a finite amplitude necking model of rifting in brittle lithosphere we formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic plastic layer overlying a fluid substrate a perfectly plastic rheology is a continuum description of faulting in rift zones two important aspects of rift evolution are illustrated by this model the evolution of the rift width as extension proceeds and the finite strain that occurs a region at yield initially develops with a width determined by the thickness of the brittle layer and the internal deformation within this yield zone is proportional to the topographic slope as extension proceeds the surface within the rift subsides and the width of the subsiding yield zone decreases at any stage of rifting material in regions just outside the yield zone is deformed but no longer deforming the width of these deformed regions increases with increasing extension vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone creating flanking uplifts the external force required to maintain active rifting increases with the amount of lithospheric stretching indicating that rifting is a quasi static stable process because the yield zone will revert to elastic behavior if the external force causing extension is removed the model predicts that the rift depression and flanking uplifts will be preserved after extension stops our simple mechanical model demonstrates the inherent relationship among graben formation lithospheric thinning and rift shoulder uplift in rift zones

investigating compositional diversity among fijian ceramics with laser ablation inductively coupled plasma mass spectrometry la icp ms implications for interaction studies on geologically similar islands this paper presents a ceramic provenance analysis of 260 fijian sherd clays by laser ablation inductively coupled plasma mass spectrometry la icp ms our analyses define three compositional groups in western fijian ceramics distributed across the 2700 year ceramic sequence frequencies of compositional groups represented in the diachronic assemblages indicate an increasing spatial sphere of interaction until approximately 1500 bp by 1000 bp interaction had spatially contracted to a significant degree these findings suggest re evaluation of current ideas about interaction in the southwest pacific is necessary

syrtis major a low relief volcanic shield viking orbiter images and earth based radar elevation data have been used to document the volcanic geology of syrtis major and to reevaluate its physiographic setting these data indicate that syrtis major is actually a very low relief simple shield with a central caldera complex arcuate segments of concentric grabens at the shield summit define the limits of a 280 km diameter depression that is thought to have been created by foundering of a magma chamber lava flows with well defined scarps are associated with two summit calderas fissures and lava tubes are recognized on the shield flanks the lava flows were deposited on volcanic ridged plains that do not display flow scarps compressional stresses acting on the shield were apparently greatest following the deposition of the ridged plains unit but continued at a somewhat reduced rate after deposition of the younger lavas the extremely low relief shield of the syrtis major as well as other volcanic plains of intermediate age in the eastern hemisphere of mars is in striking contrast to the giant shields of tharsis montes such different types of volcanic landforms may result from changes with times in the composition differentiation history eruption rates and eruption temperatures of surface igneous rocks thus directly reflecting the thermal and chemical evolution of the interior the low relief on the shields may also reflect a substantially thinner crust in these regions relative to tharsis the physiographic evidence and radar elevation data confirm the fact that syrtis major is a planum plateau rather than a planitia low plain as published maps of mars now portray the feature in addition present evidence does not support an earlier suggestion that an impact basin predated volcanism at syrtis major

ice lubricated gravity spreading of the olympus mons aureole deposits abstract gravity sliding and spreading at low strain rates can account for the general morphology and structure of the aureoles and basal scarp of olympus mons detachment sliding could have occurred around the volcano if either pore fluid pressures were exceptionally high greater than 90 or the rocks had very low resistance to shear about 1 105 pa or 1 bar because of the vast areal extent and probable shallow depth of the detachment zone development of ubiquitous high pore fluid pressures beneath aureole forming material was unlikely however a zone of sufficiently weak material consisting of about 10 interstitial or interbedded ice could have been present if so a simple rheologic model for the aureole deposits can be applied that consists of a thin ductile layer overlain by a thicker brittle layer according to this model extensional deformation would have occurred near the shield and compressional deformation in its distal parts proximal grabens and distal corrugations on aureole surfaces support this model a submarine slide at kitimat arm british columbia is a valid qualitative analogy for the observed features and inferred emplacement style of the aureole deposits ground ice processes have been considered the cause of many geologic features on mars a 3 average concentration of ground ice in the regolith is predicted by theoretical models for the ice budget and cryosphere ice may have been deposited in higher concentrations below the aureole forming material the source of the ice could have been juvenile water circulated hydrothermally by olympus mons volcanism the basal scarp of olympus mons apparently demarcates the transition between the upper stable part of the shield and its lower part that decoupled and formed the aureole deposits this transition may reflect a change in the bulk shear strength of the shield caused either by a radial dependence in the abundance of ice or fluid in the shield materials or by the concentration of intrusive dikes within the volcano other martian volcanoes exhibit virtually no evidence of similar large scale gravity spreading and basal scarps perhaps such evidence if it existed has been buried by lava flows or perhaps the smaller size of other volcanoes did not permit the development of these features

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

contourite drifts on early passive margins as an indicator of established lithospheric breakup the albian cenomanian breakup sequence bs offshore northwest iberia is mapped described and characterised for the first time in terms of its seismic and depositional facies the interpreted dataset comprises a large grid of regional 2d seismic reflection profiles complemented by industry and odp dsdp borehole data within the bs are observed distinct seismic facies that reflect the presence of a black shales and fine grained turbidites b mass transport deposits mtds and coarse grained turbidites and c contourite drifts borehole data show that these depositional systems developed as mixed carbonate siliciclastic sediments proximally and as organic carbon rich mudstones black shales distally on the northwest iberia margin mtds and turbidites tend to occur on the continental slope frequently in association with large scale olistostromes distally these change into interbedded fine grained turbidites and black shales showing widespread evidence of deep water current activity towards the top of the bs current activity is expressed by intra bs erosional surfaces and sediment drifts the results in this paper are important as they demonstrate that contourite drifts are ubiquitous features in the study area after aptian albian lithospheric breakup therefore we interpret the recognition of contourite drifts in northwest iberia as having significant palaeogeographic implications contourite drifts materialise the onset of important deep water circulation marking the establishment of oceanic gateways between two fully separated continental margins as a corollary we postulate the generation of deep water geostrophic currents to have had significant impact on north atlantic climate and ocean circulation during the albian cenomanian with the record of such impacts being preserved in the contourite drifts analysed in this work

stress generation at ridge axes by plate divergence and magma rise abstract a model is explored for the rifting process particularly at the divergent plate boundary in iceland based on direct observations of the krafla rifting episode 1975 1984 magma accumulates near the axial crust mantle transition as the plates diverge from each other and compression on the boundary decreases two dimensional finite element modelling is applied to investigate how divergence and buoyant rise of magma interact in triggering rifting both processes are found to be important but long intervals between rifting episodes require the time average deviatoric stress to be compressible normal to the axis

obsidian characterization by laser ablation icp ms and its application to prehistoric trade in the mediterranean and the near east sources and distribution of obsidian within the aegean and anatolia for geological studies interest in mass spectrometry with an inductively coupled plasma as an ion source and its association with laser ablation as a sample introduction technique la icp ms has steadily increased during the past few years and is now being developed in other fields such as archaeology after a description of the analytical procedure and the calculation method we show the potential of this technique to characterize almost non destructively archaeological artefacts among the 70 elements that could be routinely analysed by la icp ms with detection limits below the ppm level we choose to determine the more critical ones in order to evaluate the geochemical models of the magmatic process major elements rare earths and some transition elements a detailed survey of cappadocian obsidian flows is given and includes the characterization of nine different sources evidence of complex trade activities is clearly shown by the obsidian tools found at different prehistoric sites from neolithic to bronze age levels in the mediterranean and the near east new results obtained on some archaeological sites located in turkey syria and cyprus are presented they show the importance of cappadocian sources in obsidian trade our results show that la icp ms allows a non destructive analysis of archaeological objects and that it combines the advantage of the different classical methods used to characterize obsidian sources mainly xrf and inaa with high sensitivity and rapidity thus la icp ms appears to be a very powerful analytical tool and at this time this technique is the only one which can non destructively determine such an important number of elements with such low detection limits

