the accuracy of the low degree geopotential implications for ocean dynamics the low degree portion of a number of recent global gravitational fields have been tested for accuracy using precise accelerations measured on 24 hour satellites these measurements are independent of the data in the field models the results show that the most accurate of the recent solutions have degree error variances significantly less than 100 mm geoid height equivalent for all degrees less than five the broad and lasting features of ocean circulation should now be visible when comparing the geoid computed from this model with the average sea surface computed from satellite altimetry however before accurate mapping is possible the equivalent accuracy of the mean altimetric surface at long wavelengths needs to be verified

distributed deformation and block rotation in three dimensions the focus of this paper is to understand distributed deformation in particular the relationship between fault slip and rotation of faults and blocks in a three dimensional stress field regions of distributed deformation such as southern california are organized in complex arrays of contemporaneously active block faulted domains we believe that the present day orientation of faults in many domains is due to the contemporaneous slip and rotation of the faults and of the blocks they bound traditional friction models cannot explain active unfavorably oriented faults and do not consider how faults become unfavorably oriented to solve this problem we propose a three dimensional block rotation model that tracks the orientation of blocks and their bounding faults during rotation mechanically we consider coulomb criteria for rock fracture as an upper bound and slippage on a frictional surface as a lower bound the key parameter in our model is the value of 2 3 1 3 principal stress directions are assumed irrotational through time this model predicts up to 75 of vertical axis rotation along a single set of faults during rotation fault slip may change sometimes dramatically giving rise to mixed vertical as well as horizontal axis of rotation of blocks and faults for very unfavorably oriented faults the model predicts rotations about a vertical axis in both the normal and reverse stress regimes and about a horizontal axis in the strike slip stress regime therefore paleomagnetically inferred rotations may not always be directly related back to a specific stress regime combining frictional constraints of the block rotation model with paleomagnetic structural and geological data we show how only one set of faults preexisting and rotating in an irrotational strike slip stress field can account for the three major phases of deformation observed in the western transverse range domain southern california preexisting north northeast faults were reactivated as normal faults rotated and became strike slip and subsequent rotations of faults resulted in their present east west high angle reverse orientation this example demonstrates that it is not necessary to invoke complex regional and local changes in the stress regime or erratic changes in plate motion to account for alternate periods of compression and extension

a simple kinematic model for crustal deformation along two and three dimensional listric normal faults derived from scaled laboratory experiments abstract we have derived a simple kinematic model of the deformation that results from extension accommodated by movement of a crustal block along two and three dimensional listric fault surfaces the model accurately reproduces deformation observed in a series of scaled analogue models the kinematic model is based on the simple assumption that lines within the hangingwall that are normal to the fault surface before deformation remain so following deformation an additional constraint built into the model is that of incompressibility deformation in the hangingwall block as observed in the laboratory experiments and predicted by the kinematic model is characterized by 1 a key stone structure or crestal collapse graben at some finite distance from the fault tip and 2 pure solid body rotation of the hangingwall head area near the tip of the fault in three dimensions the central region of the model undergoes extension in a direction normal to the direction of imposed displacement in such a way that the direction of dip of the upper surface of the hangingwall is aligned with the direction of extension this result provides quantitative support for the use of dip analysis to infer tectonic transport direction we also show how the distribution of extension within the hangingwall is affected when the constraint of constant displacement along the fault is relaxed

dating crystalline groundmass separates of altered cretaceous seamount basalts by the 40ar 39ar incremental heating technique abstract alteration of submarine basalts compromises geochronology using conventional k ar and 40 ar 39 ar techniques to help overcome these problems we re evaluated the potential of groundmass dating techniques incremental heating on acid leached groundmass samples following an overnight bakeout at 200 c and using high resolution heating schedules eliminated most of the low temperature alteration effects in the submarine basalts studied more than 75 of the groundmass analyses n 32 display accurate age plateaus consisting of 30 70 of the total amount of 39 ar k released more than 50 of the analyses have plateau ages concordant with their total fusion ages implying minor or proportional loss of radiogenic 40 ar and 39 ar k overall we could show a high degree of coherence between ages of groundmass separates and comagmatic phenocrysts this suggests that the dating of aphyric basalts which previously has proven problematic can be accomplished with increasing confidence as well adding these rock types to the list of datable submarine basalts significantly enhances our ability to understand the eruptive history of linear submarine volcanic chains

