model for the origin of the troodos massif cyprus and other mideast ophiolites any comprehensive model for the origin of the troodos complex and other mideast ophiolite complexes must explain the geochemical evidence for subduction zone involvement the thin inferred oceanic crust the extensional environment indicated by sheeted dikes the existence of fault zones perpendicular to the inferred spreading axes and the discontinuous nature of ophiolite exposures around the arabian block the andaman sea region of the indian ocean may provide an actualistic model for the origin of these ophiolites there spreading takes place in short segments above a subduction zone in the region of active spreading the andaman sea model substantially accounts for the known geologic and geophysical data from the circum arabian ophiolite belt and it leads to some predictions about the structural and geophysical relationships to be expected in this important belt the model also predicts that andaman sea spreading center magmas may be similar in composition to those found in the mideast ophiolites

spreading structure of the troodos ophiolite cyprus some paleomagnetic constraints it has been suggested that grabens on the northern flank of the troodos ophiolite formed by listric faulting in an oceanic environment and represent axial valleys fossilized by ridge jumping this paleomagnetic survey of the solea graben was carried out to constrain the structural interpretation an analysis was devised which restores both the dikes sampled to vertical and the primary remanent magnetizations to a previously derived structurally corrected remanent magnetization direction typically two solutions for the original dike orientation and the pole and angle of net rotation are obtained for each dike sampled these solutions can be separated if a locally consistent original dike strike is assumed reasons are discussed for the selection of the solution that gives an original northwesterly dike strike results are consistent with a graben rotations are up to 78 at the sites studied rotation vectors are subhorizontal and subparallel to the original dike strike suggestive of an axial process for the development of the structure however on the basis of field observations an alternative mode of origin is proposed invoking ephemeral extreme upper crustal extension by listric faulting on the west flank of an intermediate to fast spreading axis the grabens being formed by antithetic faulting

spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the solea graben northern troodos ophiolite cyprus hydrothermal mineral zonations and o isotope patterns of the northern troodos complex do not parallel the ophiolite pseudostratigraphy but reflect the convective geometry of an upper cretaceous seawater hydrothermal system large areas of the sheeted intrusive complex sic including the subaxial region of the solea graben are composed of 18o rich subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge other areas of the sic are recrystallized to distinctive epidosite rocks granular high variance assemblages of epidote quartz chlorite that are depleted in 18o al2o3 na2o k2o zr cu and zn and are enriched in cao and sr compared with other mafic volcanic and dike rocks of the solea graben epidosite alteration occurred at temperatures of 310 370 c and involved fluids with 18o values and salinities similar to those of upper cretaceous seawater the epidosite zones are discordant with earlier mineral o isotope zonations and with the axis of spreading in the solea graben suggesting a postspreading off axis origin the seawater hydrothermal system responsible for solea graben massive sulfide deposits was probably driven by hypabyssal intrusions not exposed emplaced in a terminal failed spreading episode the geometries of o isotope surfaces within the solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits depletions in base metals show that epidosite alteration liberated cu and zn to mineralizing fluids within the fossil upflow zone

evidence for large scale rotations at spreading centers from the josephine ophiolite abstract repetition of the josephine ophiolite by large scale open folding and imbricate faulting provides an excellent opportunity to study the geometric relationships between sheeted dikes igneous layering and bedding measurements of over 600 dike margins from the sheeted dike complex and high level gabbro indicate that dikes are oriented at 20 40 to bedding in metasedimentary rocks overlying the ophiolite in addition igneous layering is discordant to gross lithologic layering and is oriented at a large angle to both sheeted dikes and bedding similar relationships may occur in several other ophiolites the inclined sheeted dikes and discordant igneous layering are best explained by a 45 rotation of new oceanic crust ophiolite immediately following formation at a spreading center rotation could have been accomplished by either subsidence or faulting further evidence for large scale rotations of oceanic crust include frequent anomalous magnetic inclinations in drill cores of layer 2 from the atlantic

