a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

spatial variability in biogenic gas accumulations in peat soils is revealed by ground penetrating radar gpr 1 we performed surface and borehole ground penetrating radar gpr tests together with moisture probe measurements and direct gas sampling to detect areas of biogenic gas accumulation in a northern peatland the main findings are 1 shadow zones signal scattering observed in surface gpr correlate with areas of elevated ch4 and co2 concentration 2 high velocities in zero offset profiles and lower water content inferred from moisture probes correlate with surface gpr shadow zones 3 zero offset profiles depict depth variable gas accumulation from 0 10 by volume 4 strong reflectors may represent confining layers restricting upward gas migration our results have implications for defining the spatial distribution volume and movement of biogenic gas in peatlands at multiple scales

gas bubble transport and emissions for shallow peat from a northern peatland the role of pressure changes and peat structure gas bubbles are an important pathway for methane release from peatlands the mechanisms controlling gas bubble transport and emissions in peat remain uncertain the effects of hydrostatic pressure and peat structure on the dynamics of gas bubbles in shallow peat were therefore tested in laboratory experiments a peat monolith was retrieved from a raised bog and maintained in a saturated state three distinct layers were identified from noninvasive permittivity measurements supported by soil physical properties porosity bulk density phase i of the experiment involved monitoring for the accumulation of gas bubbles under steady pressure and temperature conditions the data showed evidence for gas bubbles being impeded by a shallow semiconfining layer at depths between 10 and 15 cm visible gas bubbles observed on the side of the sample box were recorded over time to estimate changes in the vertical distribution of volumetric gas content porosity estimates derived using the complex refraction index model crim suggest that gas bubbles enlarge the pore space when the exerted pressure is high enough phase ii involved triggering release of trapped bubbles by repeatedly increasing and decreasing hydrostatic pressure in an oversaturated condition comparison of changes in pressure head and methane density in the head space confirmed that the increasing buoyancy force during drops in pressure is more important for triggering ebullition than increasing mobility during increases in pressure our findings demonstrate the importance of changes in hydrostatic pressure on bubble size and variations in resistance of the peat fabric in regulating methane releases from peatlands

ground penetrating radar and its use in sedimentology principles problems and progress abstract ground penetrating radar gpr also referred to as ground probing radar surface penetrating radar subsurface radar georadar or impulse radar is a noninvasive geophysical technique that detects electrical discontinuities in the shallow subsurface

mechanics of basin inversion abstract the long term consequences of rifting for the thermal state of the deep crust and upper mantle reflect in part cooling induced by the reduction in heat production in the attenuated lithosphere and heating due to the burial of this heat production beneath the basin provided that the heat production is largely concentrated in the upper half of the crust these factors result in significant increases in temperature at deep crustal and upper mantle levels because the moho depth is likely to be reduced in the long term limit of an isostatically balanced basin these same factors may lead to slight cooling or slight heating of the moho depending on the nature of the basin fill for a brace goetze lithospheric rheology i e a rheology governed by a combination of frictional sliding and power law creep significant long term lithospheric weakening up to 5 per kilometre of basin fill accompanies basin formation when the lower crust is initially relatively strong and the basin fill is characterised by appreciable heat production and low thermal conductivity in contrast initially weak lower crustal rheologies may result in long term lithospheric strengthening evidence for basin inversion in the geological record may therefore imply that heat production is strongly concentrated in the upper half of the crust and under normal continental thermal regimes the lower crust is strong

gravitational and tectonic forces controlling postcollisional deformation and the present day stress field of the alps constraints from numerical modeling 1 we perform numerical modeling to investigate the mechanisms leading to the postcollisional tectonic evolution of the alps we model the lithospheric deformation as a viscous thin sheet with vertically averaged rheology and coupled with surface mass transport the applied kinematic boundary conditions simulate the convergence between the adria indenter and the european foreland during the last 35 myr model predictions of elevation lithospheric structure erosion sedimentation pattern and vertical axis rotation are compared with observations of the planform shape of the chain topography crustal thickness distribution of rock exhumation and sediment thickness and paleomagnetic rotations thickening of the lithosphere in the alpine region is shown to be highly sensitive to the assumed viscosity law to the strength contrasts between the different regions and to the surface mass transport modeling results indicate that the large scale deformation of the alps during the postcollisional phase is mainly controlled by accommodation of convergence in a weak orogenic zone caught between a nearly rigid adria plate and a strong european foreland modeling of the present day stress field shows that 1 the present rotation of adria is responsible for the change of extension direction from strike perpendicular in the western alps to strike parallel in the east and 2 the strike perpendicular extension observed in the western alps is likely due to lateral variations of gravitational potential energy the results suggest a nne shift of about 700 km of the euler pole of adria relative to europe from its mean position during postcollisional deformation to the present day

teleseismic travel time anomalies and deep crustral structure in central australia summary teleseismic travel times have been recorded along two lines of instruments traversing the amadeus basin and southern arunta block of central australia for the purpose of examining the deep crustal and upper mantle structure underlying these features pronounced differential travel times approaching 1 5 s and with major azimuthal variation have been recorded most of the useful observations originated from events to the north japan marianas and to the east fiji tonga at epicentral distances of 40 65d a few observations of southerly azimuth events macquarie ridge and south sandwich islands were also recorded the inferred structure beneath the basin and block exhibits major lateral variations the essential aspects of which are 1 a zone of relatively high velocity material dipping steeply northwards such that the continuation of its southern edge to the surface coincides approximately with a mylonitic shear zone known as the redbank deformed zone 2 a steeply dipping zone of relatively low velocity material immediately to the south of this shear zone and partly underlying the high velocity zone if the travel times are interpreted in terms of an undulating moho variations in moho depth of 20 km occur over horizontal distances of less than 50 km with relatively shallow moho depths occurring immediately to the north of the redbank deformed zone such an interpretation is consistent with the gravity anomalies observed across the geological structures the inferred structure is out of hydrostatic equilibrium and departures from local isostasy approach 150 mpa these stress differences must have persisted during the 300 ma following the alice springs orogeny that led to the present crustal structure evidence in the form of the surface topography and seismicity suggests that some stress relaxation has occurred and still is occurring but not at the shear zone itself at the time of the orogeny the structure may have been close to isostatic equilibrium but as erosion took place complete local rebound did not occur either because horizontal compressive forces still operated or because the strength of the lithosphere increased with time

a model for the formation of intracratonic sag basins summary intracratonic sag sedimentary basins occur in the middle of stable continental or cratonic blocks they are rarely fault bounded although strike slip faulting can occur within them a simple model for the development and evolution of these basins is proposed the mechanism is driven by mildly coupled convective down welling of the asthensophere beneath the lithosphere initially a rapid alteration of the mantle convective system causes a descending plume to develop a depression which can be of the order of 600 m can be formed at the earth s surface this depression when loaded with sediment will form a sedimentary basin of the order of 2 5 km thick if the convective downwelling remains a period of thermal cooling of the lithosphere occurs which is similar to the thermal cooling subsidence phase of passive continental margins this thermal cooling occurs because of the thermal anomaly temperature decrease beneath the lithosphere caused by the convective downwelling if a change of convective pattern occurs and the descending plume is removed the basin will undergo uplift and erosion whereby a significant thickness of the sedimentary basin can be removed the ordovician and silurian tectonic development of the canning basin western australia appears to be well explained by this model

ground penetrating radar antennae frequencies and transmitter powers compared for penetration depth resolution and reflection continuity1 twelve ground penetrating radar gpr experiments were conducted on the modern wave influenced william river delta on the southern shore of lake athabasca in northern saskatchewan canada the delta is a well sorted quartzoserich clean sand dominated water saturated geomorphic feature which provided an ideal site to test gpr penetration depths resolution and continuity of reflections were compared for different antennae frequencies 25 50 100 200 mhz and transmitter powers pulser voltage 400 v 1000 v the data show significant variations in vertical resolution from 0 15 m to 0 76 m 200 25 mhz depth of penetration from 14 m 28 m 200 25 mhz and continuity of reflections increasing the transmitter power from 400 v to 1000 v increases the depth of penetration by 5 to 14 and improves the continuity of reflections with little effect on the resolution

