rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

evidence for pre folding vein development in the oligo miocene asmari formation in the central zagros fold belt iran in order to understand the interplay between vein development and folding in the carbonates of the oligo miocene asmari formation one of the main hydrocarbon reservoir rocks in iran several anticlines have been investigated in the central part of the zagros folded belt combining observations of relative chronology between veins based on calcite filling phases and crosscutting abutting relationships as well as aerial satellite image interpretation on several anticlines allowed proposing a tectonic model highlighting the widespread development of veins and other extensional micro meso structures in the central zagros folded belt our data suggest that most of the veins affecting the asmari formation predated the main miocene pliocene folding episode an early regional vein set striking n50 marked the onset of collisional stress build up in the region then n150 and n20 trending vein sets were initiated in response to local extension caused by large scale flexure drape folds above n s and n140 basement faults reactivated under the regional ne compression at the onset and during miocene pliocene folding of the sedimentary cover the early formed veins were reactivated reopened and or sheared while duplexes low angle reverse faults and thrusts formed beyond regional implications this study puts emphasis on the need of carefully considering regional local vein development predating folding as well as influence of underlying basement faults in models of folded fractured reservoirs in fold thrust belts

the generation of oceanic rhyolites by crystal fractionation the basalt rhyolite association at volcan alcedo galapagos archipelago

hf isotope geochemistry of the galapagos islands 1 we report 176hf 177hf ratios determined by multicollector inductively coupled plasma mass spectrometry on 49 basalts from the galapagos islands the hf values range from about 8 a value typical of many oceanic island basalts to about 15 a value typical of mid ocean ridge basalt the hf shows the expected correlation with nd and inverse correlations with sr and pb isotope ratios the range of values and correlations observed is consistent with galapagos mantle plume asthenosphere mixing inferred from previous geochemical studies while oceanic island basalt isotopic arrays suggest that multicomponent mixing is common in the sources of such basalts the galapagos is unusual in that depleted upper mantle appears to be a principal component thermal and dynamic interaction between the galapagos mantle plume and the lithosphere and shallow asthenosphere is the most likely cause of mixing and of the geographic pattern of geochemical variation the galapagos hf nd sr pb isotopic array diverges toward the plume end indicating the plume is heterogeneous this heterogeneity is geographic with distinct northern central and southern components the central component and to a lesser degree the northern component are compositionally similar to the common component of plumes variously called phem fozo and c while the southern component is unique with relatively high hf and 206pb 204pb compared to 87sr 86sr and nd

cover deformation above a blind duplex an example from west virginia u s a abstract pretectonic cover above blind thrust systems responds to underlying thrusting by combinations of forethrusting backthrusting and coupling the role of these responses was examined in the hanging rock cacapon mountain anticlinorium of the central appalachians this cover fold consists of middle ordovician and younger sedimentary rocks above a blind horse of cambro ordovician carbonates the cover has abundant folding and subsidiary faulting cover cleavage in carbonate and fine grained clastic rocks formed before regional folding in response to layer parallel shortening lps this 15 lps also produced grain suturing and fluid inclusion trails in quartzarenites the lps formed during cover forethrusting accommodating older blind horses in the hinterland with the formation of the underlying horse cover coupling occurred with bending folding additional secondary buckling and cleavage rotation by flexural flow with perhaps minor flattening this cover coupling was insufficient to accommodate horse formation no evidence exists for backthrusting but forelandwards abundant subsurface evidence exists for cover forethrusting with imbricate thrusting this thrusting accommodates horse formation and differs in style from the lps of the older forethrusting event so for this blind system the cover responded with a combination of coupling and forethrusting

interpreting the geologic map expression of contractional fault related fold terminations lateral oblique ramps versus displacement gradients abstract fault related folds in contractional settings do not extend indefinitely but rather commonly terminate as plunging anticlines near the tips of the genetically related fault geologists often attribute the formation of such terminations to loss of slip on the underlying fault i e a displacement gradient to changes in fault geometry in which the fault laterally changes stratigraphic position along strike i e a lateral oblique ramp or to some combination of these mechanisms discerning between these formative mechanisms solely through the interpretation of the geologic map expression of the termination can be difficult because of the subjective nature of the criteria used e g lateral oblique ramps often are interpreted at terminations where folds plunge at steep angles where hanging wall cutoff lines trend at high angles to fault strike where stratigraphic contacts trend at high angles to fault strike etc we created pseudo three dimensional model terminations of individual fault related folds using both displacement gradients and lateral oblique ramps to determine if unique characteristics in their map expressions exist we show that map patterns of folds produced by a displacement gradient along a thrust fault of constant geometry are similar to map patterns of folds produced by constant slip on a thrust fault with a lateral oblique ramp specifically our modeling results suggest that 1 simple fault bend folds that plunge less than 20 and simple fault propagation folds that plunge less than 50 at their terminations can be created by both displacement gradients and lateral oblique ramps angles greater than these values suggest that a lateral oblique ramp may be involved in forming the termination 2 hanging wall cutoff lines at terminations that trend at angles less than about 35 to fault strike for simple fault bend and fault propagation folds also may be produced by both displacement gradients and lateral oblique ramps higher angles likely indicate the presence of a lateral oblique ramp at the termination 3 the angle at which stratigraphic contacts trend to the fault strike cannot be used to uniquely identify displacement gradients or lateral oblique ramps for simple fault related folds 4 stratigraphic separation diagrams can indicate the presence of ramps in the thrust sheet but do not uniquely differentiate between frontal ramps with displacement gradients and lateral oblique ramps and 5 actual changes in fault orientation after topographic influences have been taken into account by definition indicate a lateral oblique ramp in reality most natural fault related fold terminations probably share components of both displacement gradients and lateral oblique ramps with each structure possessing contributions from each mechanism

strains developed in the hangingwalls of thrusts due to their slip propagation rate a dislocation model reply abstract fault planes propagate radially from a point source and this failure propagation process is very similar to the movement of a dislocation through a crystal lattice an elastic strain represented by an extra half plane is necessarily imparted to the lattice at an edge dislocation and this is equivalent to the ductile bead which accompanies the propagating tip of a thrust fault this ductile bead migrates with the fault tip and imparts a characteristic internal strain to the thrust sheet a two dimensional model is presented to illustrate the inter relationship between fault plane slip fault tip propagation and internal strain in multilayered sequences internal strain is usually represented by asymmetric folds verging in the thrust transport direction a simple technique the displacement distance plot utilizes the fact that displacement dies out towards the fault tip this plot can be used to quantify the relative stretch measured parallel to the fault movement which is dependent on the slip propagation rate and it may be used to define exactly the position of the fault tip on a cross section examples of fold thrust structures from devon england and pembrokeshire wales are used to illustrate the technique

modeling the tectonic development of the tucano and sergipe alagoas rift basins brazil abstract the tucano and sergipe alagoas basins of northeast brazil form part of a regional extensional basin system that was operative during the mesozoic breakup of south america and africa with both basins developing on precambrian crust the synchronous development of the rift basins suggest that they are genetically linked in space and time whereas the offshore sergipe alagoas basin is characterized by a substantial thickness of post rift sediment the tucano basin either failed to develop or at least preserve a significant thickness of post rift sediment observed primary porosities within both pre and syn rift tucano basin sediment imply that significant post rift sedimentation never occurred failure to develop significant post rift subsidence has important thermal and mechanical implications for the reaction of the lithosphere to rifting and can be explained in terms of 1 depth dependent lithospheric extension in which intracrustal detachments allow the extension of the crust to be decoupled from the thinning of the lithospheric mantle 2 small rates of extension that allow the lithosphere to cool during rifting and or 3 lithospheric rifting during which the flexural strength of the lithosphere remains high with respect to points 2 and 3 forward modeling demonstrates that finite rifting rates over a 20 25 m y period are insufficient to cool the lithosphere to the point where post rift subsidence fails to develop an interesting complication arises when the flexural strength of the lithosphere remains large during rifting it tends to suppress the vertical motions of the lithosphere such as those engendered by the cooling of the lithosphere following rifting thereby reducing significantly the amplitude of the post rift subsidence thus the lack of post rift sedimentation within a basin does not necessarily imply that extension has been limited to the crust from our kinematic modeling of the tucano basin the observed negative free air and bouguer gravity anomalies 120 mgals suggest that the flexural strength of the lithosphere has remained sufficiently large to maintain the load of the sediment we can model successfully the observed tucano basin architecture and gravity anomaly irrespective of whether we assume depth dependent or depth independent lithospheric extension primarily because the flexural strength of the lithosphere tends to buffer or suppress the amplitude of the post rift subsidence in contrast the adjacent sergipe alagoas basin is associated with low amplitude gravity anomalies which may reflect a resetting of flexural strength during rifting prior to rifting however there was no appreciable elevation difference between the tucano and sergipe alagoas regions as might be induced by a hot spot for example implying that the flexural strength of the lithosphere was similar the total thickness of the syn and post rift sediments within the sergipe alagoas basin is about the same as the thickness of rift phase sediments in the tucano basin thus the amount of crustal extension responsible for each basin was similar however since the sergipe alagoas basin contains 4 5 km of post rift sediments lithospheric mantle thinning in this region must have been significantly greater than the crustal extension to overcome the effects of flexural bufferring the need for lithospheric mantle thinning to be greater that crustal extension in the sergipe alagoas region was also a result obtained by modeling the development of the ocean continent boundary between brazil and africa from our coupled kinematic and rheological model of lithospheric extension we predict that the ocean continent boundary should form preferentially at the location of greatest crustal and lithospheric mantle thinning that is within the region of maximum depth independent lithospheric extension the ocean continent boundary formed east of the sergipe alagoas basin despite the fact that the tucano basin represents the region of greatest crustal thinning if extension had been uniform with depth beneath these basins the ocean continent boundary should have formed in the tucano basin as it did not some form of intracrustal detachment appears to have been operative to reduce lithospheric mantle thinning beneath the tucano basin while enhancing it beneath the offshore regions of the sergipe alagoas basin

