thermomagnetic characteristics in late orogenic granites and gneisses of the southern appalachian piedmont abstract thermomagnetic curves have been obtained for samples from 25 granites and gneisses in georgia south carolina and north carolina this data set can be divided into two distinct curve types type i includes rocks which exhibit no distinct curie points and a linear decrease in magnetization with increasing temperature type ii curves exhibit a distinct curie point over a range of temperatures expected for magnetite 500 580 c synthetic samples constructed from magnetite and hematite powder exhibit type i behavior when the hematite to magnetite ratio is high e g 15 1 examination of polished sections shows relatively coarse grained magnetite only in type ii rocks we interpret our data to indicate that type i thermomagnetic curves are dominated by relatively large hematite magnetite ratios while type ii granites are characterized by relatively coarse grained magnetite type i granite samples have low magnetic susceptibility values less than 4 10 4 cgs while most type ii granite samples have higher values the type i granites are invariably18o enriched whereas type ii granites typically exhibit low18o 16o ratios these relationships are consistent with previously reported correlations of susceptibility generally indicative of magnetite content and oxygen isotopic trends in the southern appalachian piedmont

miocene igneous activity in the northern subbasin offshore senegal nw africa abstract more than 70 enigmatic discordant seismic anomalies of limited lateral extent associated with eye shaped structures and folds have been imaged on 2d seismic data from the offshore northern subbasin of the senegal basin nw africa in this paper we define their distribution characterise their morphology and compare them to similar features imaged by seismic data along the ne atlantic margin that have been demonstrated to be igneous sills hydrothermal vents and intrusion related forced folds we propose that the cretaceous and paleogene strata in the northernmost part of the offshore northern subbasin were heavily intruded by igneous sills during the miocene the proposed timing of intrusion is coincident with that previously suggested for the development of the cayar seamount located 100 km nnw of dakar and onshore volcanism in the cap vert area near dakar and in the cape verde islands this stage of miocene igneous activity is interpreted to be closely associated with the coeval collision between africa and europe the interpreted igneous sills give rise to an unexpected soft seismic response the preferred explanation for the soft appearance of the sills is that the majority of sills are thin

cocorp seismic profiling of the appalachian orogen beneath the coastal plain of georgia a southeastward extension onto the coastal plain of an earlier cocorp traverse which confirmed large scale thin skinned thrusting of crystalline rocks of the southern appalachians has provided some of the most spectacular reflections yet seen in crustal seismic data most of the reflectors can be interpreted as either fault surfaces or as metamorphosed strata of late precambrian early paleozoic age they are consistent with the hypothesis that a major detachment extends eastward beneath this part of the orogen although other interpretations with a more complex pattern of detachments or sutures are also possible large scale overthrusting provides a mechanism for incorporating sedimentary rocks into the lower crust and may help to explain many of the layered features on crustal seismic data reflections from deep beneath the coastal plain indicate that the structural configuration of the rocks is complex and that the remains of a collision zone are being observed several east dipping horizons which bear strong similarities to thrust faults in valley and ridge sedimentary rocks are seen in the basement at shallow and mid crustal levels beneath the coastal plain the augusta fault for example displays a reflection which extends at a low angle some 80 km or more southeast of its surface position in conjunction with surface geologic information these new data demonstrate that late paleozoic compressive deformation was pervasive and resulted in lateral movements in the upper crust extending from the valley and ridge to the crystalline rocks beneath the coastal plain a distance of 400 km or more a large antiform cresting at about 2 3 sec or about 6 km below the surface and other structures beneath the coastal plain of georgia deserve further consideration for petroleum exploration although metamorphism may have eliminated petroleum from these rocks refracted arrivals and fault geometries indicate two triassic rift basins beneath coastal plain sedimentary rocks one of which has apparently not been recognized previously

post magmatic cooling of the elberton granite bearing on the late paleozoic tectonothermal history of the georgia inner piedmont the elberton granite is an undeformed pluton which intrudes polydeformed gneisses along the eastern margin of the georgia inner piedmont magmatic biotite and muscovite display internally concordant 40 ar 39 ar release spectra which record plateau dates of 235 241 m y biotite from adjacent gneisses and from recrystallized xenoliths yield similar plateau ages hornblende from country rock amphibolite and from recrystallized xenoliths record well defined 40 ar 39 ar plateau ages of 267 276 m y the 40 ar 39 ar ages are markedly younger than the 320 20 m y crystallization age of the pluton u pb zircon ross and bickford 1980 and are interpreted to date times of post magmatic cooling through temperatures required for argon retention in the various mineral species examined because similar ages are recorded by equivalent minerals from the country rock terrane post magmatic cooling must have closely followed post metamorphic cooling of the inner piedmont 40 ar 39 ar date

magma fingers and host rock fluidization in the emplacement of sills the golden valley sill in south africa like many similar structures imaged in three dimensional seismic data is saucer shaped with a transgressive rim this rim exhibits magma fingers tens to hundreds of meters wide radiating from a central axis and forming the outer dipping rim localized fluidization of the host rock is observed close to the margins where the fingers occur suggesting a causal link between fluidization of the country rocks and finger formation we relate the large scale formation of fingers to host rock fluidization caused by flash boiling of pore fluids following the tensional failure of the roof of the inner dish at a maximum radius once fluidization has occurred by this method the mechanical heterogeneity controlling the propagation of a concordant sill is broken and the intrusion can transgress this work emphasizes the importance of understanding the nature of magma intrusion on host rocks depending on their lithology and in particular the ability of host rocks to behave in either a brittle or nonbrittle manner during magma intrusion