geodynamics of the broad triangle area active seismic zone in asia stress field modeling abstract the broad triangle area bta is a significant active seismic zone located mainly in east asia the pamir baikal tectonic belt north south tectonic belt and himalayan tectonic belt compose the three boundaries of the bta previous studies suggested that the geodynamics of the bta may be related to coupling between the indian and eurasian plates yet quantitative analysis is lacking in this study we use the elastic finite element method in the ansys software to establish a spherical shell model and analyze the stress field of the model we obtain a best fit model that matches the geological evidence and seismicity by considering the real geological background a series of models with different initial settings are also discussed to analyze the boundary influence on the different models our modeling results indicate that the india eurasia collision the stable siberia craton and the blocking effects of the yangtze and north china cratons play the most important roles in the formation of the bta crustal heterogeneity has little influence on the distribution of seismicity within the bta further the effects of subduction of the pacific and philippine plates may not reach the bta area which is different from the results of other studies

origin and evolution of a cold based tropical mountain glacier on mars the pavonis mons fan shaped deposit 1 each of the large tharsis montes volcanoes in the equatorial region of mars has an unusual amazonian aged fan shaped deposit on its west northwestern flank on the basis of viking orbiter data the origin of these deposits has been variously ascribed to volcanic mass wasting tectonic and glacial processes using new mgs and odyssey data combined with recent developments in the study of cold based glaciers we reassess the geology and mode of origin for these deposits with particular emphasis on pavonis mons the deposits share three characteristic facies including 1 a ridged facies consisting of tens to 100 parallel concentric ridges around the margins of the deposits 2 a knobby facies composed of irregular hills and hummocks and 3 a smooth facies of broad lobate plains that superposes all other units within the deposits on the basis of morphology topography superposition relationships and close terrestrial analogs in the dry valleys of antarctica we interpret the pavonis fan shaped deposit as the depositional remains of a cold based glacier that formed on the northwestern flank in recent martian history we interpret the ridged facies as drop moraines formed around the margins of a retreating cold based glacier the knobby facies as a sublimation till derived from in situ down wasting of cold based glacial ice and the smooth facies as extant debris covered glacial ice in addition to the three main facies the fan shaped deposit at pavonis mons contains several unique features including arcuate scarps high relief flow like features and radial ridges which suggest that volcanism played a role during its formation using recent results from mars general circulation model simulations we outline a model of glacier formation involving atmospheric deposition of water ice on the northwestern flanks of the tharsis montes during periods of high mean obliquity reconstructed ice sheet profiles for each of the tharsis montes glaciers suggest that the ice sheets attained average thicknesses of 1 6 2 4 km values that are consistent with a cold based glacial origin analysis of crater size frequency distributions using new data indicates that the age of the glaciation lies within the late amazonian 10 200 ma thus our results suggest that multiple phases of tropical mountain glaciation occurred on mars within the past few hundred myr and that significant amounts of near surface equatorial ice may remain within the deposit today

the moho in extensional tectonic settings insights from thermo mechanical models the lithospheric memory is key for the interplay of lithospheric stresses and rheological structure of the extending lithosphere and for its later tectonic reactivation other important factors are the temporal and spatial migration of extension and the interplay of rifting and surface processes the mode of extension and the duration of the rifting phase required to lead to continental break up are to a large extent controlled by the interaction of the extending plate with slab dynamics the finite strength of the lithosphere has an important effect on the formation of extensional basins this applies both to the geometry of the basin shape as well as to the record of vertical motions during and after rifting we demonstrate a strong connection between the bulk rheological properties of europe s lithosphere and the evolution of some of europe s main rifts and back arc systems the thermo mechanical structure of the lithosphere has a major impact on continental break up and associated basin migration processes with direct relationships between rift duration and extension velocities thermal evolution and the role of mantle plumes compressional reactivation has important consequences for post rift inversion borderland uplift and denudation as illustrated by poly phase deformation of extensional back arc basins in the black sea and the pannonian basin region

ignimbrites of amazonis planitia region of mars a series of postulated ignimbrite units is mapped in the amazonis memnonia and aeolis quadrangles of mars the units cover about 2 2 106 km2 within a broad but discontinuous and irregular belt trending east west along the highland lowland boundary the ignimbrites overlie parts of the western and southern aureole materials of olympus mons but are embayed in places by the lava plains of the lowlands stratigraphic relations between the basalt flows from the tharsis montes region and the ignimbrites are not clearly defined crater counts suggest that the younger ignimbrites postdate the lava flows crater counts per square kilometer for the ignimbrites range from 7 29 1 95 10 4 to 6 36 2 01 10 5 for craters larger than 1 km in diameter the ignimbrite materials form thick 100 m extensive relatively flat sheets that are smooth to grooved or gently undulating grooved surfaces appear to be yardangs and in most places are not alined with prevailing wind directions the seven mapped ignimbrite units are characterized by morphologic expression stratigraphic position and crater counts similarities to ignimbrites in the pancake range of central nevada include 1 rounded patches of smooth high albedo nonwelded material superposed on jointed low albedo welded material 2 local complementary joint sets in welded materials and 3 thick flow sheets of great areal extent that follow but subdue underlying topography four major eruptive centers occur in areas where units are thickest and where a dominant nnw sse structural trend is expressed locally by unit margins elongate collapse features and normal faulting a minimum volume of 3 85 106 km3 for the deposits has been calculated from thickness estimates based on shadow measurements and crater rim height relations

the mechanism of continental splitting abstract continental splitting is the response of the continental lithosphere under favourable conditions to deviatoric tension and a hot partially fused upper mantle beneath it has commonly been suggested that rift systems such as that of east africa may possibly represent splitting in the process of occurring here the hot upper mantle has probably formed by convective upwelling from deep in the mantle allowing the escape of heat from the earth s interior by hotspot activity this causes volcanism and isostatic uplift of the heated and thinned lithosphere and the crustal tension subsequently develops in response to the surface load caused by the uplift and the upthrust of the isostatic compensation at depth it is suggested here that the crustal tension causing continental splitting does not normally originate as in east africa but rather by the suction force acting on the overriding plates at trenches at periods in earth history when subduction occurs on the opposite sides of a large continental mass such as pangaea the critical evidence on the splitting mechanism is mostly deeply buried at passive margins and inaccessible to study some margins such as the north biscay one were apparently formed under much quieter volcanic and tectonic conditions than continental rift zones whereas others such as those of the icelandic region underwent extensive continental volcanism indicating a hot upper mantle beneath the icelandic types of margin probably acted as foci for continental splitting by dyke emplacement with dykes propagating laterally from them through the more quietly developed parts of the margin to link with existing plate boundaries normal plate motions occur in response to the steady escape of heat from the deep interior through formation and cooling of the oceanic lithosphere which can be recycled whereas continental splitting is seen as the result of the spasmodic escape of heat from the deep interior through the continental lithosphere

the relation of catastrophic flooding of mangala valles mars to faulting of memnonia fossae and tharsis volcanism detailed stratigraphic relations indicate two coeval periods of catastrophic flooding and tharsis centered faulting producing memnonia fossae in the mangala valles region of mars major sequences of lava flows of the tharsis montes formation and local lobate plains flows were erupted during and between these channeling and faulting episodes first late hesperian channel development overlapped in time the tharsis centered faulting that trends north 75 to 90 e next late hesperian early amazonian flooding was coeval with faulting that trends north 55 to 70 e in some reaches resistant lava flows filled the early channels resulting in inverted channel topography after the later flooding swept through both floods likely originated from the same graben which probably was activated during each episode of faulting faulting broke through groundwater barriers and tapped confined aquifers in higher regions west and east of the point of discharge the minimum volume of water required to erode mangala valles about 5 1012 m3 may have been released through two floods that drained a few percent pore volume from a relatively permeable aquifer the peak discharges of the floods may have lasted from days to weeks the perched water discharged from the aquifer may have been produced by hydrothermal groundwater circulation induced by tharsis magmatism tectonic uplift centered at tharsis montes and compaction of saturated crater ejecta due to loading by lava flows

