chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

on the strength of oceanic fracture zones and their influence on the intraplate stress field we use the locations and source mechanisms of oceanic intraplate earthquakes to test the hypothesis that the strength of oceanic fracture zones is less than that of normal oceanic lithosphere if fracture zones are weaker than the surrounding lithosphere they should be sites of concentrated intraplate seismicity and the lithospheric stress field should be perturbed in their vicinity the 77 earthquakes selected for the study have well determined focal mechanisms and epicenters in regions where fracture zones are well mapped we have searched for dependence of faulting style fault orientation or principal stress direction on the distance from the nearest fracture zone if fracture zones were generally weaker than the surrounding lithosphere one of the principal horizontal stresses would be oriented nearly perpendicular to the fracture zone we find no evidence that principal stresses near fracture zones are oriented preferentially in this manner there is a slight tendency for earthquakes to occur near fracture zones and patterns of fault orientation and sense of slip support the view that differential cooling and horizontal contraction on fracture zones may contribute seismogenic stress these patterns are not demonstrable at high significance however if oceanic transform fault zones are weak relative to the surrounding lithosphere these results suggest that the strength of the fracture zones increases within a few million years after cessation of transform motion to levels comparable with that of normal oceanic lithosphere

on the dynamics of the juan de fuca plate the juan de fuca plate is currently fragmenting along the nootka fault zone in the north while the gorda region in the south shows no evidence of fragmentation this difference is surprising as both the northern and southern regions are young relative to the central juan de fuca plate we develop stress models for the juan de fuca plate to understand this pattern of breakup another objective of our study follows from our hypothesis that small plates are partially driven by larger neighbor plates the transform push force has been proposed to be such a dynamic interaction between the small juan de fuca plate and the pacific plate we aim to establish the relative importance of transform push for juan de fuca dynamics balancing torques from plate tectonic forces like slab pull ridge push and various resistive forces we first derive two groups of force models models which require transform push across the mendocino transform fault and models which do not intraplate stress orientations computed on the basis of the force models are all close to identical orientations of predicted stresses are in agreement with observations stress magnitudes are sensitive to the force model we use but as we have no stress magnitude observations we have no means of discriminating between force models model stresses in the north and the south of the juan de fuca plate are similar breakup along the nootka fault zone and not in the gorda region can be explained if the local strength of a lithosphere column depends on lithospheric age stresses are invariant to whether or not transform push is acting on the mendocino transform fault we therefore conclude that the introduction of this extra plate driving force is not required by observations for the juan de fuca plate fl 2001 elsevier science b v all rights reserved

reconciling late neogene pacific absolute and relative plate motion changes new models of pacific absolute plate motion relative to hot spots and models of relative plate motion involving the pacific plate all agree there was a significant change in the late neogene chron 3a 5 89 ma reflecting a more northerly absolute motion than previously determined as pacific absolute plate motion became slightly more northerly left stepping transform segments came under compression some left stepping segments became microplates with clockwise rotation others show clear evidence of compressional deformation conversely right stepping transforms came under tension and many developed intratransform spreading centers or show similar evidence for transform magmatism several large left stepping transform offsets represent portions of the nazca plate protruding into the pacific and as such act as obstacles to the more northerly pacific absolute plate motion we suggest these obstructions act to enhance the generally tensile equatorial pacific stress regime caused by distant slab pull as a consequence the greater french polynesia region has experienced diffuse volcanism that increased considerably following the chron 3a plate motion change we propose that since the hawaii emperor bend time the distant pacific slab pull and the friction between the large buoyant ontong java plateau and the northern margin of the australia plate have produced episodes of increased tensile stresses in the equatorial pacific we believe these stress excursions are responsible for much of the intraplate volcanism observed in a wide triangular region from samoa to the east pacific and pacific antarctic rises

