emplacement of martian rampart crater deposits 1 we present a basic continuum flow model for the emplacement of the distal rampart crater deposits on mars assuming cylindrical symmetry analytic solutions of the volume and momentum conservation equations yield time dependent flow thickness and velocity profiles the momentum equation has an inertial term and a frictional resistance term that is proportional to the local volumetric flow rate we find that only a few basic physical processes are necessary to form sharp distal ramparts first there must be sufficient material to form a continuum overland flow then for simple choices of boundary conditions at the source of the flow distal ramparts form naturally due to the cylindrical geometry the inertia of the flow and local frictional resistance themis images of rampart deposits show that the ejecta flow velocities were relatively slow being diverted by preexisting obstacles even within 0 2 crater radii from the rim of the parent crater to develop inferences about rampart deposit emplacement we require measurements of the crater radius and flow distance and assumptions about the maximum source flow velocity and depth we measured nine clearly expressed rampart deposits for craters with diameters of 3 4 17 0 km the model provides estimates of the emplacement time the rampart width and the shape of the radial flow depth profiles we find that modest velocities of 27 116 m s 1 maximum source flow depths of 10 40 m and emplacement durations of 12 31 min adequately explain the morphologies of selected rampart craters

paleomagnetic constraints on the late cretaceous and cenozoic tectonics of southeastern asia abstract many features of the cenozoic tectonic history of central and southeastern asia can be understood as direct consequences of the thrust and penetration of india into asia recent indentation experiments with plasticine tapponnier et al 7 have extended this idea and have led to the prediction of a pattern of large rotations and displacements of continental blocks that can be tested by paleomagnetism the available cretaceous and cenozoic paleomagnetic data from this part of the world have been reviewed and a new apwp for eurasia has been constructed for reference the negligible rotation of south china and large clockwise rotation of indochina are consistent with the model i e with an history of large scale left lateral strike slip motion along the altyn tagh and red river faults data from malaya and borneo can be reconciled with the model although in a less straightforward fashion the large counter clockwise rotation of south tibet implies that it rotated in sympathy with india during the collision and suggests that future indentation experiments should include this feature finally a middle cretaceous reconstruction of the south margin of asia is proposed one interesting result is the restored continuity of geological features in tibet and indochina with active subduction of oceanic indian plate crust taking place to the south at subtropical latitudes

tectonic geomorphology of the southernmost sagaing fault and surface rupture associated with the may 1930 pegu bago earthquake myanmar abstract the sagaing fault is a continental transform fault between the india and sunda plates that connects spreading centers in the andaman sea and the continental convergence zone along the himalayan front several m 7 earthquakes occurred along the fault in the last century and global positioning system campaigns revealed a right lateral slip rate of 18 mm yr about half of the total india sunda displacement rate of 35 mm yr however there are few fundamental geologic data on the sagaing fault such as detailed fault trace locations information on late quaternary slip rate or spatial extent of surface ruptures during historical earthquakes we conducted geologic field investigations along the southernmost 120 km long stretch of the fault zone that ruptured during the m 7 3 1930 pegu bago earthquake we found well defined tectonic geomorphic features across a deltaic lowland including fault scarps tectonic depressions stream offsets and pressure ridges the sense of displacement is predominantly right lateral strike slip with vertical motion less than 1 5 of horizontal motion based on right lateral offsets of stream channels terrace risers and property boundaries we estimate coseismic displacement during the 1930 earthquake as 3 0 m this in turn gives us a preliminary recurrence interval of surface rupturing earthquakes on the sagaing fault east of yangon as 160 yr coulomb stress change induced by the may 1930 pegu bago earthquake may have triggered the december 1930 pyu earthquake immediately to the north this study demonstrates that detailed geomorphic investigations are key for a better assessment of seismic hazard for the sagaing fault