origin of a thrust related fold geometric vs kinematic tests abstract geometric tests to determine the origin of fault related folds are common but as is typical in structural geology more than one fold origin may yield the final natural geometry thus the results of geometric tests are usually non unique in contrast kinematic tests of origin which employ both geometry and data about deformation commonly yield more constrained if not unique results unfortunately the necessary data collection requires much more work than for a geometric test in this study the thrust related barclay anticline is analyzed both geometrically and kinematically to determine which test is more effective geometric tests using angular relationships indicate three possible origins fault bend fault arrest and break thrust for the kinematic test predicted deformations for interlayer and flexural slip flexural flow simple shear and bending strains are compared to micro and mesostructural distributions solution strain and finite strain from the anticline strain measurements indicate that microscale deformation is uniformly distributed through the structure and is lithification dominated the microscale deformation does not match kinematic predictions and did not accommodate fold formation fold growth was achieved primarily through layer parallel slip restricted mostly to the forelimb and absent in the hinge which eliminates fault bend and fault arrest origins that require material transport from forelimb to backlimb the barclay anticline is therefore interpreted to be a break thrust structure interestingly a suite of contraction faults in the forelimb and hinge indicates material transport from forelimb to backlimb such transport has been discounted for break thrust folds the most important result of this study is that a kinematic test was a necessary step for distinguishing fold origin geometric testing alone was insufficient given that the barclay anticline has geometric characteristics typical of many thrust related folds kinematic testing appears necessary to determine their structural origin

variation in quartz arenite deformation mechanisms between a roof sequence and duplexes abstract microstructural abundances and histories in quartz arenite of the lower silurian tuscarora sandstone were used to determine the nature and role of microscale deformation in a cover sequence and underlying thrust system and to assess the degree to which the cover sequence accommodated emplacement of the thrust system sandstone samples are located across the transition from the central to southern appalachian foreland thrust system where the thrusts change from blind to emergent southwards and where southern deformation of early alleghanian age has been previously shown to be overprinted by central deformation of alleghanian age microstructures observed with transmitted light and cathodoluminescence microscopy indicate that grain scale deformation occurred by dislocation flow pressure solution and microfracturing with the last being generally the most important the sequence of deformation mechanisms is the same for the cover sequence and the thrust system pressure solution during sedimentary compaction dislocation flow during layer parallel shortening and localized microfracturing with limited pressure solution near major thrust ramps and in steep fold limbs a greater abundance of dislocation flow microstructures in the cover sequence from layer parallel shortening indicates grain scale accommodation in the tuscarora sandstone of some shortening associated with emplacement of the thrust system the transition zone between the central and southern appalachians contains the greatest occurrence of every microstructure which is consistent with the area having been affected by diachronous central and southern alleghanian deformations

evolution and characterization of fracture patterns insights from multi scale analysis of the buxa dolomite in the siang valley arunachal lesser himalayan fold thrust belt abstract fractures at different scales are an integral component of shallow crustal deformation associated with the progressive evolution of a fold thrust belt ftb we characterize the evolution of fractures from the buxa dolomite of the main boundary thrust mbt sheet in the frontal segment of the arunachal lesser himalayan ftb regionally the mbt sheet forms a fault bend antiform based on unfolding of strata early formed low and moderate angle fractures are interpreted to be pre folding layer parallel shortening structures while the dominant late stage high angle fractures are inferred to have formed synchronous to folding tracking the progressive deformation of the orogenic wedge the high angle fracture set is the dominant one at both outcrop and microscopic scales forming 44 and 45 of the total number of fractures respectively microfracture analysis indicates that 41 of the high angle fractures 22 of the moderate angle fractures and 10 of the low angle fractures form veins respectively cumulative plots of fracture spacing values collected from outcrops and under the microscope reveal that this parameter is best described by power law distributions that indicate scale independence of fracture spacing however the coefficients of variation cv of spacing of these high angle fractures reflect scale dependent clustering

progressive rollover fault propagation folding a possible kinematic mechanism to generate regional scale recumbent folds in shallow foreland belts the occurrence of strongly overturned to recumbent anticlines in shallow foreland thrust fold belts is not well explained by the available geometric models of thrust related folding when a very large amount of forward shear cannot be assumed and for growth anticlines when the tectonic uplift rate is greater than the sedimentation rate we have developed new geometries for fault propagation folding progressive rollover fault propagation folding that are suitable for overcoming these problems in our configuration an outer right way up sector and an overturned sector can grow simultaneously in the forelimb of the tip anticline they are linked to a recumbent leading syncline a semiquantitative computation of fracture strain during folding allowed us to define six homo eneously strained rock panels the location of the most strained panels suggests that an anticlinal breakthrough or a synclinal breakthrough is likely to affect the structure later in its evolution the outer morphology of the tip anticline in progressive rollover fault propagation folding resembles a fault bend fold this model produces geometries suitable for hydrocarbon trapping

structural evolution of the pulaski thrust system southwestern virginia the pulaski fault system is a complex series of alleghanian thrusts which has juxtaposed rocks of the pulaski thrust sheet over rocks as young as early to middle mississippian maccrady formation of the saltville thrust sheet the salem branch a later major branch of the pulaski fault system within the pulaski thrust sheet displaced the complexly deformed plate containing a broken formation a lithotectonic assemblage of complexly folded and faulted elbrook and rome carbonates and shales with associated breccias over rocks as young as early mississippian lower price formation of the salem synclinorium the classic ubiquitous max meadows breccias generally are confined to the broken formation which forms the basal part of the complexly deformed plate and which is interpreted as an exhumed lower level decollement zone formed during an earlier alleghanian stage and transported during a later alleghanian stage of deformation to a much higher structural level minimum displacement of the complexly deformed plate is on the order of 100 110 km on the basis of a new palinspastic reconstruction of the pulaski thrust sheet on the basis of this reconstruction and of two balanced and restored cross sections for the western portion of the valley and ridge minimum shortening for the pulaski thrust sheet is 80 whereas that for the western part of the valley and ridge province is only 10 20 the pulaski fault system is interpreted to have originated below the appalachian wide basal decollement zone near the contact between the cambrian rome and elbrook formations

3 d deformation pattern analysis and evolution of the anisclo anticline southern pyrenees understanding the genetic relationships of folding related deformation structures is of importance for both academic and industrial purposes we analysed the three dimensional deformation pattern of the anisclo anticline in the southern pyrenees the specifically designed statistical method includes sequential steps of statistical and spatial data analysis and classification our results indicate that pressure solution cleavage frequency exhibits a spatial distribution that depends on the position within the fold conversely such a dependence does not occur in joints and veins consequently our data suggest that pressure solution cleavage is the most appropriate deformation structure for unravelling fault fold kinematics in carbonate multilayer in particular we found that the spacing of solution cleavages s is related to the corresponding bed thickness h this supports the use of h s value for deformation intensity quantification instead of spacing in the anisclo anticline analysis of h s indicates the occurrence of three main deformation panels whose restoration suggests that fold evolution progressed from layer parallel shortening to decollement folding up to thrust breakthrough and fault propagation folding comparison of h s values with finite strain data indicates that line length was not preserved during fold evolution this evidence has important implications for the kinematic interpretation of layer parallel shortening and cross section balancing

evolution of the hawaiian emperor swell thinning of the oceanic lithosphere abstract the thinning process of the oceanic lithosphere is discussed with special reference to the evolution of the hawaiian emperor swell according to the hypothesis employed in this study the lithosphere increases its thickness depending upon the heat balance between the input heat flow from the asthenosphere cooling by thermal conduction within the lithosphere and release or absorption of latent heat at the lithosphere asthenosphere boundary the input heat flow from the asthenosphere controls this process although under ordinary conditions the lithosphere thickens with time an extra high heat flow thins the lithosphere a source of excess heat flow may be recognized as a hotspot near hawaii based on this theory thinning and thickening processes of the pacific plate beneath the hawaiian emperor swell are examined to evaluate the change in lithospheric thickness in more detail the residual gravity anomalies around the hawaiian emperor swell are calculated using available data or both gravity anomalies and crustal structures derived from explosion seismology from these data the lithospheric thickness beneath hawaii is estimated to have been decreased by about 20 km extra high heat flux from the asthenosphere in the order of 420 mw m 2 10 hfu is required in this region theoretical predictions of temporal variations of ocean bottom topography surface heat flow gravity anomalies and plate thickness along the hawaiian emperor swell fit well with the observations