crustal structure of the northwestern vizcaino block and gorda escarpment offshore northern california and implications for postsubduction deformation of a paleoaccretionary margin the vizcaino block is an anomalously shallow region of the western u s continental margin located southwest of the mendocino triple junction it originated as part of the accretionary prism of the north america plate and was transferred to the pacific plate in the miocene as the pacific north america plate boundary migrated 130 km eastward forming the gorda escarpment at its northern boundary we present hybrid crustal models for the northwestern part of the vizcaino block derived from marine seismic and gravity data the velocity and density structure of the northwestern vizcaino block are compatible with paleoaccretionary complex material similar to san simeon patton terrane overlying oceanic crust or a mafic layer the most significant result of our modeling is an abrupt increase in moho dip from 5 to 20 30 beneath the western edge of the oconostota ridge along the northwestern margin of the vizcaino block this moho dip is steeper than observed anywhere along the cascadia subduction zone indicating postsubduction deformation we suggest that the paleotrench was deformed by compression which reactivated preexisting thrust faults in the upper crust and thickened the crust within this apparent weak zone at least part of the deformation predates late pliocene pacific north america plate convergence and may result mainly from north south compression between the pacific juan de fuca plates across the mendocino transform fault north south compression continues today and may dynamically support the uplifted northern margin of the vizcaino block although the primary locus of deformation shifted to the relatively weak gorda plate sometime prior to 3 ma

diagenetic replacement controlled by force of crystallization many diagenetic replacements in sedimentary rocks occur along thin solution films between the authigenic and host phases rather than conventionally invoking bulk pore water undersaturation for host phase dissolution we propose that many diagenetic replacements occur by a force of crystallization controlled replacement mechanism whereby nonhydrostatic stresses resulting from authigenic crystal growth are principally responsible for host phase dissolution this model explains diagenetic features such as the restriction of host phase dissolution to authigenic host crystal contacts and euhedral authigenic crystal faces in planar contact with unreplaced host phases as well as the relative replacement tendencies of some common authigenic minerals

a kinematic model for the evolution of the gorda plate the magnetic lineation pattern produced by the gorda rise reflects a complex tectonic history marked by propagating ridges and nonrigid deformation of the gorda plate the pacific side lineation pattern is matched using a kinematic model which incorporates northward and southward propagating ridge segments and ridge rotation by superimposing flexural slip style deformation on the resulting gorda side lineations and allowing convergent motion at the mendocino fracture zone the gorda magnetic pattern is also successfully predicted a similar model but one which does not allow convergence at the mendocino fracture zone fails to produce the observed lineation pattern model predictions of the amount of material that would be obducted at the mendocino due to convergence between the gorda and pacific agree with bathymetric and density estimates of the amount of material located along the mendocino ridge

geochemical and age constraints on the formation of the gorda escarpment and mendocino ridge of the mendocino transform fault in the ne pacific the mendocino transform fault is an active dextral strike slip zone that separates the gorda plate from the pacifi c plate in the ne pacifi c ocean the compositions of the igneous rocks exposed along the southern margin of the mendocino transform fault include tholeiitic and alkaline basalts majorelement trace element and radiometric data suggest that the rocks were generated through fractionation of different parental melts derived by varying degrees of partial melting from different depths at or near the intersection of the mendocino transform fault with the gorda ridge there is evidence for extensive cooling and fractionation reminiscent of the transform fault effect of langmuir and bender 1984 alkaline and highal compositions also argue for melts from a deeper source than a normal mid ocean ridge environment the preferred geochemical analogue for the mendocino transform fault is a failed rift system where mid ocean ridge basalt morb compositions likely represent basalts created at a waning spreading center before its abandonment the morb compositions were subsequently buried by younger enriched e morb and alkaline basalts derived from deeper melting and or a more enriched source we suggest that a period of rift failure abandonment and continued alkaline volcanism occurred on the southernmost gorda ridge or on a series of short intratransform spreading center segments during plate reorganization thus the mendocino transform fault provides a record of ridge migration abandonment and residual volcanism of the southern gorda ridge spreading system from 23 to 11 ma