provenance of loess from the spanish central region chemometric interpretation in this work our purposes are 1 geochemical characterization of loess primary loess or true loess and secondary loess or loess like deposits located in the centre of the iberian peninsula 2 systematic study of element behaviour during pedogenesis and 3 evaluation of the suitability of using the geochemistry of loess to establish the average composition of these discontinuous aeolian sedimentary covers in central spain several analyses were carried out on the bulk sample and on the sandy and clay fractions mineralogical composition by x ray diffraction mineralogical studies of heavy minerals by petrographical microscopy and chemical composition by flame atomic absorption spectrometry loess from the spanish central region has a local origin the presence of gypsum in the loess like deposits reaches values two times higher than in true loess and true loess has a higher concentration of quartz calcite and kaolinite regarding chemical composition similar concentrations of ca k mg and na were found although it is important to note the higher concentration of na in some of the samples

electrical imaging of narmada son lineament zone central india from magnetotellurics the narmada son lineament nsl zone is the second most important tectonic feature after himalayas in the indian geology magnetotelluric mt studies were carried out in the nsl zone along a 130 km long nne ssw trending profile the area of investigation extends from edlabad 20 46 16 7 5 59 05 in the south to khandwa 21 53 51 7 6 18 05 i n the north the data shows in general the validity of a two dimensional 2d approach besides providing details on the shallow crustal section the 2d modeling results resolved four high conductive zones extending from the middle to deep crust spatially coinciding with the major structural features in the area namely the gavligarh tapti barwani sukta and narmada south faults the model for the shallow section has brought out a moderately resistive layer 30 150 m representing the exposed deccan trap layer overlying a conductive layer 10 30 m inferred to be the subtrappean gondwana sediments the latter resting on a high resistive basement upper crust the deccan trap thickness varies from around a few hundred meters to as much as 1 5 km along the traverse a subtrappean sedimentary basin like feature is delineated in the northern half of the traverse where a sudden thickening of subtrappean sediments amounting to as much as 2 km is noticed the high resistive upper crust is relatively thick towards the southern end and tends to become thinner towards the middle and northern part of the traverse the lower crustal segment is conductive over a major part of the profile considering the generally enhanced heat flow values in the nsl region coupled with characteristic gravity highs and enhanced seismic velocities coinciding with the mid to lower crustal conductors delineated from mt presence of zones of high density mafic bodies intrusives with fluids presumably associated with magmatic underplating of the crust in the zone of major tectonic faults in nsl region are inferred 2004 elsevier b v all rights reserved

a collision boundary imaged by magnetotellurics hidaka mountains central hokkaido japan magnetotelluric mt data were acquired across the hidaka mountains which are thought to have formed by collisional orogenic processes a regional electrical strike of n18 w consistent with surficial geology was derived from impedance tensor decomposition two dimensional resistivity models were constructed by forward model fitting with preference for reducing the misfit of transverse magnetic mode the final modeling imaged a collision boundary between cretaceous forearc basin rocks and continental crust the major features of the model are 1 a 5 km thick surficial conductive layer corresponding to cretaceous forearc basin and the underlying resistive basement that shallows and is exposed to the east where sorachi formations characterized by oceanic crustal rocks crop out therefore we interpret the underlying resistive layer as oceanic crust 2 a thin 500 m thick conductive 3 m anomaly corresponds to serpentinite exposure in the kamuikotan metamorphic belt 3 exposed deep crustal rocks in the hidaka belt showed high resistivity 1000 2000 m at the surface the subsurface westward extension inferred from the mt data is supported by seismic refraction data 4 in the hidaka mountains a relatively conductive 500 1000 m layer implying accretionary prism sequences was inferred at 2 to 10 km depth beneath a more resistive 1000 2000 m surface layer 5 a wedge shaped resistivity anomaly was found at 5 to 50 km depth beneath the hidaka belt the resistivity of this wedge decreases eastward from 30 000 m to 2000 m consistent with the surface decrease of metamorphic grade the spatial distribution is also consistent with a seismic high velocity anomaly 6 the western side of the resistive wedge is fringed by a dipping conductor correlations with a low velocity anomaly and high seismicity suggest that this conductor is an enhanced porosity region and that it may be working as a detachment zone at the collision boundary the high resistivity contrast between oceanic crust and high temperature metamorphosed continental crust permitted us to image the interfingered complex structure beneath the hidaka mountains from 300 m to 50 km depth