paleomagnetism of rocks from burin peninsula newfoundland hypothesis of late paleozoic displacement of acadia criticized the posttectonic st lawrence granite 360 5 ma and associated subvertical dikes have two magnetizations n and h n points north with an intermediate upward inclination declination 005 inclination 50 alpha 95 10 paleopole 12 n 120 e we consider n to have been acquired close to the time of initial cooling the h magnetization points south with low inclination h is also the dominant magnetization in the nearby permo carboniferous spanish room formation h is considered to be an overprint acquired chemically during the kiaman reversed superchron late carboniferous and permian the n magnetization corresponds to a paleolatitude of 31 7 s very different from the near equatorial values expected from the commonly accepted late devonian and earliest carboniferous tournaisian reference paleofields for cratonic north america this is generally consistent with the hypothesis of a southerly displaced acadia proposed by kent and opdyke although the displacement is much greater 30 rather than 15 however we show that all inclinations observed from middle to upper devonian and tournaisian rocks from the craton are not significantly different either from the kiaman paleofield or from the h overprints observed from newfoundland some of these cratonic studies are not very detailed but it is noteworthy that the declination observed in the most comprehensive of them catskill red beds also agrees with the kiaman paleofield and with the h overprints from newfoundland declinations in some other cratonic studies show small departures of only a few degrees with one exception based on limited sampling and two from the colorado plateau which may have been rotated which could be caused by experimental inaccuracies moreover the directions obtained from cratonic studies are 97 reversed as they are in rocks with kiaman magnetizations we suggest therefore that the commonly accepted late devonian to earliest carboniferous reference paleofield for north america is either entirely kiaman in age or has been seriously affected by kiaman overprinting consequently the 30 of southerly displacement which the n magnetization of the st lawrence granite apears to indicate and the 15 invoked by kent and opdyke we consider to be illusory three late precambrian rock units 623 to 606 ma have scattered declinations but similar inclinations yielding paleolatitudes of about 35 the scattered declinations presumable were caused by local pre carboniferous relative rotations there is little or no evidence of kiaman overprinting in precambrian rocks indicating that overprinting is comparatively superficial and is probably related to late paleozoic groundwater circulation

sill morphology and comparison of brittle and non brittle emplacement mechanisms magmatic sheet intrusions contribute significantly to the upper crustal magma transport network the emplacement mechanism of the magmatic sheets controls the final geometry of the intrusions and the characteristics of host rock deformation previous observations have highlighted the preponderance of brittle structures associated with shallow level sheet intrusions however recent studies have suggested that non brittle host rock behaviour also occurs particularly related to the formation of magma fingers during shallow level sill intrusion here we examine both brittle and non brittle intrusion mechanisms and expand upon them with field observations from a series of widespread and variable magmatic systems non brittle emplacement appears primarily associated with viscous flow of the host rock during intrusion and is therefore intimately linked to the contemporaneous host rock rheology as well as magma dynamics purely brittle and non brittle emplacement processes are found to be end members with many intrusions containing evidence of both behaviours deriving the host rock characteristics is therefore important for discerning potential diagnostic intrusion indicators and intrusion geometries both within the field and in modelling incorporation of variable host material behaviours in numerical and analogue modelling tuned using direct field observations may consequently further our understanding of the controls on shallow level intrusion

erratum modeling the growth of laccoliths and large mafic sills role of magma body forces journal of geophysical research 2011 116 b08211 doi 10 1029 2011jb008618

contrasting accreted terranes in the southern appalachian orogen basement beneath the atlantic and gulf coastal plains and west african orogens abstract a variety of lithostratigraphic units may be outlined within the pre mesozoic crystalline basement of the atlantic and gulf coastal plains these include 1 a group of metamorphic rocks of variable grade together with deformed and retrogressed granite in southwestern alabama and southeastern mississippi wiggins uplift hornblende and biotite within higher grade units record 40ar 39ar post metamorphic cooling ages of 310 300 ma 2 a suite of contrasting igneous rocks granite basalt and agglomerate and serpentinite which occurs along the brunswick altamaha magnetic anomaly in southwestern alabama and 3 an extensive apparently coherent tectonic element the suwannee terrane comprised of undeformed granite in which biotite records 40ar 39ar post magmatic cooling ages of 530 525 ma low grade felsic metavolcanic rocks a suite of high grade metamorphic lithologies st lucie metamorphic complex in which hornblende yields 40ar 39ar post metamorphic cooling ages of 515 510 ma and a succession of lower ordovician middle devonian sedimentary rocks characterized by gondwanan fauna the west african orogens mauritanides bassarides and rokelides record a locally complex polyphase tectonothermal evolution the earliest event corresponded to westward rifting of a continental fragment from the west african craton at 700 ma this led to development of a rift facies lithotectonic succession which included sedimentary units and intracontinental igneous sequences rifting was limited and a western ensialic arc began to develop by 680 ma the associated convergence culminated in an episode of folding and metamorphism at 650 ma pan african i orogenesis following widespread deposition of late paleozoic glacial and flyschoid sediments a second tectonothermal event occurred between 550 and 500 ma pan african ii orogenesis the west african orogens were marked by general tectonic quiescence throughout most of the early and middle paleozoic late paleozoic 300 ma collision of gondwana and laurentia resulted in eastward translation of previously tectonized mauritanide units over their foreland and emplacement at highest structural levels of previously imbricated nappes which include sequences with uncertain palinspastic origins the late paleozoic hercynian transport was largely intracontinental and only westernmost portions of the exposed mauritanides record an associated penetrative tectonothermal overprint characteristics of pre mesozoic basement units which comprise the suwannee terrane in the florida subsurface suggest they represent extensions of the bassaride rokelide orogen there is no apparent record of paleozoic tectonothermal activity in the suwannee terrane this contrasts markedly with penetrative late paleozoic reworking of basement units in the wiggins uplift this suggests that a major dextral transcurrent fault was active during the late paleozoic and that proximal basement units were directly involved in collisional aspects of pangea assembly the various basement units of the coastal plain are not correlative with any of the non laurentian terranes exposed in the southern appalachian orogen e g carolina terrane of the eastern piedmont which accreted in the ordovician devonian to exterior positions along the eastern margin of the north american craton these exotic appalachian terranes were transported into their present structural positions during late paleozoic collision of gondwana and laurentia they are separated from gondwanan elements of the suwannee terrane by a suture approximately marked by the brunswick altamaha magnetic anomaly