mars dual polarization radar observations with extended coverage abstract thirteen centimeter wavelength radar observations of mars made in 1982 at arecibo observatory yield accurate measurements of the full backscatter spectrum in two orthogonal polarizations the data which were obtained for several widely separated subradar longitudes at 24 n latitude provide the first global view of the distribution of small scale surface roughness on mars the diffuse component of the echo exhibits strong spatial variations areas of maximum depolarization correlate well with volcanic regions tharsis and elysium while the heavily cratered upland terrain yields relatively low depolarization parts of tharsis give near complete depolarization polaziation ratio c 1 when viewed at oblique angles of incidence northern martian plains regions tharsis elysium and amazonis may comprise the most extensive area of severe decimeter scale surface roughness in the inner solar system on the average the northern martian tropics yield higher diffuse radar cross sections d 0 05 0 12 and a higher of degree disk integrated depolarization c 0 1 0 4 than is found for the moon mercury and venus comparisons between the moon and mars using radar data ground truth and simple scattering models suggest that mars possesses a relatively high average coverage by decimeter scale rocks also discussed are several of the more interesting quasispecular scattering results the most unsual of which were obtained over the olympus mons aureole region

chemical characterization of micronesian ceramics through instrumental neutron activation analysis a preliminary provenance study abstract the development of exchange models from pacific islands archaeological data is a major task facing oceanic archaeologists chemical characterization of ceramics can assist provenance studies required for modelling past human interactions this regional instrumental neutron activation analysis inaa conducted on ceramics collected from the micronesian ceramic producing islands of belau chuuk guam kosrae pohnpei and yap allows us to form compositional groups and provenance ceramics from the coral islands of fais satawal and ulithi different island contexts which cause post depositional chemical transformations in ceramic sherds do not hinder the provenance assignments

diachronous rifting drifting and inversion on the passive margin of central eastern north america an analog for other passive margins integration of new data with existing information indicates that the tectonic development of the passive margin of eastern north america between the carolina trough and scotian basin was considerably more complex than the classic two stage rift drift model first the transition from rifting to drifting was diachronous in the southeastern united states the rift drift transition occurred after the late triassic synrift deposition and before eastern north america magmatism in the earliest jurassic 200 ma in maritime canada the rift drift transition occurred after eastern north america magmatic activity and synrift deposition in the early jurassic and before postrift deposition in the early middle jurassic 185 ma second the deformational regime changed substantially after rifting on both the southern and northern segments of the margin generally northwest southeast postrift shortening replaced northwest southeast synrift extension northeast striking reverse faults formed and many of the rift basin boundary faults had reverse displacements in the southeastern united states the change in the deformational regime occurred in the late triassic early jurassic during the rift drift transition simultaneously diabase sills and dikes many striking nearly perpendicular to the trend of the rift basins intruded the continental crust and a massive wedge of volcanic or volcaniclastic rocks developed near the continent ocean boundary in maritime canada the change in the deformational regime occurred during or after the early jurassic and before or during the early cretaceous that is during the rift drift transition or early stages of sea floor spreading

the cenozoic uplift and earthquake belt of mainland britain as a response to an underlying hot low density upper mantle a belt of hot low density uppermost mantle underlying mainland britain down to at least 200 km depth revealed by seismic tomography may be the prime cause of the cenozoic uplift and exhumation we use finite element modelling to demonstrate how isostatic uplift can occur in response to such a low density hotspot beneath continental crust to explain the narrow width of the uplift of northern britain the lower crust must be ductile power law rheology assumed and the asymmetrical uplift may be bounded at least on the west side by a pre existing fault or faults of appropriate polarity faulting has probably been reverse under nw se regional compression since the onset of the cenozoic with the assistance of continuing denudation inferred gross cenozoic exhumation of up to 3000 m can be explained british earthquakes concentrate along a similar north south belt with the strongest events in the west we suggest that the earthquakes result from the continuing tectonics associated with the hot upper mantle the uplift it causes and the weakened crust the underlying low density region gives rise to tensional loading stress in all directions and bending stresses are associated with the upper crustal flexuring accompanying uplift these large stresses supplement nw se regional compression available earthquake mechanisms are approximately consistent with this stress environment

inaa of pre contact basalt quarries on the samoan island of tutuila a preliminary baseline for an artifact centered provenance study abstract this project presents a material centered instrumental neutron activation analysis inaa characterization of 120 geologic samples selected from four fine grained basalt quarries on the samoan island of tutuila previous attempts at definitive differentiation of these tutuilan quarries have utilized x ray fluorescence xrf in this study clear differentiation of each analyzed quarry was achieved using inaa biplots of canonical discriminant analysis cda scores for the inaa data illustrate clear separation based on the variation in chemical composition between each quarry the samples analyzed not only define quarry separation but also provide the core group for a preliminary baseline necessary for future artifact centered provenance studies inclusion of these core group samples in the baseline was confirmed by stepwise discriminant analysis these findings suggest the ability to determine artifact quarry of origin on the island of tutuila which can elucidate the importance of individual tutuilan quarries in the production distribution and consumption of fine grained basalt artifacts in polynesia

igneous processes and closed system evolution of the tharsis region of mars a quantitative petrologic model for the formation of the tharsis rise of mars has been developed that is consistent with global gravity and topography data partial melting of the mantle underlying the tharsis plateau results in the depletion of basaltic chemical components in the source region iron is partitioned preferentially into the basaltic liquid so that the density of the source region is lowered relative to surrounding unmodified mantle basalt is erupted to the surface and or intruded at shallow levels in the crust where it solidifies to low pressure mineral assemblages or perhaps glass these low pressure materials are less dense than their mantle equivalents hence the net volume of the tharsis crust mantle system is increased resulting in the prominent topographic rise with no net gain or loss of mass the distribution of densities with depth is quantitatively consistent with the isostatic models of sleep and phillips 1979 1985 typical values of parameters that are constrained by the gravity and topography data are 150 km crustal thickness 250 km elastic mantle thickness 26 partial melting in the mantle source region and 32 addition of intrusive material to the crust this closed system petrologic process is strictly constructional in nature but shallow intrusion of magma into the crust can cause uplift of overlying strata as advocated by r j phillips and n h sleep submitted manuscript 1987

ancient and modern slopes in the tharsis region of mars the directions of lava flows in the tharsis region of mars are used to identify regional palaeo slopes vent areas and local topography a comparison is made between these flow directions and the present day radar measured topography good agreement between these data sets indicates that little infra regional tectonic deformation has occurred following the emplacement of the preserved lavas

plate boundary forces at subduction zones and trench arc compression abstract viscoelastic finite element modelling has been used to study the state of stress in the overriding and subducting plates meeting at a subduction zone the subduction fault is included using the dual node technique it is demonstrated that substantial horizontal deviatoric compressive stress occurs in the trench arc region as a result of the downpull of the dense slab and the associated surface depression including the trench and other downflexing of the plates this may be masked at the trench by bending stress it is the lack of significant shearing stress along an unlocked subduction fault in the presence of this compressive stress that gives rise to the slab pull and trench suction plate boundary forces slab pull and trench suction were found to be of comparable magnitude within the range 1 0 to 4 0 10 12 n m in models studied with vertical subduction and there are indications that this may also apply when the slab dips at 45 as a result of viscous flow induced by rollback when the slab dips beneath the arc backarc region it is shown that horizontal deviatoric compression can occur in this region contemporaneous with plate interior tension produced by trench suction this suggests that backarc tension associated with marianas type trench arch systems may be related to the nearly vertical slab whereas backarc compression in the chilean type may result from the low dip and small downpull of the slab it is also shown that successive locking and unlocking of the subduction fault may give rise to large variations of stress in plate interiors