musings over sedimentary basin evolution field geologists and explorationists are of necessity immersed in numerous very detailed surface and subsurface observations they are often perplexed by the choice of relatively simple geophysical models that so elegantly explain the origin and evolution of sedimentary basins geophysicists on the other hand search for a simple theme to explain the origin of sedimentary basins and much like managers are often impatient with lengthy detailed geological discourse that often uses fancy jargon to hide the very real difficulty that geologists have in separating important evidence from mere encyclopaedic description the following musings address the quality and limitations of geologic and geophysical evidence that may be used to evaluate the relative roles of stress thermal effects and gravity loading which have been so lucidly summarized by m h p bott in the preceding summary the fine papers presented during this meeting of course have led to significant modifications of some of my earlier thoughts because these have been previously published elsewhere bally snelson 1980 bally 1980 they are summarized here only for the convenience of the reader

oblique slip deformation in extensional terrains a case study of the lakes tanganyika and malawi rift zones the east african rift system ear is the archetypal continental rift and a widely proposed analogue for the early stages of evolution of passive continental margins the three dimensional structure of parts of the ear has been recently elucidated by a multifold seismic mfs survey of lakes tanganyika and malawi project probe analysis of fault geometries displayed on the probe mfs data coupled with the more extensive 28 khz echosounder data has improved understanding of the geometry and kinematics of the linked fault systems that underlie the lakes in particular it has been recognized that profiles across i e at high angles to the rift elongation commonly display fault geometries that are not readily retrodeformable there must therefore be significant displacement out of the plane of what would conventionally be regarded as dip lines that is perpendicular to rift elongation and parallel to the tectonic transport or extension direction careful determination of fault plane dip and the dip of synrift reflectors in the hanging wall demonstrates that the dip directions of the shallowest faults and the steepest hanging wall sediment dips are oriented either nw or se this defines the direction of maximum fault block rotation and therefore the direction of extension or tectonic transport as nw se oblique to the trend of the regional rift axis and to the apparent strike of many of the major border faults in the survey area the border faults and many other faults imaged on the probe mfs data must therefore have significant components of strike slip motion and the tanganyika and malawi rift zones have undergone extension oblique to and not perpendicular to their axes near dip slip normal faults steeply dipping oblique slip faults and subvertical strike slip or transfer faults have all been recognized in a complex linked fault system that accomplishes the extension the overall orientation of the rift lakes and their internal segmentation are influenced by major preexisting subvertical fault zones within the basement complex accommodation zones act principally as transfer zones that allow switches in gross polarity of the border fault system the detailed geometries of the accommodation zones result from the specific relations between juxtaposed half graben and the strike and internal geometry of the influencing basement structure the variations in fault geometry subsidence water depth and basin or rift morphology can be better explained by an oblique slip extensional model influenced by basement structures than by a simple orthogonal extensional model

the seasat altimeter data and its accuracy assessment the seasat satellite launched on june 27 1978 carried a radar altimeter designed to measure the altitude of the satellite above the ocean surface the surface wave height and the ocean surface backscatter coefficient from which wind speed can be inferred postlaunch engineering assessment indicated that the seasat altimeter met the performance specification for 10 cm precision noise in the altitude measurements however to determine the accuracy of this measurement as well as the accuracy of the wave height and wind speed measurements a significant calibration validation and model development effort was required this summary describes the instrument atmospheric and geophysical effects which influence the radar altimeter measurement accuracies and the attendant correction models adopted for the altimeter geophysical data record also summarized are the activities of the seasat altimeter precision orbit determination experiment team directed towards the validation and improvement of these models as well as the investigations required to assess the accuracy of the altimeter measurement and the computed satellite altitude ephemeris finally an accuracy assessment is made for the various altimeter measurement corrections which are included on the altimeter geophysical data record

forces associated with mantle plumes this study addresses first order aspects of the physics of continental lithosphere extension above mantle plumes developing a testable model that predicts extension rates from flow parameters radial flow in the head of a plume causes a viscous force and uplift over this head causes a buoyancy force which both drive extension the viscous force is roughly proportional to the plume s volume flux and given my best estimates for flow parameters predominates for the most vigorous plumes extension is opposed by a viscous force beyond the plume head and its rate is thus affected by the dimensions of the plate on which this force acts extension rates are 2 mm yr 1 above yellowstone 1mm yr 1 above baikal and 0 7 mm yr 1 above the east african plume volume fluxes for all these plumes are 50 m3 s 1 the extension rates depend mainly on plate dimensions