reconstructed seismic velocity structure of the lewis hills massif and implications for oceanic fracture zones the lewis hills massif of the bay of islands ophiolite complex provides a lower crustal to upper mantle cross section of an oceanic fracture zone segment and adjacent oceanic lithosphere the structural and petrologic character of the fracture zone region is distinctly different from the comparatively simple layered structure of the rest of the bay of islands complex results of detailed field mapping petrography and laboratory seismic velocity measurements have been combined in order to reconstruct the seismic velocity structure of this slice of oceanic lithosphere velocities and velocity anisotropies of each of the major rock units in the massif are restored to their preobduction geometries relative to different stratigraphic levels in the complex and proximity to the fracture zone region although all oceanic fracture zones probably have unique structural and petrologic characteristics in detail from the perspective of the present study it is suggested that in general seismic properties differing from that of normal oceanic crust and upper mantle may be expected within approximately 10 km of the axis of fracture zones in these regions lower crustal as well as upper mantle anisotropy is expected the moho might be expected to shallow due to the presence of abundant ultramafic cumulates resulting in crustal thinning and significant velocity inversions may occur with depth where large cyclic mafic layers occur within cumulate ultramafics in oceanic lithosphere which has passed or is passing through a transform fault domain enhanced mantle anisotropy might occur the middle to lower crust in these regions should be composed of highly anisotropic metamorphic rocks which grade upward into serpentinites and highly fractured gabbros diabases and basalts with relatively low velocities it is suggested that velocities as well as velocity anisotropies produced by these types of structures might be determined in situ by seismic refraction experiments and may be used to map the extent of deformation in the oceanic lithosphere near fracture zones

the east pacific rise in cross section a seismic model in 1982 we undertook a seismic refraction experiment known as the magma expedition to examine the detailed structure of the east pacific rise near 12 50 n this segment of the rise where the full spreading rate is about 110 mm yr is near the projected trace of the o gorman fracture zone and is the site of an overlapping spreading center as well as black smoker hydrothermal activity in this paper we describe the analysis of a subset of the travel time data collected during the magma expedition namely the data from profiles which were oriented normal to the rise axis these profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross sectional structure of the rise we have modeled these data using a two dimensional ray tracing program we have found that the seismic velocities in the young oceanic crust are rather high with velocity gradients of 4 0 5 5 s 1 in the uppermost crust the highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0 1 ma this decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads the decrease in velocity does not appear to penetrate deeper than about 0 5 km and may reverse itself as hydrothermal alteration fills the pores and cracks the fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits a result consistent with the high temperatures and discrete nature of the vents our best model includes a magma chamber some 4 km wide and extending from the moho to within about 1 1 km of the seafloor this magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the east pacific rise these discrepancies probably arise because the magma chamber under the east pacific rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

modes of extension at oceanic spreading centers evidence from the solea graben troodos ophiolite cyprus abstract the solea graben in the troodos ophiolite cyprus represents a preserved cretaceous age axial valley formed during a brief period of amagmatic seafloor spreading structural analysis of all crustal levels of the graben reveals two periods of deformation an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the tertiary recent amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike plutonic contact and oriented subparallel to ophiolite pseudostratigraphy individual fault segments are essentially planar but link to approximate listric geometries locally bulk structural rotation of fault blocks bounding the graben occurred about a nnw trending axis parallel to the present graben trend average dips in the sheeted complex within the central portions of the graben suggest 40 50 of structural rotation although this is a minimum figure because present dike dips are not a simple function of rotation magnitude tensor analysis of fault populations associated with extensional deformation shows that 3 was oriented approximately perpendicular to the graben axis late near axis extensional deformation segmented the detachment fault along nnw trending normal faults obduction related uplift and doming of the troodos ophiolite is expressed in the solea graben by minor thrust reverse and strike slip faults principally developed during reactivation of pre existing low angle fault zones formed during seafloor deformation tensor analysis of this later phase of faulting shows that 1 was oriented approximately n s and that 2 and 3 alternated position during the deformation the style of deformation documented in the solea graben supports recent speculation for basin and range style detachment faulting at slow spreading ridge crests with low magma budgets such as parts of the mid atlantic ridge near large offset transform faults natural cross sectional exposure of the solea graben shows how extensional structures might link at depth beneath such slow spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large scale ore forming hydrothermal circulation cells