continental mafic dyke swarms as tectonic indicators an example from the vestfold hills east antarctica abstract the two main types of magma that were emplaced as mafic dyke swarms in the vestfold hills are high mg tholeiite hmt and fe rich tholeiite frt the former magma type is less dense than the vestfold hills felsic crust whereas the latter is more dense at deep to intermediate crustal levels therefore frt dyke emplacement requires a higher deviatoric extensional stress as is reflected in a more constant orientation palaeoproterozoic hmt frt bimodal dyke swarms were emplaced around 2 4 ga and 2240 ma the geometry and orientation of 2240 ma hmt dykes imply a low deviatoric extensional stress field inconsistent with a mantle plume origin for magmatism current petrogenetic models for hmt magmatism suggest generation at active convergent plate boundaries although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2 5 ga the exact mechanisms for melting and the generation of crustal extensional stress are unknown a possible scenario involves the sinking of gravitationally unstable oceanic crust a major mesoproterozoic magmatic extensional event in the vestfold hills started with normal faulting followed by lamprophyric dyke emplacement and by two distinct episodes of frt dyke emplacement at 1380 and 1245 ma a consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift by comparison with other mafic dyke swarms this is interpreted to be due to a sheet of upwelling asthenosphere elongated plume around 1 4 ga rather than a low angle detachment this upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 ma erosion during a long period of uplift prior to 1380 ma was followed by sedimentation during thermal subsidence as suggested by segmentation of 1245 ma dykes the lithospheric structure inherited from the mesoproterozoic magmatic extensional event was probably reactivated during two high grade tectono metamorphic events around 1 0 and 0 5 ga the vestfold hills formed part of a relatively stable craton peripheral to the mobile belt that was formed during these events

dike complexes in american samoa abstract coherent dike complexes in which dikes make up more than 50 of the total rock occur in american samoa in two volcanoes on tutuila and in ofu olosega volcano in the manu a islands the dikes are remarkably narrow median width 0 24 0 36 m average width 0 46 0 56 m based on 860 measured dikes significantly narrower than dikes in the koolau volcano oahu hawaii the most closely comparable volcano for which quantitative data are available the vesicularity of these narrow dikes indicates that they were injected at shallow depths below the land surface depths of 200 900 m are inferred from the topography many dikes have prominent bands of vesicles parallel with their margins they tend to form clusters where the weakness of the vesicular rock favored center injection of one dike up the center of another examples are found of center injection repeated up to eight times local breccias often composed largely of dike fragments evidently brecciated in situ occur where irregular dikes were accommodated by deforming the host rocks we see evidence for two dike facies 1 narrow and vesicle banded early formed and shallow injected clustered dikes in the dike complexes which are cut by 2 scattered broad and almost nonvesicular deep injected solitary dikes surface eruptions were fed by solitary dikes the ene trending dikes and elongation of tutuila island contrast with the wnw trend of rift zones and island elongation elsewhere in samoa tutuila is nearly in line with the north fiji transform fracture zone suggesting that at 1 5 1 0 ma when tutuila was formed the volcanism of tutuila was guided by a temporary extension of this fault zone into the pacific plate fault movement offset the samoan hotspot trace by 45 km the en echelon arrangement of dike exposures on tutuila is consistent with left lateral transcurrent motion above this concealed fracture zone

structural attitudes of large scale layering in valles marineris mars calculated from mars orbiter laser altimeter data and mars orbiter camera imagery abstract valles marineris located on the flank of the tharsis ridge uplift on mars exposes layering within the canyon walls interpreted to be volcanic flood lavas by combining 1 128 1 128 mars orbiter laser altimeter elevation data with wide angle mars orbiter camera images using orion structural analysis software we computed the attitude of some of this large scale layering multilinear regression was used to fit planes to three dimensional coordinates of points selected along exposed layer traces giving the plane attitude and various fitting statistics by measuring the same layer using different images we found the measurements to be quite reproduceable errors in dip angle were typically only a few degrees or less analysis of the data indicates that most layers dip gently into the adjacent chasma we interpret this orientation to be the result of the crustal subsidence probably related to the formation of the early collapse basins rather than the result of rotations produced by extensional faulting since the dip is consistent far away from the edge of the current chasmata we suggest that the scale of the depressions was on the order of hundreds of kilometers exceeding the dimensions of the current chasmata

volcanoes and volcanic provinces martian western hemisphere the recognition of some martian landforms as volcanoes is based on their morphology and geologic setting other structures however may exhibit classic identifying features to a varying or a less degree these may be only considered provisionally as having a volcanic origin regional geologic mapping of the western hemisphere of mars from viking images has revealed many more probable volcanoes and volcanotectonic features than were recognized on mariner 9 pictures these abundant volcanoes have been assigned to several distinct provinces on the basis of their areal distribution although the olympus tharsis region remains as the principal center of volcanism on mars four other important provinces are now also recognized the lowland plains tempe terra plateau southern highlands in the phaethontis and thaumasia quadrangles and a probable ignimbrite province situated along the highland lowland boundary in amazonis planitia volcanoes in any one province vary in morphology size and age but volcanoes in each province tend to have common characteristics that distinguish that particular group

radar images of mars full disk images of mars have been obtained with the use of the very large array vla to map the radar reflected flux density the transmitter system was the 70 m antenna of the deep space network at goldstone california the surface of mars was illuminated with continuous wave radiation at a wavelength of 3 5 cm the reflected energy was mapped in individual 12 minute snapshots with the vla in its largest configuration fringe spacings as small as 67 km were obtained the images reveal near surface features including a region in the tharsis volcano area over 2000 km in east west extent that displayed no echo to the very low level of the radar system noise the feature called stealth is interpreted as a deposit of dust or ash with a density less than about 0 5 gram per cubic centimeter and free of rocks larger than 1 cm across the deposit must be several meters thick and may be much deeper the strongest reflecting geological feature was the south polar ice cap which was reduced in size to the residual south polar ice cap at the season of observation the cap image is interpreted as arising from nearly pure co 2 or h 2o ice with a small amount of martian dust less than 2 percent by volume and a depth greater than 2 to 5 m only one anomalous reflecting feature was identified outside of the tharsis region although the elysium region was poorly sampled in this experiment and the north pole was not visible from earth

combined escape tectonics and subduction rollback back arc extension a model for the evolution of tertiary rift basins in thailand malaysia and laos the tertiary rift basins of thailand and adjacent countries show considerable variability in the timing of rift initiation termination the timing and magnitude of thermal subsidence and the timing and intensity of inversion episodes the rift basins developed on continental blocks that were extruded southeastwards hence their development must be tied into himalayan extrusion tectonics current tectonic models propose that the tertiary basins opened up as pull apart basins associated with strike slip faults published geochronology of strike slip fault zone rocks mapping of fault patterns in the tertiary basins and mapped releasing restraining bend geometries all indicate that in thailand major sinistral strike slip motion ceased at about 30 ma prior to the formation of most rift basins the thailand the effects of later dextral slip were minor and probably a result of reactivation during episodes of inversion during nw se to ne sw himalayan compression dextral slip was not responsible for opening most of the rift basins in thailand an alternative mechanism to open the rift basins is subduction rollback of the indian plate to the west of thailand it is proposed that subuction rollback can help explain some of the characteristics of the rift basins such as non uniform lithospheric extension deep sag basins and the diachronous onset and termination of rifting

tectonic collision processes after plate rupture abstract rupture of a continental plate subducted below a forearc produces a fold and thrust mountain belt with fast overthrusting of nappes post rupture plate unflexing leading to reflexure provides a mechanism for foreland basin formation application to the australia banda arc collision accounts for the origin of the timor trough its imbrication and contemporaneous extension in outer arc as well as reversal of subduction direction after the emplacement of nappes

stresses and plate boundary forces associated with subduction plate margins primary tectonic stress in the lithosphere is predominantly caused by lateral density variations within the earth and associated topographical loading when such a stress system caused by major sublithospheric density anomalies is intersected by a weak zone which cuts across the lithosphere plate boundary forces develop and modify the plate interior stresses in this paper unite element analysis is used to model the stresses and plate boundary forces associated with subduction plate boundaries with dipping and vertical slabs extending to about 270 and 400 km depths having the subduction fault both locked and unlocked in such regions several types of horizontal deviatoric stress may occur including 1 local compression in the trench arc region caused by the dense sinking slab and the associated surface downflexure 2 plate interior tension which occurs when this compression is intersected by a weak subduction fault 3 local tension associated with thickening of the crust at the arc and elsewhere 4 local tension in the back arc region produced by the underlying low density upper mantle and 5 downbending stresses in the subducting slab thermal stresses and transmitted ridge push which are not included in the modelling here a gradient from compression in the forearc to tension in the back arc can be modelled in terms of these stresses when the fault is partially locked it is however the intersection of the local compression 1 by an unlocked or partially locked subduction fault that modifies the plate interior stresses and gives rise to the slab pull and trench suction plate boundary forces the state of stress in the interior of the overriding plate is also crucially influenced by back arc spreading where this occurs plate boundary forces have been evaluated for each of the models it is shown that slab pull and trench suction may be of comparable magnitude