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

paleoclimate history of galapagos surface waters over the last 135 000 yr abstract records of planktonic and benthic foraminiferal 18 o and planktonic mg ca from core tr163 22 just northwest of the galapagos islands reveal a detailed 250 450 year resolution climate history of the region over the last 135 thousand years kyr sea surface temperatures ssts reconstructed from g ruber mg ca averaged 24 3 0 4 c during the holocene 22 6 0 6 c during marine isotope stage mis 2 3 and 4 and 26 0 0 9 c during mis 5e changes in sst lead changes in both planktonic and benthic 18 o by an average 3 kyr suggesting that sst changes in this region predated continental ice volume changes changes in sst display clear millennial scale variability especially in marine isotope stage 3 with behavior somewhat similar to antarctic proxy air temperature and south pacific sst records removal of the temperature component from the planktonic 18 o record demonstrates that glacial interglacial 18 o water changes at this site were 1 0 0 2 similar to estimates for mean ocean shifts implying that salinity changes due to regional hydrological variation between the last glacial maximum and holocene in the galapagos region were minimal comparison between the tr163 22 sst record and an sst record from core tr163 19 north of the equatorial front reveals a largely similar broad scale climate history suggesting that changes in the galapagos region were caused by large scale forcing rather than by local or regional dynamical changes changes in atmospheric greenhouse forcing are the most plausible explanation for the observed large scale climate changes in the eastern equatorial pacific

paleomagnetic estimates of temperatures reached in contact metamorphism a paleomagnetic study of some silurian devonian lavas from the lorne plateau scotland locally reheated by the intrusion of small tertiary dikes has shown that a laboratory equivalent of the actual peak temperature reached in the reheating can be accurately determined theoretically derived corrections are applied to the experimentally determined temperatures to obtain an estimate of the actual peak temperatures reached in a series of samples across the contact zones these corrected experimental profiles can be compared with peak temperature profiles calculated from heat conduction models the two profiles fit well when the temperatures of the lavas immediately prior to the intrusion are taken into account ambient temperature estimates of 75 25 c and 150 25 c have been obtained for the lavas before intrusion by a normally magnetized dike and a reversely magnetized dike respectively

a complete oxygen isotope profile through the lower oceanic crust odp hole 735b abstract odp hole 735b located on the southwest indian ridge at 57 e is an in situ sampled long continuous section of lower oceanic crust oxygen isotope compositions of constituent minerals of leg 176 gabbros have been measured by uv laser oxygen isotope microprobe together with existing data from leg 118 a complete oxygen isotope profile through the lower oceanic crust has been obtained most clinopyroxenes and olivines have normal mantle values of 5 5 and 5 2 respectively while plagioclases show slight 18 o enrichment relative to its mantle value of 6 1 down hole variations of hole 735b gabbro indicate a downward decreasing 18 o profile with a kink at a depth of about 800 m below sea floor above this depth gabbros are depleted in 18 o relative to unaltered basalts while below 800 m they show nearly unmodified 18 o values abundant seawater penetration appears to be limited to the upper part of the lower crust at odp site 735 800 m into the gabbroic layer and 2 2 5 km into the oceanic crust from the top of pillow basalts mass balance calculations show that the lower crust formed under this ultra slow spreading ridge has an average 18 o value of 5 5 the whole crust at site 735 has an overall 18 o enrichment with 18 o values of 6 0 to 7 8 depending on the possible variation of the 18 o values of the upper pillow basalts and sheeted dykes the apparent difference in oxygen isotope compositions of ocean crusts formed with different spreading rates has important implications on the buffering of ocean water over geological time as well as on the oxygen recycling between crust and mantle through subduction the difference of seawater penetration between fast and slow spreading ridges could be related to their particular magmatic tectonic history during the formation and aging of the crust however more analyses on continuous sections through oceanic and ophiolitic crust in different tectonic settings are required to derive any predictive models

petrology of lavas from sierra negra volcano isabela island galapagos archipelago sierra negra volcano is a voluminous active basaltic shield situated in the western galapagos archipelago a hotspot related chain adjacent to the galapagos spreading center the volcano has erupted fe rich tholeiitic basalts of very limited compositional range comparison of lavas from each of five stratigraphic units covering the entire volcano indicates that the lavas are mineralogically similar and compositionally restricted 4 4 to 6 9 wt mgo the lavas are fe rich and hypersthene normative and plot in both the alkaline and tholeiite fields of the alkali silica diagram rare earth element ree patterns are steep and parallel the major and trace element data indicate a comagmatic relationship by fractional crystallization of the observed phenocryst phases projections into pseudoternary phase diagrams suggest that the magmas cooled and crystallized at pressures between 1 and 3 kbar calculated melt densities range from 2 73 to 2 77 g cm3 which are higher than those of typical mid ocean ridge basalt morb hawaiian tholeiite and other galapagos lavas the high calculated melt densities may be caused by fractionation at greater depths than is ordinary for the other basaltic magmas suppression of plagioclase and olivine crystallization in favor of augite may eliminate the minimum density window that has been proposed for morb fractionation sierra negra lavas have the most radiogenic lead and strontium isotopic ratios in the western galapagos indicating that the magmas have a relatively large contribution of plume material and have been minimally contaminated by entrainment of morb producing mantle magmatic 3he 4he isotopic ratios from sierra negra are approximately 15 times the atmospheric ratio although these ratios clearly indicate plume helium they are not the highest 3he 4he in the archipelago suggesting that helium is decoupled from the heavier isotopes high sm yb ratios light rare earth element lree enrichment and a steep ree slope are consistent with an origin by moderate extents 5 15 of partial melting of a garnet lherzolite source with ree characteristics that are between chondrite and depleted earth mantle sources sierra negra lavas are compositionally monotonous because homogeneous plume rich material has experienced long term exposure to a well regulated sublithospheric thermal environment with additional compositional restrictions imposed in the lithosphere by high magma flux

oxygen isotope trajectories of crystallizing melts insights from modeling and the plutonic record elevated oxygen isotope values in igneous rocks are often used to fingerprint supracrustal alteration or assimilation of material that once resided near the surface of the earth the 18 o value of a melt however can also increase through closed system fractional crystallization in order to quantify the change in melt 18 o due to crystallization we develop a detailed closed system fractional crystallization mass balance model and apply it to six experimentally and naturally determined liquid lines of descent llds which cover nearly complete crystallization intervals melt fractions of 1 to 0 1 the studied llds vary from anhydrous tholeiitic basalts to hydrous high k and calc alkaline basalts and are characterized by distinct melt temperature sio 2 trajectories as well as crystallizing phase relationships our model results demonstrate that melt fraction temperature sio 2 relationships of crystallizing melts which are strongly a function of magmatic water content will control the specific 18 o path of a crystallizing melt hydrous melts typical of subduction zones undergo larger increases in 18 o during early stages of crystallization due to their lower magmatic temperatures greater initial increases in sio 2 content and high temperature stability of low 18 o phases such as oxides amphibole and anorthitic plagioclase versus albite conversely relatively dry tholeiitic melts only experience significant increases in 18 o at degrees of crystallization greater than 80 total calculated increases in melt 18 o of 1 0 to 1 5 can be attributed to crystallization from 50 to 70 wt sio 2 for modeled closed system crystallizing melt compositions as an example application we compare our closed system model results to oxygen isotope mineral data from two natural plutonic sequences a relatively dry tholeiitic sequence from the upper and upper main zones uumz of the bushveld complex south africa and a high k hydrous sequence from the arc related dariv igneous complex mongolia these two sequences were chosen as their major and trace element compositions appear to have been predominantly controlled by closed system fractional crystallization and their llds have been modeled in detail we calculated equilibrium melt 18 o values using the measured mineral 18 o values and calculated mineral melt fractionation factors increases of 2 3 and 1 1 5 in the equilibrium melts are observed for the dariv igneous complex and the uumz of the bushveld complex respectively closed system fractional crystallization model results reproduce the 1 increase observed in the equilibrium melt 18 o for the bushveld uumz whereas for the dariv igneous complex assimilation of high 18 o material is necessary to account for the increase in melt 18 o values assimilation of evolved supracrustal material is also confirmed with sr and nd isotope analyses of clinopyroxene from the sequence beginning with a range of mantle derived basalt 18 o values of 5 7 pristine mantle to 7 0 heavily subduction influenced mantle our model results demonstrated that high silica melts i e granites with 18 o of up to 8 5 can be produced through fractional crystallization alone lastly we model the zircon melt 18 o fractionations of different llds emphasizing their dependence on the specific sio 2 t relationships of a given crystallizing melt wet relatively cool granitic melts will have larger zircon melt fractionations potentially by 1 5 compared to hot dry granites therefore it is critical to constrain zircon melt fractionations specific to a system of interest when using zircon 18 o values to calculate melt 18 o

oxygen isotope composition of a section of lower oceanic crust odp hole 735b 1 the oxygen and hydrogen isotope compositions of vein minerals and 18o values of 64 whole rocks from the deeper kilometer of the 1 5 km lower crustal section of odp hole 735b in the sw indian ocean were analyzed in order to investigate seawater penetration into and reaction with the lower oceanic crust vein amphiboles have d values of 35 to 65 indicating formation from seawater fluids and hydrothermal fluids containing a component of low d magmatic fluid crystal plastic shear zones reacted with 18o enriched fluids at high temperatures and have 18o values of 6 0 7 0 indistinguishable from surrounding rocks reaction with seawater dominated fluids over a range of temperatures 350 700 c led to local 18o depleted rocks but much of the section exhibits 18o enrichment 18o up to 7 6 as the result of static alteration of plagioclase over a range of temperatures 250 to 600 c and fluid compositions chloritic veins and associated alteration by 18o enriched fluids occurred at temperatures of 150 250 c and late carbonates and fe oxyhydroxides formed from cold 10 seawater associated with open fault related fractures bulk rock 18o values of the lower 1 km range from 3 7 to 10 2 but are mostly 5 6 7 6 and the section has a mean bulk 18o of 6 5 1 0 in contrast rocks from the shallower 500 m exhibit net 18o depletion 5 1 1 0 reflecting high temperature deformation and seawater hydrothermal alteration stakes 1991 the weighted average 18o for the entire gabbro section of hole 735b is 6 0 exhibiting little or no change compared to unaltered crust although tectonism at slow spreading rates may allow exposure of lower crust to greater amounts of low temperature alteration and 18o enrichment there is no evidence for a consistent difference in the bulk 18o of the plutonic sections of oceanic crust formed at fast versus slow spreading rates