migration of widespread long lived volcanism across the galapagos volcanic province evidence for a broad hotspot melting anomaly abstract the well studied galapagos archipelago is a small part of the much larger galapagos volcanic province gvp consisting of the cocos carnegie coiba and malpelo aseismic ridges and related seamount provinces although these aseismic ridges and seamounts dominate the morphology of the region little is known about their origin due to a lack of direct age and geochemical information in order to establish how well the gvp fits with the predictions of the standard fixed hotspot and mantle plume hypotheses we conducted a first reconnaissance dredge grab sampling of submerged regions of the gvp we present here 40ar 39ar ages for many of these new basement samples and evaluate their implications for the various models put forward to explain the origin of the gvp correlating new and published sample site ages with distance from the western side of the galapagos islands show that volcanism has not progressed in narrow time progressive lines of seamounts and ridges as predicted by the conventional fixed hotspot and mantle plume hypothesis rather volcanism apparently migrated time progressively across the gvp in broad regions of long lived and possibly concurrent volcanism we propose that the most viable explanation for these observations is that the gvp is the product of cocos and nazca plate motions across a broad hotspot melt anomaly the complex spreading history of the cocos nazca spreading centre likely controlled the relative distribution of gvp volcanism between the cocos and nazca plates while creating lithosphere of variable age thickness across the region while the notion of a broad galapagos hotspot melting anomaly linked to a complex regional tectonic history requires significant testing it nevertheless highlights the need to test alternative mantle upwelling shapes and sizes compared to the widely accepted notion of a narrow continuous long lived galapagos mantle plume conduit defined by the size and location of a galapagos island

the somalia plate and the east african rift system present day kinematics summary the motion of the somalia plate relative to the nubia africa arabia and antarctica platcs is re evaluated using a new inversion method based on a monte carlo technique and a least absolute value misfit criterion a subset of the nuvel 1 data set with additional data along the levant fault and in the red sea is used the results confirm that the motion of arabia with respect to africa is significantly different from the motion relative to somalia it is further shown that the data along the sw indian ridge are compatible with a pole of relative motion between africa and somalia located close to the hypothetical diffuse triple junction between the ridge and thc east african rift the resulting africa somalia motion is then compatible with the geological structures and seismological data along the east african rift system assuming a separate somalia plate thus solves kinematic and geological problems around the afar triple junction and along the east african rift

reconstructing the architectural evolution of volcanic islands from combined k ar morphologic tectonic and magnetic data the faial island example azores the morpho structural evolution of oceanic islands results from competition between volcano growth and partial destruction by mass wasting processes we present here a multi disciplinary study of the successive stages of development of faial azores during the last 1 myr using high resolution digital elevation model dem and new k ar tectonic and magnetic data we reconstruct the rapidly evolving topography at successive stages in response to complex interactions between volcanic construction and mass wasting including the development of a graben we show that 1 sub aerial evolution of the island first involved the rapid growth of a large elongated volcano at ca 0 85 ma followed by its partial destruction over half a million years 2 beginning about 360 ka a new small edifice grew on the ne of the island and was subsequently cut by normal faults responsible for initiation of the graben 3 after an apparent pause of ca 250 kyr the large central volcano cv developed on the western side of the island at ca 120 ka accumulating a thick pile of lava flows in less than 20 kyr which were partly channelized within the graben 4 the period between 120 ka and 40 ka is marked by widespread deformation at the island scale including westward propagation of faulting and associated erosion of the graben walls which produced sedimentary deposits subsequent growth of the cv at 40 ka was then constrained within the graben with lava flowing onto the sediments up to the eastern shore 5 the island evolution during the holocene involves basaltic volcanic activity along the main southern faults and pyroclastic eruptions associated with the formation of a caldera volcano tectonic depression we conclude that the whole evolution of faial island has been characterized by successive short volcanic pulses probably controlled by brief episodes of regional deformation each pulse has been separated by considerable periods of volcanic inactivity during which the faial graben gradually developed we propose that the volume loss associated with sudden magma extraction from a shallow reservoir in different episodes triggered incremental downward graben movement as observed historically when immediate vertical collapse of up to 2 m was observed along the western segments of the graben at the end of the capelinhos eruptive crises 1957 58