dimensions of the late cretaceous paleocene northeast atlantic rift derived from cenozoic subsidence abstract the distribution of cenozoic subsidence across northeast atlantic volcanic margins have been evaluated to define the width of the rift zone and magnitude of extensional deformation the subsidence profiles are corrected for the effects of lower crustal magmatic bodies emplaced during continental break up the dimensions of the bodies have been derived from the crustal velocity structure the width of the late cretaceous paleocene northeast atlantic rift zone was more than 300 km and the lithospheric extension factor increases gradually towards the line of continental separation a large number of high quality seismic reflection data tied to scientific and commercial wells reveals that the initiation of extensional deformation preceded continental separation by 18 m y on the voring margin off norway these results show that the northeast atlantic volcanic margins commonly considered as typical volcanic margins indeed have similar dimensions as non volcanic margins and as continental rifts thus these margins contrast significantly with previously suggested evolutionary models based on narrow rift zones and formation during rapid lithospheric failure the wide rift is compatible with volume of igneous rocks observed along these margins and with a thermal anomaly similar to that associated with production of northeast atlantic oceanic lithosphere

tectonic evolution of the fundy rift basin canada evidence of extension and shortening during passive margin development seismic and field data show that the fundy rift basin of southeastern canada experienced two distinct episodes of deformation during mesozoic time the first episode during middle triassic to early jurassic time was extensional rifting associated with nw se extension reactivated ne trending paleozoic compressional structures as normal faults forming the northwestern boundary faults of the fundy basin displacements on the low angle boundary faults locally exceeded 10 km rifting also reactivated east trending paleozoic compressional structures as oblique slip faults with normal and sinistral strike slip components forming the northern boundary faults of the fundy basin several kilometers of sediments and lava flows filled the basin during rifting the second deformational episode occurred during or after early jurassic time and probably before or during early cretaceous time inversion associated with nw se shortening occurred along all faulted margins of the fundy basin the northwestern boundary faults experienced several kilometers of reverse displacement broad anticlines developed within their hanging walls and the fundy basin acquired its synclinal form the northern boundary faults of the fundy basin became oblique slip faults with reverse and dextral strike slip components gentle synclines tight anticlines and faults with reverse separation deformed the synrift strata near the northern margin of the fundy basin neither collision nor subduction zones existed near the fundy basin during mesozoic time hence we believe that tectonic processes associated with seafloor spreading e g incipient ridge push forces continental resistance to plate motion produced the shortening in the fundy basin shortening occurred during the transition from rifting to drifting as north america separated from northern africa and or during the early stages of drifting as the seafloor spreading centers of the north atlantic propagated northward

plate moving mechanisms constraints and controversies the nature of the driving forces for plate tectonics has been a subject for intense often heated debate for more than a quarter of a century on 15 april 1992 peter ziegler delivered the annual william smith lecture to the geological society on the subject of plate moving mechanisms followed by a series of lectures on related themes this thematic set of papers resulting from the meeting covers some of the major issues discussed for more than a quarter of a century earth scientists have debated the nature of the forces which drive the lithospheric plates one of the major problems has been whether mantle convection including the activity of mantle plumes dominates or whether boundary and plate forces such as slab pull and ridge push exert the most important controls on plate motions clearly the relative importance of the different forces involved may vary both in space and time and they may either reinforce or counterbalance each other depending upon the tectonic setting on 15 april 1992 peter ziegler delivered the annual william smith lecture to the geological society on the subject of plate moving mechanisms this was followed by a further eight presentations on related themes by an international group geoscientists from europe and north america of the nine papers presented at the meeting six are included in this thematic set ziegler bott wilson searle et al doglioni pavoni the other speakers were randall m richardson university of arizona tuscon usa john f dewey oxford and sierd cloetingh free university

the breakup sequence and associated lithospheric breakup surface their significance in the context of rifted continental margins west iberia and newfoundland margins north atlantic regional 2d seismic reflection profiles and borehole data are used to characterise the syn to post rift transition in the shallow offshore porto basin and in deep offshore regions of west iberia and newfoundland east canada the interpreted data highlight the development of a regional stratigraphic surface at the time of complete lithospheric breakup between west iberia and newfoundland this surface usually called breakup unconformity is renamed in this work as lithospheric breakup surface lbs on the basis that 1 it is not always developed as an unconformity and 2 all lithosphere is involved on the breakup process not only the continental crust depositional changes occur across the lbs in association with late aptian lithospheric breakup which is marked by the deposition of a breakup sequence bs rather than a single stratigraphic surface stratigraphic correlations between strata in shallow and deeper parts of the two margins lead us to propose the breakup sequence bs as representing the transitional period between lithospheric breakup and the establishment of thermal relaxation as the main process controlling subsidence on divergent continental margins the results in this work are important for other continental margins as they demonstrate that during lithospheric breakup significant quantities of sediment bypassed the inner proximal margins of west iberia and newfoundland on their way to the outer proximal margin in addition the interpreted data show that complete lithospheric breakup between conjugate margins is recorded by similar tectono stratigraphic events in iberia and newfoundland these events are associated with reservoir successions in sediment overfilled basins and with carbon rich strata black shales in sediment starved basins

modelling the plate driving mechanism this paper demonstrates how sub lithospheric loading gives rise to plate interior stresses and plate boundary forces the stressing of the lithosphere resulting from plume heads in the upper mantle hot spots from low density asthenospheric upwelling beneath ocean ridges and from dense subducting slabs has been modelled by finite element analysis it is first demonstrated how a sub lithospheric load exemplified by a sub continental plume head produces local deviatoric stress in the strong upper lithosphere within a plate interior region it is shown that a 500 km wide region of anomalous density averaging 8 kg m 3 between 100 km and 400 km depths gives rise to a deviatoric tension of 75 mpa in a 20 km thick strong layer forming the upper lithosphere above the deep buoyant load the large stresses in the strong layer are caused by the shear drag and excess pressure exerted by the buoyant load when such a stress system is cut across by a zone or plane of weakness then plate boundary forces and distant plate interior stresses are produced a weak zone beneath the crest is incorporated in a model of a normal ocean ridge and this yields a ridge push force of 2 5 10 12 nm 1 referenced to old ocean floor and deviatoric compression of about 40 mpa in old ocean floor a model of a ridge underlain by an anomalously low density upper mantle plume head yields a much larger ridge push force and large deviatoric compressions of nearly 100 mpa extend into the bordering continents subduction pull slab pull and trench suction of about 4 5 10 12 nm 1 is modelled for a 300 km deep slab separated from the overriding plate by a weak fault and collision pull is shown to result from downbulging lower lithosphere beneath collision mountain ranges

valles marineris mars wet debris flows and ground ice abstract detailed study of the valles marineris equatorial troughs suggests that the landslides in that area contained water and probably were gigantic wet debris flows one landslide complex generated a channel that has several bends and extends for 250 km further support for water or ice in debris masses includes rounded flow lobes and transport of some slide masses in the direction of the local topographic slope differences in speed and emplacement efficiency between martian and terrestrial landslides can be attributed to the entrainment of volatiles on mars but they can also be explained by other mechanisms support that the wall rock contained water comes from the following observations 1 the water within the landslide debris must have been derived from wall rock 2 debris appears to have been transported through tributary canyons 3 locally channels emerged from the canyons 4 the wall rock apprarently disintegrated and flowed easily and 5 fault zones within the troughs are unusually resistant to erosion the study further suggests that in the equatorial region of mars material below depths of 400 800 m was not desiccated during the time of landslide activity within the last billion years of martian history therefore the martian ground water or groundice reservoir if not a relic from ancient times must have been replenished