australia s southern margin a product of oblique extension abstract recently developed detachment models of continental margin formation interpret the southern margin of australia to have formed when the lower plate australian margin was pulled out from beneath the upper plate antarctic margin data now available and summarised in this paper point very strongly to a generally nw se direction of initial continental extension for the southern margin in contrast to the widely held picture of simple n s rifting the evidence for this extension direction comes from the analysis of deep seismic data acquired by the bureau of mineral resources in 1986 in the central great australian bight gab the gravity field of the gab seismic and magnetic basement structures in the eyre sub basin polda trough ceduna depocentre and duntroon basin and from the analysis of the magnetic seafloor spreading anomalies produced during the slow first phase of drifting between australia and antarctica further it is now believed that the formation of the southern margin of australia can be described in terms of three phases of continental extension el to e3 and two phases of drifting d1 and d2 in summary these phases were as follows 1 e1 approximately 300 km of late jurassic or older to early cretaceous nw se oriented extension in the gab with strike slip motion in the nascent otway basin and along the tasmanian margin 2 e2 120 km of early cretaceous nne ssw oriented extension which formed the basins of southeastern australia otway bass gippsland and which probably produced a structural overprinting in the gab basin 3 e3 d1 minor continental extension and the first 500 km of slow drift between australia and antarctica on an azimuth of 165 wrenching on the tasmanian margin 4 d2 2600 km of fast drifting between australia and antarctica on a n s azimuth

weathering effects on the magnetic properties of the milledgeville granite georgia abstract one of the major problems in paleomagnetic sampling of granites in the southeastern united states is finding fresh outcrop since most outcrops are weathered to some extent it is important to quantify the effect of weathering on the magnetic properties of individual samples the lake sinclair dam site near milledgeville georgia was chosen for this study where a very fresh stable granite outcrops in the excavated spillway lying immediately above is the weathered equivalent drilled samples were obtained from fresh and weathered portions at one outcrop and the remanent and induced magnetizations measured with af demagnetization it was possible to obtain the stable remanent directions exhibited by fresh samples from all weathered samples the induced and remanent magnetic behavior can be explained by maghemitization during weathering and the development of a secondary low oxidation magnetic phase and the reduction in domain size of primary magnetite

analytical model of surface uplift above axisymmetric flat lying magma intrusions implications for sill emplacement and geodesy abstract in this paper we develop a new axisymmetric analytic model of surface uplift upon sills and laccoliths based on the formulation of a thin bending plate lying on an elastic foundation in contrast to most former models also based on thin bending plate formulation our model accounts for i axi symmetrical uplift ii both upon and outside the intrusion the model accounts for shallow intrusions i e the ratio a h 5 where a and h are the radius and depth of the intrusion respectively the main parameter of the model is the elastic length l which is a function of the elastic properties of the bending plate and of the elastic foundation the model exhibits two regimes depending on the ratio a l when a l a l 5 the uplift is mostly restricted upon the intrusion when the elastic foundation is very stiff our model converges towards that of a clamped plate we provide as supplementary material a matlab function that calculates the surface uplift from the set of system and control parameters we discuss three possible applications of our model i the model can be used to describe sill propagation by introducing a propagation criterion for realistic values our model reproduces well the behavior of horizontal intrusions simulated in experiments ii the model can also be used to compute the critical size of saucer shaped sills it shows for instance that a soft elastic foundation favors the horizontal spreading of sills before they form inclined sheets iii we show that the classical mogi point source model cannot be used to constrain sill properties from the surface uplift we thus propose that our model can be used as a valuable alternative to both simple analytical models like mogi s and more complex numerical models used to analyze ground deformation resulting from sill intrusions in active volcanoes

paleomagnetic evidence for the continuity and independent movement of a distinct major crustal block in the southern appalachians the magnetization of 22 granitic and gneissic southern appalachian rock units with estimated cooling ages of 415 250 m y has been determined included are 842 samples from 114 sites within 19 granites 100 sites and 3 gneisses 14 sites located in north carolina south carolina and georgia data for units which cooled to temperatures 20 000 km sup 2 in size a mean paleopole for this group with corrections for maximum tilt estimates exhibits good precision but has a paleosite latitude of approx 10 sup 0 n without tilt correction the paleopole for group b still exhibits an anomalous paleositemore latitude of approx 4 sup 0 n these data indicate that the group b block elberton sparta crustal block lying to the se has an apparent paleosite latitude corresponding to magnetization at a location to the north of the zone containing group a units less

magnetization of the austell gneiss northwest georgia piedmont the austell gneiss is located in the piedmont 16 km west of atlanta georgia and 8 km northwest of the brevard fault zone this metamorphosed granite gneiss lies refolded in the nose of the austell frolona antiform a doubly plunging structure of regional extent subsequent to intrusion the unit has undergone four episodes of deformation the first and second generation folds f1 and f2 are coaxial but not coplanar the two later fold events were less intense and have little effect on local outcrop patterns in the study area the age of the unit has been estimated to be greater than 325 m y but less than 365 m y the induced and remanent magnetic properties for at least six cores drilled at each of 10 sites within the austell gneiss have been determined the magnetic foliation for the unit is approximately parallel with field estimates for s1 foliation planes exhibited by mineral orientations within the body indicating post emplacement magnetic grain mobility the stable remanent magnetization exhibited by the body was therefore probably acquired during or slightly after the f1 or f2 deformational events as a result of cooling from metamorphic temperatures 600 c or alternatively from secondary magnetite grain nucleation and growth a paleopole position calculated for these data without structural correction 126 2 34 4 dp 6 6 dm 13 2 can be explained by magnetic acquisition in at least three possible geologic settings from simple to complex these are the following first since a large oval of confidence is observed for the unit and minor tilting can not be resolved the data may be explained by magnetization of the austell at its present site during cooling from metamorphic temperatures second a northwestward thrust event may have resulted in bringing the austell to its present location third the data can be explained by two major translational events the austell may have initially been magnetized and then moved to the northeast by transcurrent strike slip faulting 1000 km into juxtaposition with the southern appalachians followed by thrusting of the unit to the northwest to its present location in support of this last interpretation is the similarity between the paleopole for the austell and paleopoles determined for the maritime provinces of north america which were suggested to have moved 1500 km to the northeast during the mid to late paleozoic against such an interpretation is the lack of geologic evidence in support of large scale left lateral strike slip faulting in the southern appalachians