variation of depth to the brittle ductile transition due to cooling of a midcrustal intrusion the depth to the brittle ductile transition in the crust is often defined by the intersection of a shear resistance relation in the brittle upper crust that increases linearly with depth and a power law relation for ductile flow in the lower crust that depends strongly on temperature transient variation of this depth caused by a magmatic intrusion at a depth near the regional transition can be modeled by a heat conduction model for a rectangular parallelepiped superimposed on a linear geothermal gradient when parameters appropriate for the southeastern united states are used a moderate sized intrusion is found to decrease the transition depth by as much as 7 km significant variations last about 10 m y since the base of the seismogenic zone is identified with the brittle ductile transition and since significant depth variations provide loci for strain energy concentration these results imply that intrusions of late tertiary age or younger could be important sources of clustered seismicity

displacement transfer at thrust terminations the saltville thrust and sinking creek anticline virginia u s a abstract the saltville thrust terminates in the core of the sinking creek anticline changes in the amplitude of the anticline may preserve profile shortening as the thrust displacement decreases towards the fault termination consequently thrust displacement would be transferred into the fold we combine newly published maps and new strain and mesostructural data from the tuscarora sandstone to argue that the sinking creek anticline did perform this kinematic role we propose that the anticline developed as a fault propagation fold that experienced both decollement and anticlinal breakthrough by the saltville thrust thrust displacement was accommodated by the development of the modified fault propagation fold and by transfer to a roof flat from the decollement breakthrough new strain and mesostructural data indicate no fixed pin lines in either the hinge or backlimb throughout folding consequently layer slip occurred through the hinge during folding and the forelimb deformed internally during breakthrough towards the plungeout of the sinking creek anticline the displacement on the saltville thrust is transferred to a floor thrust we apply this interpretation to the termination of the st clair thrust which has a similar surface geometry where the thrust terminates into an anticline a testable implication of this proposed interpretation for the st clair thrust is that the appalachian plateau must absorb over 20 km of thrust displacement as horizontal shortening

the problem of strain marker centers and the fry method abstract the fry method and its variants will underestimate finite strain when applied to strain markers deformed heterogeneously at marker scale despite homogeneous behavior at a population scale this difference results from the post deformation centers not coinciding with the original pre deformation centers the amount of error may rapidly approach 50 for small strains in ideal cases but is likely to be in the range of 20 30

the kinematics of break thrust folds abstract the kinematics of asymmetric open to close often overturned thrust related folds are commonly explained with fault propagation or detachment fold models field observations of thrust related folds exposed in tennessee virginia wyoming and montana that exhibit this geometry indicate that models of fault propagation and detachment folds do not adequately describe the kinematics of these structures existing models of fault propagation and detachment folding employ migrating kink shaped hinges and relate fold geometry entirely to fault geometry slip and to the thickness of the basal detachment layers the models exclude the relation between fold shape and amplitude and they do not predict a correlation between competent layer thickness and fold wavelength as expected from buckling theory thrust related folds examined in this study exhibit unique distributions of fabrics in the hinges and forelimbs which demonstrate a lack of hinge migration furthermore the ratios of dominant member thickness to fold wavelenght agree with those expected for folds controlled by a single layer and they are within the expected range for folds with multiple evenly spaced harmonically deforming layers our analyses suggest these folds evolved from an initial brief stage of sinusoidal buckling to a later stage of fixed hinge kinking and thrusting thrust related folds which exhibit a geometry and mesofabric distribution suggesting this type of evolution are defined as break thrust folds

postintrusive thermal evolution of continental crust a moving boundary approach intrusion of magmatic bodies into continental crust has been extensively used to explain high heat flow anomalous seismic velocity structure and gravity anomalies these intrusions significantly control the crustal deformation by modifying the heat and mass budget of the crust however thermal modeling results based on simple cooling of intrusives ignore the effect of moving boundaries during magma solidification in the present work we discuss a mathematical formulation of the thermal evolution of continental crust in presence of such magma solidification process the solution is obtained by using the fourier spectral approach in the space domain and a modified finite difference scheme in time domain this solution provides the cooling history and thermal evolution within an intrusion unlike the solutions proposed earlier to adjust the effect of latent heat of intrusion in the form of apparent specific heat or apparent temperature of intrusion and is a general one in the sense that this includes the radiogenic heat sources distribution and an initial depth dependent temperature distribution in lithosphere we apply this solution to study the thermo rheological evolution of continental lithosphere after magmatic underplating we conclude that this effect keeps the intrusive body and the surrounding rocks at a higher temperature for a longer duration than when the effect is ignored or is adjusted in the form of increased specific heat for an initially ductile lower crust and brittle upper mantle magmatic underplating causes a change in the mode of deformation from brittle to ductile in the mantle layer and the latent heat effect included in the form of moving boundaries prolongs the duration of ductile deformation in this layer in comparison to the latent heat effect adjusted by increasing the specific heat of intrusion the prolonged duration of ductility in lower crust upper mantle due to the slow cooling of intrusion results in a weak lithosphere and is important in addressing the issues concerning crustal dynamics

evolution of galapagos magmas mantle and crustal fractionation without assimilation summary of helium isotopic data from the gala pagos islands and dredged samples from the gala pagos platform data from graham et al 1993 and kurz et al 1993 data are normalized to the atmospheric value r ra most gala pagos samples are intermediate betweenmorb value of 8 1 and the fernandina value 22 material in those magmas the exception is floreana structure first the depth to the magma chambers mayisland which ranges from 14 5 to 7 7 ra it is possible be due to the rate at which magma is supplied from thethat the low value at floreana re ects assimilation of mantle as the volcano is carried away from the focus ofmaterial with excess radiogenic 4 he by a magma with the hotspot the volcanoes in a juvenile growing stage 3 he 4 he ra 14 most floreana lavas contain abundant of evolution those that have had numerous historicdisaggregated debris from ultrama c xenoliths bow eruptions are dominated by lavas with mgo con geist 1992 a potential source of helium contamination centrations between 5 and 6 e g reynolds geist 1995 a narrow deep caldera characterizes the juvenilestage e g cerro azul wolf and fernandina and iteventually broadens and shallows because of lling with

spatial arrangement of decollements as a control on the development of thrust faults abstract we used two dimensional finite element models to explore different configurations of weak layers in undeformed sedimentary sequences to investigate the occurrence of three characteristic types of thrust configurations ramp flat imbricate and duplex in our models we embedded two low friction weak layers with a finite spatial extent in a sequence of stronger rock these two weak layers were initially horizontal were separated vertically by 1 km and were arranged in three different relative positions to each other when the models were deformed and these weak layers developed into decollements they interacted to produce one of the three types of thrust faults as a function of their initial configurations when the tips of weak layers were separated by a large gap 10 km only the lower level decollement became active producing imbricate thrusts when the two weak layers overlapped for a large distance 10 km they simultaneously became active decollements producing duplexes in the overlapped zone when the gap or overlap was small

crustal structure beneath the galapagos archipelago from ambient noise tomography and its implications for plume lithosphere interactions 1 to constrain the seismic velocity structure of the crust beneath the galapagos archipelago we conducted a tomographic study using high frequency rayleigh waves obtained from cross correlations of ambient noise we analyzed waves with periods between 5 and 8 5 s sensitive to shear wave velocity vs structure between about 3 and 10 km depth after accounting for the effect of water depth crustal velocities are up to 25 lower than those of very young crust at the east pacific rise and are comparable to those of hawaii we attribute the lower than normal velocities to the combined effect of heating and the presence of melt in the crust above the galapagos plume as well as the construction of a highly porous volcanic platform emplaced atop preexisting oceanic crust on average vs between 3 and 10 km depth beneath the western archipelago is up to 15 lower than beneath the eastern archipelago we attribute the west to east velocity increase to a decrease in porosity of the volcanic platform and to cooling of the crust after its passage above the galapagos plume the results of this study in combination with previous work indicate that many of the unusual aspects of the galapagos archipelago are the result of variations in the thickness and internal structure of the chemical and thermal lithospheres our findings indicate that observed variations in the flexural response to loading observed in the galapagos cannot be explained by the current thermal state of the lithosphere instead the flexural response likely represents varying elastic strength at the time of loading we also propose that the northwest and northeast trending alignments of volcanic centers found throughout the archipelago the darwinian lineations may be associated with preexisting zones of weakness in the lithosphere formed during earlier episodes of ridge jumping and ridge propagation that were later reactivated by stresses generated by plume lithosphere interactions

strain variations and three dimensional strain factorization at the transition from the southern to the central appalachians abstract the transition between the southern and the central appalachian foreland thrust system is a region subjected to at least two non coaxial deformations where typical two dimensional analyses such as cross section balancing or plane strain retro deformation cannot be applied three dimensional factorization can simulate the non coaxial strain history simulation results for compaction and layer parallel shortening lps were compared to finite strains of 16 quartz arenite samples from the tuscarora sandstone successful simulations indicate in the southern appalachians 30 35 total compaction by volume loss including a 4 5 9 e w horizontal shortening during lithification and 5 lps along 150 330 by plane strain or axial symmetric flattening in the central appalachians 30 35 total compaction by volume loss including a 3 7 n s horizontal shortening during lithification and 10 lps along 120 300 by plane strain or axial symmetric flattening in the transition zone 30 compaction by volume loss including a 3 e w horizontal shortening during lithification 5 southern lps along 150 330 by axial symmetric flattening and 5 10 central shortening along 130 or 140 by plane strain or axial symmetric flattening at 10 to the normal central trend from the simulations horizontal shortening in the southern appalachians during lithification in the mississippian was greater than the later alleghanian deformation this shortening probably represents a very early lps developed in unlithified sand that required less deviatoric stress to deform than did the cemented quartz arenites during later tectonic deformation also the oblique and weaker central tectonic lps in the transition zone probably represents behavior at the limits of central appalachian deformation finally the lps simulations indicate that axial symmetric flattening oblate strain is more successful than plane strain as the representative behavior so caution is recommended in assuming plane strain as the dominant strain behavior in orogenic forelands