arecibo radar imagery of mars the major volcanic provinces abstract we present earth based radar images of mars obtained with the upgraded arecibo s band 12 6 cm radar during the 2005 2012 oppositions the imaging was done using the same long code delay doppler technique as for the earlier pre upgrade imaging but at a much higher resolution 3 km and for some regions a more favorable sub earth latitude this has enabled us to make a more detailed and complete mapping of depolarized radar reflectivity a proxy for small scale surface roughness over the major volcanic provinces of tharsis elysium and amazonis we find that vast portions of these regions are covered by radar bright lava flows exhibiting circular polarization ratios close to unity a characteristic that is uncommon for terrestrial lavas and that is a likely indicator of multiple scattering from extremely blocky or otherwise highly disrupted flow surfaces all of the major volcanoes have radar bright features on their shields although the brightness distribution on olympus mons is very patchy and the summit plateau of pavonis mons is entirely radar dark the older minor shields paterae and tholi are largely or entirely radar dark which is consistent with mantling by dust or pyroclastic material other prominent radar dark features include the fan shaped deposits possibly glacial associated with the three major tharsis montes shields various units of the medusae fossae formation a region south and west of biblis patera where stealth deposits appear to obscure tharsis flows and a number of dark halo craters with radar absorbing ejecta blankets deposited atop surrounding bright flows several major bright features in tharsis are associated with off shield lava flows these include the olympus mons basal plains volcanic fields east and south of pavonis mons the daedalia planum flows south of arsia mons and a broad expanse of flows extending east from the tharsis montes to echus chasma the radar bright lava plains in elysium are concentrated mainly in cerberus and include the fluvio volcanic channels of athabasca valles grjota valles and marte valles as well as an enigmatic region at the southern tip of the cerberus basin some of the cerberus bright features correspond to the distinctive platy ridged flows identified in orbiter images the radar bright terrain in amazonis planitia comprises two distinct but contiguous sections a northern section formed of lavas and sediments debouched from marte valles and a southern section whose volcanics may derive in part from local sources this south amazonis region shows perhaps the most complex radar bright structure on mars and includes features that correspond to platy ridged flows similar to those in cerberus

a simple model of whole mantle convection isotopic evidence of the chemical variability of basalts implies the existence of distinct long lived chemical reservoirs in the mantle this has commonly been interpreted as evidence that mantle convection is layered since known models of whole mantle convection have too much shearing to allow preservation of these reservoirs however the geochemical evidence requires at least three reservoirs and models of layered convection have dynamical difficulties casting serious doubt on the viability of these models this difficulty has been resolved in the present paper by the development of a simple model of whole mantle convection which allows the preservation of chemically distinct reservoirs in this model heating of the mantle from below produces axisymmetric plumes which carry heat upward through the mantle without causing overturning hence the lower mantle is much less sheared than in the usual two dimensional model mantle convection is driven by the penetration of cold lithospheric slabs from above it is argued that the pattern of convection is governed primarily by the dependence of viscosity on temperature and this dependence drives the mantle toward an isoviscous state these ideas are quantified in a simple one dimensional model of mantle convection in which the slab mass flux as a function of radius is governed by the requirement that the temperature profile remains isoviscous the resulting convection cycles material through the upper mantle more rapidly than through the lower mantle this provides a mechanism for an initially homogeneous mantle to become chemically stratified with a depleted upper mantle and primitive lower mantle

the spatial distribution of rocks on mars abstract the spatial distribution of rocks exposed at the surface of mars has been mapped using viking infrared thermal mapper irtm observations overall there are no regions on the surface of mars at a scale of 1 1 in latitude and longitude that are rock free the modal value of rock abundance is 6 areal coverage with no areas having more than 30 35 rock cover the model developed to determine rock abundance relates the thermal emission in each of the four surface sensing irtm bands to temperature contrasts within the field of view non unit thermal emissivity due to absorption bands in the surface materials and the absorption and scattering of the outgoing energy by atmospheric dust and water ice each of these effects produce characteristic spectral and diurnal signatures allowing them to be separated the temperature contrasts provide a means to determine both the abundance of exposed rocks and the fine component thermal intertia rock abundance alone does not produce the observed variation in bulk thermal inertia of the surface low inertia fine bright surfaces have fewer rocks exposed than do high inertia coarse dark surfaces rock abundance does not correlate well with the rms slope nor reflectivity determined from delay doppler radar measurements there is however a possible correlation between rms slope determined from continuous wave radar observations dual polarization radar measurements which provide the best radar measure of small scale roughness indicate that the tharsis volcanic region is very rough whereas thermal measurements indicate very few rocks and a covering of dust together these observations suggest an approximately 1 m thick mantle of fines covering a very rough subsurface this fine material may be an aeolian deposit of dust storm fallout valles marineris together with several major channels and chaotic terrains have high rock abundances as does the acidalia planitia basin this observed distribution may reflect the initial deposition of coarse material associated with channel formation syrtis major is unique in having a high inertia but a very low rock abundance together with low radar roughness these observations are consistent with mantling of rocks by sand the observed distribution of rocks and fines indicates that aeolian processes both erosional and depositional play a dominant role in shaping the present martian surface

mars stratigraphy and gravimetry of olympus mons and its aureole olympus mons and its surroundings have been investigated using impact crater statistics crater retention ages of all major geological units show that their emplacement occurred during three time intervals the underlying fractured plains are part of the northern plains in general n 1 4 10 3 km 2 this time is followed by the time interval represented by crater retention ages n 1 8 10 4 to 3 10 3 km 2 some lava flows in the fractured plains the smooth plains and the aureole materials were emplaced during this time the aureole probably developed by some sort of collapse of the outer parts of an ancient volcano proto olympus mons underlying the present shield its remnants may show in the scarp and small parts of the shield not covered by younger lava flows the third interval covers crater retention ages n 1 5 10 5 to 3 10 4 km 2 this volcanic activity followed a period of dormancy a series of lava flows forming caldera and present shield surface and flooding a depression around the shield are distinguishable the regional stratigraphy is discussed as part of the global geological history the question of the genesis of the aureole material is addressed by gravity models the topographic mass of olympus mons contributes about 80 to the aureole gravity high northwest of the mountain the gravity high of the entire aureole can be explained by a density model consisting of the topographic masses of olympus mons and the aureole materials it does not require additional high density materials underneath the aureole thus aureole materials of an allochthonous origin by collapse are in agreement with the gravity data however the gravity data do not preclude an autochthonous and volcanic origin of the aureole materials in the latter case possible subsurface intrusions should have no density contrasts with the surroundings

mars paleostratigraphic restoration of buried surfaces in tharsis montes volcanism in the tharsis province of mars occurred in several different areas and was generally continuous without large time intervals between eruptive episodes major lava flow units are numerous and extensive but relatively thin in many places impact craters on buried surfaces project above younger flows that overlie them a new application of crater dating methods has been developed to aid in the identification of these buried surfaces and to determine their lateral extent the technique is especially adaptable to the tharsis region where the stratigraphic succession of major flow units has been established by detailed geologic mapping knowledge of the overall stratigraphy allows correlations to be made between known and unknown surfaces by comparing their crater frequencies at diameters large enough to insure their recognition on the buried unit the method has been applied to aid in the restoration of buried rock units and to construct a series of paleostratigraphic maps showing the sequence of major eruptive events in the tharsis region

tectonic controls on magmatism associated with continental break up an example from the parana etendeka province the high and low ti basalts of the parana etendeka province were primarily derived from old trace element enriched source regions in the lithospheric mantle and they are associated with dyke swarms of different orientations these swarms appear to reflect different amounts of extension and it is inferred that the high and low ti magma types were characterised by different melt generation rates of 0 15 km3 yr 1 and 0 4 km3 yr 1 respectively stewart et al earth planet sci lett 143 1996 95 109 there is probably a gap of 2 myr between the end of the main phase of cfb magmatism and the oldest rocks on the adjacent ocean floor a simple numerical model has been used to constrain the amounts and rates of melt generated from the continental lithosphere and asthenosphere under finite duration extension melting in the mantle is assumed to be controlled by the dry peridotite solidus in the asthenosphere and the hydrous 0 2 h2o peridotite solidus in the lithosphere for a maximum of 4 and a duration of extension of 10 myr the derivation of melt from the asthenosphere by dry peridotite melting depends primarily on potential temperature tp and is relatively insensitive to the thickness of the mbl while the converse is the case for melt derived from the lithosphere by hydrous peridotite melting for a tp of 1450 50 c inferred from the crustal thickness estimates along the rio grande rise and walvis ridge the model successfully generates 2 4 km of lithosphere derived melt before producing significant volumes of asthenosphere derived melt it is concluded that increases of melt volume with time can be generated by decompression melting of the mantle lithosphere critically in areas of significant melt generation within the mantle lithosphere during extension and break up there is likely to be a gap in the volcanic record between the end of melt generation in the lithosphere and the onset of melting in the underlying asthenosphere no such gap is present if all melts are generated within the mantle plume and thus these models may in principle be tested in the geologic record