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

210pb 226ra and 32si in pavin lake massif central france abstract measurements of cosmogenic 32si and the u decay series nuclides 210pb and 226ra in waters and sediments of lake pavin are reported both 210pb and 226ra are enriched in the anoxic deep waters compared to the oxic surface waters respectively by a factor of 4 and 10 whereas 32si is depleted by a factor of2 redox conditions in the lake appear to have no marked effect on the 32si using a steady state box model it is shown that the deep water 32si concentration is controlled by the underground lacustrine springs the residence times of 210pb 32si and 226ra are1 10 and80 a respectively in the case of 32si where more data are available the assessed inventory data from the overhead atmospheric fallout and that measured in the sediments agree very well as expected the 210pb and 32si based deposition rates during the past100 a ranged from 0 8 to 1 9 mm a 1 earlier these were a factor of3 5 faster the geochemistry of 32si and 210pb in lake pavin in many ways resembles that in the ocean only the time scales of the processes involved are faster

seismotectonics of the southern boundary of anatolia eastern mediterranean region subduction collision and arc jumping the pattern of seismicity and fault plane solutions of earthquakes are used to outline the tectonic features of the southern boundary of anatolia in the eastern mediterranean and southeastern turkey the results of this study show that this boundary is composed of two distinct parts one in southeastern turkey and syria is a wide and complex zone of continental collision the other in the levantine basin of the eastern mediterranean is a zone of oceanic subduction in the region of continental collision three zones of seismicity are observed most of the seismic activity in this region follows the bitlis zone and is associated with a zone of thrusting and mountain building this appears to be the zone of most active deformation and plate consumption in the plate boundary region between arabia and turkey a less active zone of seismicity to the north of the bitlis zone is interpreted to have been more active in the past whereas another active zone of seismicity to the south is interpreted to be a zone which may be more active in the future as the main zone of plate consumption jumps to the south in the subduction zone of the eastern mediterranean the depth of the subducted slab and the rate of seismicity generally increase from east to west the zone of present day convergence between africa and turkey in the levantine basin can be best outlined by the northern edge of the mediterranean ridge the subduction zone in this area sequentially jumps to the south as small continental fragments collide with existing zones of subduction deep seismic activity near the gulf of antalya is associated with a detached subducted slab north of the anaximander mountains that is distinctly different from the seismic trend which is associated with present day active subduction the plate boundary between africa and turkey at the center of the levantine basin appears to have shifted to the south of the anaximander mountains and florence rise most of the focal mechanisms of the earthquakes along the entire southern boundary of anatolia indicate that n to nnw thrusting is the dominant mode of seismic deformation the present set of data is inconsistent with a major transform fault in the cyprean arc east of cyprus and in southeastern turkey

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

geometry conditions and timing of off axis hydrothermal metamorphism and ore deposition in the solea graben the recent discovery of fossil spreading axes along the northern margin of the troodos ophiolite1 provides a unique opportunity to compare a fossil seawater hydrothermal system with systems of modern oceanic spreading centres2 6 whereas previous accounts of ophiolite hydrothermal systems7 13 including troodos14 16have elucidated many aspects of sea floor hydrothermal metamorphism earlier workers have not placed ophiolite hydrothermal features within any specific spreading centre structural framework most ophiolites exhibit complex histories of post accretion deformation that mask or obliterate early structures related to seafloor spreading the troodos ophiolite is apparently still in the process of being emplaced 17 and most structural elements at troodos appear to have originated beneath a cretaceous sea floor1 we have discovered an extensive area of intense seawater hydrothermal alteration within the sheeted dyke complex of the solea graben fig 1 the metamorphic petrology and oxygen isotope geochemistry of 100 dyke samples from this graben reveal the size and three dimensional geometry of the hydrothermal system and yield estimates of temperature salinity oxygen isotope composition and recharge discharge geometry for fluids that transported the constituents of massive sulfide orebodies geometric discordance between the trends of the hydrothermally altered area and of earlier seafloor spreading structures compels us to consider a post spreading off axis origin for the orebodies of the solea graben