fracture patterns in successive folding in the western sichuan basin china abstract a detailed field survey of fractures was carried out in the pingluoba and adjacent anticlines in the western sichuan basin to discuss fracture development during progressive folding including the relative timing of fracture formation the successive deformation in a rock layer the distribution of fracture sets in folds and the variation of fractures along with the subsequent deformation of rock layers based on a comparison of the diverse fracture patterns in folded strata at different stages of fold growth horizontal low angle dipping high angle dipping and vertical layers a model of fracture development related to fold evolution was built up it is suggested that the strata were initially under uniform compression and two sets of planar conjugate shear fractures were formed the orientations of which would vary along with the subsequent folding of strata then during progressive folding the deformation of rheologically hard layers was controlled by tangential longitudinal strain inducing the development of a set of tensile fractures that struck parallel to the fold axis layer parallel shear was predominant in the soft layers and two sets of bedding oblique dextral shear fractures intersecting with the bedding at angles of 25 40 and 160 were developed in the progressive folding process the shear fractures were developed later than the tensile fractures a polar stereonet of fractures was generated and applied to improve the efficiency and accuracy of fracture identification in the field providing precise evidence for the exact analysis of fracture patterns and rock deformation

fracture mechanics model of a fault termination zone a fault termination zone from middle eocene chalks near beer sheva israel is analysed via a fracture mechanical technique the zone consists of the end of the primary fault three secondary faults and joints associated with these structures we demonstrate that the shape of the first secondary fault can be obtained from theoretical fracture mechanical calculations this shape also enables us to obtain the set of conditions which induced the observed structures the technique reveals the relative importance of the different variables that appear in the theory the most significant parameters in determining the shape of the first secondary fault are the vertical dimension of the primary fault the ratio of the two horizontal differential stresses and the initiation angle of the secondary fault results indicate that the fault termination zone was created under an almost pure shear load the true lateral displacement of the primary fault is unknown hence exact calculation of the stresses leading to the secondary fault is hardly possible however an estimation based on a partial conversion of this displacement to an uplift yields 1 and 3 values at initiation of between 3 8 and 7 6 mpa and 3 65 and 7 3 mpa respectively

oxygen osmium isotope systematics of west maui lavas a record of shallow level magmatic processes new 18 o and 187 os 188 os data from late shield building stage lavas from west maui volcano hawai i indicate that a range of distinct shallow level processes can affect compositions of plume generated ocean island magmas the relative ages of the samples studied were determined either on the basis of field relationships or from collection depth in a water well the stratigraphically lower lavas have os 61 399 ppt initial 187 os 188 os 0 1311 0 1324 and 18 o olivine 4 53 4 88 the stratigraphically higher lavas have os 20 273 ppt initial 187 os 188 os 0 1323 0 1390 with the exception of one low os sample with 187 os 188 os 0 1517 and 18 o olivine 4 73 5 21 these compositions are generally similar to those of kea type hawaiian lavas from mauna kea and kilauea volcanoes but in detail there are some significant differences between the west maui lavas and other kea type lavas the stratigraphically lower lavas have 187 os 188 os similar to those previously observed for mauna kea and kilauea lavas and define a nearly flat 87 sr 86 sr vs 187 os 188 os trend that is consistent with assimilation by these magmas of small degree melts of pacific oceanic crustal gabbros the stratigraphically higher west maui lavas extend to 187 os 188 os higher than observed at mauna kea or kilauea this probably results from interaction of these magmas with the hydrothermally altered volcanic edifice during the waning phases of the shield building stage this process is likely to affect the isotope systematics of os and o but will have little effect on those of hf nd pb or sr

displacement transfer at thrust terminations in southern appalachians saltville thrust as example transition from faulting to noncylindrical folding involving rapid plunge change at the fold closure is the primary displacement transfer mechanism at the northern end of the south appalachian foreland fold and thrust belt regional thrust faults including the saltville narrows and st clair terminate in regional overturned asymmetric anticlines this mechanism of displacement transfer is distinct from thrust terminations where fold complexes of smaller noncylindrical folds converge toward the end of the fault trace the saltville thrust transition to the sinking creek anticline is representative of the southern appalachian structures deformation intensity patterns reflected by cleavage development mesoscopic folding and faulting suggest that the sinking cree anticline formed due to upwarping from shear thrusting at depth laboratory deformation analogs of folds formed above ramps support this contention experimentally produced folds are asymmetric overturned structures with high strains on overturned limbs and very low strains on upright limbs a deformation sequence involving shear thrusting with subsequent break thrusting may characterize fault propagation at thrust termini this will be important for layered sedimentary sequences where thick rigid members low in the sedimentary pile are overlain by thinner interbedded mechanically anisotropic strata

the northern samail ophiolite an oxygen isotope microprobe and field study geological petrological and oxygen isotopic data are presented for 228 whole rock and mineral samples collected from a 100 20 km area of the northern samail ophiolite in oman most of these samples are from three detailed profiles through the pillow lavas sheeted dikes and layered gabbros of this laterally heterogeneous fragment of cretaceous oceanic crust down to and across the petrologic moho the profiles encompass a range of petrologic and tectonic styles and each profile exhibits distinctive 18 16 o variations compared to one another and to mid ocean ridge basalts as a result of pervasive seawater hydrothermal interaction that varied in intensity along strike in the ophiolite in general 18 o depletions are observed in the layered gabbros and 18 o enrichments in most of the sheeted dikes and pillow lavas similar to results previously observed in the southern part of the ophiolite ibra area where 18 o depletions within the gabbroic section are quantitatively balanced by 18 o enrichments in the shallower parts of the oceanic crust the wadi hilti profile selected as an example of relatively intact crust differs from ibra in having more uniform and slightly higher 18 o in the gabbros 5 4 to 6 3 as well as in containing more hydrous alteration minerals amphibole epidote chlorite and prehnite the profiles in the wadi kanut shafan and wadi rajmi sections are much more complex and reveal the impact of off axis intrusions and deep crustal shearing plagiogranite wehrlite intrusions in the shafan kanut area superimposed a local hydrothermal aureole on the ophiolite evident in dikes highly depleted in 18 o quartz sulfide veins abundant epidote thullite and chlorite in shallower rocks and low temperature hydrous alteration of deeper gabbroic rocks the latter produced an overall increase in whole rock 18 o 6 2 to 6 9 such late stage intrusions are found throughout the northern half of the samail ophiolite the wadi rajmi area which is a possible fossil transform or propagating rift represents the most complex of the three profiles it also contains the most abundant highly deformed and hydrothermally altered rocks together with the deepest and largest zone of 18 o depletion yet found in any ophiolite locally 18 o 2 0 conduits for large volumes of high temperature hydrothermal fluids were provided by fractures now occupied by low 18 o gabbro pegmatites and low 18 o dikes material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk 18 o averages 5 9 to 6 3 that are within sampling error almost identical to the average morb basalt value of about 5 8 if both vertical and lateral crustal heterogeneities are integrated into a three dimensional model this supports and amplifies the conclusion of earlier workers that the 18 o of seawater is buffered and controlled by hydrothermal interaction with oceanic crust as long as the cumulative effects both spatial and temporal of all seafloor magmatic hydrothermal processes are considered the very slight out of balance enrichment of the integrated crustal average 18 o compared to morb may be explained by the ubiquitous mineralogical and isotopic evidence for a late low temperature alteration event in the basal gabbros these effects are prominent in the vicinity of the petrologic moho and may indicate exchange with low temperature aqueous fluids during or after detachment of the obducted slab

structure and tectonics of central and southern appalachian valley and ridge and plateau provinces west virginian and virginia the valley and ridge and eastern allegheny plateau from pennsylvania into tennessee is subdivided into distinct allochthonous sheets emplaced over an essentially featureless precambrian basement surface sheet development is related to variations in stratigraphic thickness and lithology as well as regional basement configuration three sheets control central appalachian structural style the massanutten blue ridge sheet is easternmost and is bounded on the west by the little north mountain pulaski fault system continuing westward into the plateau the martinsburg sheet contains anticlinoria and synclinoria uncut by major thrust faults both massanutten blue ridge and martinsburg sheets are seated in upper ordovician martinsburg reedsville shales the underlying waynesboro sheet is rooted in lower cambrian waynesboro rome shales and its imbrication has controlled the locations of major anticlinoria and synclinoria in the upper sheet within the southern appalachians greater imbrication of the lower sheet and increased thrust fault displacement at longitudinal ramps have destroyed the lateral continuity along strike of the central appalachian waynesboro and overlying martinsburg sheets there imbrication to the surface has formed the saltville sheet continuous into alabama as well as the st clair narrows copper creek cove mountain and st paul honaker sheets southwest of the trend change the holston stone mountain sheet adjacent to the blue ridge is rooted primarily in lower and middle cambrian strata and is underlain by the pulaski sheet in the westernmost sector of this region the pine mountain and richlands sheets are seated in devonian shales the pulaski and massanutten blue ridge sheets rooted primarily in upper ordovician shales form a continuous structural unit and include blue ridge and piedmont rocks