tectonic tests of proposed polar wander paths for mars and the moon abstract we have tested the polar wander paths recently proposed for mars by schultz and lutz garihan and for the moon by runcorn through a comparison of the lithospheric stress field predicted for rapid global reorientations against observed tectonic features we have employed the theory of vening meinesz and of melosh to calculate the reorientation stresses and we argue that the formation of normal faults or graben in broad regions surrounding the former rotation poles should be the minimum tectonic signature of a reorientation that generates lithospheric stresses in excess of the extensional strength of near surface material such regions of normal faults are not present in the vicinity of the most recent proposed paleopoles for mars despite the large magnitude of the predicted shear stress 1 2 kbar the minimum tectonic criterion would not be relaxed by invoking gradual polar wander or by considering the superposition of stresses associated with the global lithospheric response to the tharsis rise we conclude that polar wander of the magnitude and timing proposed by schultz and kutz garihan did not occur it follows either that tharsis has always been located near the martian equator or that tharsis began to dominate the nonhydrostatic figure prior to the end of heavy bombardment so that any tectonic signature of reorientation has since been obliterated by cratering the predicted directions of stresses that would result from the most recent episode of proposed polar wander on the moon including stresses produced by reorientation of both the rotational and tidal figures show little or no correspondence to observed tectonic features in the vicinity of the postulated nearside paleopole the magnitude of the predicted reorientation stress is at most a few tens of bars however so that the tectonic test of polar wander on the moon is inconclusive

aeromagnetic data from the southern weddell sea embayment and adjacent areas synthesis and interpretation a new regional aeromagnetic anomaly map covering the southern weddell sea and surrounding areas was produced by combining russian and british magnetic anomaly data acquired between 1973 and 1989 the map is divided into four zones on the basis of characteristic anomaly wavelength antarctic peninsula weddell sea embayment haag nunataks and the western coast of east antarctica over the southern weddell sea and the ronne and filchner ice shelves the magnetic field is relatively smooth indicating a thick sedimentary cover although there are some distinctive anomalies a broad positive anomaly is clearly visible running w e across the north of the area which is interpreted as representing the continent ocean boundary the most prominent magnetic feature is the elongate explora anomaly which runs parallel to the coast of east antarctica and continues over berkner island terminating at the dufek massif we interpret the source of this anomaly as rift related volcanic rocks probably emplaced around the same time as the dufek intrusion the rift itself is interpreted as running south between the andenes escarpment and east antarctica and terminating south of berkner island russian gravity profiles show that berkner island does not have an associated bouguer anomaly although a large positive anomaly approx 80 mgal is seen to the east of the island over the filchner ice shelf suggesting the presence of a second rift structure gravity modelling across these features shows thinned crust below the ronne and filchner ice shelves indicating that the whole area has undergone significant extension a gravity model across the flanks of berkner island is used to estimate the ratio of unstretched to stretched crustal thickness the resulting value of approximately 2 7 indicates that enough extension has taken place to induce melting supporting the idea that the source of the magnetic anomaly over berkner island is rift related volcanic rocks

martian volcanic materials preliminary thickness estimates in the eastern tharsis region mars displays a variety of volcanic materials from high standing constructs to smooth plains these materials are evaluated in terms of their susceptibility to thickness determination and a representative area is analyzed the martian ridged plains are much like lunar maria in both surface expression and apparent mode of emplacement flooded craters within the ridged plains materials provide an estimate of the thickness distribution in the tharsis region ridged plains materials occur as part of the layered sequence on the east flank of the tharsis dome partially buried craters indicate that the ridged plains materials range from zero meters thick at their eastern limit to over 1 5 km thick westward on the flank of the dome where they are overlain by younger materials these materials are responsible for only a small portion of the total relief of the tharsis dome

modelling the loading stresses associated with active continental rift systems abstract the loading stress associated with an ideal active continental rift system underlain by a hot low density upper mantle such as that of baikal or east africa is modelled at various possible stages of evolution using elastic viscoelastic finite element analysis the first set of models excludes faulting an incipient hot upper mantle with anomalously low density of only 6 kg m 3 between 400 and 200 km depths produces a small uplift of 300 m and deviatoric tension of up to 70 mpa possibly sufficient to initiate a rift system the stress is transferred upwards into the upper elastic lithosphere by a combination of excess pressure and shearing stress produced by the deep load much larger deviatoric tension of up to 230 mpa occurs when the hot low density region extends upwards to the base of the lithosphere so as to produce 1 5 km isostatic uplift the tension is further increased by an asthenospheric upwelling but a small opposing compression is caused by crustal thinning in contrast local loading stresses associated with simple passive rifting are small symmetrical faulting is incorporated in the next set of models and a graben forms in response to the deviatoric tension the rate of faulting is critically influenced by the underlying viscosity distribution faulting slightly reduces the deviatoric tension except within the graben where it is generally slightly increased it is shown that the local loading tension can produce subsidence of a sediment loaded rift graben of 8 km or more by constraining the distant edge of a model with a faulted or otherwise weak central rift it is shown that a substantial rift push force can develop which may be of importance in the continental splitting mechanism the modelling explains the occurrence of tension in the present day uplifted rift systems which were initiated in the tertiary in presence of predominant continental compression it suggests that a hot upper mantle produced by a plume can initiate continental rifting in association with directional plate interior stresses and basement weaknesses

tectonic history of the syria planum province of mars we attribute most of the development of extensive fractures in the tharsis region to discrete tectonic provinces within the region rather than to tharsis as a single entity one of these provinces is in syria planum faults and collapse structures in the syria planum tectonic province on mars are grouped into 13 sets based on relative age areal distribution and morphology according to superposition and fault crosscutting relations and crater counts we designate six distinct episodes of tectonic activity in the following chronologic order stage i is an early to late noachian deformation forming mostly east west structures fault set ia some large volcanoes also formed faults were produced possibly by flexural uplift also arcuate north trending grabens set ib indicate that faulting is transitional to the next stage stage ii is late noachian to early hesperian radial faulting centered in syria planum possibly due to isostatic uplift in late noachian set iia to early hesperian set iib time stage iii is early to late hesperian faulting tangential to syria planum that was related to local centers of uplift sets iii1 iii3 on the periphery of syria planum stage iv is a late hesperian graben formation that was circumferential to syria planum set iva caused either by collapse associated with eruption of magma or by flexure of the lithosphere due to volcanic loading in association with volcanism minor faulting occurred producing an oval pattern of faults in southwest syria planum set ivbl and a radial pattern south of the planum set ivb2 that apparently rejuvenated buried stage ii faults stage v is a late hesperian to early amazonian development of grabens and troughs of noctis labyrinthus and western valles marineris set v that was probably instigated by local uplift exposure of groundwater or ground ice zones may have produced further collapse and trough enlargement stage vi is early amazonian northwest trending faulting in noctis fossae set vil perhaps due to tharsis monies centered tectonism and north northwest normal faulting along the eastern side of the claritas rise set vi2 that was due to tectonic subsidence the duration of tectonic activity in the syria planum province was perhaps 2 3 b y photoclinometric topographic profiles across 132 grabens and fault scarps show that syria planum grabens have widths average of 2 5 km and most range from 1 to 6 km similar to lunar grabens but the martian grabens have slightly higher side walls average about 132 m and gentler wall slopes average of 9 and range of 2 25 than lunar grabens 93 m high and 18 slopes scarp degradation on mars has progressed through quakes impact shaking and dry slope processes the lower slopes may be due to mars higher gravity estimates of the amount of extension for individual grabens range from 20 to 350 m most estimates of the thickness of the faulted layer range from 0 5 to 4 5 km average is 1 5 km this thickness range corresponds closely to the 0 8 to 3 6 km range in depth for pits troughs and canyons in noctis labyrinthus and along the walls of valles marineris we propose that the predominant 1 to 1 5 km values obtained for both the thickness of the faulted layer and the depths of the pits troughs and theater heads of the canyons reflect the initial depth to the water table in this region as governed by the depth to the base of ground ice maximum depths for these features may indicate lowered groundwater table depths and the base of ejecta material