geometry and controls on the development of igneous sill related forced folds a 2 d seismic reflection case study from offshore southern australia emplacement of magma in the shallow subsurface can result in the development of dome shaped folds at the earth s sur face these so called forced folds have been described in the fi eld and in subsur face data sets although the exact geometry of the folds and the nature of their relation ship to under lying sills remain unclear and in some cases controversial in this study we use high quality two dimensional 2 d seismic refl ection and borehole data from the ceduna sub basin offshore southern australia to describe the structure and infer the evolution of igneous sill related forced folds in the bight basin igneous complex thirty three igneous sills which were em placed 200 1500 m below the paleo seabed in upper cretaceous rocks are mapped in the ceduna sub basin the intrusions are expressed as packages of high amplitude refl ections which are 32 250 m thick and 7 19 km in diameter we observe fi ve main types of intrusion type 1 strata concordant sills type 2 weakly strata discordant trans gressive sills type 3 saucer shaped sills type 4 laccoliths and type 5 hybrid intru sions which have geometric characteristics of intrusion types 1 3 these intrusions are overlain by dome shaped folds which are up to 17 km wide and display up to 210 m of re lief the edges of these folds coincide with the margins of the underlying sills and the folds display the greatest relief where the underly ing sills are thickest the folds are therefore interpreted as forced folds that formed in response to emplacement of magma in the shallow subsurface the folds are onlapped by lutetian middle eocene strata indicat ing they formed and the intrusions were em placed during the latest ypresian ca 48 ma we demonstrate that fold amplitude is typi cally less than sill thickness even for sills with very large diameter to depth ratios suggest ing that pure elastic bending forced folding of the overburden is not the only process ac commodating magma emplacement and that supra sill compaction may be important even at relatively shallow depths based on the ob servation that the sills intruded a shallowly buried succession the discrepancy between fold amplitude and sill thickness may refl ect loss of host rock volume by fland pore fl uid expulsion from poorly lithifi ed water rich beds this study indicates that host rock composition emplacement depth and deformation mechanisms are important controls on the style of deformation that oc curs during intrusive igneous activity and that forced fold amplitude may not in all cases refl ect the thickness of an underlying igneous intrusion in addition the results of this study suggest that physical and numeri cal models need to model more complex host rock stratigraphies and rheologies if they are to capture the full range of deformation mechanisms that occur during magma em placement in the earth s shallow subsurface

anisotropy of magnetic susceptibility data indicating remagnetization in diabase dikes induced and remanent magnetic analyses have been performed on drilled samples from 2 generations of unweathered diabase dikes a breccia dike and the host tonalite which make up the basement on which the russell dam foundation was poured the maximum magnetic foliation plane for most samples defined by anisotropy of magnetic susceptibility ams data is vertical and is oriented perpendicular to maximum stresses developed in the region during mid late paleozoic time the excellent ams data consistency between units of different ages is interpreted to be indicative of stress controlled magnetite recrystallization at the site it is inferred that this recrystallization is responsible for the multicomponent remanent magnetization rm in these samples which was probably acquired over long periods of time

paleomagnetism of some mafic intrusions in the south carolina piedmont i magnetic systems with single characteristic directions abstract four mafic and two dioritic igneous bodies intruding along the flanks of the charlotte belt within the king s mountain belt and near the slate kiokee charlotte belt boundaries in the south carolina piedmont were studied paleomagnetically the results suggest that these mafic rocks with a single characteristic magnetization are broadly contemporaneous a mean paleomagnetic pole position of 38 9 n 120 8 e has been calculated for the six bodies this pole position falls near the 300 ma old point of irving s apparent polar wander path apwp for north america the 95 circle of confidence 10 2 includes points on the apwp between 250 360 ma anomalously old k ar apparent ages greater than one billion years are suggestive of excess 40 ar contamination for the mafic buffalo pluton whereas apparent ages in the range of 360 395 ma old are interpreted as a maximum age for the other bodies due to the possibility of small amounts of excess 40 ar being present a 10 westerly tilt correction suggested by dooley and smith for early mesozoic diabase brings the mafic pole position of this study to more nearly coincide with the 350 ma virtual geomagnetic position of irving s curve but the test is inconclusive awaiting better definition of radiometric ages the simplest interpretation of the data is that the mafic pole position reflects the direction of the geomagnetic field in late devonian carboniferous times the similarity of this pole position with the points on the apwp for north america provides little evidence for displaced terrains and with the precision of this pole position horizontal displacements on the order of that suggested by the consortium for continental reflection profile cocorp results or by subsequent studies are not detectable this and other pole positions from granitic rocks in the southern appalachian orogen suggest that if displaced terrains exist the evidence must be found in older rocks or in other geologic belts

confirmation of the carolina slate belt as an exotic terrane an assemblage of middle cambrian atlantic faunal province trilobites has been found in the rocks of the carolina slate belt near batesburg south carolina geologic and paleomagnetic data suggest that the carolina slate belt and the adjacent charlotte belt constitute an exotic terrane that was accreted to north america in early to middle paleozoic time