carbon and oxygen isotopic composition of holocene reefal carbonates carbon and oxygen isotopic compositions have been determined for holocene reefal carbonate grains and cements in samples from enewetak bikini bermuda and belize to identify components that exhibit systematic isotopic fractionation relationships these findings are compared with fractionation relationships determined from experimentally precipitated low magnesium calcite lmc aragonite and high magnesium calcite hmc to determine if these relationships are observed in naturally precipitated carbonates compositional variation within individual samples is extreme up to 6 in 13 c pdb and 3 in 18 o pdb experimental fractionation relationships fail to predict the range of observed compositions aragonite cements are enriched 1 0 to 1 4 in 13 c and 1 5 to 2 0 in 18 o relative to calcite equilibrium compositions hmc cements are enriched by 0 3 to 0 4 in 13 c and by 1 8 to 1 9 to 18 o for carbon these results are in agreement with the experimental work of rubinson and clayton for oxygen our results suggest that fractionation factors of both aragonite and hmc may have been underestimated skeletal components exhibit inconsistent fractionation relative to inorganically precipitated marine cements suggesting that marine cement compositions are more reliable indicators of the physicochemical conditions of precipitation than co occurring skeletal carbonates

three dimensional geometry of the gorda plate beneath northern california the isostatic residual gravity field over northern california displays a gravity gradient interpreted to reflect the south edge of the gorda plate where it is subducted eastward beneath the north american plate the locus of points of maximum slope defines a line trending s60 e from a coastal point approximately 20 km south of cape mendocino a point where the buried plate boundary is inferred from magnetic and seismicity data southeast from the coast for a distance of 120 km the gravity anomaly parallels the strike of the blanco fracture zone and the present direction of relative motion between the pacific and northern gorda plates calculations from the form of the anomaly yield depth estimates that fit an east southeast plunge of approximately 9 for the top of the gorda south edge the sense of the anomaly higher gravity to the south supports the hypothesis that a window developed in the subducted slab east of the san andreas fault and south of the gorda plate south of the gorda boundary the base of the north american plate is thus in contact with hot material from the asthenosphere that invaded the window because the overlying north american plate has been moving relatively south across the gorda boundary the north american plate beneath the coast ranges east of the san andreas fault in central california may be decoupled from the underlying material at a depth slightly deeper than the depth to the top of the boundary at the time the north american plate passed over it

a kinematic model for the gorda deformation zone as a diffuse southern boundary of the juan de fuca plate several lines of evidence indicate that the space problem caused by southeastward motion of the juan de fuca plate toward the east west mendocino transform is resolved by distributed deformation of the oceanic crust north of the transform a kinematic model for this deformation based on the assumption of nondivergent flow conservation of area within the area known as the gorda plate and on the assumption of no subduction along the mendocino transform successfully matches the isochron pattern inferred from magnetic anomalies the model assumes strain rate decreasing to zero approaching the rigid juan de fuca plate with no sharp boundary or separate rigid area that would imply a discrete gorda plate this interpretation allows magnetic anomalies from south of the blanco fracture zone to be used to constrain the kinematics of the juan de fuca plate these anomalies imply a substantial change in the distance to the rotation pole at roughly 3 ma in addition to the clockwise shift in motion direction at 5 ma