on the reactivation of extensional fault systems in terranes that have undergone substantial extension three sets of faults dominate a shallow to steep dipping commonly rotational normal faults b a really extensive shallow dipping normal detachment faults and c steep dipping transfer faults that strike at high angles to the normal faults these fault systems may extend through a large fraction of the crust reactivation of these fault systems will depend primarily on the relative strengths of the faults shear zones and their host rock and their orientation in the prevailing stress field it is concluded that reactivation is generally mechanically favoured but that it will probably only take place when the fault shear zones are in near ideal orientations consideration of the tectonic setting of extended terranes and of the limited number of well described examples suggests that reverse thrust reactivation of the normal and detachment faults and wrench reactivation of transfer faults are the most likely styles examples of these styles are described from the bass strait basins of southeastern australia because extended terranes commonly underlie sedimentary basins for example on passive continental margins reactivation of extensional faults may be a key control on the tectonic evolution of such basins i e basin inversion

a refined gravity model from lageos gem l2 a refined gravity field model goddard earth model gem l2 has been derived using the lageos orbital data yielding better baseline measurements for the analysis of tectonic plate motion this field also contributes to an improved understanding of long wavelength features such as the sea slope across broad ocean basins through its significant improvement of the long wavelength geoid through degree and order 4 the geoid for these terms has an accuracy estimated at 8 cm in gem l2 gem l2 as in all recent goddard earth models relies heavily on the precise near earth satellite laser ranging data in this case provided by nasa s crustal dynamics program two and a half years of lageos laser data acquired from over 20 well distributed stations were combined with the existing data from the best satellite derived model gem 9 to develop the new lageos model testing shows that the lageos gravity field error at long wavelengths is less than half that for gem 9 independent tests using well determined longitude accelerations of 24 hour satellites have verified the improved accuracy of the new model a comparison of global laser base stations from independent data sets of alternating 15 day data segments over the 2 1 2 years of lageos show total inter station positioning to 1 8 cm when using this new field the same comparison using the 1979 versus the 1980 lageos data yields 5 2 cm this difference in agreement reflects the change in data distribution and other systematic errors along with the tectonic motion which has occurred between these chronologically distinguishable data sets five day average polar motion values with a precision of 10 cm and change in length of day values accurate to better than 5 msec have been derived in the solution the adjustment of these parameters are necessary to achieve the accurate stations and geopotential results in gem l2

morpho structural evolution of a volcanic island developed inside an active oceanic rift s miguel island terceira rift azores the evolution of volcanic islands is generally marked by fast construction phases alternating with destruction by a variety of mass wasting processes more specifically volcanic islands located in areas of intense regional deformation can be particularly prone to gravitational destabilisation the island of s miguel azores has developed during the last 1 myr inside the active terceira rift a major tectonic structure materializing the present boundary between the eurasian and nubian lithospheric plates in this work we depict the evolution of the island based on high resolution dem data stratigraphic and structural analyses high precision k ar dating on separated mineral phases and offshore data bathymetry and seismic profiles the new results indicate that 1 the oldest volcanic complex nordeste composing the easternmost part of the island was dominantly active between ca 850 and 750 ka and was subsequently affected by a major south directed flank collapse 2 between at least 500 ka and 250 ka the landslide depression was massively filled by a thick lava succession erupted from volcanic cones and domes distributed along the main e w collapse scar 3 since 250 kyr the western part of this succession furnas area was affected by multiple vertical collapses associated plinian eruptions produced large pyroclastic deposits here dated at ca 60 ka and less than 25 ka 4 during the same period the eastern part of the landslide scar was enlarged by retrogressive erosion producing the large povoacao valley which was gradually filled by sediments and young volcanic products 5 the fogo volcano in the middle of s miguel is here dated between ca 270 and 17 ka and was affected by at least one southwards flank collapse 6 the sete cidades volcano in the western end of the island is here dated between ca 91 and 13 ka and experienced mutliple caldera collapses a landslide to the north is also suspected from the presence of a subtle morphologic scar covered by recent lava flows erupted from alignments of basaltic strombolian cones the predominance of the n150 and n75 trends in the island suggest that the tectonics of the terceira rift controlled the location and the distribution of the volcanism and to some extent the various destruction events

