a better understanding of deformation within accretionary complexes shear zones and rifts in this report to the i u g g we review the progress resulting from paleomagnetic studies during the last four years in the cordillera of north america the appalachian orogen and the circum pacific region

middle cambrian paleomagnetism of the avalon terrane in cape breton island nova scotia interbedded volcanics and sediments of the bourinot group mid cambrian in central cape breton island were studied as part of a paleomagnetic investigation to quantify the geologically plausible displacements of the avalon terrane with respect to the north american craton both volcanics and sediments reveal prefolding magnetizations with steep to intermediate inclinations dual polarity magnetizations in different volcanic flows are probably primary and they yield a mean direction of d 293 i 66 n 9 k 41 8 95 8 1 the sediments reveal upon detailed thermal demagnetization complex multivectorial remanences however a few samples show characteristic components and a large number of samples 38 have intersecting great circle remagnetization trajectories when combined this yields a mean direction of d 267 i 65 k 16 mean radius of error ellipse is 8 5 the mean directions for the bourinot rocks correspond to a paleolatitude of 49 s 11 which is different from the range of possible values for the predicted mid cambrian paleolatitudes for nova scotia 29 s to 18 n inferred from the north american apparent polar wander path this discrepancy between predicted paleolatitudes and the observed paleolatitude implies that nova scotia was not part of north america during the mid cambrian but rather they were separated by an ocean with a minimum width of 1000 km 20 10 this interpretation of the relative positions of nova scotia and north america based on paleomagnetism is consistent with the geological record in the northern appalachians and supports previous interpretations of the dissimilar early paleozoic faunas in avalon and north america it is paleomagnetically quite plausible that the avalon terrane formed part of a larger continent comprising africa gondwana and armorica during the cambrian

cambro ordovician paleogeography of the southeastern new england avalon zone implications for gondwana breakup paleomagnetic measurements and u pb geochronology of alkalic igneous rocks intruding the ediacaran cambridge argillite and the lower cambrian weymouth formation north of boston at nahant massachusetts yield a well constrained paleopole the oldest intrusive phases are a syenodiorite sill and a granite dike with 206 pb 238 u crystallization ages of 490 19 0 90 ma and 489 03 0 82 ma respectively this sequence along with undated mafic sills penetrating both the argillite and weymouth strata are cut by 488 53 0 81 ma gabbro locally associated with 488 48 0 79 ma syenite forming a steep sided intrusive plug ten paleomagnetic sampling sites within the mafic sills and the gabbro contain a stable a magnetic component with a mean direction of d 278 5 i 76 7 a 95 3 9 n 10 this is considered to be the primary magnetization direction because it passes both fold and reversals tests the corresponding paleopole at 320le 34 n a 95 7 2 n 10 places the southeastern new england avalon zone at a southerly paleolatitude of 65 slightly removed from the african margin of gondwana and suggests that bimodal nahant magmatism is linked with latest cambrian early ordovician rifting of west avalonia to form the rheic ocean stable b and c magnetic components in nahant syenodiorite and syenite give rise to virtual geomagnetic poles that track late silurian through carboniferous segments of the north american apparent polar wander path indicating docking of west avalonia by mid paleozoic time

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

petrophysical characterization of oil bearing shales by low field nuclear magnetic resonance nmr abstract shales are important components of petroleum systems and it is necessary to reveal their petrophysical properties as reservoirs low field nuclear magnetic resonance nmr has proven to be a non invasive and effective technique for petrophysical characterization of sandstone carbonate and coal however little study has been done using this technique to systematically investigate the petrophysical properties of oil bearing shales which is essential for characterizing shale oil reservoirs in this study two sets of nmr measurements were performed to reveal the porosity permeability movable fluid volume and pore size distribution psd of oil bearing shales the results show the following 1 the nmr t2 spectrum strongly relates to the content of quartz and the amount of dissolution pores within shale three t2 spectrum peaks were identified by the relaxation times at 0 03 1 ms 1 20 ms and 20 ms corresponding to micropores 1000 nm respectively 2 by combining nmr with centrifugation both nmr porosity and movable porosity mp were calculated nmr porosity is well consistent with helium porosity and the mp values strongly relate to the development of mesopores in shales 3 based on the t2 spectrum an effective permeability model was proposed which can conveniently estimate the permeability of shales moreover two classical permeability models coates and sdr were also used to estimate shale permeability and the permeability values determined by these models agrees well with helium permeability 4 in combination with sem images analyses an nmr based psd was constructed that agrees well with the psd determined by sem the results demonstrate the applicability of nmr relaxation in a systematic investigation of the petrophysical properties of shales which has potential applications for geophysical logging in shale oil exploration