structural segmentation inversion and salt tectonics on a passive margin evolution of the inner kwanza basin angola the kwanza basin angola is divided into the inner and outer kwanza salt basins separated by a chain of synrift basement highs on which aptian 112 122 ma salt is thin or absent north to northwest trending basement structures in the inner kwanza basin have repeatedly been reactivated since neocomian 144 127 ma rifting reactivation formed three northwest striking fold and thrust belts near basement uplifts the thrust belts are bounded by northeast striking rift related transfer fault zones that were apparently reactivated during subsequent shortening three episodes of postrift basement involved shortening are documented in the inner kwanza basin 1 albian early cenomanian 112 96 ma 2 senonian 89 65 ma and 3 oligocene holocene 34 0 ma we relate the albian early cenomanian event to ridge push the senonian event to global plate reorganization and oligocene holocene shortening to uplift of the african superswell structural segmentation of the inner kwanza basin controlled the evolution of salt structures adjacent to basement uplifts diapirs were initiated as buckle folds some anticlines were unroofed by erosion and evolved into passive salt walls elsewhere broad salt walls were triggered by either detached extension or basement block uplift these walls grew until they exhausted their supply of salt thereafter dissolution rates exceeded rates of salt inflow so the walls began to subside withdrawal of salt from the walls produced the elongate sedimentary troughs for which the basin is famous trough fill ranges in age from cenomanian to pliocene and this age varies greatly from trough to trough and along strike within troughs

the surface of syrtis major composition of the volcanic substrate and mixing with altered dust and soil syrtis major is an old low relief volcanic plateau near the equatorial regions of mars it is a persistent low albedo feature on the planet and is thought to contain a high abundance of exposed bedrock and or locally derived surface material and debris spatially resolved variations in surface spectral properties and therefore composition are investigated with data from the imaging spectrometer for mars ism instrument ism obtained 128 wavelength channel spectra from 0 76 to 3 16 m for contiguous pixels approximately 22 22 km in size across much of the plateau the value and spatial distribution of four primary spectral variables albedo continuum slope wavelength of the ferric ferrous band minimum area of the ferric ferrous absorption are mapped and coregistered to viking digital photomosaics analysis of these maps shows that although there is a high degree of overall spectral variability on the plateau the key indicators of mafic mineralogy are relatively homogeneous detailed examination of reflectance spectra from representative areas across the plateau indicate the volcanic surface is dominated by augite bearing basalts and the pyroxene composition in the basalts is estimated to be 0 275 0 075 ca ca fe mg and 0 3 0 1 fe fe ca mg additional mineral components may include olivine feldspar and glass most of the spectral variability on the plateau is interpreted to result from mixing of volcanic bedrock and or locally derived surface material and debris with highly altered dust and soil in western syrtis major the altered material is a transient component on the surface or occurs in large spatially coherent patches e g crater rims in eastern syrtis major it is apparent that the dust components are firmly fixed to the basaltic substrate as a stable oxide rind or coating

on the rifting dynamics of plate divergence and magma accumulation at oceanic ridge axes rifting dynamics at spreading axes is governed by two processes the large scale plate divergence and the local magma accumulation in the crust mantle transition layer both evolve simultaneously a model is developed particularly for the situation in iceland where a well studied rifting episode occurred in the krafla volcanic system 1975 1984 both the divergence and the buoyant rise of magma create tensile deviatoric stress in the axial region but while divergence generates an altogether extensional stress field uprising of buoyant melt produces tension only near the axis but compression of the sides the buoyant rise is driven by the differential pressure gradient in rock and melt the processes are studied with a two dimensional finite element routine presently thermal effects are neglected the model parameters are density difference size of the buoyant body externally applied stress field mechanical properties of rock and melt relatively small amounts of divergence and small increases of buoyancy are shown to generate axial tension which can overcome the tensile strength axial tension produced by buoyant bodies can even overcome lateral compression observed long intervals of quiescence require either large rock strength quasi continuous stress relaxation small buoyant bodies and or a compressive deviatoric stress normal to the axis during much of the time between rifting episodes buoyant rise and injection of melt must be important in generating compression

dual polarization radar observations of mars tharsis and environs abstract observations of the tharsis region of mars with the 12 6 cm radar at arecibo observatory have yielded radar echoes which are highly depolarizes and which are in terms of total echo power dominated by diffuse rather than quasi specular backscattering the observations were made on february 7 8 and 9 1980 and the subradar track extended from 77 to 126 w longitude at 22 n latitude dual polarized reception was employed i e the echo was received in the same sense of circular polarization as transmitted depolarized sense as well as in the opposite polarized sense the disk integrated ratio of depolarized power to polarized power averages 0 37 and the ratio of diffuse power to quasi specular power averages 3 2 the depolarized spectra are dominated by a broad enhancement identified primarily with echoes from the tharsis ridge implying that extensive areas of tharsis are rough on decimeter scales the major tharsis shield volcanoes are candidates for sources of strong depolarization although they alone cannot account for the entire depolarization enhancement

investigating the production and distribution of plain ware pottery in the samoan archipelago with laser ablation inductively coupled plasma mass spectrometry la icp ms this paper presents a provenance study of 170 ceramic artifacts and 21 ceramic tiles from three islands in the samoan archipelago using laser ablation inductively coupled plasma mass spectrometry la icp ms our analyses confirm that la icp ms can be used to differentiate between clay formations on a single island we identify different distribution patterns for pottery recovered from lowland and highland sites on tutuila island we also examine evidence for movement of pottery between islands and find only limited evidence for such movement our findings suggest dynamic patterns of prehistoric interaction and site use that need to be evaluated with further data from across the archipelago

young volcanic deposits in the valles marineris mars abstract a study of the interior deposits of the central valles marineris has led to the discovery of a sequence of deposits that cover the chasma floors and range in thickness from that of thin dust to several kilometers the emplacement of the deposits was the last major event in the history of the valles marineris following deposition of older layered interior beds warping faulting erosion and landslide emplacement the young deposits are of three major types 1 dark patches typically occurring along faults 2 light colored deposits locally associated with craters and 3 rugged mottled deposits with in places light colored lobate fronts these young materials may be of volcanic origin as suggested by the low albedo and spectrally gray colors of some their association with faults and possible volcanic craters and calderas and their embayment relations and lobate margins no other mechanism explains all the observed features and relations as well as volcanism if these deposits are volcanic implications are that 1 volcanism was associated with rifting in the valles marineris 2 the volcanism was explosive in places and 3 the volcanism may be as young as the late tharsis volcanism and locally may well be recent

orientation and origin of ridges in the lunae palus coprates region of mars numerous wrinkle ridges similar in appearance to lunar mare ridges occur on volcanic plains units surrounding the tharsis region of mars where they are oriented circumferential to the center of the plateau by analogy with lunar mare ridges and from morphologic evidence such as the smooth slopes of arches these features are interpreted to result primarily from compressional folding of the volcanic plains although the uppermost ridge may represent faulting of the near surface materials east of the tharsis region ridges are best developed in the lunae palus and coprates quadrangles where they can be used to indicate the degree of compressional stress occurring there comparison of ridge orientations in these quadrangles to a tharsis center at 1 n 122 w indicates that more than 60 of the total length of ridges is orthogonal to a normal from this center non orthogonal ridges are seen to have general northerly orientations after removal of the tharsis orthogonal population trends of graben in surrounding terrain indicate that non orthogonal ridges are not greatly affected by regional scale structures comparison of the frequency of orthogonal and non orthogonal ridges further suggests that loading due to the tharsis plateau was effective in producing compressional features as far as 4000 km from the center of the load additional sources for compressional stress are necessary to account for ridge systems at greater distances from the tharsis plateau