roof assimilation at fast spreading ridges an investigation combining geophysical geochemical and field evidence 1 the role of assimilation at the roof of the axial magma chamber amc at fast spreading mid ocean ridges is investigated using field geophysical and geochemical data field observations from ophiolites indicate that roof assimilation is a common process but provide little constraint on the amount of material assimilated arguments based on geophysical data that include the subsidence of the layer 2a 2b boundary across the width of the amc and the depth of large axial summit troughs require at least 50 100 m of assimilation however significant variations in the depth of the amc on the east pacific rise epr over short along axis distances suggest up to 500 m fluctuations in the depth of the amc may be common on short timescales geochemical modeling of the over enrichment of chlorine in epr basalts suggests that 20 of the oceanic crust may go through a cycle of crystallization alteration then assimilation this is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation assimilation most likely occurs on a number of different timescales ranging from decadal due to perturbations in the thermal regime related to diking to longer period fluctuations controlled by magma supply from the mantle large scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation and for the interpretation of the geochemistry of the oceanic crust

upper mantle structure beneath the galapagos archipelago from surface wave tomography 1 we present a rayleigh wave tomographic study of the upper mantle beneath the galapagos archipelago we analyze waves in 12 separate frequency bands 8 50 mhz sensitive to shear wave velocity vs structure in the upper 150 km average phase velocities are up to 2 and 8 lower than for 0 to 4 my old and 4 to 20 my old pacific seafloor respectively laterally averaged vs is 0 05 0 2 km s lower between 75 and 150 km depth than for normal pacific mantle of comparable age corresponding to an excess temperature of 30 to 150c and 0 5 melt a continuous low velocity volume that tilts in a northerly direction as it shoals extends from the bottom of our model to the base of a high velocity lid which is located at depths varying from 40 to 70 km we interpret this low velocity volume as an upwelling thermal plume that flattens against the base of the high velocity lid the high velocity lid is 30 km thicker than estimated lithospheric thickness beneath the southwestern archipelago above the main region of plume upwelling we attribute the thicker than normal high velocity lid to residuum from hot spot melting the thickness of the lid appears to control the final depth of melting and the variability of basalt composition in the archipelago at depths less than 100 120 km plume material spreads in directions both toward and against eastward plate motion indicating that plume buoyancy forces dominate over plate drag forces and suggesting a relatively high plume buoyancy flux b 2000 kg s

submarine fernandina magmatism at the leading edge of the galapagos hot spot new multibeam and side scan sonar surveys of fernandina volcano and the geochemistry of lavas provide clues to the structural and magmatic development of galapagos volcanoes submarine fernandina has three well developed rift zones whereas the subaerial edifice has circumferential fissures associated with a large summit caldera and diffuse radial fissures on the lower slopes rift zone development is controlled by changes in deviatoric stresses with increasing distance from the caldera large lava flows are present on the gently sloping and deep seafloor west of fernandina fernandina s submarine lavas are petrographically more diverse than the subaerial suite and include picrites most submarine glasses are similar in composition to aphyric subaerially erupted lavas however these rocks are termed the normal series and are believed to result from cooling and crystallization in the subcaldera magma system which buffers the magmas both thermally and chemically these normal series magmas are extruded laterally through the flanks of the volcano where they scavenge and disaggregate olivine gabbro mush to produce picritic lavas a suite of lavas recovered from the terminus of the sw submarine rift and terraces to the south comprises evolved basalts and icelandites with mgo 3 1 to 5 0 wt this evolved series is believed to form by fractional crystallization at 3 to 5 kb involving extensive crystallization of clinopyroxene and titanomagnetite in addition to plagioclase high k lavas were recovered from the southwest rift and are attributed to hybridization between normal series basalt and evolved series magma the geochemical and structural findings are used to develop an evolutionary model for the construction of the galapagos platform and better understand the petrogenesis of the erupted lavas the earliest stage is represented by the deep water lava flows which over time construct a broad submarine platform the deep water lavas originate from the subcaldera plumbing system of the adjacent volcano after construction of the platform eruptions focus to a point source building an island with rift zones extending away from the adjacent buttressing volcanoes most rift zone magmas intrude laterally from the subcaldera magma chamber although a few evolve by crystallization in the upper mantle and deep crust

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

how preexisting weaknesses control the style of continental breakup continental breakup is a highly variable process differences occur in the relative timing and extent of volcanism uplift and graben formation as well as in the mode and amount of continental extension before breakup we propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends continental lithosphere is viewed as a composite material composed of two strong layers one in the upper mantle and one in the middle crust finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust this leads to early melt segregation and volcanism margin uplift during the late stages of the extension process and relatively narrow symmetrical extended margins in contrast failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle this produces initial graben formation cooling in the lower crust and upper mantle and broad asymmetrical extended margins volcanism only occurs late in the process failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup

lithosphere response to externally and internally derived stresses a viscoelastic stress guide with amplification summary lithosphere subjected to an externally derived horizontal stress undergoes creep in the lower lithosphere resulting in the decay of lower lithosphere stress and the associated amplification of stress within the upper lithosphere this stress response of lithosphere has been investigated for a lithosphere model with power law stress and temperature dependent viscoelastic properties the rate and extent of stress decay and associated stress amplification is greatly dependent on the lithosphere geotherm oceanic lithosphere subject to an applied stress of o 1 kb undergoes upper lithosphere stress amplification of 1 5 1 8 and 2 0 at 104 106 and 108 yr respectively at 106 yr the effective lithosphere thickness is reduced to approximately 40 km the stress decay and amplification proceeds more rapidly for an applied stress of 1 0 kb and in the case of a tensile applied stress results in some upper lithosphere fracture for continental and basin and range type lithosphere the comparable stress amplification at 106 yr for a stress of 0 1 kb is 2 0 and 6 5 with effective lithosphere thicknesses of 60 and 20 km respectively the large values of stress amplification for the basin and range lithosphere result in complete upper lithosphere fracture which gives rise to a cyclic process of upper lithosphere faulting and lower lithosphere creep in which extensive lithosphere deformation can occur the stress amplification process also occurs for stresses generated by lateral density contrasts for an isostatically compensated plateau uplift structure deviatoric stresses of the order of 1 kb can be generated in the upper lithosphere by this process and are sufficient to cause tensile fracture of the upper lithosphere the response of viscoelastic lithosphere to constant geometry bending stresses has also been examined and results in substantial but not complete reduction of the bending stresses

oxygen isotope geochemistry of oceanic arc lavas variations of oxygen isotope ratios in arc related lavas can constrain the contributions of subducted crustal igneous rocks sediments and fluids to the sub arc mantle we have measured oxygen isotope ratios in 72 arc and back arc lavas from five ocean ocean subduction zone systems using laser fluorination analyses of olivine and other phenocrysts and glass eighty percent of our samples have delta 18o values for any given phase olivine plagioclase glass or biotite within 0 2 per thousand of the average value for that phase in upper mantle peridotites and mid ocean ridge basalt morb the range for each phase is 1 0 per thousand this result contrasts with previous studies of whole rock samples which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation and demonstrates that most arc related lavas contain 1 2 of 18o enriched crustal oxygen from any source i e assimilation or subducted contributions elevations in delta 18o that do occur in these basic arc derived magmas relative to values most common for mantle derived lavas are associated both with enriched radiogenic isotope signatures and even more strongly with chemical indices consistent with high integrated extents of melting of their peridotite sources we interpret these relationships as evidence that melting in the sources of the high delta 18o lavas we have studied was fluxed by addition of high delta 18o aqueous fluid or perhaps a hydrous melt from the subducted slab such that sources that contain relatively large components of slab derived fluid or melt are both relatively 18o enriched and also experienced relatively large amounts of melting we have developed a quantitative model linking the amount of melting to the extents of 18o radiogenic isotope and trace element enrichment in a mantle source being fluxed by addition of aqueous fluid comparison of this model with observed variations in the geochemistry of lavas from the vanuatu fiji new caledonia region the suite of related samples showing the greatest range in delta 18o observed in this study constrains the amounts and chemical and isotopic compositions of slab derived phases in the sources of these arc related lavas assuming a delta 18o value of 20 per thousand for the slab derived fluid 0 5 1 0 wt is added to the sources of most mantle derived arc magmas the maximum amount of slab derived flux in the sources of arc magmas according to our results is 2 5 wt

broad top thrust sheet an extensive blind thrust in the central appalachians interpretation of seismic data from the central appalachians valley and ridge province reveals that a large blind thrust the broad top thrust is much more extensive than previously realized this thrust sheet extends across the 15 mi 24 km width of the nittany anticlinorium a dominate structural element that lies along the western margin of the valley and ridge province adjacent to the plateau province in the appalachian foreland it extends for a distance along strike of at least 100 mi 160 km and duplicates a 7800 ft 2380 m thick sequence of cambrian and ordovician limestone and dolomite referred to as the cambrian ordovician lithotectonic unit thinning of the duplicated cambrian ordovician sequence along the length of the nittany anticlinorium by as much as 2500 ft 760 m observed on seismic lines is attributed to removal of less brittle intervals from the base of the upper sheet and top of the lower sheet the missing section from this inter sheet detachment zone may have been transported toward the foreland and incorporated into the leading parts of the thrust sheet the tectonic thinning solves space problems associated with duplication of the lithotectonic unit beneath what have previously been considered abnormally deep synclines of the anticlinorium the abnormal depth of these synclines occurs where tectonic thinning is located on the seismic lines structural traps in a triangle zone along the allegheny structural front have been attributed to the intense shortening associated with emplacement of the broad top thrust sheet the presence of the thrust sheet along the length of the structural front in west virginia and virginia enhances the probability of successful exploration for triangle zone structures along the entire length of the structural front structures produced by variable tectonic thinning in the inter sheet detachment zone may also produce additional traps within the anticlinorium in the form of closure or associated fracture systems