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

exploration for unconformity type uranium deposits with audiomagnetotelluric data a case study from the mcarthur river mine saskatchewan canada unconformity type deposits supply a significant amount of the world s uranium and consist of uranium that is generally codeposited with graphite in a fault zone the low resistivity of the graphite produces a significant contrast in electrical resistivity which can be located with electromagnetic em methods the athabasca basin in western canada hosts significant uranium deposits and exploration in deeper parts of the basin has required the application of new em methods this paper presents an evaluation of the audiomagnetotelluric amt exploration method at the mcarthur river mine in the athabasca basin amt data were collected at 132 stations on a grid and two dimensional 2d and three dimensional 3d inversions were used to generate resistivity models these models showed two major results 1 a significant conductor coincident with a major basement fault p2 and the uranium deposits this conductor begins at the unconformity at a depth of 550 m and extends to a depth of at least three km an

along strike variations in the electrical structure of the san andreas fault at parkfield california magnetotelluric exploration has been used to image along strike variations in the electrical resistivity structure of the san andreas fault at parkfield california a low resistivity wedge extending to a depth of several kilometers is continuous over a horizontal distance of 8 km the base of the wedge is coincident with the shallowest microearthquakes a change in the electrical and fluid connection of the san andreas fault with a low resistivity zone in the franciscan formation is observed along the parkfield segment

structure and significance of s wave velocity and poisson s ratio in the crust beneath the eastern side of the qinghai tibet plateau the receiver functions of body waves of distant earthquakes obtained for the regions beneath 41 digital stations lhasa and ganz in tibet mandalay and rangoon in myanmar shio in india chto in thailand and station network in sichuan and yunnan were used to invert for s wave structure in the crust and upper mantle in sichuan yunnan and their surrounding areas meanwhile the distribution characteristics of the poisson s ration and the crustal thickness in sichuan and yunnan areas were also obtained results indicate that the depth of moho beneath the eastern side of qinghai tibetan plateau varies strikingly it is obvious that the greatest changes in crustal thickness occur in a north south direction the crustal thickness decreases from north to south being as thick as 70 km in eastern tibet the northern portion of our area of interest and less than 30 km in chaing mai and rangoon the southern portion of our area there are however exceptions regarding the trend the thickness exhibits an east west variation trend in the area from ma erkong kongding in sichuan to lijiang in yunnan in general the jinpingshan longmenshan fault and anninghe fault can be taken as the boundaries of this exception area the thickness in kongding in the west is 68 km while it is only 39 km in yongchuan in the east moreover the poisson s ratio values in the blocks of central sichuan and sichuan yunnan diamond are high and a low velocity layer in the crust of this area can be obviously detected the distribution characteristics of the high poisson s ratio and the low velocity of the crust in this block correspond to the tectonic structure being in contrast with the surrounding areas combining with the distribution features of the modern tectonic stress field it is deduced that the sichuan yunnan area is probably the channel through which the materials of the lithosphere flow eastward

the phuket terrane a late palaeozoic rift at the margin of sibumasu abstract it is widely accepted that sibumasu rifted from gondwana in the late palaeozoic but the rifts themselves have not previously been documented in southeast asia this paper identifies the pre middle permian kaeng krachan group of upper peninsular thailand as the infill of one such rift which is given the name phuket terrane indirect evidence suggests the rift infill is several kilometres thick and glacially influenced diamictites are conspicuous in the succession there are significant similarities with the 3 km thick pre middle permian rift infill of the carnarvon basin of western australia east of the khlong marui fault belt the succession is thinner and diamictites are a minor component a tectono stratigraphic model is proposed involving gondwana glaciers dropping their load at the present western margin of the phuket terrane from where it was re sedimented in the rapidly subsiding marine rift basin it is suggested that the khlong marui fault formed part of the eastern boundary of the rift system the three pagodas fault belt similarly juxtaposes different pre middle permian successions rifting ceased in the early permian and a passive margin formed as the mesotethys ocean widened the upper part of the kaeng krachan group and the overlying ratburi limestone representing the post rift sequence