why intracontinental basins subside longer 3 d feedback effects of lithospheric cooling and sedimentation on the flexural strength of the lithosphere

continuous and simultaneous measurement of reflector depth and average soil water content with multichannel ground penetrating radar ground penetrating radar is a fast noninvasive technique that can monitor subsurface structure and water content distribution to interpret traveltime information from single common offset measurements additional assumptions such as constant permittivity usually are required we present a fast ground penetrating radar measurement technique using a multiple transmitter and receiver setup to measure simultaneously the reflector depth and average soil water content it can be considered a moving minicommon midpoint measurement for a simple analysis we use a straightforward evaluation procedure that includes two traveltimes to the same reflector obtained from different antenna separations for a more accurate approach an inverse evaluation procedure is added using traveltimes obtained from all antenna separations at one position and its neighboring measurement locations the evaluation of a synthetic data set with a lateral variability in reflector depth and an experimental example with a large variability in soil water content are introduced to demonstrate the applicability for field scale measurements the crucial point for this application is the access to absolute traveltimes which are difficult to determine accurately from common offset measurements

geophysical evidence for the lateral distribution of free phase gas at the peat basin scale in a large northern peatland 1 northern peatlands contribute to the global carbon cycle by way of gas emissions in the form of biogenic carbon dioxide and methane originating in the subsurface determining how much gas is present and where it is trapped is essential to understanding the role of northern peatlands on the global carbon cycle particularly in regard to methane cycling in this study spatial variability in free phase gas content was estimated using ground penetrating radar gpr along a 1 4 km transect crossing distinct peat landforms including a bog crest midslope lawn and fen variations in gas content and distribution were observed in landforms crossed by this transect estimated gas content up to 25 by volume was observed in landforms dominated by woody surface vegetation in areas of the midslope lawn the estimated gas content was greater than 15 while estimated gas contents between 0 and 7 were found in the fen changes in estimated gas content of up to 20 were observed over distances of 100 m across a transition between a stand of 10 m tall trees and an adjacent fen the mean of the estimated errors in gas content is 1 3 suggesting that these large changes in estimated gas content between landforms can be clearly determined using geophysical methods a geophysical image of the distribution of free phase across this transect reveals evidence for gas storage in deep peat in intervals as thin as 0 35 m these data support conceptual models based on accumulation and storage of free phase gas in deep peat

ground penetrating radar derived measurements of active layer thickness on the landscape scale with sparse calibration at toolik and happy valley alaska active layer thickness alt is an important parameter for studying surface energy balance ecosystems and hydrologic processes in cold regions we measured alt along 10 routes with lengths ranging from 0 7 to 6 9 km located on the alaska north slope near toolik lake and the happy valley airstrip between 68 475 and 69 150 n and 149 512 and 148 769 e using a ground penetrating radar gpr system in a common offset configuration we measured the two way traveltimes from the surface to the bottom of the active layer at the end of summer when the thaw depth was greatest we used 500 and 800 mhz antennas the 500 mhz antenna provided suitable vertical resolution while producing more unambiguous active layer reflections in the presence of nonideal antenna coupling and active layer inhomogeneity we derived alt measurements and their uncertainties from gpr two way traveltimes with mechanical probing for velocity calibration using an empirical relationship between the wave velocity and soil volumetric water content vwc we found that the velocities were consistent with soil vwcs ranging from 0 46 to 0 63 in 31 of traces the permafrost table horizon was identifiable resulting in alt measurements with uncertainties of generally less than 25 the average alt was 48 1 cm with a standard deviation of 16 1 cm we found distinct patterns of alt spatial variability at different sites and different length scales at some sites the alt at one point was effectively uncorrelated with alt at other points separated by lag distances as small as tens of meters for other sites there was correlation at lag distances up to approximately 400 m the alt statistics were similar to nearby long term in situ alt measurements from the circumpolar active layer monitoring network through which yearly alt measurements have been made since 1990

ohmic dissipation in the earth s outer core resulting from the free inner core nutation abstract the diurnal tidal forces can excite a normal mode of the earth s core the free inner core nutation ficn which is characterized by a tilt of the rotation axis of the inner core with respect to the rotation axis of the outer core the differential rotation between the inner core and the outer core induces fluid motions in the outer core and gives rise to ohmic dissipation in the presence of the earth s internal magnetic field nutation measurements can reflect such dissipation if it is sufficiently strong and thus can provide insights into the properties and dynamics of the earth s core in this study we perform a set of numerical calculations of the linear perturbations in the outer core induced by the ficn at very low ekman numbers as small as 10 11 our numerical results show that the back reaction of the magnetic field notably alters the structure and length scale of the perturbations induced by the ficn and thus influences the ohmic dissipation resulting from the perturbations when the ekman number is sufficiently small ohmic dissipation tends to be insensitive to the fluid viscosity and to the magnetic diffusivity which allows us to estimate the ohmic dissipation associated with the ficn without relying on an extrapolation in contrast to the results of buffett 2010b the estimated ohmic dissipation based on our numerical calculations is too weak to account for the observed damping of the ficn mode this also implies that nutation measurements cannot provide effective constraints on the strength of the magnetic field inside the earth s outer core

controls on the locus of intraplate deformation in central australia abstract the locus of intraplate deformation in central australia changed from the presently preserved southern margin of the amadeus basin during the late neoproterozoic to early phanerozoic petermann orogeny to the northern margin in the late phanerozoic alice springs orogeny immediately prior to each event the thickness of the amadeus basin sediments varied from as little as 1 5 km in the central parts of the basin to as much as 7 km in the vicinity of the presently preserved margins of the basin with the locus of deformation mimicking the locus of maximum sedimentary thicknesses at the onset of each orogenic event as such these orogenies represent extreme examples of basin inversion a process observed in many intracratonic basins worldwide we show that differential burial of the central australian proterozoic basement complexes beneath the amadeus basin is capable of producing variations in moho temperature of up to 30 c prior to the petermann orogeny and up to 110 c prior to the alice springs orogeny for a brace goetze model of lithospheric rheology the variations in moho temperature equate to variations in effective lithospheric strain rate of 1 4 orders of magnitude implying that variations in thickness of the sedimentary blanket may have played a primary role in localising phanerozoic intraplate deformation in central australia an appealing aspect of this model lies in its corollary that the removal of the sedimentary blanket during denudation of the orogen will be accompanied by dramatic cooling and strengthening of the lithosphere this may provide a plausible explanation for the long term persistence of the extraordinary gravity anomalies 150 mgal developed during these central australian intraplate orogenies

ground penetrating radar reflection profiling of groundwater and bedrock in an area of discontinuous permafrost we have used ground penetrating radar to profile the depth of permafrost to groundwater beneath permafrost and to bedrock within permafrost in alluvial sediments of interior alaska we used well log data to aid the interpretations and to calculate dielectric permittivities for frozen and unfrozen materials interfaces between unfrozen and frozen sediments above permafrost were best resolved with wavelet bandwidths centered at and above 100 mhz the resolution also required consideration of antenna configuration season and surface conditions depths to subpermafrost groundwater were profiled where it was in continuous contact with the bottom of the permafrost except near transitions to unfrozen zones where the contact appeared to dip steeply the complexity of the responses to intrapermafrost bedrock detected at a maximum depth of 47 m appears to distinguish these events from those of subpermafrost saturated sediments the relative dielectric permittivity ranged between 4 4 and 8 3 for the permafro