faulting and extension rate over the last 20 000 years in the offshore whakatane graben new zealand continental shelf oblique rifting in the offshore taupo volcanic zone new zealand is expressed in widely distributed active normal faulting in the 20 km wide whakatane graben active faults are identified along seafloor scarps and displacements of the post last glacial transgressive ravinement surface 400 faults we derive an extension rate at seismogenic depths 6 10 km across the graben of 13 6 mm yr by summing surface measurements assuming an average crustal fault dip of 45 15o and correcting for the discrepancies between surface and deep crustal extension estimates structural and kinematic data implies an extension direction 20o oblique to the rift axis resulting in up to 4 6 2 1 mm yr of dextral motion parallel to the rift axis the strike slip motion is accommodated by dip slip displacements on oblique faults in the centre of the graben and oblique slip faulting along the rift margins pure dip slip in the graben centre represents 50 of the total slip with the rangitaiki fault accommodating 25 of the total extension in the graben

the 2011 2012 submarine eruption off el hierro canary islands new lessons in oceanic island growth and volcanic crisis management forty years after the eruption of the teneguia volcano on la palma 1971 the last volcanic event in the canary islands a submarine eruption took place in 2011 off shore el hierro the smallest an

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

radial variations of a satellite orbit due to gravitational errors implications for satellite altimetry the linear perturbations of the radius of a satellite orbit due to the geopotential are derived from these estimates are made of the radial orbit error due just to geopotential errors from current models employing only conventional satellite tracking data the estimates show that about an order of magnitude improvement in the satellite geopotential will be necessary to utilize the full accuracy of independent satellite altimeter measurements the linear perturbations are used to show that 1 a substantial improvement in the low degree geopotential is possible from the indirect use of seasat altimetry at track crossovers and 2 a separation of permanent sea topography and geoid may be possible from direct use of the heights along the track

high resolution 40 ar 39 ar study of mount avital northern golan reconstructing the interaction between volcanism and a drainage system and their impact on eruptive styles we present a high resolution 40ar 39ar data set of a volcanic complex at mt avital northern golan which experienced a transition from strombolian to phreatomagmatic activity previous studies attributed this transition to a change in the drainage basin of a nearby stream due to the damming by a lava flow which resulted in flooding of the eruption site in this study we determined the age of different volcanic phases and events in the history of mt avital as well as that of the damming flow and examined the cause of the transition in light of the new 40ar 39ar ages the 40ar 39ar results show that the history of mt avital includes two main phases of volcanic activity an early phase at ca 800 600 ka and a late phase at 120 95 ka most of the activity of the late phase was continuous within the 40ar 39ar error including the transition to phreatomagmatic explosions which occurred sometime between 115 107 ka probably between 115 113 ka the age of the damming flow is 115 6 3 1 ka which suggests that the volcanic activity immediately reacted to the change in the drainage basin dynamics the activity culminated with lava flow eruptions the latest at 100 4 ka either due to the establishment of the tuff ring levees which prevented access of water to the eruption site or due to the migration of activity to the northern part and then to the southern part of the complex

grim gravity model improvement using lageos grim3 l1 improved modeling of the earth s geopotential is one of the major tasks of satellite geodesy the availability of a new set of precise lageos satellite laser ranging slr data as well as seasat acquired observations enabled us to compute an improved solution in the grim model series the new geopotential grim3 l1 recombined the satellite perturbation normals of the former grim3 solution with new normal equation subsystems derived from an additional 16 months of lageos slr data oceanic 1 1 mean free air anomalies recovered from seasat altimetry an updated set of terrestrial 1 1 mean free air anomalies and several condition equations for zonal and resonant harmonics the combined normal equations were solved for a complete set of spherical harmonics up to degree and order 36 and the coordinates of 109 tracking stations comparisons with altimetry derived quantities terrestrial gravity anomalies from satellite to satellite tracking longitude accelerations of 24 hour satellites and satellite orbit fits show a substantial improvement over previous grim solutions and indicate that the lageos data have contributed considerably to the reliable estimation of the long wavelength portion of the gravity field

segmentation and along strike asymmetry of the passive margin in socotra eastern gulf of aden are they controlled by detachment faults on the island of socotra the southern passive margin of the gulf of aden displays along its strike two different types of asymmetric structures western socotra is made up of a series of southward tilted blocks bounded by consistently northward dipping normal faults eastern socotra consists of a broad asymmetric anticline with a steep northern limb and a gently dipping southern limb a zone of ne sw striking strike slip and normal faults separates the two areas the overall structure is interpreted as representing two rift segments separated by a transfer zone the along strike juxtaposition of crustal scale asymmetric structures on the southern margin of the gulf of aden is complemented by the asymmetry of the conjugate margins on either side of the gulf whereas the western socotra margin is narrow and characterized by oceanward dipping normal faults the conjugate oman margin is broader and dominated by horsts and graben considering that asymmetric structures in the upper crust are often associated with synthetic shear zones at deeper ductile levels we propose that the western and eastern socotra margin segments were controlled at depth by two detachment faults with opposite dips and senses of shear the normal faults of western socotra would sole out into a top to the north ductile shear zone whereas the asymmetric anticline of eastern socotra would be associated with a top to the south detachment fault