magnetotelluric imaging beneath the taiwan orogen an arc continent collision 1 the taiwan orogen has formed since the late miocene by oblique collision between the luzon volcanic arc on the philippine sea plate and the eurasian continental margin this oblique collision has produced an orogen that decreases in age from north to south and permits study of the temporal evolution of an arc continent collision these factors make taiwan a favorable location to study the process of arc continent collision the first long period magnetotelluric mt measurements were recorded in taiwan as part of the taiwan integrated geodynamics research taiger project in 2006 7 measurements were made at 82 sites on three transects across south central and north taiwan that span the breadth of the orogen and cross all major tectonic boundaries robust remote reference processing of the mt time series data resulted in high quality soundings that were modeled in both 2 and 3 dimensions these mt models support predictions of lithospheric deformation i e thick skinned tectonics beneath the central ranges in south and central taiwan but are inconsistent with predictions of orogen scale thin skinned models the mt resistivity model for northern taiwan is consistent with dewatering of the subducting philippine slab and with deformation described by the subducting indenter tectonic model modeling the taiger mt data has definitively shown that conductive and seismically active crustal structures exist to 30 km beneath the orogen these conductive regions interpreted as interconnected fluid map pervasive zones of collisional deformation that are lithospheric in scale

electromagnetic heterogeneity of the seismogenic region of 1962 m6 5 northern miyagi earthquake northeastern japan magnetotelluric surveys were carried out to in vestigate the electrical resistivity structure of the seismo genic region of the 1962 northern miyagi earthquake m6 5 in miyagi prefecture northeastern japan a 2 d inversion reveals the existence of a deep conductive zone and overly ing re sistivezonein theuppe r crust a positiveae romag netic anomaly is also recognized above the resistive zone which is interpreted as a buried kitakami granitoid pluton by comparison with these ismicity wefound that themi croearthquakes occur just above the deep conductor and in the resistive zone and that several s wave reflectors are just above the deep conductor since the s wave reflectors sug gest the existence of fluid beneath them the deep conduc tor can be interpreted as a fluid filled zone moreover we suggest that the seepage of the fluid from the conductive fluid filled zone to the resistive granitoid pluton can become a trigger of the earthquakes

lithospheric characterization of the central pyrenees based on new magnetotelluric data terra nova 23 213 219 2011 abstract the pyrenees resulted from the continent continent collision between the iberian and european plates during the alpine orogeny although the subduction of the iberian lower crust is currently accepted the physical processes that occur at the boundary between the plates are still a topic of open discussion today as illustrated by the debate about the presence or absence of partial melting in the iberian subducted lower crust long period magnetotelluric data acquired for the first time in the central pyrenees constrain the low electrical resistivity structure in the iberian subducted lower crust better than previous studies plotting their upper limit 9 km deeper this new upper limit reinforces the hypothesis of partial melting additionally long period magnetotelluric data constrain the lithosphere asthenosphere boundary at 90 and 130 km deep below the iberian and european plates respectively and are compatible with electrical resistivity values between 10 and 18 ohm m for the asthenosphere

electrical structure of the tectonically active kalabsha fault aswan egypt in this work we use the magnetotelluric mt method to detect geoelectrical conductivity anomalies in the earth s crust and link them to local seismic activity this application affords the unusual opportunity to study the percolation of water from a lake into a fault system and its effect on the induced seismicity mt measurements were carried out in the period range 0 0046 420 s at nine sites along a 15 km long north south profile crossing the kalabsha fault on the western bank of lake aswan data were analysed by 2d simultaneous inversion of both polarisations the resulting model is compared with the local seismicity map and reveals the conductive signature of the fault as well as geological and tectonic stresses prevailing in the aswan area our mt investigations show the following features the measured mt strike aligns with the seismic epicentre axis corresponding to the kalabsha fault while crossing the fault enhanced conductivity is found down to depths of 5 km on a 1 2 km profile segment at mid crustal depths 20 km a very high conductive body is found to coincide with the main seismic cluster in the aswan area these observations indicate that seismic activity and high electrical conductivity are related the link between them is the presence of crustal fluids which are presumably the cause of the high conductivity observed their presence is also required to trigger the observed seismicity in addition we explain the lower conductivity of the local upper crust in terms of stress modulated rock porosity we believe that these results are of general significance as they could explain the mid crustal seismicity of tectonically active zones d 2005 elsevier b v all rights reserved