paleomagnetism of the callander complex and the cambrian apparent polar wander path for north america the 7 km2 circular callander alkaline complex was emplaced into anorthositic and granitic gneisses of the grenville province in the canadian shield about 575 5 ma ago at the start of the cambrian the complex has not been subsequently metamorphosed or tilted detailed alternating field and thermal step demagnetization of 252 specimens from 29 sites led to the identification of a characteristic a magnetization component with a direction of d 82 2 i 82 7 95 3 1 k 83 n 26 sites in 5 sites of mesocratic to leucocratic syenite from the core of the complex in 5 sites of fenitized host rock from its aureole and in 16 sites of lamprophyre from radiating dikes isothermal remanent magnetization tests show that the a component is retained by both magnetite and hematite in a complete spectrum of domain sizes a reversals test suggests and a contact test shows the a component to be primary its pole position at 46 3 s 121 4 e dp 5 9 dm 6 1 does not fall on published but poorly defined

remagnetized cratonic cambrian strata from southern nevada stratigraphic sections of cambrian strata in southeastern nevada are akin to thin cratonic facies exposed in the grand canyon their structural setting is much more complicated however from mesozoic and cenozoic tectonism paleomagnetic samples from two sections through these strata appear to have been completely remagnetized coarse grained lightcolored sandstone from the tapeats sandstone yields scattered magnetizations residing in hematite that appear to reflect protracted diagenetic acquisition of remanence a hematitic sandstone in the bright angel shale yields relatively consistent paleozoic directions of magnetization but petrographic study shows that the hematite results from diagenetic oxidation and stratigraphic arguments suggest that the oxidation was not penecontemporaneous gray limestones of the jangle and muav limestones yield a magnetization residing in magnetite that may reflect late tertiary remagnetization being imposed during uplift related to the onset of basin and range deformation in any case this magnetization differs greatly in direction from a hematite magnetization reported from slightly younger muav limestone in the grand canyon both the sampling sites also appear to have been tectonically rotated but whether this rotation is true or a geometric artifact of the tilt correction cannot be determined from the present data these results suggest that paleomagnetic data from rocks as old as cambrian must be scrutinized very carefully before their magnetizations can be accepted as penecontemporaneous and such scrutiny must include geologic data

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

a review of paleomagnetic results from the carboniferous of north america the concept of carboniferous geomagnetic field horizon markers results from the east coast carboniferous basin contain an important time stratigraphic horizon of possible global significance late carboniferous units are found to have mostly reversed magnetizations while older carboniferous units may have normal reverse or most frequently dual polarity magnetizations this feature records the change in geomagnetic field from a period of common polarity changes to a period of mainly reversed polarity the kiaman magnetic interval recognition of the stratigraphic position of this polarity feature provides a time line for correlation of units within and among carboniferous basins in the east coast carboniferous basin of atlantic canada this horizon has now been located at the riversdale group level early pennsylvanian namurian westphalian boundary since remanence acquisition of red sediments is often a lengthy process possibly spanning field reversal time scales red bed units pre dating and post dating this time line can in most instances be readily distinguished the former generally recording both polarities and the latter just one reverse polarity a polar displacement of at least 16 is recorded in the results from the east coast carboniferous basin paleolatitude maps show that the area was located near the equator and was displaced northwards by about 8 during the pennsylvanian it cannot be ascertained if those observations are representative of the whole of north america or merely the atlantic seaboard poles derived from other areas of north america do not all fit the regular pattern observed on the atlantic coast using the horizon marker as a tool it is suggested that those poles are post time line in age so that none of them is representative of the position of north america prior to the time line