40ar 39ar ages and paleomagnetism of sao miguel lavas azores we present new 40 ar 39 ar ages and paleomagnetic data for sao miguel island azores paleomagnetic samples were obtained for 34 flows and one dike successful mean paleomagnetic directions were obtained for 28 of these 35 sites 40 ar 39 ar age determinations on 12 flows from the nordeste complex were attempted successfully ages obtained are between 0 78 ma and 0 88 ma in contrast to published k ar ages of 1 ma to 4 ma our radiometric ages are consistent with the reverse polarity paleomagnetic field directions and indicate that the entire exposed part of the nordeste complex is of a late matuyama age the duration of volcanism across sao miguel is significantly less than previously believed which has important implications for regional melt generation processes and temporal sampling of the geomagnetic field observed stable isotope and trace element trends across the island can be explained at least in part by communication between different magma source regions at depth the 40 ar 39 ar ages indicate that our normal polarity paleomagnetic data sample at least 0 1 myr 0 0 1 ma and up to 0 78 myr 0 0 78 ma of paleosecular variation and our reverse polarity data sample approximately 0 1 myr 0 78 0 88 ma of paleosecular variation our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long term geomagnetic field behavior negative inclination anomalies are observed for both the normal and reverse polarity time averaged field within the data uncertainties normal and reverse polarity field directions are antipodal but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction 1998 elsevier science b v all rights reserved

analogue experiments of propagation of oblique rifts abstract we present the results of analogue models of oblique rifting which were carried out using a isostatically compensated two layered small scale models the experiments show that structures formed in the brittle layer under oblique extension comprise a series of lenticular rifted basins set en echelon along the axial zone of the experiment box in time the basins propagate lengthways to merge and form a unified composite rift however traces of the initial en echelon rifts expressed in deeper axial basins separated by shallower threshold zones are clearly discernible in the mature rifts both the basins and the threshold zones are located within the composite rift so that the rift margins are abrupt and steep along the basins and more diffuse along the threshold zones the thinning in the brittle layer along the rift is enhanced by diapirs that ascend from the ductile layer and penetrate the rift floor model experiments of nearly orthogonal extension showed a single unified rift which differed conspicuously from the composite oblique rifts the results of the experiments suggest that oblique extension forms en echelon rift segments the axes of which are perpendicular to the extension direction separated by threshold zones therefore there is ground to presume that occurrence of a series of alternating basins and threshold zones inside the axial section of a major rift could suggest a regime of oblique tectonic extension

the auriga nunataks shear zone mesozoic transfer faulting and arc deformation in northwest palmer land antarctica the auriga nunataks shear zone places new tectonic and temporal constraints on the mesozoic evolution of west antarctica the shear zone is a long lived arc orthogonal ductile transfer fault that preserves a history of regional mesozoic compressional basement deformation and extensional arc pluton emplacement in the antarctic peninsula magmatic arc it forms an east west trending positive flower structure 2 4 km wide exposed along 5 km of strike marble graphite bearing pyroxene granulite granodiorite diorite amphibolite and gabbro are deformed to mylonite and marble hosted tectonic breccia the mesozoic history of the shear zone is interpreted as follows emplacement of granodiorite diorite at circa 206 ma during early jurassic dextral transtension and metamorphism that peaked at circa 188 ma d1 brecciation of marble and mylonitization of gneiss by late jurassic to early cretaceous sinistral shear during the peninsula wide palmer land orogeny d2 early cretaceous dextral transtension with emplacement of gabbro and garnet leucogranite between 140 and 135 ma d3 and mid cretaceous ocean vergent thrusting and sinistral transpression between 125 ma and 80 ma with a peak at circa 110 ma that folded mylonitized and brecciated preexisting plutonic and metamorphic rocks d4 this is responsible for the current geometry of the shear zone the auriga nunataks shear zone transferred motion between arc parallel compressional and extensional structural elements and by hosting plutons appears to have acted like a leaky transform fault during episodes of regional extension