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

genetic relationship between faults and folds and determination of laramide and neotectonic paleostress western colorado plateau transition zone central utah faults in the western part of the colorado plateaus and the adjacent eastern part of the structural transition zone to the basin and range in central utah represent two stages of deformation laramide compressional faults on the plateau eastern side and late cenozoic extensional faults on the transition zone western side the faults range in displacement from a few centimeters to several hundred meters this study of minor structures provides strong evidence for regional ne sw horizontal maximum compressive stress as a dynamic component of monoclinal flexure the results further suggest that it is inaccurate to interpret the monoclines simply as passive drape folds formed during vertical block up lift the laramide faults on the plateau side cut two types of large scale folds 1 a broad warp called the teasdale anticline and 2 two monoclines that flank the teasdale anticline called the waterpocket and cocks comb monoclines the faults that cut the monoclines dip steeply and have mostly strike slip displacements strike azimuths of dextral and sinistral faults tend to occupy separate sectors suggesting that they are conjugate shears activated in a stress field with a subhorizontal maximum compressive stress 1 oriented northeast southwest this orientation is confirmed by computations of mean paleostress characteristics by inversion of fault slip data the monoclines differ from one another in trend by 30 so it is possible to compare these trends with the results of separate paleostress computations for faults from each monocline four separate computed 1 axes three from the cocks comb monocline are approximately horizontal and cluster within 10 of the n 65 e normal to the axial trace of the waterpocket monocline suggesting that the waterpocket at the latitude of our study formed approximately perpendicular to the local laramide compressive stress direction the maximum compressive stress 1 is located clockwise about 30 from the normal to the cocks comb monocline we interpret this discordance as evidence that the cocks comb trend was influenced by and in large part inherited from the trend of an ancient buried fault rooted in the basement whereas the exposed faults from which 1 was computed are structures that formed in and are probably restricted to the sedimentary cover rocks the exposed faults probably formed fresh in the cover rocks during laramide compression and are generally more reliable indicators of compressional kinematics than fold orientations part of the cocks comb monocline shows a 13 distortion of its axial trace by cumulative dextral slip on hundreds of small displacement transverse faults we interpret this distortion as a late stage brittle compressional strain 20 related to the buttressing by mechanically resistant rocks in the most structurally elevated part of the adjacent teasdale anticline fault slip data were gathered from the thousand lake fault the easternmost main fault zone of the transition zone faults that parallel the main fault show mainly dip slip but many faults in the zone strike oblique to it and show strike slip or dip slip analysis of the predominantly dip slip faults indicates a west northwest east southeast 285 least principal stress strain axis that we interpret as the neotectonic extension direction for the area

detachment faulting and amagmatic extension at mid ocean ridges the josephine ophiolite as an example lithospheric extension at the josephine paleo spreading center occurred by a combination of magmatic and amagmatic processes the amount of extension by amagmatic processes appears at least as large as that from magmatic processes the structural processes responsible for amagmatic extension in the absence of a magma chamber appear to be normal faulting and block rotation in the brittle upper lithosphere and ductile flow in the lower lithosphere an extensive detachment shear zone occurs beneath the fault blocks approximately 1 km below the base of the crustal sequence the amount of amagmatic extension can be approximated from a simple geometric model relating extension to rotations of sheeted dikes similarly attenuated crustal thicknesses can be related to fault block rotations the results suggest that thin crustal sequences commonly observed in ophiolites and near fracture zones may result from attenuation during amagmatic extension furthermore detachment faulting and block rotation may be characteristic of spreading centers where the magma budget is low

the geometry of listric normal faults and deformation in their hangingwalls abstract in cross sections containing listric extensional faults area balancing techniques for depth to decollement are usually based on either bed length conservation or displacement conservation listric fault geometry may be constructed from a hangingwall roll over profile using the chevron construction this construction based on conservation of heave necessitates a reduction in fault displacement with decreasing fault dip a modification of this construction utilizing conservation of fault displacement predicts a listric fault that detaches at a shallower depth a new construction based on slip lines uses fault perpendicular displacement segments to generate listric fault shape fault propagation strain may be responsible for anomalous hangingwall geometries and these can be predicted by forward modelling using either a modified chevron construction or a slip line construction

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

microstructure of serpentinite mylonites from the josephine ophiolite and serpentinization in retrogressive shear zones california the josephine peridotite has experienced deformational episodes over a wide range of temperatures deformation within the stability field of a serpentine group mineral appears invariably accompanied by retrogressive metamorphism serpentinization two general types of deformed serpentinites have been identified 1 incohesive serpentinites most common which appear to have experienced low temperature predominantly brittle deformation and 2 serpentinite mylonites which have experienced plastic flow near the upper limit of antigorite stability the distinction between these two is based on textural and mineralogical criteria serpentinite mylonites are cohesive homogeneously foliated and typically strongly lineated rocks composed primarily of antigorite and magnetite they occur in planar bounded zones enclosed within the josephine peridotite that were the loci of both noncoaxial deformation and serpentinization we discuss the development of two important shear sense indicators porphyroclast systems and oblique shear band foliations as observed in these rocks porphyroclast systems in serpentinite mylonites are often comprised of cores and mantles of different mineralogy cores are usually composed of primary pyrogenetic minerals whereas mantles including tails are composed of products of the serpentinization reaction these systems are therefore termed porphyroctasts with neocrystallization tails oblique shear bands are common micro structures in serpentinite mylonites shear bands which offset the mylonitic foliation along narrow discrete fractures are termed f type whereas those which offset the mylonitic foliation along narrow ductile micro shear zones are termed d type the latter are considered more reliable for shear sense determinations strains accomplished by movement on shear bands are probably small strain resulting from a volume increase dur ing serpentinization while holding the shear zone width constant is sufficient to produce the shear bands therefore we suggest that oblique shear bands may develop to accom modate volume increases associated with deformation