intergranular pressure solution in four quartzose sandstones abstract cathodoluminescence petrography and scanning electron microscopy reveal that several geologic variables significantly influenced intergranular pressure solution and quartz cementation in the bromide ordovician oklahoma upper minnelusa permian wyoming nugget jurassic utah and tuscarora silurian pennsylvania sandstones a linear relationship exists between mean grain size and intergranular pressure solution with finer grained samples having experienced more intergranular pressure solution than coarser grained ones samples that display pervasive illite grain coatings have also experienced more intergranular pressure solution than those with no or poorly developed grain coatings sorting and limited framework grain compositional variability have exerted little influence o intergranular pressure solution silica budgets indicate that in the bromide upper minnelusa and nugget sandstones more quartz has been dissolved by intergranular pressure solution than is present as cement these sandstones have exported silica during burial diagenesis in the tuscarora the amount of quartz dissolved by intergranular pressure solution is about equal to the amount of quartz cement present implying approximate mass balance however petrographic evidence suggests that most of the quartz cement in at least three of these sandstones predates most intergranular pressure solution implying that they acted as silica importers during shallow diagenesis and silica exporters during deeper diagenesis intergranular pressure solution has played varying roles in determining the ultimate porosity of these four sandstones in the nugget intergranular pressure solution was the single most important diagenetic process that influenced porosity evolution in the bromide intergranular pressure solution was slightly more important than mechanical compaction and considerably more important than cementation in the tuscarora intergranular pressure solution was slightly more important than mechanical compaction and of about equal importance to quartz cementation in the upper minnelusa intergranular pressure solution was less important than both mechanical compaction and gypsum anhydrite cementation

oxygen isotope compositions of lavas from the galapagos archipelago geochemical contributions from modern crustal sources new oxygen isotope compositions of olivine phenocrysts collected across the galapagos archipelago show a larger range of 18 o olivine values than previously reported 4 74 5 40 in the region olivines from fernandina floreana and pinta which represent the main radiogenic isotope end members of the galapagos hotspot have 18 o values of 5 02 0 08 1 within the accepted range of the oxygen isotope composition of mantle olivines in general 18 o olivine values do not correlate with radiogenic isotope compositions of hosting lavas instead the span of 18 o olivine values is more consistent with widespread lithospheric contamination 18 o olivine values at the higher end of the range in the galapagos are correlated with indices of crustal assimilation including sr sr values below the normal mantle range can be explained by assimilation and fractional crystallization processes olivines that have 18 o values below the normal mantle range come from the western part of the archipelago with the thickest lithosphere this is consistent with melt interacting with crust that underwent hydrothermal alteration at elevated temperature causing a decrease in 18 o values in contrast the highest 18 o values of the galapagos come from areas underlain by thin lithosphere in the eastern part of the archipelago this is consistent with shallow crust melt interaction that is generally associated with 18 o values above the normal mantle range these results suggest that while the three end member components of the galapagos mantle have a generally homogenous 18 o value indistinguishable from normal upper mantle there is a more widespread effect of lithospheric contamination in melts than previously thought

structural and microstructural evolution of the rattlesnake mountain anticline wyoming usa new insights into the sevier and laramide orogenic stress build up in the bighorn basin the rocky mountains in western us provide among the best examples of thick skinned tectonics following a period of thin skinned tectonics related to the sevier orogeny the compressional reactivation of basement faults gave birth to the so called laramide uplifts arches the bighorn basin located in wyoming is therefore a key place to study the transition from thin to thick skinned tectonics in orogenic forelands especially in terms of microstructural and stress strain evolution our study focuses on a classic laramide structure the rattlesnake mountain anticline rma wyoming usa a basement cored anticline located in the western part of the bighorn basin stress and strain evolution analysis in folded sedimentary layers and underlying faulted basement rocks were performed on the basis of combined analyses of fractures fault slip data and calcite twinning paleopiezometry most of the fractures are related to three main tectonic events the sevier thin skinned contraction the laramide thick skinned contraction and the basin and range extension serial balanced cross sections of rma and displacement profiles suggest that all thrust faults were coeval evidencing strain distribution in the basement during faulting the comparison of rma with another structure located in the eastern edge of the bighorn basin i e the sheep mountain anticline sma allows to propose a conceptual model for the geometric and kinematic evolution of laramide related basement cored anticlines finally the stress evolution is reconstructed at both the fold scale and the basin scale we show that the evolution of stress trends and magnitudes was quite similar in both structures rma and sma during laramide times thick skinned tectonics in spite of different stress regimes during sevier thin skinned tectonics and post laramide times stress trends and fracture patterns were different in these two structures these results suggest that the distance to the orogenic front influenced the fracture patterns but not the foreland stress magnitudes which were likely controlled by the structural style

wolf volcano galapagos archipelago melting and magmatic evolution at the margins of a mantle plume wolf volcano an active shield volcano on northern isabela island in the galapagos archipelago has undergone two major stages of caldera collapse with a phase of partial caldera refilling between wolf is a typical galapagos shield volcano with circumferential vents on the steep upper carapace and radial vents distributed in diffuse rift zones on the shallower sloping lower flanks the radial fissures continue into the submarine environment where they form more tightly focused rift zones wolf s magmas are strikingly monotonous estimated eruptive temperatures of the majority of lavas span a total of only 22 c this homogeneity is attributed to buffering of magmas as they ascend through a thick column of olivine gabbroic mush that has been deposited from a thin shallow 2 km deep subcaldera sill that is in a thermochemical steady state wolf s lavas have the most depleted isotopic compositions of any historically active intraplate ocean island volcano on the planet and have isotopic compositions except for he he indistinguishable from midocean ridge basalt erupted from the galapagos spreading center gsc 250 410 km away from the peak of influence of the galapagos plume wolf s lavas are enriched in incompatible trace elements and have systematic major element differences relative to gsc lavas however wolf s magmas result from lower extents of melting deeper melt extraction and a greater influence of garnet compared with gsc magmas but wolf and the gsc share the same sources these melt generation conditions are attributed to melting in a thermal and mechanical boundary layer of depleted asthenosphere at the margins of the galapagos plume the lower degrees of melting and extraction from deeper levels result from a thicker lithospheric cap at wolf than exists at the gsc

late paleozoic deformation of the cratonic carbonate cover of eastern north america paleozoic carbonates that cover the eastern half of the north american craton preserved a bedding parallel shortening fabric up to 800 km away from the appalachian ouachita orogenic front as recorded by mechanical twins in calcite a strain fabric is present in 29 of 31 samples from three traverse within the flat lying cratonic cover sequence jasper arkansas to minneapolis bluefield west virginia to madison wisconsin and albany new york to hamilton ontario twinning is found only in paleozoic carbonates and is absent in cretaceous limestones in the two southernmost traverses the orientation of the principal shortening axis e1 is perpendicular to the thrust front and parallel to the thrusting direction of the southern appalachians and the ouachitas the magnitude of calcite shortening strain 6 and the calculated differential stress 90 mpa decrease exponentially away from the orogenic front until twinning is absent the northernmost traverse shows greater complexity probably because of its location in the bend of the appalachian trend the authors conclude that the regional strain fabric preserved in the cratonic cover sequence of eastern north american was produced by late paleozoic orogenic stresses of the appalachian ouachita orogeny and that compressive stresses were transmitted over distances greater than 1200 km awaymore from the active plate margin less

geochemical constraints on the role of oceanic lithosphere in intra volcano heterogeneity at west maui hawaii 87 sr 86 sr 206 pb 204 pb variation in the stratigraphically lowest lavas is orthogonal to the all hawaii variation indicating that it is not the result of mixing between components normally sampled by hawaiian shield stage magmas we compare our west maui data with observed compositions of pacific oceanic basaltic and gabbroic crust and predicted compositions for 2ga basaltic and gabbroic oceanic crust the observed fine scale compositional variability in the stratigraphically lowest west maui lavas is consistent with 10 15 mixing of small degree 2 partial melts of the pacific gabbroic oceanic crust with plume derived kea type magmas mass balance considerations indicate that this geochemical signal can be present in no more than 5 of the total volume of lavas erupted at west maui

deformation history of the roanoke recess appalachians usa the evolution of the roanoke recess is controversial because agreement is lacking about the timing and continuity of deformation increments because it may or may not have behaved like an adjacent salient and recess because while many salients have been investigated few recesses have and because the role of blind thrusts is unclear to gain insight into these controversies we estimated shortening directions shortening magnitudes and relative timing in the recess from map scale folds outcrop scale stylolites and grain scale solution structures and twins the key results from combining these new data with previous work are as follows 1 the deformation history around the roanoke recess is most simply described as temporally continuous with deformation migrating northeastward while the primary shortening direction rotated counterclockwise from n directed to almost w directed 2 consequently the roanoke recess did not deform in the symmetrical manner of the adjacent pennsylvania salient nor with the two step history of the kingston recess further to the north in the appalachians 3 the timing and magnitude of shortening increments are sufficient to necessitate a blind thrust under the present surface rocks of the inner recess throughout the deformation history

fault related folding in sandbox analogue models of thrust wedges abstract the nucleation and growth of thrust fault related folds has been simulated using high resolution sandbox coulomb wedge type analogue models thrust related anticlines grew progressively from initial layerparallel shortening to decollement folding thrust tip folding and eventually thrust ramp folding thrust ramps nucleated in the central sector of the overturned forelimb of decollement folds as a consequence of limb thinning thrust tip folding occurred during outward migration of the ramp tips the fault propagation rate was greater at the lower ramp tip which quickly joined the basal thrust flat while folding continued beyond the upper tip the development of the analogue model thrust fault related folds by progressive changes in kinematics from decollement folding to thrust tip folding and finally thrust ramp folding compares well with analyses of naturally occurring thrust fault related folds