polar wander and paleomagnetic reference pole controversies the purpose of this paper is to review the progress made over the past four years in studies of phanerozoic and precambrian polar wander with an emphasis on the contributions made by u s scientists we perceive a general theme embracing the notion that the accuracy of cratonic reference poles and therefore of apparent polar wander apw paths is not as good as was generally believed four years ago a renewed interest in research concerning cratonic reference poles is largely the result of paleomagnetic studies in orogenic belts namely the western cordillera and appalachians of north america the andes of south america and elsewhere which seek to document the existence or non existence of displacements of suspect terranes i e rotation and or translation with respect to their presently associated cratons some of the results of this research are reviewed in this volume by hillhouse and mcwilliams an important realization of which paleomagnetists have long been aware but which was demonstrated recently by a number of sobering examples for both the paleozoic and mesozoic apw paths for north america is that the reliability of concordance discordance parameters that define terrane displacement is dependent not only on the precision and accuracy of the individual paleomagnetic study but also on the precision and accuracy of the appropriate cratonic reference pole

paleomagnetism of lower cambrian volcanics and a cross cutting cambro ordovician diabase dyke from buckingham quebec the buckingham area quebec is characterized by cambrian volcanic flows 573 32 ma cut by a cambro ordovician diabase dyke which is dated at 497 14 ma by k ar method the volcanics yield multicomponent magnetizations a hard characteristic magnetization and a relatively soft one these components could be separated easily and the intensity decay curves of the different components could also be obtained separately using zijderveld s method three different groups of site mean characteristic magnetizations can be distinguished each representing a different stratigraphic level the paleopoles derived from these magnetizations are from oldest to youngest 186 e 10 s 173 e 01 n and 154 e 06 n this is interpreted as a rapid motion of the paleopole for the north american craton during the early cambrian the paleopole derived from the hard characteristic dyke direction is 123 e 03 s this magnetization direction was also found at one of the sites in the volcanics this means that the contact test

dual polarity early carboniferous remagnetization of the fisset brook formation cape breton island nova scotia summary red siltstones and volcanic flows of the fisset brook formation of cape breton island nova scotia were folded or tilted in two phases one of late tournaisian and the other of mid namurian age upon thermal demagnetization both rock types yield three components of magnetization herein denoted as l i and h the l component has low unblocking temperatures and a direction conforming to that of the present day geomagnetic field l is inferred to be of recent origin the intermediate i component carried by haematite is of synfolding origin directions could be obtained through vector subtraction for the demagnetization interval of 300 550 c and yield a mean of d i 160 38 k 90 8 ag5 s lo after 39 per cent of the tilt correction is applied the h component has even higher unblocking temperatures which overlap to a large degree with those of the i component but analysis of intersecting great circles appears to yield a best fit direction for h which is nearly antipodal to that of the i component h is also synfolding and yields a mean direction of d i 342 38 k 120 9 ag5 6 9 after 60 per cent of tilt correction both components are interpreted as secondary chemical remanent magnetizations of late tournaisian to early namurian age a comparison of all available carboniferous results from the craton and the northern appalachians indicates that palaeolatitudes for nova scotia changed from about 20 s to about 10 s in the interval between the early and late namurian and that they changed again from about 10 s to 0 between the late namurian and the stephanian declinations show good agreement between newfoundland nova scotia new brunswick and the craton with the exception of the meguma terrain which underwent a counterclockwise rotation with respect to the mainland in the late carboniferous as noted previously by spariosu kent keppie 1984

geoelectromagnetic measurements across the moine thrust and the great glen in northern scotland summary results are presented for a magnetotelluric and magnetovariational study in northern scotland for nine of the 20 sites at which observations were made the results of the analysis are presented fully together with one dimensional 1 d monte carlo inversions for representative sites from the lewisian foreland the north west highlands the great glen region and the grampian highlands for the remaining sites the results are summarized the 1 d models of all sites situated in a traverse extending from the lewisian foreland to the neighbourhood of the highland boundary fault have been used as the basis for preliminary 2 d modelling maps of magnetic field induction vectors and transfer functions for an inducing field perpendicular to the average geological strike are presented for periods of 23 819s the 2 d conductivity models were constrained by both the magnetotelluric and magnetovariational responses the period range of observation in this study was too limited to resolve structural differences in the upper crust but adequate to reveal a moderately good conductor at upper mantle lower crustal depths in the caledonian metamorphic zone in contrast the whole of the crust in the lewisian foreland is highly resistive and there is no evidence of an intermediate conducting layer between the crust and mantle in this region