the eagle pass detachment southeastern arizona product of mid miocene listric normal faulting in the southern basin and range mid miocene low angle normal faulting is an integral part of the regional tectonic strain of the southern basin and range the distinguishing characteristic is the presence of detachments allochthons of moderately to steeply dipping hanging wall rocks resting in very low angle fault contact on structurally deeper generally older footwall rocks rotation of detachment strata was accomplished by imbricate listric normal faulting and coordinated tear faulting regional structural domains each thousands of kilometres in area contain detachments marked by uniform sense of detachment strata rotation the eagle pass detachment 70 km northeast of tucson arizona reveals informative structural details of parts of the system steeply southwest dipping mid miocene volcanic and sedimentary rocks several thousands of metres in thickness rest in very low angle 10 fault contact on precambrian granitic basement detachment strata were faulted into position by ne directed translation accompanied by rotation of strata to steep sw dips the detachment fault which underlies the 25 km2 detachment has the form of a large northeast trending mullion trough with relief of at least 500 m along the keel of the detachment precambrian quartz monzonite was transformed into a hydrothermally altered selvage of microbreccia the steep 80 northwest margin of the mullion trough served as a tear fault along which quartz monzonite was crushed volcanics were dragged and fanglomerate was locally sheared into flattened pebble conglomerate detachments like that at eagle pass resemble slump blocks on a grand scale they are elements of a regional system of extensional deformation which coincides with the belt of cordilleran metamorphic core complexes mid miocene detachment faulting post dates the formation of exposed mylonitic tectonites of core complex affinity but the direction of translation of the detachments is parallel to penetrative lineation in nearby and or underlying tectonite moreover in southeastern arizona examples the sense of translation of detachments matches the sense of simple shear movements disclosed by tectonite fabrics we speculate that large crustal displacements achieved during extension induced formation of tectonite produced a geometric and or dynamic condition which strongly influenced subsequent detachment faulting consequently with renewed or continued extension in mid miocene time deep upper crustal slumping was triggered and detachment rocks were translated along listric normal faults in the direction and sense of the earlier displacements which had accommodated the formation of tectonite

interaction between the mid atlantic ridge and the azores hot spot during the last 85 myr emplacement and rifting of the hot spot derived plateaus 1 multiple and single beam bathymetric data are compiled over the azores plateau to produce a 1 km 1 km grid between latitudes 32 n and 49 n and longitudes 22 w and 43 w mantle bouguer anomalies are then calculated from this grid and the satellite derived gravity these grids provide new insights on the temporal and spatial variations of melt supply to the ridge axis the elevated seafloor of the azores plateau is interpreted as resulting from the interaction of a mantle plume with the mid atlantic ridge mar the presence of a large region of elevated seafloor associated with a thick crust between the great meteor seamounts and the azores platform on the africa plate and less developed conjugate structures on the north america plate favors genetic relations between these hot spot derived structures this suggests that a ridge hot spot interaction has occurred in this region since 85 ma this interaction migrated northward along the ridge axis as a result of the sse absolute motion of the africa plate following a direction grossly parallel to the orientation of the mar kinematic reconstructions from chron 13 35 ma to the present allow a proposal that the formation of the azores plateau began around 20 ma and ended around 7 ma a sharp bathymetric step is associated with the beginning of important melt supply around 20 ma the excess of melt production is controlled by the interaction of the ridge and hot spot melting zones the geometry and distribution of the smaller scale features on the plateau record episodic variations of the hot spot melt production the periodicity of these variations is about 3 5 myr following the rapid decrease of widespread volcanism the plateau was subsequently rifted from north to south by the mid atlantic ridge since 7 ma this rifting begins when the mar melting zone is progressively shifted away from the 200 km plume thermal anomaly these results bear important consequences on the motion of the africa plate relative to the azores hot spot they also provide an explanation to the asymmetric geochemical signature of the azores hot spot along the mid atlantic ridge