the deccan tholeiite lavas and dykes of ghatkopar powai area mumbai panvel flexure zone geochemistry stratigraphic status and tectonic significance abstract mumbai city situated on the western indian coast is well known for exposures of late stage deccan pillow basalts and spilites pyroclastic rocks rhyolite lavas and trachyte intrusions these rock units and a little studied sequence of tholeiitic flows and dykes in the eastern part of mumbai city constitute the west dipping limb of a regional tectonic structure called the panvel flexure here we present field petrographic major and trace element and sr nd isotopic data on these tholeiitic flows and dykes best exposed in the ghatkopar powai area the flows closely resemble the mahabaleshwar formation of the thick western ghats sequence to the east in sr nd isotopic ratios and multielement patterns but have other geochemical characteristics e g incompatible trace element ratios unlike the mahabaleshwar or any other formation the flows may have originated from a nearby eruptive center possibly offshore of mumbai two dykes resemble the ambenali formation of the western ghats in all geochemical characteristics though they may not represent feeders of the ambenali formation lavas most dykes are distinct from any of the western ghats stratigraphic units some show partial e g sr nd isotopic similarities to the mahabaleshwar formation and these include several dykes with unusual concave downward ree patterns suggesting residual amphibole and thus a lithospheric source the flows and dykes are inferred to have undergone little or no contamination by lower continental crust most dykes are almost vertical suggesting emplacement after the formation of the panvel flexure and indicate considerable east west lithospheric extension during this late but magmatically vigorous stage of deccan volcanism

insight into vent opening probability in volcanic calderas in the light of a sill intrusion model the aim of this paper is to discuss a novel approach to provide insights on the probability of vent opening in calderas using a dynamic model of sill intrusion the evolution of the stress field is the main factor that controls the vent opening processes in volcanic calderas on the basis of previous studies we think that the intrusion of sills is one of the most common mechanism governing caldera unrest therefore we have investigated the spatial and temporal evolution of the stress field due to the emplacement of a sill at shallow depth to provide insight on vent opening probability we carried out several numerical experiments by using a physical model to assess the role of the magma properties viscosity host rock characteristics young s modulus and thickness and dynamics of the intrusion process mass flow rate in controlling the stress field our experiments highlight that high magma viscosity produces larger stress values while low magma viscosity leads to lower stresses and favors the radial spreading of the sill also high rock young s modulus gives high stress intensity whereas low values of young s modulus produce a dramatic reduction of the stress associated with the intrusive process the maximum intensity of tensile stress is concentrated at the front of the sill and propagates radially with it over time in our simulations we find that maximum values of tensile stress occur in ring shaped areas with radius ranging between 350 m and 2500 m from the injection point depending on the model parameters the probability of vent opening is higher in these areas

southward extrusion tectonics during the carboniferous africa north america collision abstract from the devonian to the permian north america and africa shared the same orogenic history involving plate convergence followed by continent continent collision that closed the proto atlantic ocean conventionally it is held that this resulted in thrust and nappe tectonics with a westward vergence in the appalachians and an eastward vergence in the mauritanides in contrast to the movement picture suggested by the thrusting model medium to late carboniferous displacements along major fault zones are mainly dextral strike slip in the southeastern appalachians and sinistral in the western mauritanides an alternative kinematic model is presented here it is suggested that the southern appalachian piedmont and western senegal mauritania provinces moved southward relative to north american and african cratons this escape tectonics during continental collision would result from 1 1 a plate convergence vector oriented at a medium oblique angle to preexisting zones of weakness within the north american margin and 2 2 indentation of the appalachians by the reguibat uplift of west africa

evaluation of southern eastern overthrust belt beneath blue ridge piedmont thrust seismic reflection surveys in the southern part of the eastern overthrust reveal that crystalline rocks of the blue ridge and piedmont were thrust westward burying a large segment of sedimentary rocks of the valley and ridge surface and subsurface data suggest that the buried segment includes cambrian and lower ordovician shallow shelf clastic and carbonate rocks overlain by middle ordovician foredeep black shale because eastward increasing regional thermal patterns existed prior to thrusting westward movement of thrust sheets disrupted and telescoped that pattern by placing more thermally mature eastern rocks over less mature western rocks the original thermal pattern can be reconstructed by restoring the thrust sheets to their prethrust position the reconstruction mphasizes that rocks less metamorphosed than greenschist facies are contained in only two thrust sheets all of the saltville and part of the pulaski because subgreenschist rocks of the saltville and pulaski thrust sheets appear to be confined to the subsurface of the blue ridge and westward it is most likely that rocks with commercial gas potential will not be present in the subsurface of the piedmont east of the brevard fault nevertheless the concealed area of the southern eastern overthrust with possible gas potential is about as wide as its exposed part thus the combined surface and subsurface area for future exploration in the southern appalachians is about doubled in size

oxygen isotopic compositions of the late orogenic granites in the southern piedmont of the appalachian mountains u s a and their relationship to subcrustal structures and lithologies abstract data were acquired from 143 whole rock samples from 20 late orogenic post metamorphic hercynian age granitic plutons from the piedmont of the southern appalachians principally in georgia and south carolina these plutons exhibit a regional gradient in oxygen isotopic compositions in which the granites confined to the inner piedmont to the northwest are 18 o enriched 11 4 to 7 9 whereas those toward the southeast within the charlotte slate and portions of the kiokee belts have distinctly lower 18 o 16 o compositions 8 2 to 5 5 one body that lies along the southeastern edge of the piedmont in south carolina however appears to be anomalously 18 o enriched 8 9 most plutons display 18 o 16 o variations of 18 o enriched plutons occupy areas characterized by negative anomalies whereas low 18 o bodies are invariably restricted to regions of positive anomalies 2 87 sr 86 sr data where granites with 18 o values 18 o enriched plutons 9 have ratios 0 710 3 contrasting chemical and accessory mineral compositions in which many 18 o depleted granites have a number of i type characteristics whereas several of the most 18 o enriched plutons exhibit a number of s type features it can be inferred from these data that the 18 o enriched granites were formed from continental crustal protoliths that underlie much of the inner piedmont and portions of the kiokee belt whereas the low 18 o plutons were derived from more mafic sources beneath the charlotte slate and portions of the kiokee belt the overall correspondence between the regional 18 o 16 o patterns exhibited by the granites and gravity data implies that these grantes may be essentially rooted to their protoliths in turn suggesting that the large scale translational movement recently proposed for the southern piedmont may have occurred prior to intrusion of these granites 320 m y ago