an alternative interpretation for slip vector residuals of subduction interface earthquakes a case study in the westernmost ryukyu slab abstract slip partitioning along oblique subduction zones has conventionally been examined by slip vector residuals determined from interface earthquakes it is measured through contrasting the slip vector with respect to the local plate convergence direction defined on the surface of the earth by the euler vector interpretation of regional plate kinematics based on slip vector residuals thus defined may be misleading besides the apparent discrepancy attributable to the surface projection of vectors on an obliquely plunging surface a more fundamental problem is the construction of the reference subduction flow field for the subducted slab based upon the known plate convergence it can be shown that for complicated slab geometry the conventional practice of intuitively rotating the euler kinematics onto the subducted slab leads to unrealistic intraplate deformation that invalidates the mechanical integrity of the oceanic lithosphere alternatively we propose to calculate the velocity field for the specific slab geometry based on the rationale that the subduction flow field should be the one that endures the least amount of intraplate deformation the calculated flow field for the westernmost ryukyu slab that is in transition to the philippine sea plate psp eurasian plate ep collision near taiwan reveals quite distinct subduction flow field the particle paths associated with the calculated flow field are in better accordance with the observed slip vectors while the corresponding strain rate field is consistent with the previously reported lateral compression seismogenic zone furthermore it is speculated that the plate kinematics of the northwestern corner of psp that is impinging the vicinity of the collision might have also undergone significant readjustment to accommodate the potential intraplate deformation it is noted that the westernmost subducted ryukyu slab and the northwestern corner of psp belong to a mechanically coherent tectonic unit the 3d geometric constraints set up by the subducted slab and the unfriendly kinematic constraints imposed by the collision boundary both play important roles in configuring the intraplate deformation within psp manifestation in the local kinematics might indicate deviation from the general psp ep convergence and is thus of crucial importance to tectonic interpretations of geophysical observations in this area

a comparison of brittle deformation models for the gorda plate abstract the gorda plate is an actively deforming subplate of the juan de fuca system located off the west coast of north america the site of intense seismicity and unusually curved magnetic lineations the gorda has defied rigorous explanation since it was originally mapped in 1961 a variety of models for its deformation mechanism have been proposed two of which are analyzed in depth here the right lateral shear and flexural slip models are compared using constraints imposed by geometry seismicity stress and basement fabric also examined are the questions of convergent motion between the pacific and gorda plates at the mendocino transform fault and shortening by means of pure shear of the magnetic lineations while both deformation models fit most of the constraints neither fits all but the flexural slip model best fits the most well known of the constraints if in fact flexural slip is the preferred deformation mechanism lineation shortening must be ruled out in favor of convergent motion

a model for large scale plastic yield of the gorda deformation zone the lithosphere east of the gorda ridge often called the gorda plate but referred to here as the gorda deformation zone gorda dz is constricted as it spreads eastward to be subducted beneath the north american plate for the plane strain conditions defined by the seismicity the distortion of the gorda dz is analogous to the ironing of metal cups here the punch boundary is the blanco fracture zone and the die that does the cutting is the mendocino transform fault from the gorda ridge to the mendocino triple junction this analogy is useful in analyzing this problem but is not identical due to differences in boundary constraints the deformation of the gorda dz is determined for an elastoplastic lithosphere with a von mises yield criterion plate motion is constrained to be parallel both to the blanco fracture zone and to the mendocino transform fault west of 1250w the gorda ridge is considered a free boundary unlike the ironing analogy and the lithosphere is pulled through the constriction from the trench any nonlinear feedback between distortion and ridge push forces is not considered traction due to slip along the mendocino transform is varied to fit both seismicity and plate distortion for these conditions the amount of plate movement required before a constant resistance is achieved depends upon the ratio of elastic moduli to plastic yield stress and may vary from hundreds of meters to kilometers the minimum slab pull for steady movement occurs when the transform is weak lubricated and is 60 of the average plate yield stress the yield stress is determined using the temperature structure inferred from plate age and the laboratory constraints on brittle and ductile strength for olivine for an average plate age of 3 m y the yield stress is about 550 mpa so the pull required is 330 mpa when the transform is about as strong as the lithosphere the pull required is twice this value the stress directions in the gorda dz can be inferred from both deformation and seismicity the current seismic data are not precise enough to differentiate between a strong and weak transform but the deformation data are sufficient the constraints placed by distortion of magnetic isochrons on the modeling results indicate a mixed history about 5 ma changes in the position of the pole of rotation of the juan de fuca system began to increase the convergence of the gorda dz with the mendocino transform fault increasing the effective normal stress and brittle shear stress across the fault a strong fault is evident in the deformation data until about 2 ma and this corresponds to the time when convergence stabilized with a stable configuration the analysis indicates that about a kilometer of relative slip reduced the apparent strength of the transform to negligible values compared to the strength of the plate