internal deformation of the shan thai block inferred from paleomagnetism of jurassic sedimentary rocks in northern thailand abstract paleomagnetic samples of red sandstones were collected at 16 sites from the lower to upper jurassic phu kradung formation and the upper jurassic phra wihan formation around nan city 19 2 n 101 0 e in the southern region of the shan thai block after stepwise thermal demagnetization a high temperature component with an unblocking temperature of about 680 c is isolated from 11 sites we interpret this to be a prefold characteristic remanent magnetization because of a positive fold test at the 95 confidence level the tilt corrected mean paleomagnetic direction from 11 sites is d 32 3 i 33 3 95 12 2 k 15 0 which gives a paleomagnetic pole for this area of 60 1 n 186 5 e with a95 11 7 compared with a jurassic paleopole for the stable part of the yangtze block we observe a clockwise tectonic rotation of 12 8 which is similar to the observed rotation 18 28 of the northern region of the shan thai block this amount contrasts with a large rotation of more than 45 that is estimated from the narrow zone extending from luxi to mengla in the central region of this block this declination aspect suggests that a coherent whole block rotation by about 20 of the shan thai block took place in an earlier phase during indentation of india into asia and that an internal deformation with additional local rotation between 25 and 77 followed within a limited zone in the central part the internal deformation of the shan thai block absorbed the stress on the asian continent induced by indentation of india as well as its coherent whole block motions of clockwise rotation and southward displacement

the central india tectonic zone a geophysical perspective on continental amalgamation along a mesoproterozoic suture abstract the central india tectonic zone citz is a prominent divide and a major suture zone between the north indian and south indian crustal blocks the resistive upper crust as modeled in the magnetotelluric data from citz suggests a dominant tonalite trdondhjemite granodiorite composition associated with an accretionary complex characterized by mainly felsic rock components the highly conductive bodies in this zone might represent mafic ultramafic layered intrusives derived from a deeper reservoir of underplated basaltic magma related to the formation of the cretaceous deccan flood basalts the uniformly thick mafic lower crust below the cratons on both sides of the suture is interpreted as the accreted remnants of archaean and paleoproterozoic subducted slabs we redefine the nature of deep faults traversing the citz which were described as steep and penetrating the moho by previous workers and classify them as listric faults with gentle dips at depth seismic reflection data from the eastern side of the suture suggest a northwestward subduction of the bhandara craton reflection data from the central part of the citz show northerly dip in the southern part suggesting northward subduction of the dharwar craton however an opposite trend is observed in the northern part of the suture with a southward dip of the bundelkhand craton based on these features and in conjunction with existing magnetotelluric models we propose a double sided subduction history along the citz this would be similar to the ongoing subduction accretion process in the western pacific region which possibly led to the development of paired collision type and pacific type orogens one important feature is the domal structure along the central part of the suture with a thick felsic crust occurring between mafic and intermediate crust the high resistivity felsic domain suggests underplated sediments felsic crust that would have caused the doming our model also accounts for the extrusion of regional metamorphic belts at the orogenic core and the occurrence of high pressure ultrahigh temperature paired metamorphic belts within the suture

an alternative plate tectonic model for the palaeozoic early mesozoic palaeotethyan evolution of southeast asia northern thailand burma abstract an alternative model for the geodynamic evolution of southeast asia is proposed and inserted in a modern plate tectonic model the reconstruction methodology is based on dynamic plate boundaries constrained by data such as spreading rates and subduction velocities in this way it differs from classical continental drift models proposed so far the different interpretations about the location of the palaeotethys suture in thailand are revised the tertiary mae yuam fault is seen as the emplacement of the suture east of the suture we identify an indochina derived terrane for which we keep the name shan thai formerly used to identify the cimmerian block present in southeast asia now called sibumasu this nomenclatural choice was made on the basis of the geographic location of the terrane eastern shan states in burma and central thailand and in order not to introduce new confusing terminology the closure of the eastern palaeotethys is related to a southward subduction of the ocean that triggered the eastern neotethys to open as a back arc due to the presence of late carboniferous early permian arc magmatism in mergui burma and in the lhasa block south tibet and to the absence of arc magmatism of the same age east of the suture in order to explain the presence of carboniferous early permian and permo triassic volcanic arcs in cambodia upper triassic magmatism in eastern vietnam and lower permian middle permian arc volcanites in western sumatra we introduce the orang laut terranes concept these terranes were detached from indochina and south china during back arc opening of the poko song ma system due to the westward subduction of the palaeopacific this also explains the location of the cathaysian west sumatra block to the west of the cimmerian sibumasu block