interactions between continental breakup dynamics and large scale delta system evolution insights from the cretaceous ceduna delta system bight basin southern australian margin 1 the interpretation of two regional seismic reflection profiles and the construction of a balanced cross section through the southern australian margin bight basin are designed to analyze the influence of the australia antarctica continental breakup process on the kinematic evolution of the cretaceous ceduna delta system the data show that the structural architecture of this delta system consists of two stacked delta systems the lower white pointer delta system late albian santonian is an unstable tectonic wedge regionally detached seaward above late albian ductile shales sequential restoration suggests that the overall gravitational sliding behavior of the white pointer delta wedge 45 km of seaward extension i e 27 is partially balanced by the tectonic denudation of the subcontinental mantle we are able to estimate the horizontal stretching rate of the mantle exhumation between 2 and 5 km ma 1 the associated uplift of the distal part of the margin and associated flexural subsidence in the proximal part of the basin are partially responsible for the decrease of the gravitational sliding of the white pointer delta system lithospheric failure occurs at 84 ma through the rapid exhumation of the mantle the upper hammerhead delta system late santonian maastrichtian forms a stable tectonic wedge developed during initial slow seafloor spreading and sag basin evolution of the australian side margin lateral variation of basin slope related to the geometry of the underlying white pointer delta wedge is associated with distal raft tectonic structures sustained by high sedimentation rates finally we propose a conceptual low angle detachment fault model for the evolution of the australian antarctic conjugate margins in which the antarctic margin corresponds to the upper plate and the australian margin to the lower plate

implications of simple flexure theory for the formation of saucer shaped sills saucer shaped dolerite sills have been identified in several sedimentary basins they are characteristically sub circular in plan with an inner sill connected to a steeply inclined arcuate sheet extending upwards from the edge of the inner sill and in some cases have raggedly developed flat outer rims by applying elastic plate theory to the overburden we obtain an expression for the maximum radius of a circular sill in terms of emplacement depth excess magma pressure elastic constants of the overburden and the tensile strain at failure this expression predicts that the radius of the flat base of a saucer shaped sill increases with emplacement depth as observed in the western karoo basin south africa some saucer shaped sills display an elliptical morphology from consideration of the flexural strains around the periphery of a horizontal elliptical sill we infer that saucer shaped elliptical sills have been fed from below by feeder dykes oriented along the major axis of the ellipse flat outer rims would have developed at the final stage of each intrusive episode when the excess magma pressure was decreasing towards equilibrium with the overburden load consequently flexural strains would also have decreased so continued intrusion might preferentially occur along a bedding plane

paleomagnetic evidence for post devonian displacement of the avalon platform newfoundland the possibility that the avalon platform where the avalonian lithotectonic belt is best developed was involved in late paleozoic displacement was tested by paleomagnetic study of red sandstones of the upper devonian terrenceville formation of eastern newfoundland two magnetization directions were identified by thermal demagnetization analysis of 60 oriented samples from 10 sites a high blocking temperature thermally discrete a component of normal and reversed polarity and an intermediate blocking temperature thermally distributed b component of reversed polarity the b component d 185 9 i 3 3 a95 7 2 for n 8 sites is interpreted as a postfolding secondary magnetization and gives a paleomagnetic pole position latitude 43 6 n longitude 117 1 e near early to late permian paleopoles for north america the a component d 181 6 i 28 0 a95 10 1 for n 9 sites is interpreted as the characteristic magnetization possibly dating from near the time of deposition of the terrence ville formation the corresponding paleomagnetic pole position latitude 27 4 n longitude 123 5 e falls within a group of late devonian early carboniferous paleopoles obtained from the acadia displaced terrain encompassing the coastal areas of new england and the canadian maritimes which form another part of the avalonian belt these paleopoles are systematically offset by 15 to 20 in latitude from coeval pole positions obtained from cratonic north america thus the avalonian belt of the northern appalachians which is thought to represent a remnant of a precambrian and early paleozoic microcontinent on tectonostratigraphic considerations appears to correspond to a late paleozoic displaced terrain on the basis of paleomagnetic evidence

the chogat chamardi subvolcanic complex saurashtra northwestern deccan traps geology petrochemistry and petrogenetic evolution abstract the saurashtra region in the northwestern part of the deccan continental flood basalt province india is notable for its many volcano plutonic complexes compositional diversity and the abundance of rhyolite and granophyre in this first detailed study of the chogat chamardi subvolcanic complex in eastern saurashtra we report microgranite and granophyre plutons gabbro plutons cut by basaltic andesite dykes as well as rhyolite plugs bulk rock geochemical including nd sr isotopic data suggest a strong contribution from ancient rb rich basement crust to the silicic magmas the mafic rocks are also crustally contaminated but less than the silicic rocks in conformity with observations elsewhere e g the skye and skaergaard granophyres the chogat chamardi silicic rocks have initial 87sr 86sr for t 65 ma as high as 0 72914 and initial end values as low as 13 9 trends defined by the chogat chamardi and other deccan rhyolitic suites on plots of sr concentration vs sr isotopic ratio are modeled with concurrent assimilation and fractional crystallization afc processes involving a basaltic magma and granitic basement rocks considering both nd and sr isotopic variations the chogat chamardi silicic rocks notably require crustal end members very different from those inferred in most previous isotopic studies of deccan rocks