origin of dolomitization on the barbwire terrace canning basin western australia a thick areally extensive subsurface sequence of upper devonian carbonates occurs on the barbwire terrace in the canning basin of western australia it is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized slightly deeper water mark have remained as limestones the major regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite up to 600 m with no primary calcite a small volume of evaporitic supratidal dolomite is present at one location this dolomite is derived from highly saline fluids developed in an arid supratidal environment replacement dolomite of the regional dolomite type has a xenotopic form with undulose extinction and irregular crystal boundaries in addition saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite this suggests the regional dolomite type was precipitated at slightly elevated temperatures dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution geochemically the synsedimentary supratidal and regional dolomite types are quite distinctive supratidal dolomites have 6 o values which are significantly higher 6180 2 to i pdb than the regional dolomite type 6l80 9 to 2 pdb assuming the lowest 6 o values for the sabkha dolomite represent replacement in marine waters the oxygen isotopic composition for upper devonian canning basin marine dolomite would be around 6l80 2 pdb the petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin however sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the barbwire terrace therefore it is suggested that dolomitization may have taken place in a near surface environment with a major recrystallization event superimposed during burial diagenesis

deformation of the so called gorda plate a new compilation of magnetic anomaly identifications recent side scan sonar data and other data provide constraints on the mechanism of deformation of the area commonly known as the gorda plate a sharp boundary between the rigid juan de fuca plate and deformed material trends southeast from the northern part of the gorda ridge in material older than 2 3 ma the boundary is gradational and trends more to the east previous observations that the anomalies east of the ridge are shorter than their counterparts on the pacific plate are confirmed there is no evidence for subduction or obduction along the mendocino transform adjacent to the ridge the deformation is interpreted to be distributed nw se right lateral simple shear possibly controlled by ductile deformation of the lower crust deformation of this sense is capable of producing the observed shortening the anomalies away from the ridge apparently in crust of age 2 3 ma there must be a transition to deformation on observed ne sw left lateral faults parallel to the original spreading fabric

s wave velocity structure of the upper mantle beneath the arctic region from rayleigh wave dispersion data abstract a new model of the upper mantle structure of the arctic region based on a representative dataset of rayleigh wave group velocity dispersion curves from 10 to 250 s period is presented a surface wave tomography procedure is used to image the group velocity distributions local dispersion curves are constructed and inverted to vertical s wave velocity sections up to the depth of 500 km results are presented in two forms as horizontal slices of s wave velocity variations relative to the average velocities at several mantle depths and as 2d velocity sections along three profiles crossing different tectonic units the model shows lateral and vertical velocity variations over the whole depth range within the study area the most prominent velocity heterogeneities are localized in the uppermost mantle up to the 200 km depth in this depth range the velocity anomalies are strongly correlated with the main tectonic structures the calculated s wave velocity distribution demonstrates that the upper mantle of the continental shields is characterized by high s wave velocities and contains regional velocity anomalies associated with small scale blocks low mantle velocities are attributed to the areas with high level of tectonic and seismic activity significant difference in deep velocity structure of the marginal seas and deep water oceanic basins of various ages is revealed the obtained model suggests some insights into the geodynamical evolution of the lithosphere of the arctic region

master event relocation of gorda block earthquakes implications for deformation we have used the master event technique to relocate 73 earthquakes in the northern half of the gorda block the revised epicenters indicate a systematic eastward bias of 30 km in epicenters as published in the isc bulletin the new seismicity pattern reflects a more symmetric distribution of earthquakes about the gorda rise and a reduced degree of seismicity within the gorda block although the level of intraplate activity has apparently been reduced a significant number of events 10 20 still appears to be related to intraplate deformation further relocation studies coupled with focal mechanism determinations are necessary to better constrain deformation models for this region