nan river suture zone northern thailand the nan river suture zone consists of a belt of ophiolitic mafic and ultramafic rocks formed in a back arc or inter arc setting and metasedimentary rocks including epidote crossite blueschists that may represent an inner trench sequence these rocks are probably pre permian in age the nan river belt forms part of the suture between the indosinian and shan thai cratonic blocks and is inferred to correlate with the changning shuangjiang suture zone in southern china

application of 3d inversion to magnetotelluric profile data from the deccan volcanic province of western india abstract using synthetic data siripunvaraporn et al 2005b demonstrated possible advantages of interpreting single profile mt data with a three dimensional 3d inversion program here we explore this idea further using real mt data from two profiles on the indian subcontinent the first profile 330 km long cuts across the deccan volcanic province of peninsular india the second 130 km long is in the narmada son lineament zone approximately 100 km further north using the data space occam inversion code of siripunvaraporn et al 2005a 3d inversion is carried out on each of these profiles independently and results are compared with previously published two dimensional 2d interpretations in addition to inversion of the full impedance tensor we consider 3d inversion of only the off diagonal components we also experiment with variants on the model covariance in particular allowing for longer smoothing length scales along the geoelectric strike not surprisingly the 3d inversion finds models that fit the data better than had been possible with the 2d programs many of the features inferred from these previous 2d interpretations are also present in the 3d inverse solutions but the positions and amplitudes of individual conductive features are in some cases changed the 3d models suggest substantial non uniqueness in the single profile data even without explicit or special treatment we find that the relatively modest near surface distortion effects in these datasets were well fit by the 3d inversion by inserting small scale conductive and resistive features in surface layers mostly off profile

fluid and deformation regime of an advancing subduction system at marlborough new zealand newly forming subduction zones on earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes the transpressional subduction regime of new zealand which is advancing laterally to the southwest below the marlborough strike slip fault system of the northern south island is an ideal setting in which to investigate these processes here we acquired a dense high quality transect of magnetotelluric soundings across the system yielding an electrical resistivity cross section to depths beyond 100 km our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops massive fluid release just inland of the trench induces fault fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates narrow strike slip faults in the shallow brittle regime of interior marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones deep subduction generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high angle thrusts that are poorly oriented for failure under dry conditions the fluid deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models

tectonic framework and phanerozoic evolution of sundaland abstract sundaland comprises a heterogeneous collage of continental blocks derived from the india australian margin of eastern gondwana and assembled by the closure of multiple tethyan and back arc ocean basins now represented by suture zones the continental core of sundaland comprises a western sibumasu block and an eastern indochina east malaya block with an island arc terrane the sukhothai island arc system comprising the linchang sukhothai and chanthaburi blocks sandwiched between this island arc formed on the margin of indochina east malaya and then separated by back arc spreading in the permian the jinghong nan uttaradit and sra kaeo sutures represent this closed back arc basin the palaeo tethys is represented to the west by the changning menglian chiang mai inthanon and bentong raub suture zones the west sumatra block and possibly the west burma block rifted and separated from gondwana along with indochina and east malaya in the devonian and were accreted to the sundaland core in the triassic west burma is now considered to be probably cathaysian in nature and similar to west sumatra from which it was separated by opening of the andaman sea basin south west borneo and or east java west sulawesi are now tentatively identified as the missing argoland which must have separated from nw australia in the jurassic and these were accreted to se sundaland in the cretaceous revised palaeogeographic reconstructions illustrating the tectonic and palaeogeographic evolution of sundaland and adjacent regions are presented