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

palaeomagnetism of late precambrian cambrian volcanics and intrusives from the armorican massif france samples of the keratophyre tuffs of treguier 640 12 myr of the upper brioverian series and the microgranite dykes of loguivy 548 24 myr have been collected from the north coast of brittany france the samples were progressively demagnetized with af and predominantly thermal methods blocking temperatures coercivities and acquisition of isothermal remanent magnetization irm indicate that the keratophyre remanence is variably carried in magnetite hematite or occasionally in both after removal of a soft viscous component two stable palaeomagnetic directions were found one in magnetite the other in hematite the characteristic magnetite direction tb 300 580 c is a pre folding primary thermoremanent magnetization trm which after structural correction has a mean declination inclination of 217 9 30 4 the younger hematite component has a mean in situ direction of 320 4 20 3 the microgranite natural remanent magnetization nrm intensity distribution is distinctly bimodal forming high and low 1 6 a m 1 intensity groups upon demagnetization a single stable remanence in magnetite was removed between 300 and 580 c or 20 and 100 mt two characteristic directions were found to be statistically related to the intensity groups at the 99 per cent confidence level low intensity samples have a mean direction of 201 9 2 3 and probably resulted from a late devonian early carboniferous local magnetization high intensity microgranites have a mean direction of 200 50 8 and this remanence is shown to be a primary trm palaeopoles obtained from the two keratophyre magnetizations magnetite 44 7 n 116 4 e hematite 39 5 n 233 6 e as well as the high intensity microgranite remanence 5 8 s 339 9 e show good agreement with similar age poles from the apparent polar wander path for the armorican massif comparison of these data to recent palaeomagnetic data from southern england and wales gives additional evidence to link that region with the armorica plate as proposed by van der voo and hagstrum et al

palaeomagnetism of early cambrian brigus red shale eastern newfoundland abstract studies of the early cambrian brigus br red shale exposed in conception bay yielded a single stable mean remanence direction of d 265 i 58 reverse polarity before tilt correction and d 279 i 48 k 47 95 6 9 forn 7 sites 25 samples after correction for gentle tilt the corresponding north pole palaeopole position is 27 s 51 e dp dm 6 9 this magnetization is characterized by a linear vector trajectory at higher unblocking temperature t ub spectra 400 680 c the magnetization directions at the low t ub range 400 c are more variable from sample to sample although they typically have steep downward inclinations with no evidence of kiaman overprinting marginal but not statistically significant improvement k 44 77 in the grouping of site means after tilt correction suggests that the predominant univectorial westerly remanance was probably acquired after deposition based on both geological and magnetic evidence the brigus shale formation appears to be remagnetized during siluro devonian times the position of the far sided antipole br 27 n 129 w located in the vicinity of the pole for the cambrian morrison river formation mr at 20 n 146 w which is discordant from the younger palaeozoic late ordovician carboniferous poles reported for the avalon zone is fortuitous since the magnetization of br is most likely secondary the magnetization is younger than the iapetus closure date determined geologically late ordovician consequently the displacement of the avalon zone with respect to the north american craton cannot be estimated the width of the proto atlantic ocean cannot be calculated

a paleomagnetic investigation of the lower ordovician st george group port au port peninsula newfoundland implications for the iapetus ocean and evidence for late paleozoic remagnetization abstract to obtain an early ordovician paleomagnetic pole position for north america and to determine the paleolatitude of its margin of the lapetus ocean prior to the taconic orogeny a study of the lower st george group tremadocian arenigian of western newfoundland was undertaken analysis of 84 oriented core samples from six localities on port au port peninsula indicates that the nrm of the st george rocks is the resultant of three distinct paleomagnetic directions the first is a steep downward component commonly observed in the low temperature range of demagnetization room temperature 250 c in virtually all samples that roughly corresponds to the present day axial dipole field direction in situ d 350 i 68 the second is a stable component with southerly declinations and shallow intermediate negative inclinations d i in situ 176 20 d i tilt corrected 173 5 respectively usually seen in the range of 450 600 c and found in approximately 46 of the specimens the third is a stable southeasterly shallow intermediate positive component d i insitu 161 16 d i tilt correcied 163 34 respectively generally unblocked within the 350 550 c or 450 600 c range which was observed in 40 of the specimens these components of remanence are herein referred to as the type 1 type 2 and type 3 components respectively the remaining 14 of the specimens were found to be either directionally unstable to thermal demagnetization or totally overprinted by the type 1 component the type 2 component is interpreted to represent a post folding late paleozoic remagnetization possibly associated with the alleghanian orogeny on the other hand the type 3 component is interpreted as an early ordovician possibly pre taconic remanence and in tilt corrected coordinates yields a paleolatitude for this region of the north american craton of 19 s 6