three dimensional numerical simulations of crustal systems undergoing orogeny and subjected to surface processes as several modeling studies indicate the structural expression and dynamic behavior of orogenic mountain belts are dictated not only by their rheological properties or by far field tectonic motion but also by the efficiency of erosion and sedimentation acting on its surface until recently numerical investigations have been mainly limited to 2 d studies because of the high computational cost required by 3 d models here we have efficiently coupled the landscape evolution model cascade with the 3 d thermomechanically coupled tectonics code fantom details of the coupling algorithms between both codes are given we present results of numerical experiments designed to study the response of viscous plastic crustal materials subjected to convergence and to surface processes including both erosion and sedimentation in particular we focus on the equilibration of both the tectonic structures and on the surface morphology of the orogen we show that increasing the efficiency of fluvial erosion increases the frontal thrust angle which in turn decreases the width of the orogen in addition the maximum summit elevation of the orogen during transient evolution is significantly higher in those models showcasing surface processes than those that do not this illustrates the strong coupling between tectonics and surface processes we also demonstrate that an along strike gradient of erosion efficiency can have a major impact upon the landscape morphology and the tectonic structure and deformation of the orogen in both the across strike and along strike directions overall our results suggest that surface processes by enhancing localization of deformation can act as a positive forcing to topographic building

postextensional mantle healing and episodic extension in the canning basin regions of the lithosphere that undergo finite crustal extension but are subsequently allowed to thermally relax during a period of reduced tectonic activity may become regions of relative strength and therefore resist any further extension the strengthening is a consequence of the temperature dependence of rock rheology and the local thinning of the crust bringing strong upper mantle olivine rich rocks closer to the surface where they replace weaker lower crustal plagioclase feldspar amphibole bearing rocks this concept which i call postextensional mantle healing is illustrated by the predictions of a numerical model although the model is based on the results of rock testing in the laboratory the results do not depend on a particular choice of creep parameters for the lower crust the palaeozoic tectonic history of the canning basin northwestern australia is characterized by a series of extensional episodes separated by periods of relative quiescence in this paper i suggest that postextensional mantle healing is responsible for the continuous shifting through time of tectonic activity between the several major depocenters the tectonic development of the east african rift system follows a scenario similar to that of the canning basin extensional structures inherited from two bursts of extension in the early mesozoic and cretaceous have not been reactivated during the tertiary rifting which is responsible for the present day rift morphology

geophysical evidence for thick skinned crustal deformation in central australia until now there has been no clear evidence from deep seismic reflection profiles of crustal faults that displace the moho new data from central australia show such a structure in a region characterized by large thrust structures large gravity anomalies and abrupt changes in teleseismic travel times here the principal thrust the redbank deformed zone cuts through the crust and the moho is significantly displaced across it the geological and geophysical evidence leads to a model of thick skinned crustal deformation which occurred in an intra cratonic environment in late devonian time

quaternary sediment architecture in the orkhon valley central mongolia inferred from capacitive coupled resistivity and georadar measurements abstract detailed information on shallow sediment distribution in basins is required to achieve solutions for problems in quaternary geology geomorphology neotectonics geo archaeology and climatology usually detailed information is obtained by studying outcrops and shallow drillings unfortunately such data are often sparsely distributed and thus cannot characterise entire basins in detail therefore they are frequently combined with remote sensing methods to overcome this limitation remote sensing can cover entire basins but provides information of the land surface only geophysical methods can close the gap between detailed sequences of the shallow sediment inventory from drillings at a few spots and continuous surface information from remote sensing however their interpretation in terms of sediment types is often challenging especially if permafrost conditions complicate their interpretation here we present an approach for the joint interpretation of the geophysical methods ground penetrating radar gpr and capacitive coupled resistivity ccr drill core and remote sensing data the methods gpr and ccr were chosen because they allow relatively fast surveying and provide complementary information we apply the approach to the middle orkhon valley in central mongolia where fluvial alluvial and aeolian processes led to complex sediment architecture the gpr and ccr data measured on profiles with a total length of about 60 km indicate the presence of two distinct layers over the complete surveying area i a thawed layer at the surface and ii a frozen layer below in a first interpretation step we establish a geophysical classification by considering the geophysical signatures of both layers we use sedimentological information from core logs to relate the geophysical classes to sediment types this analysis reveals internal structures of orkhon river sediments such as channels and floodplain sediments we also distinguish alluvial fan deposits and aeolian sediments by their distinct geophysical signature with this procedure we map aeolian sediments debris flow sediments floodplains and channel sediments along the measured profiles in the entire basin we show that the joint interpretation of drillings and geophysical profile measurements matches the information from remote sensing data i e the sediment architecture of vast areas can be characterised by combining these techniques the method presented here proves powerful for characterising large areas with minimal effort and can be applied to similar settings

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

multiple post svecofennian 1750 1560 ma pegmatite dykes in archaean palaeoproterozoic rocks of the west troms basement complex north norway geological significance and regional implications abstract the west troms basement complex north norway consist of meosarchaean neoarchaean and palaeoproterozoic crust which likely represent the continuation of the karelian craton of the fennoscandian shield beneath and across the caledonian nappes the complex is transected by several meta sedimentary volcanic belts with ages from ca 2 7 to 1 9 ga and linear ductile shear zones that record polyphase deformation after emplacement of several late svecofennian plutonic complexes 1 80 1 79 ga the astridal supracrustal belt of the senja shear zone was intruded by various generations of granite pegmatite dykes which yield zircon and titanite u pb ages ranging between 1758 and 1562 ma with evidence of a weak caledonian overprint at about 400 ma that formed or reset some but not all titanites the age range indicates that formation of the pegmatites occurred after termination of the main svecofennian orogeny 1 92 1 79 ga of the karelian craton comparison within the larger scale context of the fennoscandian shield suggests that emplacement of these pegmatites may have been caused by late stage reactivation of intracratonic shear zones possibly reflecting distal effects of deeper magmatic processes such as intrusion of coeval aged rapakivi granite suites ca 1 65 1 5 ga in finland and or the mesoproterozoic gothian orogeny and magmatism affecting the southwestern parts of fennoscandia the mesoproterozoic ages however also correspond to late laxfordian ca 1 7 1 6 ga tectonic events in the lewisian basement on the laurentian margin of scotland suggesting a potential early link between these basement complexes

development of the late proterozoic to mid paleozoic intracratonic amadeus basin in central australia a key to understanding tectonic forces in plate interiors the intracratonic amadeus basin in central australia is a complex composite basin covering 17 000 km with at least nine distinct episodes of evolution between 900 and 300 ma these have been identified in neighboring central australian basins and are characterized by intervals of renewed subsidence followed by intervals of erosion and changes in basin shape several unconformities separating the tectonostratigraphic sequences represent periods of mild regional uplifts and correlate with major episodes of compressive tectonism at the evolving margin of the early australian plate in the absence of direct geological evidence for thermal events in the amadeus basin after the initial period of subsidence ended possibly at about 800 ma the correlations found between events in the amadeus basin those in the neighboring interconnected basins and tectonic events at the continental or plate margins emphasize that regional horizontal stress fields contribute significantly to primary basin forming mechanisms this suggests that stresses generated at continental or plate margins may propagate to the plate interior with the lithosphere acting as a stress guide throughout the history of the amadeus basin both periods when compressional stresses were dominant and when extensional tectonics controlled subsidence can be identified there were also transitional periods when neither compression nor extension dominated so that other processes masked the effects of weak horizontal stresses the most recent compressional thrust belt concentrated at the northern margin between 300 and 400 ma and has partly obscured the record of earlier basin shape and size