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

two stage growth of laccoliths at elba island italy at elba island italy nine shallow level late miocene granite porphyry layers connected by feeder dikes built up three nested christmas tree laccoliths detailed mapping and reconstruction of tectonic history led to restoration of the original 5 km thick sequence and determination of the dimensional parameters of each intrusive layer the laccolith layers were emplaced at depths between 1 9 and 3 7 km exploiting physical discontinuities that served as crustal magma traps inside a stack of nappes the intrusive layers are 50 700 m thick with diameters between 1 6 and 10 km length to thickness relationships for individual laccolith layers show a power law correlation that does not fit the known dimensional distribution for laccoliths but instead fits a line with a slope typical of the theoretical vertical inflation stage of laccolith development this is interpreted as the first reported natural example of the occurrence of a vertical inflation stage during laccolith growth the dimensional data for elba intrusive layers also suggest that laccoliths and plutons commonly form by amalgamation of smaller sheet like bodies while multilayer laccoliths form when coalescence fails possibly owing to the large availability of crustal magma traps

the mount pavagadh volcanic suite deccan traps geochemical stratigraphy and magmatic evolution abstract the patterns of eruption and dispersal of flood basalt lavas on the surface or as magmas in dykes and sills within the crust determine the volcanological and stratigraphic development of flood basalt provinces this is a geochemical and sr isotopic study of lavas of varied compositions that outcrop around mount pavagadh 829 m deccan traps an important outlier north of the main basalt outcrop most of the 550 m thick exposed section at pavagadh is made up of subalkalic basalts rich in the incompatible elements particularly nb ba and sr picrite and rhyolite dacite flows also occur the latter capping the sequence the relatively high initial 87sr 86sr ratios up to 0 7083 and chemical characteristics of the rhyolitic rocks of pavagadh are consistent with a small but significant involvement of the granitic basement crust in their genesis an assimilation fractional crystallization afc model involving the picrite lava and either a southern indian or a western indian granite as the contaminant explains the geochemical and sr isotopic variation in the basalts and the rhyolites quite well a systematic comparison of the basaltic lavas with binary plots normalized multielement patterns and discriminant function analysis to the well established lava stratigraphy of the western ghats 400 500 km to the south precludes any chemical genetic relationships between the two basalts exposed in sections closer to pavagadh 150 200 km in the toranmal navagam and barwani mhow areas have several flows with some similar chemical characteristics however the pavagadh sequence is significantly different from all of these sequences geochemically petrogenetically and in magnetic polarity to be considered independently built this result is significant in terms of eruptive models for the deccan traps as it is increasingly apparent that there were separate but possibly coeval eruptive centers with their own distinctive chemistries developed in various areas of this vast province

the eastern piedmont fault system and its relationship to alleghanian tectonics in the southern appalachians alleghanian continental collision across the southern appalachians resulted in northwestward over thrusting in the blue ridge and valley and ridge and emplacement of a large crystalline nappe in the piedmont however fault patterns in the eastern piedmont fault system are consistent with the geometry expected in a strike slip fault network formation of the eastern piedmont fault system mainly postdates the last regional metamorphism in the piedmont and is interpreted to be an alleghanian structure penecontemporaneous with thrusting in the western part of the appalachian orogen this discrepancy in tectonic style across the orogen is attributed to lateral movement of parts of the raleigh and kiokee belts and of the precambrian paleozoic basement of northern florida out of the principal collision zone during latter stages of the alleghanian collision a modern analogy for such lateral movement is the present strike slip regime in turkey which represents post collisional lateral escape of the anatolia

correlation of magnetitic susceptibility with18o 16o data in late orogenic granites of the southern appalachian piedmont initial magnetic susceptibility generally indicative of magnetite content has been determined for 445 samples from 17 granites located in the southern appalachian piedmont of georgia and south carolina these values have been correlated with whole rock 18o data from the same plutons yielding a pronounced inverse relationship it has previously been shown for the southern piedmont that low oxygen isotopic 18o enriched values usually occur in s type granites wenner 1 this issue it follows then that i type granites are characterized by high susceptibilities 1 10 4 g oe and s type granites by low susceptibilities 1 10 4 g oe an interesting result of this work has been the observation that some s type granites exhibit good within site clusters of remanent magnetic directions while i type granites generally do not

seismic imaging of the elberton granite inner piedmont georgia using cocorp southern appalachian data seismic imaging of the elberton granite in the georgia inner piedmont was undertaken to define its shape and to determine its relation to the southern appalachian thrust the recent consortium for continental reflection profiling cocorp lines in the southern appalachians suggest a major thrusting of a crystalline block over sediments although the extent of this thrust is subject to interpretation the fine grained elberton of late devonian age is the only major pluton crossed by the seismic traverse and is not well defined on the unmigrated common depth point cdp seismic section migration by beam forming reveals considerable variation in the direction of returning signal and provides a clearer image of the laminated reflections ubiquitous in the southern appalachians which have been interpreted as the equivalents of the valley and ridge sediments beneath an allochthonous sheet the upper 5 km portions of the migrated and cdp sections lack events probably because of the necessary stripping mute of a large amplitude early arrival from the seismic records before further processing filtering and transformation of the data to the intercept slowness domain to eliminate this interfering arrival are attempted to enhance the upper portion of the seismic sections although these approaches do not give a consistent picture of the shallow section a pattern of scatterers located at depths of 2 3 5 km emerges these may correlate with the base of the intrusive body this identification of the granite base along with the presence of the thrust related reflectors beneath the pluton suggests a prethrust or synthrust emplacement of the granite however tectonic interpretations of the inner piedmont based on the elberton granite are severely limited by assumptions that the pluton age corresponds to time of cooling that the pluton is not fed by a narrow seismically elusive feeder and that the thrust has not been reactivated after emplacement of the intrusive body if all these assumptions hold then the late devonian age of the granite is the earliest possible time of thrusting