origin and evolution of silicic magmatism at yellowstone based on ion microprobe analysis of isotopically zoned zircons the origin of large volume yellowstone ignimbrites and smallervolume intra caldera lavas requires shallow remelting of enormous volumes of variably o depleted volcanic and sub volcanic rocks altered by hydrothermal activity zircons provide probes of these processes as they preserve older ages and inherited do values this study presents a high resolution oxygen isotope examination of volcanism at yellowstone using ion microprobe analysis with an average precision of 0 2o and a 10 mm spot size we report 357 analyses of cores and rims of zircons and isotope profiles of 142 single zircons in 11 units that represent majoryellowstone ignimbrites and post caldera lavas many zircons from these samples were previously dated in the same spots by sensitive high resolution ion microprobe shrimp and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination we additionally report oxygen isotope analyses of quartz crystals in three units the results of this work provide the following new observations 1 most zircons from post caldera low do lavas are zoned with higher do values and highly variable u pb ages in the cores that suggest inheritance from pre caldera rocks exposed on the surface 2 many of the higher do zircon cores in these lavas have u pb zircon crystallization ages that postdate caldera formation but pre date the eruption age by 10 20 kyr and represent inheritance of unexposed post caldera sub volcanic units that have do similar to the lava creek tuff 3 young and voluminous 0 25 0 1ma intra caldera lavas which represent the latest volcanic activity atyellowstone contain zircons with both high do and lowdo cores surrounded by an intermediate do rim this implies inheritance of a variety of rocks from high do pre caldera and low do post caldera units followed by residence in a common intermediate do melt prior to eruption 4 major ignimbrites of huckleberry ridge and to a lesser extent the lava creek and mesa falls tuffs contain zoned zircons with lower do zircon cores suggesting that melting and zircon inheritance from the lowdo hydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse 5 the do zoning in the majority of zircon core rim interfaces is step like rather than smoothly inflected suggesting that processes of solution reprecipitation were more important than intracrystalline oxygen diffusion concave downward zircon crystal size distributions support dissolution of the smaller crystals and growth of rims on larger crystals this study suggests that silicic magmatism atyellowstone proceeded via rapid shallow level remelting of earlier erupted and hydrothermally alteredyellowstone source rocks and that pulses of basaltic magma provided the heat for melting each postcaldera yellowstone lava represents an independent homogenized magma batch that was generated rapidly by remelting of source rocks of various ages and do values the commonly held model of a single large volume super solidus mushy state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data rather the source rocks for theyellowstone volcanism were cooled below the solidus hydrothermally altered by heated meteoric waters that caused low do values and then remelted in distinct pockets by intrusion of basic magmas each packet of new melt inherited zircons that retained older age and do values this interpretation may have significance for interpreting seismic data for crustal low velocity zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side new basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas we also suggest that the lava creek tuff

fracture patterns in the zagros simply folded belt fars iran constraints on early collisional tectonic history and role of basement faults pre early folding fracture patterns were recognized in several anticlines from three structural domains in the simply folded belt of the fars arc these fracture sets were characterized in terms of opening mode and relative chronology and used to reconstruct the main compressional trends related to the early zagros collisional history the palaeostress reconstructions based on these fracture sets were further refined by combination with newly collected or already available fault slip and calcite twin data as an alternative to previous models of rigid block rotations or regional stress rotation we propose that the complex pattern of pre folding fractures and the contrasting palaeostress orientations through time in the different domains investigated are related to the presence of basement faults with n s and wnw trends above which basement and cover were variably coupled during stress build up and early deformation of the arabian margin beyond regional implications this study draws attention to the need to carefully consider pre existing fractures possibly unrelated to folding to build more realistic conceptual fold fracture models

structural chronology of the alleghanian orogeny in southeastern west virginia the trend change between central and southern appalachian structures is sharply defined in southeastern west virginia there n30 35 e trends coincident with major central appalachian folds such as the browns mountain anticline end abruptly at n60 e trending southern appalachian structures along the st clair fault analysis of local and regional folds cleavage bedding perpendicular stylolite seams and bedding and fault slickenlines reveals that layer parallel shortening directed n10 30 w occurred in nonfolded greenbrier group mississippian carbonates well into the present appalachian plateau area this structural event is early and is associated with the evolution of southern appalachian folds and faults south of the st clair fault central appalachian folds and mesoscopic structures were superimposed on this early layer parallel shortening fabric this structural chronology indicates that southern appalachian folds and faults predated the development of central appalachian structures in the region

lithospheric structure and compensation mechanisms of the galapagos archipelago volcanic islands of the galapagos archipelago are the most recent subaerial expression of the galapagos hotspot these islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young 10 m y oceanic lithosphere just south of the active galapagos spreading center the 91 w fracture zone crosses the platform and creates an estimated 5 m y age discontinuity in the lithosphere major tectonic features of the galapagos include an unusually broad distribution of volcanic centers pronounced structural trends such as the nw se wolf darwin lineament wdl and a steep escarpment along the western and southern margins of the archipelago we use shipboard gravity and bathymetry data along with geosat geoid data to explain the tectonic and structural evolution of the galapagos region we model the gravity anomalies using a variety of compensation models including airy isostasy continuous elastic flexure of the lithosphere and an elastic plate with embedded weaknesses and we infer significant lithospheric strength variations across the archipelago the outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of 12 km this area includes the large shield volcanoes of fenandina and isabela islands where the lithosphere regionally supports these volcanic loads the central platform is weaker with an elastic thickness of 6 km or less and close to airy isostasy the greatest depths to the moho are located beneath eastern isabela island and the central platform thinner lithosphere in this region may account for the broad distribution of volcanoes the extended period of eruption of the central volcanoes and their reduced size the transition from strong to weak lithosphere along the southern escarpment appears to be abrupt within the resolution of our models and can be best represented by a free end or faultlike discontinuity also modeling the wdl as a lithospheric fault increases the match to the observed gravity anomalies the primary feature is the weak central platform whose existence is a logical convergence of several factors the 91 w transform age offset the possible increase in age of the lithosphere along the southern part of the platform due to ridge jumping and the formation of the central platform at a spreading ridge

plagioclase ultraphyric basalts of the galapagos archipelago abstract basalts containing as much as 50 plagioclase phenocrysts that attain lengths of several centimeters are common on the northern islands of the galapagos archipelago mineralogic and geochemical evidence requires that the large anorthitic phenocrysts nucleated and grew from a less evolved magma than that in which they were erupted we suggest that plagioclase ultraphyric basalts pubs form when a buoyant phenocryst laden magma enters a chamber containing a denser more evolved magma as the new magma mixes with the old anorthitic plagioclase phenocrysts may accumulate near the roof in a hybrid liquid thinner crust under the northern islands increases the probability that viscous plagioclase rich magmas can ascend to shallow magma chambers and may account for the fact that pubs are more common on the northern islands than in the rest of the archipelago

mechanisms of thrust propagation some examples and implications for the analysis of overthrust terranes abstract evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move the mechanisms are 1 failure of a stiff layer to form ramp flat geometry imbricated thrust sheets 2 detachment of a layer by folding decollement thrust sheets 3 differential layer parallel shortening lps thrust sheets the mechanisms are independent but may operate interactively to form families of hybrid thrust sheets since lps thrust sheets have not been previously described or documented data from the new york plateau deformed almost exclusively by differential lps is presented to demonstrate the physical characteristics of this thrust mechanism information from the plateau indicates that finite strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial finite strain data are used to construct a set of iso strain maps in both the deformed and undeformed states the iso strain maps are in turn used to determine the displacement field for the thrust sheet the displacement field allows visualization of the effects of both lagrangian and eulerian transformations on an initially orthogonal grid utilizing sections from a number of overthrust belts it is shown that the three mechanisms occur universally although in various proportions lps is regionally developed throughout the central and northern appalachians the presence of the lps thrust sheets probably accounts for the failure of structural cross sections to bed length balance in these blind thrust terranes finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross sections

duplex structures and imbricate thrust systems geometry structural position and hydrocarbon potential duplexes and imbricate thrust systems form some of the most complex hydrocarbon traps in overthrust belts the geometry of a duplex is controlled by the ramp angle thgr and height hr the final spacing between adjacent thrusts a prime and the relative displacements on them d1 d2 for constant thgr and hr three different classes are recognized 1 independent ramp anticlines and hinterland sloping duplexes 2 true duplexes and 3 overlapping ramp anticlines imbricate thrust systems consist of several thrust faults each of which loses displacement upsection and eventually dies out by progressively transferring its displacement to a fold at its tip or by distributing it among several splays hybrid duplexes are ormed from systems of fault propagation folds that are subsequently carried over ramps so they have a more complex geometry and a greater structural relief these different types of thrust systems occur as outcrops and prospect scale structures and they define the regional structural styles of most fold and thrust belts second and third order duplexes and imbricate thrust systems occur in specific structural positions such as footwalls hanging walls and frontal zones of major thrust ramps and in cores of major anticlines several types of duplexes and imbricate thrust systems form important hydrocarbon traps examples include the system of independent anticlines of the turner valley and highwood oil and gas fields the hinterland sloping duplex consisting of the chestnut ridge sandy ridge system of the ben hur oil field the partly overlapping anticlines of the waterton and savanna creek gas fields and the completely overlapping anticlines of the rose hill oil field