rheology of some continental lower crustal rocks abstract to gain insight into the rheology of selected feldspar bearing rocks from the continental lower crust clc three granulites from exposed lower crustal terrains and a microgabbro have been deformed to ca 10 strain at confining pressures of 0 8 1 0 gpa temperatures of 600 900 c and constant strain rates of 10 4 10 7 s 1 the most extensive work was done on the mafic pikwitonei and felsic adirondack granulites the pikwitonei mg hornblende plag an70 2 px has only a limited t range of apparent steady state behavior and strain softens at t 800 c at 10 5 10 6s 1 apparently associated with developing shear zones this strain softening is pressure insensitive from pc 0 8 1 0 gpa at t 850 c 10 6s 1 even though the dominant strain accommodation mechanisms change from semibrittle cataclastic to crystal plastic including dynamic recrystallization with increasing pressure plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs kapuskasing plag cpx gt granulite deformed at 800 c and at 850 c 10 6 s 1 has deformed in the steady state at stress levels only slightly greater than the pikwitonei the stillwater microgabbro and adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady state behavior over the entire range of conditions investigated for the adirondack 2 felds 2 px gt qtz granulite quartz 21 appears to control the creep rate at these low strains and steady state mechanical data fit well a creep power law with a 8 10 3 mpa ns 1 qc 243 kj mol and n 3 1 extrapolation of this flow law and of preliminary power law creep relations for the pikwitonei and microgabbro to a range of physical conditions assumed to encompass those at the continental moho supports geological and geophysical inferences that a mechanical discontinuity occurs at this transition zone with the lower crust generally being the weaker material in addition the stain softening exhibited by the pikwitonei may bear directly on the development of detachment zones at mid to lower crustal depths during prograde metamorphism

constraints on magnetic energy and mantle conductivity from the forced nutations of the earth observations of the earth s nutations are used to explore the possible presence of a conducting layer at the base of the mantle the existence of such a layer is suggested by recent experiments on iron and silicate mixtures at high pressures and temperatures knittle and jeanloz 1986 1989 which indicate that iron may react with silicate minerals to produce metallic compounds nutations provide an ideal tool for investigating the existence of such a layer because they are sensitive to the electrical properties within a thin magnetic boundary layer along the core mantle boundary cmb evidence in favor of a conducting layer is found in the effects of ohmic dissipation which cause the amplitude of the earth s nutation to be out of phase with tidal forcings although out of phase components in the earth s nutations are conventionally attributed to the dissipative influences of ocean tides and mantle anelasticity these effects appear incapable of explaining nutation amplitudes determined from very long baseline interferometry vlbi observations the additional effects of ohmic dissipation enhanced by the presence of a thin conducting layer at the cmb are sufficient to reconcile theory and observation the largest mismatch between observations and theory taking account of ocean tides and mantle anelasticity is a discrepancy of 0 39 0 04 milliarcseconds in the out of phase component of the retrograde annual nutation this discrepancy can be eliminated if the lowermost 200 m of the mantle has a conductivity of 5 105s m 1 and the magnetic energy in the spherical harmonic components of the field at the cmb in degrees l 12 is approximately 4 times greater than the energy inferred in degrees l 12 from surface observations unfortunately the vlbi observations do not uniquely resolve the mantle conductivity and magnetic energy so that trade offs exist between these two parameters nevertheless a high mantle conductivity is favored on the grounds that the alternative involving high levels of magnetic energy yields an excessive amount of ohmic heating other interpretations of the vlbi discrepancy are also discussed one such alternative involves viscous dissipation at the cmb requiring a kinematic viscosity of 0 1m2s 1

a theory for plastic deformation and textural evolution of olivine polycrystals seismic anisotropy in the upper mantle is due primarily to preferred orientation of olivine crystals induced by progressive deformation in order to understand better the origin of seismic anisotropy we present a simple theory for plastic deformation and textural evolution of olivine polycrystals each crystal in the aggregate is assumed to deform by intracrystalline slip on three major slip systems whose hardnesses and stress exponents are known from experiments the evolving grain orientation distribution in the aggregate is calculated by minimizing the difference between the local crystal deformation and the global aggregate deformation subject to the constraint of global strain compatibility the axial compression texture predicted by our model agrees well to first order with that determined experimentally by nicolas et al 1973 although there are significant discrepancies in the details our results suggest that the orientation texture developed during progressive plane strain deformation is a nearly unique function of the finite strain such that the crystallographic axes 100 010 and 001 are concentrated around the finite strain axes a c and b respectively a b c this result may allow the state of finite strain at depth to be estimated from observations of seismic anisotropy

palaeozoic arc growth deformation and migration across the lachlan fold belt southeastern australia abstract the palaeozoic lachlan fold belt lfb is a low pressure metamorphic belt characterized by greenschist facies rocks and infolded volcanic sequences which indicate that the belt has not undergone substantial uplift at any stage porphyroblast matrix relationships in one of the highest grade metamorphic zones of the lfb the omeo complex indicate that the thermal peak was reached before or was synchronous with the earliest deformation similar to that in high grade metamorphic zones of the northern arunta inlier central australia in both areas the metamorphism is of low p high t type centred on major granite intrusions as regional aureoles and characterized by anti clockwise p t t paths however in the outer lower grade parts of the aureoles peak metamorphic conditions post date the earliest deformation this diachronous relationship indicates that 1 outward migration of a thermal front was centred on major granitoid intrusions batholiths 2 deformation propagated outward more rapidly than migration of the thermal front and 3 plastic and ductile deformation began only after heating around the batholiths thus orogeny was initiated after and localized by heat focusing in the mid crust associated with batholith emplacement therefore the early deformation is a result of thermal softening it is typically subhorizontal ductile shear at mid crustal levels but is characterized by upright to inclined folds at upper crustal levels in the latter environment the typically discordant contact aureole plutons are often interpreted as post tectonic granitoids but they were part of the ongoing mid crustal thermal perturbation that induced regional greenschist facies metamorphism which overprinted the early formed upright folds as it migrated to upper crustal levels orogeny migrated generally eastward through the lfb it began with the early silurian benambran orogeny centred on the wagga metamorphic belt wmb and terminated with the carboniferous kanimblan orogeny which was most intense in the hill end trough eastward migration of orogeny across central victoria part of the west lfb began in the ballarat bendigo zone during the early devonian and terminated in the melbourne zone in the middle devonian orogeny was centred on the meridional batholiths of the lfb each of which are considered to represent the transitory axis of an ancient magmatic arc stepwise but generally eastward arc migration caused the eastward migration of orogeny a tectonic model based on the modern southwest pacific arc system can be applied to the lfb following cambrian intra oceanic arc growth associated with west dipping subduction an ordovician marginal sea developed partly on stretched cambrian crust and was flooded by an extensive turbiditic wedge closure of the eastern part the east lfb began in the early silurian after a magmatic arc developed over the wmb possibly associated with an east dipping subduction zone after re establishment of west dipping subduction in the middle silurian absolute motion retreat of the upper australian plate caused by oblique plate convergence resulted in dextral transtension and the generation of siluro devonian inter arc and back arc basins transient compression resulted in deformation along the magmatic arc and induced formation of a new outboard eastward arc system periodic compression within the evolving arc and back arc system resulted in a series of east younging east verging linear siluro devonian fold and thrust belts which were localized adjacent to elongate batholiths that represent the relict arc systems local westward jumps in deformation and plutonism occurred the most significant is the 600 km westward jump into the west lfb caused by closure of the remainder of the passive ordovician back arc basin melbourne and bendigo ballarat zones west dipping subduction beneath the basin produced a low pressure fold thrust belt similar to the east lfb though the granitoids have a more primitive isotopic character indicating closure of thin dominantly oceanic crust in the early to middle devonian localized deformation induced by thermal softening of the crust within magmatic arcs that have developed on earlier passive margins is likely to produce anticlockwise p 7 t paths and may be a typical feature of southwest pacific style tectonism