geothermal springs of the west florida continental shelf evidence for dolomitization and radionuclide enrichment abstract on the sea bed of the west florida continental shelf about 45 km ssw of ft myers florida an 85 km 2 area has been discovered in which six thermal springs discharge warm chemically altered seawater from vents and seepage zones the spring water apparently originates in the subsurface ocean around the florida platform and penetrates the highly porous strata of the platform about 500 1000 meters below sea level it percolates toward the interior of the platform and is geothermally heated to about 40 c en route then it rises along more vertical flow channels and is discharged in warm submarine springs beneath the platform several chemical processes alter the percolating seawater one process seems to be a secondary dolomitization of the limestone of the platform because in the discharging seawater magnesium is lower by 2 7 mmole kg and calcium higher by 3 6 mmole kg than in normal seawater with the same chlorinity other reactions within the sediments of the platform enrich the spring effluents 1000 fold in 226 ra 10 000 fold in 222 rn and 90 fold in 228 ra compared to the seawater surrounding the platform thus the springs may be important sources of radionuclides for the gulf of mexico the percolating seawater also loses all of its oxygen and nitrate to reduction processes loses most of its phosphate and 40 of its 238 u and roughly quadruples its silica content coastal carbonate platforms are fairly common geological features thus processes like those beneath the west florida shelf may function on a world wide basis to play an important role in the diagenesis of carbonate sediments

transform push oblique subduction resistance and intraplate stress of the juan de fuca plate the juan de fuca plate is a small oceanic plate between the pacific and north america plates in the southernmost region referred to as the gorda deformation zone the maximum compressive stress 1 constrained by earthquake focal mechanisms is n s off oregon and possibly off washington nw trending left lateral faults cutting the juan de fuca plate indicate a 1 in a ne sw to e w direction the magnitude of differential stress increases from north to south this is inferred from the plastic yielding and distribution of earthquakes throughout the gorda deformation zone to understand how tectonic forces determine the stress field of the juan de fuca plate we have modeled the intraplate stress using both elastic and elastic perfectly plastic plane stress finite element models we conclude that the right lateral shear motion of the pacific and north america plates is primarily responsible for the stress pattern of the juan de fuca plate the most important roles are played by a compressional force normal to the mendocino transform fault a result of the northward push by the pacific plate and a horizontal resistance operating against the northward or margin parallel component of oblique subduction margin parallel subduction resistance results in large n s compression in the gorda deformation zone because the force is integrated over the full length of the cascadia subduction zone the mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike normal compressive stresses internal failure of the gorda deformation zone potentially places limits on the magnitude of the fault normal stresses being transmitted and correspondingly on the magnitude of strike parallel subduction resistance transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

regional epigenetic dolomitization in the bonneterre dolomite cambrian southeastern missouri a dolomite bed averaging 6 m in thickness forms the base of a limestone and shale facies of the bonneterre dolomite cambrian southeastern missouri the dolomite immediately overlies the lamotte sandstone and covers an area of more than 16 600 km 2 it is proposed that this basal dolomite formed as a result of interaction with warm basin derived water circulating through the underlying lamotte sandstone and that the dolomitization event coincided with the emplacement of mississippi valley type sulfide ores in the nearby viburnum trend this hypothesis is supported by 1 petrographic evidence that suggests an origin at temperatures 50 c 2 cathodoluminescent microstratigraphy of dolomite cements which is correlative with that in gangue dolomite cements in the nearby orebodies and 3 stable oxygen and carbon isotope values that are consistent with a basinal origin of the dolomitizing water the basal dolomite bed may represent one of the most extensive epigenetic dolomites yet documented this implies that a basinal water can alter rocks over a wide area at a considerable distance from its source

the transverse tectonic features in the himalaya abstract the seismological evidence in the form of spatial distribution of earthquakes has been investigated to ascertain zones of tectonic deformation that are transverse to the himalaya arc it is discovered that two sets of cross trends are present that are roughly at forty five degrees locally with respect to the arc some trends normal to the arc have also been discerned these features correspond well with the projections of the tectonic style of deformations obtained on the basis of slip line field theory by tapponnier and molnar 1976