highly heterogeneous precambrian basement under the central deccan traps india direct evidence from xenoliths in dykes abstract crustal or mantle xenoliths are not common in evolved tholeiitic flood basalts that cover huge areas of the precambrian shields yet the occasional occurrences provide the most direct and unequivocal evidence on basement composition few xenolith occurrences are known from the deccan traps india and inferences about the deccan basement have necessarily depended on geophysical studies and geochemistry of deccan lavas and intrusions here we report two basalt dykes rajmane and talwade dykes from the central deccan traps that are extremely rich in crustal xenoliths of great lithological variety gneisses quartzites granite mylonite felsic granulite carbonate rock tuff because the dykes are parallel and only 4 km apart and only a few kilometres long the xenoliths provide clear evidence for high small scale lithological heterogeneity and strong tectonic deformation in the precambrian indian crust beneath measured 87sr 86sr ratios in the xenoliths range from 0 70935 carbonate to 0 78479 granite mylonite the rajmane dyke sampled away from any of the xenoliths shows a present day 87sr 86sr ratio of 0 70465 and initial at 66 ma ratio of 0 70445 the dyke is subalkalic and fairly evolved mg no 44 1 and broadly similar in its sr isotopic and elemental composition to some of the lavas of the mahabaleshwar formation the xenoliths are comparable lithologically and geochemically to basement rocks from the archaean dharwar craton forming much of southern india as several lines of evidence suggest the dharwar craton may extend at least 350 400 km north under the deccan lava cover this is significant for precambrian crustal evolution of india besides continental reconstructions

a kinematic model for the collision and complete suturing between gondwanaland and laurussia in the carboniferous maps of middle to late carboniferous strike slip faults in western europe northern africa and eastern north america have been compiled from geological gravity and magnetic onshore and offshore data a large majority of these faults can be interpreted in terms of a logical syn to post collisional pattern superimposed on the initial suture between the african i e gondwana and north american european laurussian continental margins the pre collision positions of the major continents are derived from paleomagnetic data and when combined with the general distribution of the strike slip faults lead to a kinematic model of the complex middle late carboniferous collision in this model the west african precambrian craton acted as a rigid promontory or die with respect to the partially rigid plastic zones between it and the large north american craton these partially rigid plastic zones comprise the external appalachian and canadian maritime margins the mauritanide and hercynian margins of north

geology petrochemistry and genesis of the bimodal lavas of osham hill saurashtra northwestern deccan traps abstract the saurashtra region in the northwestern deccan continental flood basalt province india is notable for compositionally diverse volcano plutonic complexes and abundant rhyolites and granophyres a lava flow sequence of rhyolite pitchstone basaltic andesite is exposed in osham hill in western saurashtra the osham silicic lavas are ba poor and with intermediate zr contents compared to other deccan rhyolites the osham silicic lavas are enriched in the light rare earth elements and have end t 65 ma values between 3 1 and 6 5 and initial 87sr 86sr ratios of 0 70709 0 70927 the osham basaltic andesites have initial end values between 2 2 and 1 3 and initial 87sr 86sr ratios of 0 70729 0 70887 large ion lithophile element concentrations and sr isotopic ratios may have been affected somewhat by weathering notably the sr isotopic ratios of the silicic and mafic rocks overlap however the nd isotopic data indicate that the silicic lavas are significantly more contaminated by continental lithosphere than the mafic lavas we suggest that the osham basaltic andesites were derived by olivine gabbro fractionation from low ti picritic rocks of the type found throughout saurashtra the isotopic compositions and the similar al2o3 contents of the osham silicic and mafic lavas rule out an origin of the silicic lavas by fractional crystallization of mafic liquids with or without crustal assimilation as previously proposed for some icelandic rhyolites and supported here by melts modelling the osham silicic lavas may have been derived by partial melting of hot mafic intrusions emplaced at various crustal depths due to heating by repetitively injected basalts the absence of mixing or mingling between the rhyolitic and basaltic andesite lavas of osham hill suggests that they reached the surface via separate pathways

volcano growth mechanisms and the role of sub volcanic intrusions insights from 2d seismic reflection data abstract temporal and spatial changes in volcano morphology and internal architecture can determine eruption style and location however the relationship between the external and internal characteristics of volcanoes and sub volcanic intrusions is often difficult to observe at outcrop or interpret uniquely from geophysical and geodetic data we use high quality 2d seismic reflection data from the ceduna sub basin offshore southern australia to quantitatively analyse 56 pristinely preserved eocene age volcanogenic mounds and a genetically related network of sub volcanic sills and laccoliths detailed seismic mapping has allowed the 3d geometry of each mound to be reconstructed and distinct seismic facies within them to be recognised forty six continental basaltic shield volcanoes have been identified that have average flank dips of 3 parallel seismic reflections within the shield volcanoes are interpreted to represent interbedded volcanic and clastic material suggesting that a series of temporally separate eruptions emanated from a central vent the shield volcanoes typically overlie the lateral tips of sills and we suggest that the intermittent eruption phases correspond to the incremental emplacement of discrete magma pulses within the laterally extensive sill complex eight volcanogenic hydrothermal vents which are also associated with the lateral tips of sills were also recognised and these appear to have formed from the seepage of intrusion related hydrothermal fluids onto the seafloor via emplacement induced fractures this work highlights that deformation patterns preceding volcanic eruptions may i be offset from the eruption site ii attributed to intrusions with complex morphologies and or iii reflect magma movement along pre existing fracture systems these complexities should therefore be considered in eruption forecasting models that link pre eruption ground deformation to subterranean magma emplacement depth and volume more generally our study highlights the key role that seismic reflection data can play in understanding the geometry distribution and evolution of ancient and modern volcanic systems