assessing the geochemical and tectonic impacts of fluid rock interaction in mid crustal shear zones a case study from the intracontinental alice springs orogen central australia the reynolds anmatjira ranges central australia form part of a high grade basement terrane dissected by intensely metasomatized transpressional shear zones active during the ordovician carboniferous alice springs orogeny unlike typical retrograde structures associated with discrete fluid flow the mid crustal setting and intracontinental nature of these shear systems present significant problems for the source and ingress mechanism of the fluid involved in their rehydration to address these issues we describe two detailed traverses across deformed and metasomatized basement rocks in this region and interrogate their record of fluid rock interaction from various perspectives both traverses combine structural and petrological observations with zr in rutile and ti in quartz thermobarometry oxygen and hydrogen stable isotope analysis and major trace and rare earth element mobility trends each technique is critically evaluated for its utility in this study and its more widespread applicability to alternative field areas providing a strategic framework for the general investigation of fluid affected shear zones ultimately the integrated data sets specify pressure temperature conditions of 530 c and 4 5 kbar implying average apparent thermal gradients of 29 36 c km 1 and depths of 14 18 km other characteristic features to emerge include strongly variable element mobilities and pronounced isotopic depletion fronts consistent with the alteration effects of an externally derived non equilibrium fluid this is confirmed by calculated fluid compositions indicative of contributions from a fluid of meteoric origin with estimated 18o and d values as low as 2 3 and 59 8 respectively we propose that these surficial fluid signatures are imposed on the mid crust by the prograde burial and dehydration of hydrothermally altered fault panels produced during pre orogenic basin formation progressive fluid release with continued subsidence then leads to the accumulation of increasing fluid volumes in the vicinity of the brittle ductile transition promoting extensive hydration metasomatism and reaction softening at the locus of stress transmission from plate boundary sources the sustained injection of externally derived fluids into refractory crustal material may thus stimulate a critical reduction in the long term strength of the lithosphere providing strong impetus for the initiation and advancement of intracontinental orogenesis

variations in free phase gases in peat landforms determined by ground penetrating radar 1 the spatial variability of biogenic gas produced by methanogenic archaea is difficult to assess within saturated peat soils and is often poorly quantified this study uses ground penetrating radar to noninvasively estimate the vertical distribution of biogenic free phase gas fpg in two distinct peat landform types in the glacial lake agassiz peatland minnesota a near crest bog and a midslope lawn i e both as defined by surface vegetation communities ground penetrating radar velocities retrieved from common midpoint surveys were modeled using the complex refractive index model to estimate free phase gas volumes along one dimensional vertical profiles near crest bog landforms are characterized by vertical variability in gas content with accumulations along the peat column up to 24 by volume of fpg localized within the deep peat e g 2 4 m depth midslope lawn sites show lower total volumes up to 12 free phase gas and less variability that result in a more even free phase gas distribution throughout the vertical profile high resolution data also suggests the presence of thinner layers 1 m containing up to 40 free phase gas at a near crest site our results suggest that spatial distribution of free phase gas along the peat column may vary in the glacial lake agassiz peatlands depending on peat landform statistical analysis supports vertical variability of ground penetrating radar velocity data with a 95 confidence limit our results could potentially be upscaled using surface vegetation communities to estimate landscape scale gas storage potential in peat landforms

chronology and tectonic framework of turbidite hosted gold deposits in the western lachlan fold belt victoria results abstract 40 ar 39 ar data constrain the history of metamorphism deformation and mineralization in the western subprovince of the lachlan fold belt rocks in the mount stavely volcanic complex and the bushy creek pluton give 40 ar 39 ar ages of about 500 ma for hornblende and biotite the dates from the mount stavely volcanic complex limit the age of mineralization in the metavolcanic rocks of the stavely belt whereas the date for the bushy creek pluton extends the area of cambrian ordovician plutons of the delamerian orogeny to east of the grampians mountains and helps define the surface expression of the western border of the paleozoic lachlan fold belt 40 ar 39 ar dating of metamorphic mica growth phengite in cleaved slates and phyllites within the landsborough fault avoca fault zone heathcote fault zone and mount wellington fault zone gives ages of 453 2 440 2 426 4 and 410 390 ma respectively these data indicate a progression of deformation from west to east starting in late ordovician early silurian time in the stawell and bendigo ballarat structural zones the major phase of deformation in the western subprovince of the lachlan fold belt is therefore silurian and not devonian sericites from major gold deposits in the stawell zone and bendigo ballarat zone give 40 ar 39 ar dates of about 440 ma that are coincident with folding and thrusting a second episode of mineralization and fault reactivation at about 420 to 410 ma is found in some deposits in these zones based on data from a previous study in the melbourne zone including the woods point dike swarm and locally in the bendigo ballarat zone some gold deposits give sericite dates of about 380 360 ma these ages are consistent with the close association of many melbourne zone gold deposits with devonian dikes and plutons the results indicate that gold mineralization in the stawell zone bendigo ballarat zone and melbourne zone was episodic and both temporally and spatially associated with crustal heating linked to regional metamorphism and plutonism initial stages of mineralization in the province comprising many of the large deposits in western and central victoria e g bendigo ballarat stawell wattle gully tarnagulla accompanied metamorphism folding and thrusting subsequent mineralization is widespread but produced mainly smaller deposits

thin sheet modelling of lithospheric deformation and surface mass transport we study the effects of incorporating surface mass transport and the gravitational potential energy of both crust and lithospheric mantle to the viscous thin sheet approach recent 2d cross section numerical models show that surface erosion and sediment transport can play a major role in shaping the large scale deformation of the crust in order to study these effects in 3d planform view we develop a numerical model in which both the dynamics of lithospheric deformation and surface processes are fully coupled deformation is calculated as a thin viscous layer with a vertically averaged rheology and subjected to plane stresses the coupled system of equations for momentum and energy conservation is solved numerically this model accounts for the isostatic and potential energy effects due to crustal and lithospheric thickness variations the results show that the variations of gravitational potential energy due to the lateral changes of the lithosphere asthenosphere boundary can modify the mode of deformation of the lithosphere surface processes incorporated to the model via a diffusive transport equation rather than just passively reacting to changes in topography play an active role in controlling the lateral variations of the effective viscosity and hence of the deformation of the lithosphere d 2005 elsevier b v all rights reserved

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

tectonic evolution of proterozoic australia proterozoic australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic this paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes the assembly was completed between 1300 and 1100 ma when the crustal fragments were combined as an early component of the rodinian supercontinent rifting and fragmentation of archaean continents began in the late archaean and continued into the proterozoic passive margin deposits such as those of the hamersley basin accumulated on isolated fragments of archaean crust these numerous fragments were subsequently assembled into three cratons by 1830 ma a west australian craton was established by collision of the archaean pilbara and yilgarn cratons which were joined along the capricorn orogen similarly a south australian craton developed by amalgamation of the proto gawler and proto curnamona cratons along the kimban orogen a north australian craton appears to have formed by accretion of numerous crustal fragments including the kimberley pine creek lucas and altjawarra cratons with sutures marked by the king leopold halls creek tennant creek and proto isan orogens the southern margin of the north australian craton was the site of repeated terrane accretion and orogenic activity between 1880 ma and 1400 ma this included an orogenic event at 1880 1850 ma the strangways 1780 1730 ma argilke 1680 1650 ma and chewings 1620 1580 ma orogenies and the intracratonic anmatjira uplift 1500 1400 ma intracratonic rifting at 1750 to 1710 ma and 1640 to 1600 ma produced the mcarthur basin and related minor basins parts of which were deformed by the isan orogeny at 1600 and 1530 ma rifting along the line of the capricorn orogen led to deposition in the overlying intracratonic bangemall basin between 1630 and 1300 ma along the eastern margin of the south australian craton the 1670 to 1600 ma olarian orogeny marks interaction with now obscured continental crust to the east tectonic activity between 1300 and 1100 ma led to the assembly of proterozoic australia as an early component of the supercontinent of rodinia this first involved the amalgamation of the west australian and north australian cratons followed by collision with the south australian craton the centralian superbasin developed over the junction of the north south and west australian cratons between 830 and 750 ma rifting to the east formed the adelaide geosyncline at 830 ma this was followed by the breakup of rodinia with the rifting apart of laurentia and gondwanaland along the eastern margin of proterozoic australia at 750 ma and the subsequent formation of the palaeo pacific ocean after the breakup of rodinia a series of northeast southwest compressional events followed by periods of relaxation reflect the assembly of a new supercontinent old lines of weakness were reactivated culminating in the intracratonic king leopold paterson petermann ranges and pinjarra orogenies between 620 and 540 ma subsequent reactivation continued into the phanerozoic with the widespread eruption of continental flood basalts and the formation of intracratonic basins 540 530 ma