multichannel seismic images of the oceanic crust s internal structure evidence for a magma chamber beneath the mesozoic mid atlantic ridge near the base of the mesozoic oceanic crust of the western north atlantic we recognize a discontinuous diffractive layer horizon r moho is a distinct uniform and more continuous horizon beneath variations in seismic thickness of the crust occur primarily through changes in the r to moho layer the maximum thickness occurring midway between fracture zones we propose that horizon r may mark the top of a layered cumulate sequence that formed on the floor of a relatively persistent crustal magma chamber the crust having minimum thickness is spatially restricted to a small strip near fracture zones and comprises only the super r unit we further propose that this implies the sustained accretion of normal oceanic crust at the mesozoic mid atlantic ridge required the presence of a magma chamber

microphysical characteristics of mjo convection over the indian ocean during dynamo the microphysical characteristics of precipitating convection occurring in various stages of the madden julian oscillation mjo over the indian ocean are determined from data obtained from the national center for atmospheric research dual polarimetric doppler s band radar s polka deployed as part of the dynamics of the mjo dynamo field experiment active mjo events with increased rainfall occurred in october november and december 2011 during each of these active mjo phases in addition to enhanced rainfall convection became deeper and ice phase microphysics played a greater role s polka consistently showed nonoriented small ice particles dominating the radar echoes at altitudes of 9 10 km dry aggregates concentrated between 7 and 9 km and wet aggregates and graupel near the melting level 5 km graupel occurred mainly in actively convective towers while the wet aggregates occurred almost exclusively in the stratiform regions of mesoscale convective systems mcss during each of the three multiweek mjo active phases the maximum rainfall occurred in short bursts lasting a few days each multiday rainy period began with deepening convective elements and a concurrent increase in occurrence of dry aggregates which maximized just prior to organization into mcss the peak rainfall occurrence coincided with the maximum coverage of the radar domain by mcss reflecting large stratiform regions that exhibited the most frequent occurrence of wet aggregates during the december active mjo phase however the mcss were shallower and had a slightly lower tendency for wet aggregates in the stratiform regions and therefore generally weaker brightbands

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

the geometry of planar domino style normal faults above a dipping basal detachment abstract geometrical analysis of planar domino style normal faults rooted into a dipping basal detachment fault allows derivation of equations which relate 1 horizontal extension within the upper plate 2 dip of the detachment 3 final fault dips 4 rotation that faults and beds undergo and 5 net slip on domino style faults past geometrical models have focused on extremely idealized and non unique geometries in which domino style faults are parallel and similar cut off points are all at the same elevation after faulting this corresponds to evenly spaced domino style faults above a horizontal detachment considering non parallel faults and dipping detachments introduces unique geometries which allows calculation for example of permissible depth ranges to the detachment horizontal extension varies significantly 1 with dip of the detachment and 2 for synthetic and antithetic cases in which the domino style faults dip in the same or opposite direction respectively as the detachment when other factors e g rotation and fault dips are held constant horizontal extension greatly increases for moderate detachment dips and moderately decreases for antithetic detachment dips as compared to the horizontal detachment case this is important because the synthetic case has been widely reported

fault tectonics of the baja california peninsula and the opening of the sea of cortez mexico abstract a detailed field analysis of neogene and quaternary faults in baja california has enabled us to reconstruct the stress pattern and the tectonic evolution of the central and southern parts of the peninsula the deformation which is related to the opening of the gulf affects the whole peninsula but decreases from east to west most observed faults normal and or dextral strike nnw sse to wnw ese their mechanisms include both strike slip and dip slip as well as intermediate motions compressional events have occurred since late neogene times but were probably of minor quantitative importance because reverse faults are rare and small the principal fault pattern includes dextral nnw sse riedel shears and n s tension faults induced by dextral strike slip along two main nw se fault zones bordering the peninsula the gulf of california to the east which is the most important and the tosco abreojos fault to the west the resulting pattern of deformation shows that the eastern part has been a complex transform extensional zone since late miocene early pliocene times