magnetotelluric observations across the juan de fuca subduction system in the emslab project a magnetotelluric mt transect has been obtained near latitude 45 n from the active juan de fuca spreading center across the subduction zone and cascades volcanic arc and into the back arc deschutes basin region this paper presents the mt data set and describes its major characteristics as they pertain to the resistivity of the subduction system in addition we discuss the measurement and processing procedures employed as well as important concerns in data interpretation broadband audiomagnetotelluric amt mt soundings approx 0 01 500 s period were collected on land with considerable redundancy in site location and from which 39 sites were selected which constrain upper crustal heterogeneity but sense also into the upper mantle fifteen long period mt recordings about 50 10 000 s on land confirm the broadband responses in their common period range and extend the depths of exploration to hundreds of kilometers on the juan de fuca plate offshore 33 out of 39 sea floor instruments at 19 locations gave good results of these locations five magnetotelluric soundings plus two additional geomagnetic variation sites covering the period range 200 105 s approximately constitute the ocean bottom segment of our profile the feature of the land observations which probably relates most closely to the subduction process is a peak in the impedance phase of the transverse magnetic mode around 30 50 s period this phase anomaly with a corresponding inflection in the apparent resistivity is continuous eastward from the seacoast and ends abruptly at the high cascades it signifies an electrically conductive layer in otherwise resistive lower crust or upper mantle with the layer conductance decreasing eastward from the coast to a minimum under the coast range but increasing suddenly to the east of the central willamette basin the higher conductance to the east is corroborated by the vertical magnetic field transfer function whose real component shows negative values in the period range 100 1000 s over the same distance the transverse electric mode apparent resistivity and phase on the land display a variety of three dimensional effects which make their interpretation difficult conversely both modes of the ocean floor soundings exhibit a smooth progression laterally from the coastal area to the spreading ridge indicating that the measurements here are reflecting primarily the large scale tectonic structures of interest and are little disturbed by small near surface inhomogeneities the impedance data near the ridge are strongly suggestive of a low resistivity asthenosphere beneath resistive juan de fuca plate lithosphere approaching the coastline to the east both impedance and vertical magnetic field responses appear increasingly affected by a thick wedge of deposited and accreted sediments and by the thinning of the seawater

deep electrical conductivity structures of the appalachian orogen in the southeastern u s long period magnetotelluric data across the southeastern appalachians image deep crustal and upper mantle relics of ancient orogenic events inversions of the responses show 1 beneath the appalachian mountains there is a sub horizontal conductor at 15 20 km depth which dips to the southeast at the surface trace of the brevard fault 2 at the location of the central piedmont suture there is a crustal conductor which dips towards the southeast interpreted as a structure related to the acadian suture 3 upper mantle conductors were found at 80 km depth northwest of the blue ridge and at 140 km depth southeast of the eastern piedmont between these there is a northwest dipping resistive gap possibly representing the remnant structure of the alleghanian collision

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

evidence for middle triassic to miocene dual subduction zones beneath the shan thai terrane western thailand from magnetotelluric data abstract kanchanaburi province western thailand is in the middle of shan thai terrane paleomagnetic and geological data support the hypothesis of dual subduction zones beneath the shan thai terrane during the late triassic there was a westward subduction in the east where the lampang chiang rai block subducted beneath the shan thai terrane in addition in the early tertiary the western burma terrane subducted underneath the shan thai in the west resulting in an eastward subduction a pioneer survey of this region was therefore conducted using magnetotelluric mt techniques with the aim of finding deep structures associated with these ancient subduction zones 39 mt stations were deployed to cover most of the province resistivity structures are obtained from the 3 d inversion the near surface resistivity structures correspond well with the surface geology the mid and lower crusts are conductive and this is interpreted as the crusts being composed of mafic granulites of 3 porosity in which the remnant dehydrated fluid was accumulated during the subduction in addition the westward and eastward subduction zones generated mafic ultramafic intrusions which appear as two conductive zones on the east and west respectively these two conductors and the conductive lower crust support the hypotheses of the middle triassic to miocene dual subduction zones