intraplate deformation in central australia the link between subsidence and fault reactivation abstract central australia has experienced two intraplate orogenic events involving significant north south shortening the late neoproterozoic to early cambrian petermann orogeny and the devonian to carboniferous alice springs orogeny in each event pre existing structures inherited from mesoproterozoic terrain amalgamation were reactivated and basement rocks exhumed from beneath thick sedimentary successions accumulated in the centralian superbasin the pattern of fault reactivation during these events shows a striking similarity to the pattern of subsidence in the overlying basin immediately prior to the petermann orogeny the centralian superbasin was thickest in the vicinity of the musgrave block the region in which deformation was subsequently localised at the same time crustal scale faults elsewhere in central australia that were covered by a relatively thin sheet of sediment remained inactive despite being favourably oriented to accommodate the north south shortening between the petermann and alice springs orogenies subsidence patterns shifted such that fault systems in the arunta block and also those in the southern musgrave block were buried by significant thicknesses of sediment whereas the major structures that were exhumed during the petermann orogeny were not significantly buried during the alice springs orogeny reactivation once again occurred along the most deeply buried faults even in the instances where those faults had remained inactive during the earlier petermann orogeny importantly the major petermann aged structures that were not buried during renewed subsidence remained inactive during the alice springs orogeny the record of reactivation implies that the presence of pre existing crustal scale faults alone was insufficient to localise deformation rather fault reactivation appears to have required a priming process that modulated the strength of the lithosphere on a regional scale the correspondence between the distribution of basement fault reactivation and subsidence patterns during both the petermann and alice springs orogenies implies a link between relatively thick sedimentation and long term lithospheric weakening we show that this link is compatible with the thermal effects of a thick sedimentary blanket in the context of central australia the mechanical impact of basin formation is likely to be enhanced by the presence of regionally elevated heat production in the proterozoic basement

gas bubble size estimation in peat soils from em wave scattering observed with ground penetrating radar the size of biogenic gas bubbles in peatlands is believed to regulate ebullition of carbon gases to the atmosphere the measurement of electromagnetic em wave travel times using ground penetrating radar gpr is a proven field scale method for indirect estimation of volumetric gas content however there is also the possibility that information on the size of the gas bubbles can be determined from the analysis of the spectral content of gpr signals as scattering attenuation possesses a frequency dependence for bubbles smaller than the em wavelength rayleigh type scattering theoretical modeling shows that gpr data acquired with typical antenna frequencies are likely to be affected by bubble size in peat soils analysis of gpr data from two recent studies on peat monoliths where biogenic gas production was documented produced results consistent with the model predictions using the approach zero offset cross borehole gpr data in a northern peatland suggest that large bubble clusters i e 0 05 m radius occur in peat these findings broaden the utility of gpr for providing information on biogenic gas dynamics in peatlands

velocity variations and water content estimated from multi offset ground penetrating radar the common midpoint cmp processing technique has been shown to be effective in improving the results of ground penetrating radar gpr profiling when radar data are collected with the cmp multioffset geometry stacking increases the signal to noise ratio of subsurface radar reflections and results in an improved subsurface image an important aspect of cmp processing is normal moveout velocity analysis our objectives are to show the effect of multiple velocity analyses on the stacked radar image and particularly to demonstrate that this velocity information can also be used to determine subsurface water content most gpr surveys are very limited in spatial extent and assume that within the survey range radar velocity structure in the shallow subsurface can be adequately approximated by a single velocity function in data processing in this study we show that variation in radar velocity can be quite significant and that the stacked profile improves as the number of velocity analysis locations is increased interval velocities can be calculated from the normal moveout velocities derived in the cmp velocity analysis with some reasonable assumptions about subsurface conditions necessary for radar propagation interval velocity can be converted to an estimate of volumetric water content therefore by collecting gpr data in the multioffset cmp geometry not only is the radar profile improved but it also allows for an interpretation of subsurface variation in water content we show the application of these techniques to multioffset gpr data from the chalk river test area operated by atomic energy of canada limited

atmospheric pressure drives changes in the vertical distribution of biogenic free phase gas in a northern peatland 1 atmospheric pressure patm is known to regulate methane emissions from northern peatlands however recent conceptual models differ in how gas production and release occurs in shallow defined here as less than 1 m depth versus intermediate to deep peat soils i e more than 1 m depth we used ground penetrating radar gpr measurements to non invasively estimate the vertical distribution of free phase gas and the dependence of this distribution on atmospheric pressure in a northern peatland variations in the travel time of the electromagnetic wave to three interfaces in the peat column were used in conjunction with deformation rod data and a petrophysical model relating electromagnetic wave velocity to free phase gas content to model changes in the vertical distribution of free phase gas over time we found a negative linear relation between changes in free phase gas content and changes in patm for shallow peat soils and a positive linear relation for deeper soils our results suggest that 1 free phase gas content confined in deep peat soils is larger and less variable to changes in patm than gas in shallow intermediate peat soils 2 increases in patm result in gas release from shallow peat soils into the atmosphere i e rapid ebullition and 3 decreases in patm result in upward gas movement from intermediate layers to replenish shallow layers our results suggest that changes in patm drive changes in the vertical distribution of free phase gas in peat soil and regulate methane ebullition from peat soils to the atmosphere our data shows a relationship between free phase gas and depth that may be due to changes in peat properties or increasing water pressure with depth

evolution of biogenic gases in peat blocks inferred from noninvasive dielectric permittivity measurements 1 peatlands contain methanogenic archea responsible for generating significant amounts of free phase biogenic gases for example methane and carbon dioxide but considerable uncertainty still exists regarding the mechanisms of formation and spatial distribution of these gases within the soil matrix we demonstrate the effectiveness of a new method to record noninvasively the evolution spatial distribution and emission patterns of biogenic gases in a peat soil a peat block 0 022 m3 was extracted from a large freshwater peatland in maine the bulk dielectric permittivity at 1 2 ghz for multiple slices of the block was measured noninvasively 1 as temperature was increased 2 c d 1 from 5 c to 21 c and 2 for a subsequent 2 month period during which temperature was held constant at 21 1 c methane emissions at the surface and peat surface deformation were monitored concurrently using a portable methane detector and a grid of surface elevation rods respectively our results demonstrate that 1 the measurement of electromagnetic wave traveltimes across a peat block offers a unique and more accurate when compared to surface deformation measurements way to monitor gasdynamics and spatial gas distribution within a peat block without any disturbance to the natural gas regime 2 the ebullition under our experimental conditions seems to preferentially occur from the near surface peat and shows some correspondence with changes in atmospheric pressure and 3 the ebullition flux exhibits periodicity suggesting that it may be predictable and quantifiable which could assist climate modeling efforts our findings are consistent with previous studies based on gasdynamics in peat soils including gas volumes and fluxes associated with biogenic gas ebullition

application of ground penetrating radar to measure near saturation soil water content in peat soils 1 the presence and flux of biogenic methane rich free phase gas that is eventually released to the atmosphere influence water content of peat soils below the water table small variations in gas content in peat soils at near saturation could be inferred by changes in dielectric permittivity but detailed measurements in that range of needed to develop calibration functions are lacking our experiment uses a new method for varying in the sample using elevated pressure to reduce the naturally occurring volumetric gas content in a manner similar to what occurs in situ under atmospheric pressure change which relevant to understanding carbon gas cycling in peatlands we recorded dielectric permittivity using a 1 6 mhz ground penetrating radar antenna at multiple water contents between 0 87 and 0 95 m3 m 3 on four peat monoliths with varying levels of humification and with 5 gas content as is commonly observed in the field we identified empirical equations that were linear over the range of investigated and optimized a dielectric mixing model for estimating from gpr these results indicate that there are differences in the permittivity relationships developed between peat samples and suggest that variability in dielectric relationships may be attributed to peat structure and particle orientation the empirical relationships developed in this study confirm the utility of previously applied multiphase mixing models for estimating peat properties using methods sensitive to dielectric permittivity this work is relevant to studies of gas content in peat and the role of peatlands in the carbon cycle