isotopic and trace element composition of volcanic glasses from the akaki canyon cyprus implications for the origin of the troodos ophiolite abstract we report major elements k rb cs sr ba sc cr ni hf ta th and ree concentrations and isotopic compositions of sr nd pb and o of carefully handpicked volcanic glasses from the akaki river section of the troodos ophiolite complex on the basis of sr and o isotopic composition and fe 2 o 3 feo ratios nine of the ten glasses analyzed are considered to be fresh even with respect to those elements that are easily affected by alteration the glasses range in composition from basaltic andesite to dacite incompatible element concentrations in the mafic compositions are low 10 chondritic and the light ree are strongly depleted however the ratios of low field strength elements k rb cs sr th to high field strength elements ta hf ti and rare earth elements are higher than those in n morb nd isotopic compositions show a very small range with e nd t 80ma varying from 6 5 to 7 5 initial 87 sr 86 sr ratios range from 0 7033 to 0 7040 pb isotope ratios have a restricted range with 206 pb 204 pb 18 60 18 70 207 pb 204 pb 15 55 15 59 and 208 pb 204 pb 38 08 38 51 the oxygen isotopic composition for fresh glasses falls within the range reported for fresh mafic mantle derived volcanics with 18 o varying from 5 4 to 6 5 all geochemical characteristics of the troodos volcanic glasses are similar to those observed in supra subduction zone volcanics but we cannot distinguish between an early stage island arc or back arc origin for the troodos ophiolite on a geochemical basis only however considering our data and previously published geologic and geochemical data we suggest that the troodos ophiolite formed in the earliest stages of island arc development

a study of white mica microstructure and microchemistry in a low grade mylonite a suite of mylonites rich in white mica taken from the betic movement zone sierra alhamilla spain shows progressive grain refinement involving a two stage recrystallization history stage one is isochemical and possibly works by nucleation and growth of new crystals at sites of fracture and dilatancy stage two is nonisochemical and may be driven by differences in strain energy and chemical free energy between the two phases deformation is probably governed by brittle fracture and diffusional mass transfer along grain boundaries and cracks the two stages of recrystallization are thought to represent a single progressive episode of deformation under changing metamorphic conditions

fault mechanics and the kinematics of block rotations in many strike slip tectonic settings large rotations up to 100 of crustal blocks have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which accommodate the relative motion between the blocks as regional deformation progresses simple geometrical considerations require that the faults must also rotate in this paper we show that on the basis of mechanical considerations the amount of fault rotation permissible under a stationary stress field is limited to 20 to 45 consequently block rotations that are larger than 40 or 45 require more than one set of accompanying faults to accommodate the block rotation examples of such multiple sets with 40 to 45 between them as predicted by the model were recognized in sistan iran in yerington the lake mead area nevada and in southern california

tectonic rotations in extensional regimes and their paleomagnetic consequences for oceanic basalts paleomagnetic samples of many deep sea drilling project dsdp basalts have shallower inclinations than would be inferred from the paleolatitude at which the basalts were extruded at mid ocean ridges rather than arising from peculiarities of the geomagnetic field the anomalous magnetic directions may result from tectonic rotations along the listric normal faults which often characterize extensional tectonic regimes several lines of evidence support this conclusion observations of the basin and range province an extensional tectonic regime occurring in continental crust observations of the troodos complex cyprus an oceanic extensional tectonic regime now exposed on land and analysis of possible fault patterns associated with the magma chambers postulated to occur at some mid oceanic ridges the tectonic rotations resulting from movement along the listric normal faults occur about a horizontal axis generally perpendicular to the extension direction and typically amount to 30 to 50 although rotations as large as 70 to 90 are not uncommon the paleomagnetic consequences of these rotations can be calculated for dsdp basalts if the original strike and latitude of a ridge are known at four dsdp sites for which adequate data are available namely sites 319 332 410 and 417 rotations ranging from 50 to 70 are required to explain the paleomagnetic results if the original strike and location of the ridge are not known as is the case for most of the older parts of the pacific ocean the paleomagnetic data and the inferred rotation determine a relationship between possible ridge strikes and locations this information combined with a similar but independent relationship derived from the marine magnetic anomalies puts severe constraints on the original strike and location of the ridge