eocambrian paleomagnetism of the boston basin evidence for displaced terrane a paleomagnetic study of the late precambrian roxbury formation consisting of extrusives and clastic sediments has yielded a steeply downward directed and pre folding characteristic magnetization d i 219 71 95 5 k 311 paleopole at 13 n 267 e n 4 sites which indicates a significantly higher paleolatitude 55 than would be expected if the boston basin were part of the equatorial north american craton in the latest precambrian and earliest paleozoic this characteristic magnetization reveals dual polarities and is further supported by a positive conglomerate test a ubiquitous post folding late paleozoic overprint is present in nine sites d i 183 14 95 8 k 42 with a paleopole at 41 s 285 e the pre folding magnetization resides in hematite which is inferred to have formed during early oxidation of the rocks the high stability of this hematite may have prevented its magnetization from being reset during the late paleozoic chemical event responsible for the magnetic overprint the boston basin has a marked geological similarity to the avalon basement terranes in nova scotia and newfoundland as well as the armorican massif in france and the high paleolatitudes observed for all these terranes suggest a common paleogeographical affinity a likely paleolocation is near the northwestern margin of gondwana which was located at the southpole in the latest precambrian and early cambrian

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

a cambrian pole for the moroccan coastal meseta summary middle cambrian volcanics of sidi said maachou east of el jadida western morocco 158 cores 23 sites sampled in two localities north of sidi said maachou were subjected to progressive alternating field and thermal demagnetization thermomagnetic cycles isothermal remanent magnetization acquisition experiments as well as microscopic observations the magnetic mineralogy of these volcanic rocks is dominated by magnetite and titanomagnetites the palaeomagnetic analysis has revealed two characteristic components of magnetization the first direction d 300 i 31 95 6 pole position at 14 n 231 e p 4 m 7 palaeolatitude 16 7 s is carried by magnetite and yields a positive fold test it is interpreted using the results of a baked contact test as the original magnetization of middle cambrian age 520 ma the second direction d 121 i 20 95 6 pole position at 18 s 55 e p 3 m 6 is nearly anti parallel to the first magnetization but the fold test is not conclusive and it is interpreted as an overprint that occurred a short time after this volcanism this study provides the first detailed palaeomagnetic result from cambrian rocks in the moroccan meseta

the evolution of the avalonian plate abstract the avalon platform of newfoundland is the type locality of the avalonian plate that is characterized by late precambrian volcanic and clastic rocks marine at the bottom and terrestrial at the top all are intruded by 650 550 ma granites late precambrian rocks in the southern british isles are similar in both areas northeastern parts of the belt are affected by deformation and metamorphism that have orogenic aspects the orogeny is referred to as the avalonian in newfoundland and monian in britain to the southeast of the avalonian rocks in europe lies another late precambrian cadomian orogenic belt of rocks as a part of the pan african orogenic belt the cadomian and avalonian belts extend into north africa morocco since these belts are ophiolite bearing it is suggested that the plate was bordered by oceanic domains it is therefore proposed that a microcontinental volcanic arc that was generated in the late precambrian was orogenically deformed by collision with north america europe and africa and survived as a coherent terrain until the late carboniferous

provenance and tectonic evolution of ganderia constraints on the evolution of the iapetus and rheic oceans we provide estimates for the width timing and rates of opening and closing of the iapetus and rheic oceans the evolution of which profoundly infl uenced paleozoic global paleogeography these estimates are primarily derived from the transfer of ganderia and avalonia from gondwana to laurentia which led to closure of the iapetus ocean and opening of the rheic ocean ganderia a long lived arc terrane separated from the paleo caribbean margin of amazonia at 505 ma with a latitudinal speed of 9 cm a northward initiating the rheic ocean as a backarc basin ganderia s trailing edge was reduced to 5 cm a following opening of a 600 800 km wide backarc basin within ganderia at 475 ma opening and closing of the iapetus ocean was largely driven by far fi eld stresses slab pull in some places and slab rollback in others