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

intracontinental orogeny enhanced by far field extension and local weak crust the accommodation of intraplate stresses in preexisting weak regions of plate interiors is here investigated using thin viscous sheet numerical models the intraplate stresses are governed by multicomponent and multidirectional stresses originating at plate boundaries the modeled scenarios mimic plate boundary conditions during the intraplate alice springs orogeny aso central australia and include 1 a northwest southeast zone of weak lithosphere within strong continental blocks to the north and southand 2 a principal south directed stress condition at the northern boundary that causes minor clockwise rotation of the northern block alternative tectonic environments are investigated in additional models that include 1 secondary compressional or extensional stresses acting at the eastern boundary representing the temporally variable stress conditions during the tasmanides orogeny and 2 an eastern wedge shaped zone of rheologically weak lithosphere mirroring rift fill of the irindina subbasin our results highlight that a low angle between major crustal features e g orogenic elongation and preexisting regional structures and the principal transmitted stresses is highly relevant in the concentration of elevated levels of differential stress and subsequent localization of deformation in plate interiors secondary stresses orthogonal to the principal acting stresses may introduce effects that explain the episodic orogenic activity in the case of the aso the combination of secondary extensional stresses at the eastern boundary of australia and weak lithosphere of the preexisting irindina subbasin strongly influences the observed spatial strain intensity localization and kinematics of deformation during the aso

length scales for continental deformation in convergent divergent and strike slip environments analytical and approximate solutions for a thin viscous sheet model the deformation of a thin viscous layer that has a moving boundary is investigated for comparison with zones of deformation in the continental lithosphere exact analytical solutions for the case of a newtonian fluid and approximate solutions for the case of fluid with power law rheology show that when the imposed velocity vector is normal to the boundary congressional or extensional regime the deformation field decays away from the boundary with a characteristic length scale 1 3 to 1 10 the wavelength of the imposed boundary velocity distribution for n between 1 and 10 where n is the stress strain exponent in the rheology in contrast when the imposed velocity vector is parallel to the boundary transcurrent regime the length scale of the deformation field is approximately 4 times smaller in each case these length scales decrease approximately as n the difference in length scales arises even in the absence of any buoyancy forces acting on thickened or thinned crust such forces would modify the ratio of length scales but not sufficiently to affect this result

thermal contraction and flexure of intracratonal basins a three dimensional study of the michigan basin summary cooling of continental lithosphere by thermal conduction to the surface following a heating event imposes a load on the lithosphere and causes subsidence as the basement rocks contract accumulation of sediments in the resulting depression forms a sedimentary basin studies of the geometry of sedimentary basins with horizontal dimensions of a few hundred kilometres suggests the lithosphere responds to loads by regional flexure of a strong elastic or viscoelastic upper lithosphere two and three dimensional models for flexure of the lithosphere owing to thermal contraction are applied to the michigan basin 2 d flexural models are compatible with previously estimated rheological parameters for an elastic lithosphere an effective flexural rigidity of about 5 1029 dyne cm can explain observed gravity anomalies and structural contours across the michigan basin a viscoelastic lithosphere requires an effective flexural rigidity of about 1032 dyne cm for a viscosity of 1025 poise for a 3 d elastic or viscoelastic lithosphere effective flexural rigidity must be approximately an order of magnitude smaller 2 1028 and 1031 dyne cm respectively larger values leave an excessive remaining load at the centre of the basin which is not indicated by observed gravity anomalies the decrease in effective flexural rigidity is attributed to greater difficulty in equi dimensional bending of a 3 d plate the magnitude and spatial distribution of the computed thermal contraction load are consistent with the presumed mechanism as the thermal anomaly of the initial heating event was present mainly at great depths in the lithosphere the lack of geological evidence for an initial heating event prior to subsidence is not a fatal objection to the thermal contraction mechanism of subsidence a driving load presumably from subsurface processes including crustal stretching and emplacement of dense rocks into the crust is necessary to explain the net subsidence of the basin from the time before the heating event until the lithosphere beneath the basin cooled without this driving load the thermal expansion would produce uplift but the surface would subside only back to sea level after the lithosphere cooled theoretical gravity results indicate that the driving load is centred at a depth of approximately 15 km deviations in subsidence curves from exponentials associated with thermal contraction can be explained by changes in sediment supply spatial variations in sediment load caused by regional facies changes produce migration of the centre of maximum deposition water and basement depths are determined for sequential time intervals during basin development the predicted depositional environments are consistent with lithofacies maps of the middle devonian

chemical stratification at the top of earth s core constraints from observations of nutations abstract dissipation due to tidally driven flow in the earth s liquid core is detected in observations of nutations one source of dissipation is due to electromagnetic core mantle coupling but this mechanism requires a high electrical conductivity on the mantle side of the boundary and a strong radial magnetic field an alternative mechanism is viable in the presence of fluid stratification at the top of the core stratification causes the fluid close to the core mantle boundary to be trapped by the effects of topography further from the boundary the stratified fluid is swept past the mantle with the underlying tidal flow shear in the flow induces electric currents where the fluid is permeated with a radial magnetic field the resulting dissipation is only weakly dependent on the electrical conductivity of the mantle and the required strength of the radial magnetic field can be lowered for a representative calculation we explain the observed dissipation with a radial field of 0 5 mt and a mantle conductivity of 1000 s m provided the buoyancy frequency in the stratified layer is 0 09 s 1 such a strong stratification has recently been proposed on the basis of chemical interactions between the core and the mantle nutation observations support the presence of stratification to the extent that the resulting dissipation mechanism is more compatible with conventional estimates for the radial magnetic field and the mantle conductivity

initiation and propagation of shear zones in a heterogeneous continental lithosphere continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities such a deformation pattern is displayed by the borborema shear zone system in northeastern brazil it involves continental scale curvilinear e w trending right lateral transcurrent shear zones that branch off from a major ne trending right lateral strike slip deformation zone formed at the northern termination of the sao francisco craton and that finally terminate in transpressive metasedimentary belts we suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain craton and rheologically weaker domains basins the effect of a low viscosity domain on the deformation of a continental plate and in particular the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling the low strength domain induces an enhanced strain localization and the geometry of the shear zone is significantly modified it is either split forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low viscosity domain or completely rotated toward the weak block the perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation which is controlled by its initial viscosity contrast relative to the surrounding lithosphere its orientation relative to the convergence direction and its distance from the shear zone initiated at the termination of the stiff block the interaction between imposed boundary conditions tectonic forces and plate geometry and the internal structure of the plate may give rise to highly heterogeneous strain fields as exhibited by the borborema shear zone system in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones a heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity with shear zones bordering nearly undeformed blocks and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories

the role of compressive forces in intracratonic basin formation and mid plate orogenies lithospheric buckling due to compressive forces requires excessively large stresses with some modifications of the elastic plate buckling problem significant deformation can occur at compressive stresses of a few kbars an order of magnitude less than required to induce elastic buckling three modifications are proposed i the plate is considered as an effective viscoelastic layer subjected to an initial perturbation ii when a compressive force is applied these perturbations are magnified and then grow with time iii uplifted areas are eroded and sediments are deposited in the downwarped areas this mechanism appears to be relevant to the formation of some intracratonic basins such as those in central australia the mechanism can lead to significant orogenies away from plate boundaries such as the petermann ranges orogeny and the alice springs orogeny of central australia

short pulse radar wavelet recovery and resolution of dielectric contrasts within englacial and basal ice of matanuska glacier alaska u s a wavelets transmitted by short pulse radar are recovered from continuous profiles and used to determine interfacial dielectric contrasts within the englacial and basal ice at the terminus area of matanuska glacier alaska u s a the field studies were in the ablation region where radar horizons could at some point be identified with interfaces between clear ice and air water or basal ice and were performed in early spring before drainage fully developed the profiles usedclosely spaced antennas with bandwidths centered near 50 and 400 mhz transmitted wavelets reflected from interfaces of known dielectric contrasts are used to establish a phase reference for other events from interfaces between unknown contrasts migration and fourier transform filtering are then applied to the profiles and shown to recover these wavelets from diffractions and reflections interfacial dielectric contrasts are determined from the relative phase of the wavelets near the terminus lake some basal ice events above 8 m depth are interpreted as voids further up glacier most englacial events are interpreted as voids but deeper localized reflectors and horizons to 90 m depth and within the basal zone are interpreted as voids water or debris phase cannot be determined for the basal substrate transition reflections recommendations are made for improving the wavelet recovery process and the quality of gpr migration

convergence and intraplate deformation in the lachlan fold belt of southeastern australia abstract the lachlan fold belt of southeastern australia is dominated by a widespread oceanic association including cambrian submarine mafic volcanics and an overlying widespread ordovician quartz rich turbidite and black shale succession with scattered mafic to andesitic volcanic centres these rocks are overlain by or in fault contact with deep marine to continental silurian to early carboniferous successions with mafic to silicic volcanics and abundant plutonic rocks major deformation achieved by thrusting and folding affected the fold belt during the silurian to middle devonian and the early carboniferous shortening estimates vary throughout the belt but the widespread ordovician quartz turbidite succession usually has at least 60 shortening which implies an original width of the fold belt of about 1700 km strike slip faulting was important on a local to regional scale but large scale strike slip displacements appear unlikely the large values of shortening are consistent with development of the belt in a convergent margin setting at least for the silurian to carboniferous the nature of the lower crust of the lachlan fold belt has always presented a problem of interpretation but we favour a late proterozoic quasi continental lower crustal layer that has deformed independently of the upper crust by either homogeneous flattening or crustal duplexing during silurian carboniferous convergence and provided a source for silurian devonian granites

ordovician high grade metamorphism of a newly recognised late neoproterozoic terrane in the northern harts range central australia granulite facies rocks from the northernmost harts range complex arunta inlier central australia have previously been interpreted as recording a single clockwise cycle of presumed palaeoproterozoic metamorphism 800 875 c and 9 10 kbar and subsequent decompression in a kilometre scale e w striking zone of noncoaxial high grade c 700 735 c and 5 8 6 4 kbar deformation however new shrimp u pb age determinations of zircon monazite and titanite from partially melted metabasites and metapelites indicate that granulite facies metamorphism occurred not in the proterozoic but in the ordovician c 470 ma the youngest metamorphic zircon overgrowths from two metabasites probably meta volcaniclastics yield 206pb 238u ages of 478 4 ma and 471 7 ma whereas those from two metapelites yield ages of 463 5 ma and 461 4 ma monazite from the two metapelites gave ages equal within error to those from metamorphic zircon rims in the same rock 457 5 ma and 462 5 ma respectively zircon and possibly monazite ages are interpreted as dating precipitation of these minerals from crystallizing melt within leucosomes in contrast titanite from the two metabasites yield 206pb 238u ages that are much younger 411 5 ma 417 7 ma respectively than those of coexisting zircon which might indicate that the terrane cooled slowly following final melt crystallization one metabasite has a second titanite population with an age of 384 7 ma which reflects titanite growth and or recrystallization during the 400 300 ma alice springs orogeny the c 380 ma titanite age is indistinguishable from the age of magmatic zircon from a small late and weakly deformed plug of biotite granite that intruded the granulites at 387 4 ma these data suggest that the northern harts range has been subject to at least two periods of reworking 475 460 ma 400 300 ma during the palaeozoic detrital zircon from the metapelites and metabasites and inherited zircon from the granite yield similar ranges of proterozoic ages with distinct age clusters at c 1300 1000 and c 650 ma these data imply that the deposition ages of the protoliths to the harts range complex are late neoproterozoic or early palaeozoic not palaeoproterozoic as previously assumed

mesozoic contractional deformation in the middle of the asian tectonic collage the intraplate western ordos fold thrust belt china abstract intraplate deformation can occur at great distances from synchronous plate boundaries and is most likely the result of strain concentrations into weak zones these weak zones may be pre existing crustal heterogeneities such as faults thermally weakened zones due to magmatism and or the effects of a thick sedimentary cover or mechanical contrasts between adjacent crustal blocks intraplate deformation is not exclusively related to continent continent collision it can be the result of a change in subduction dynamics or the interaction of two convergent plate boundaries asia has many examples of active intraplate deformation that are thought to be related to the cenozoic indo asian collision although much attention is placed on active examples discerning controls on pre cenozoic asian intraplate deformation will lead to not only a better understanding of the processes involved in intraplate orogens but also the tectonic evolution of asia situated on either side of the yellow river along the western margin of the ordos plateau the intraplate western ordos fold thrust belt involves units as old as archean basement and as young as late jurassic the fold thrust belt is spectacularly exposed in two ranges the helan shan and the zhuozi shan cross sections drawn through the north trending fold thrust belt suggest 30 minimum shortening a paleocurrent reversal within the lower middle jurassic section is interpreted to mark the onset of contraction and mountain building a late jurassic synorogenic boulder conglomerate records the final deformation in the fold thrust belt synchronous with contraction and cutting across the fold thrust belt is an e w striking strike slip fault that displays right lateral drag and sub horizontal striae we interpret this fault as an accommodation structure in the thrust belt following e w contraction the thrust belt was dismembered by an n15 w trending left lateral strike slip fault the left lateral fault displaces the frontal portion of the thrust belt the zhuozi shan 62 km to the north relative to a more internal portion the helan shan we propose that the anomalous orientation and intraplate location of the western ordos fold thrust belt is a function of mechanical contrasts between a previously deformed area and a stable crustal block ordos to the east pre existing crustal weakness or anisotropy along the western margin of the ordos block is most likely related to either a late proterozoic early paleozoic aulacogen a triassic fault system or to both although its intraplate location and the complexities of mesozoic asian plate interactions make it difficult to relate deformation in the western ordos fold thrust belt to a specific contemporaneous plate boundary its n s trend may suggest a link with paleo pacific subduction along the eastern margin of asia

continental interiors and cratons any relation abstract continental interior was originally defined as a geographic concept on the example of central asia later james dwight dana used it in a geological context when he argued that north america was an ideal continent with a low old stable interior and higher younger more active periphery this picture was thought satisfactory from the viewpoint of fixist tectonics for more than a century although it was clear that it did not account for the structure of eurasia neither in asia nor in europe cratons and or areas of gentle deformation outside orogenic belts coincide with the continental interior mobilist tectonics and especially its plate tectonics version made clear that in a world where continents are constantly united and re dispersed such non coincidence with continental interior however it may be defined is exactly what one would expect if however after isolation by rifting and or transform fault displacement no plate boundary cuts across a continent the lithosphere beneath it would cool and thicken if this lithosphere is also made up of high mg mg fe residual material left after making basalt it would be lighter than pristine mantle lithosphere such lithosphere would be resistant to subduction and to deformation it would thus protect the portion of the continent overlying it giving rise to a craton cratonic keels of 300 km or deeper can be generated by shortening a depleted mantle cushion that normally reaches down to some 150 km by 50 in the archaean high geothermal gradients would eclogitise the mafic bottom of a continent more mafic than those that are younger and prevent its upper surface from rising during such a shortening this would keep the tops of archaean cratons unmetamorphic or at low grades if a continent with such a deep keel is left alone for a time period on the order of 1 ga continuous cooling will render it very strong only very large strike slip systems can remain active as plate boundaries for a long time within a continental interior therefore if a continental interior can be kept away from such a boundary for about 1 ga it inevitably will turn into a craton with low relief probably a high percentage areawise of internal drainage and high climatic continentality late palaeozoic gondwana land was one such region pre miocene africa was perhaps another with many features inherited from gondwana land if india stops pushing it is likely that central asia will turn into yet another one plume controlled active rifting is the only way to destroy such a consolidated continental interior it is thus the composition and the thermal state plus the state of stress of the lithosphere underlying a continent that determines its tectonic behaviour and not its crustal structure nor its geography craton formation is largely a lithospheric process not a crustal one by the very nature of the process of craton formation cratons commonly but not always originate within continental interiors but continental interior alone can be assigned no tectonic connotation whatever