a pennsylvanian paleomagnetic pole from the mineralized late cambrian bonneterre formation southeast missouri one hundred eighty eight oriented samples of the late cambrian bonneterre carbonates of southeastern missouri were taken from viburnum trend mines and local surface outcrops initial natural remanent magnetization directions have south southeasterly declinations and downward inclinations progressive demagnetizations using thermal and af techniques give two directions one is a viscous present day field direction and the other is of reversed polarity with south southeasterly declinations and very shallow inclinations the resulting paleopole at 43 n 126 e is located near the pennsylvanian to early permian segment of the north american apparent polar wander path a negative conglomerate test supports this assignment for the magnetization age magnetic extracts from the mineralized as well as unmineralized zones of the bonneterre contain spherical crystal aggregates of pure magnetite of late diagenetic origin the secondary magnetization resides in the magnetite spheroids and we infer that the spheroids formed prior to sulfide mineralization

a plate tectonic scenario for the iapetus and rheic oceans abstract the tectonics dynamics and biogeographic landscape of the early paleozoic were dominated by the opening and expansion of one large ocean the rheic and the diminution to terminal closure of another iapetus an understanding of the evolution of these oceans is thus central to an understanding of the early paleozoic but their chronicle also presents a rich temporal profile of the wilson cycle illustrating continental scale rifting microcontinent formation ocean basin development arc accretion and continent continent collision nevertheless contemporary paleogeographic models of the iapetus and rheic oceans remain mostly schematic or spatiotemporally disjointed which limits their utility and hinders their testing moreover many of the important kinematic and dynamic aspects of the evolution of these oceans are impossible to unambiguously resolve from a conceptual perspective and the existing models unsurprisingly present a host of contradictory scenarios with the specific aim to resolve some of the uncertainties in the evolution of this early paleozoic domain and a broader aim to instigate the application of quantitative kinematic models to the early paleozoic i present a new plate tectonic model for the iapetus and rheic oceans the model has realistic tectonic plates which include oceanic lithosphere that are defined by explicit and rigorously managed plate boundaries the nature and kinematics of which are derived from geological evidence and plate tectonic principles accompanying the presentation and discussion of the plate model an extensive review of the underlying geological and paleogeographic data is also presented

petrogenesis and geological history of a uranium source rock a case study in northeastern washington u s a abstract a small 4 km 2 drainage basin in northeastern washington contains highly uraniferous groundwater and highly uraniferous peaty sediments of holocene age the u is derived from granitic bedrock that underlies the entire drainage basin and that contains 9 16 ppm u this local bedrock was studied by petrographic chemical and isotopic methods to determine conditions of its petrogenesis and post emplacement history that may have contributed to its present high u content and source rock capability the original magma was derived by anatexis of precambrian continental crust of probable mixed metaigneous and metasedimentary character mineral melt partitioning controlled the enrichment of u in chemically evolved phases of the crystallizing melt following emplacement in the upper crust at 100ma the pluton interacted with meteoric hydrothermal water at ambient temperatures 300 c locally intense fracturing promoted alteration and fracturing and alteration probably continued during later regional uplift in the eocene regional uplift was followed by low temperature alteration and weathering in the middle to late tertiary the combined result of hydrothermal alteration and low temperature alteration and weathering was the redistribution of u from primary mineral hosts such as allanite to new sites on fracture surfaces and in secondary minerals such as hematite zones of highly fractured and altered rock show the most obvious evidence of this process a model is proposed in which high angle fractures beneath the drainage basin were the sites of tertiary supergene enrichments of u recent glacio isostatic uplift has elevated these older enriched zones to shallow levels where they are now being leached by oxidizing groundwater the chemistry mineralogy texture and geological history of this u source rock suggest criteria for locating other granitic terrane that may contain uraniferous waters and associated young surficial u deposits the details of u distribution and mobility at this site also apply to the general topic of u mobility in granitic rocks

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

avalonian proximity of the ordovician miramichi terrane northern new brunswick northern appalachians paleomagnetic evidence for rifting and back arc basin formation at the southern margin of iapetus a paleomagnetic investigation of the middle ordovician tetagouche group in northern new brunswick was undertaken to determine the paleogeo aphi position of the miramichi terrane stepwise thermal demagnetization of pillow basahs reveals a high temperature characteristic gnet tion with a mean direction of b w z 69 k 22 aas 13 tih corrected n 7 sites 73 samples a positive fold test and the presence of antipodal normal and reversed polarity directions indicate that this ancient direction is ordovician in age with a paleopole position of 52 n 352 e the corresponding pateolatitude of 535 places these vokanic rocks near the southern margin of the iapetus ocean at paleolatitudes similar to those revealed by avalon for the middle to late ordovician the mafic and felsic volcanic rocks and marine sedimentary rocks of the tetagouche and fournier groups have been interpreted to be remnants of a rifted continental margin and a middle ordovician back arc basin our results show that the process of rifting and back arc basin formation occurred at the avalonian margin of iapetus which implies that ordovician subduction was not restricted to the laurentian margin but also marks the southern margin of japetus

palaeomagnetism of palaeozoic rocks from the cabo de penas asturia spain summary ordovician volcanic samples of the cabo de penas area yielded a primary remanent magnetization d 202 i 78 95 6 which is partially or entirely overprinted by a late palaeozoic remagnetization late devonian early carboniferous sedimentary samples reveal a pre or syntectonic carboniferous remanent magnetization similar to the secondary one of the volcanic samples a soft post tectonic component appears also in few samples for which a late carboniferous age is inferred comparison of the carboniferous remanent magnetizations and structural parameters suggests that in a late stage of the deformation the sites have undergone minor rotations around a vertical axis a strike correction is therefore proposed to take such an effect into account the average of the corrected mean site directions d 178 i 19 95 9 is considered to be a good approximation of the early or middle carboniferous palaeomagnetic direction in asturia

paleomagnetism of siluro devonian and cambrian granitic rocks from the avalon zone in cape breton island nova scotia as part of a major study of the avalon zone in cape breton island we describe the paleomagnetism of radiometrically well studied granitic rocks of two generations representing the acadian 350 45

paleomagnetism of the cambro ordovician mcclure mountain alkalic complex colorado the middle to late cambrian loop in the north american apparent polar wander path apwp has been variously attributed to tectonic rotations remagnetizations and primary magnetizations although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated the temporally self consistent nature of the loop has been used as an argument for primary magnetizations we have studied 535 5 ma nepheline syenites and syenites of the mcclure mountain alkalic complex as well as 495 10 ma red trachyte dikes which intruded the complex in an effort to find a primary trm because zijderveld analysis yielded consistent results for only one trachyte dike remagnetization great circle analysis was employed giving a pole for the trachyte dikes at the tip of the loop 43 n l14 e while the syenites and nepheline synenites gave a pole at the base of the loop 18 n 142 e the magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration k ar whole rock dating of the trachytes suggests a pennsylvanian age for the alteration and thus a late paleozoic remagnetization of the trachytes thus the low latitude cambrian pole is confirmed but we find no evidence in this study to support the primary nature of the cambrian apwp

morphology genesis and distribution of nanometer scale pores in siliceous mudstones of the mississippian barnett shale research on mudrock attributes has increased dramatically since shale gas systems have become commercial hydrocarbon production targets one of the most significant research questions now being asked focuses on the nature of the pore system in these mudrocks our work on siliceous mudstones from the mississippian barnett shale of the fort worth basin texas shows that the pores in these rocks are dominantly nanometer in scale nanopores we used scanning electron microscopy to characterize barnett pores from a number of cores and have imaged pores as small as 5 nm key to our success in imaging these nanopores is the use of ar ion beam milling this methodology provides flat surfaces that lack topography related to differential hardness and are fundamental for high magnification imaging nanopores are observed in three main modes of occurrence most pores are found in grains of organic matter as intraparticle pores many of these grains contain hundreds of pores intraparticle organic nanopores most commonly have irregular bubblelike elliptical cross sections and range between 5 and 750 nm with the median nanopore size for all grains being approximately 100 nm internal porosities of up to 20 2 have been measured for whole grains of organic matter based on point count data from scanning electron microscopy analysis these nanopores in the organic matter are the predominant pore type in the barnett mudstones and they are related to thermal maturation nanopores are also found in bedding parallel wispy organic rich laminae as intraparticle pores in organic grains and as interparticle pores between organic matter but this mode is not common although less abundant nanopores are also locally present in fine grained matrix areas unassociated with organic matter and as nano to microintercrystalline pores in pyrite framboids intraparticle organic nanopores and pyrite framboid intercrystalline pores contribute to gas storage in barnett mudstones we postulate that permeability pathways within the barnett mudstones are along bedding parallel layers of organic matter or a mesh network of organic matter flakes because this material contains the most pores