40ar 39ar geochronology of neogene phreatomagmatic volcanism in the western pannonian basin hungary abstract neogene alkaline basaltic volcanic fields in the western pannonian basin hungary including the bakony balaton highland and the little hungarian plain volcanic fields are the erosional remnants of clusters of small volume possibly monogenetic volcanoes moderately to strongly eroded maars tuff rings scoria cones and associated lava flows span an age range of ca 6 myr as previously determined by the k ar method high resolution 40 ar 39 ar plateau ages on 18 samples have been obtained to determine the age range for the western pannonian basin neogene intracontinental volcanic province the new 40 ar 39 ar age determinations confirm the previously obtained k ar ages in the sense that no systematic biases were found between the two data sets however our study also serves to illustrate the inherent advantages of the 40 ar 39 ar technique greater analytical precision and internal tests for reliability of the obtained results provide more stringent constraints on reconstructions of the magmatic evolution of the volcanic field periods of increased activity with multiple eruptions occurred at ca 7 95 ma 4 10 ma 3 80 ma and 3 00 ma these new results more precisely date remnants of lava lakes or flows that define geomorphological marker horizons for which the age is significant for interpreting the erosion history of the landscape the results also demonstrate that during short periods of more intense activity not only were new centers formed but pre existing centers were rejuvenated

dip analysis as a tool for estimating regional kinematics in extensional terranes abstract in basin analyses that rely almost solely on seismic reflection data to configure the underlying structure it has become common practice to use extensional fault geometries to infer kinematic histories this procedure relies on the existence of fault geometries predicted by ideal orthogonal extension and assumes the structure is well constrained in areas where data are sparse or of equivocal quality this often leads to circular analysis that is ideal orthogonal extension is assumed to have been operative then under constrained fault geometries are mapped according to model predictions fault geometries so derived are subsequently used as evidence that orthogonal extension was operative and that the direction of extension is determinable we develop an alternative tool for inferring kinematics which is independent of fault geometries the method relies on the empirical evidence that the dip of the hangingwall s is systematic and predictable in extensional systems even when fault geometries are non ideal although not without significant limitations dip analysis can provide additional constraints for structural and kinematic interpretations especially where seismic reflection data are the main source of information and dip domains define discrete crustal blocks

40ar 39ar constraints on the temporal evolution of graciosa island azores portugal lava flows spanning the eruptive record of graciosa island azores archipelago and a gabbro xenolith were dated by 40ar 39ar in order to constrain the pleistocene and holocene volcanic evolution of the island the results range from 1 05 ma to 3 9 ka whereas prior published k ar and 14c ages range from 620 to 2 ka the formation of the serra das fontes shield volcano started at minimum 1 05 ma and the magmatic system was active for ca 600 ky as suggested by the formation of the gabbro xenolith by magmatic differentiation evolved magmas making up the serra das fontes serra branca composite volcano were generated at ca 450 ka after a period of ca 110 ky of volcanic inactivity and erosion of volcanic edifices volcanism was reactivated with the formation of the vitoria unit nw platform later the development of the vulcao central unit started with the formation of monogenetic cones located to the south of the serra das fontes serra branca vitoria unit this volcanism became progressively more evolved and was concentrated in a main eruptive center forming the vulcao central stratovolcano with an age older than 50 ka the caldera related to this stratovolcano is older than 47 ka and was followed by effusion of basaltic magmas into the caldera resulting in the formation of a lava lake which ultimately spilled over the caldera rim at ca 11 ka the most recent eruptions on graciosa formed two small pyroclastic cones within the caldera and the pico do timao cone within the vitoria unit at ca 3 9 ka

deep origin of hotspots the mantle plume model many volcanoes are associated with subduction zones or mid ocean ridges but other areas of unusually high volcanism or hotspots do not have such a direct connection to plate tectonic processes depaolo and manga argue in their perspective that some hotspots are caused by narrow upwelling plumes originating in the lower mantle while others are caused by broad hot upwellings or superswells yet others are not obviously associated with either deep plumes or superswells direct evidence for mantle plumes will require seismic imaging at higher resolution than is currently accessible in a related perspective foulger and natland take the opposing view that hotspot volcanism is a by product of plate tectonics and does not require deep plumes

construction and destruction of a volcanic island developed inside an oceanic rift graciosa island terceira rift azores abstract there is a great lack of knowledge regarding the evolution of islands inside active oceanic rifts in particular the meaning of the different evolutionary steps therefore we conducted an investigation in graciosa island which lies at the northwestern end of the terceira rift in the azores triple junction with the objective of constraining the evolution of the island in terms of volcanic growth and mass wasting in particular the meaning and age of the destruction events from digital elevation model dem analysis stratigraphic and tectonic observations k ar dating on key samples and available bathymetry and gravity data we propose that graciosa comprises five main volcanic complexes separated by major unconformities related to large scale mass wasting 1 the older volcanic edifice serra das fontes complex grew until ca 700 ka and was affected by a major flank collapse towards the southwest which removed the whole sw flank the summit and a part of the ne flank 2 the baia do filipe complex developed between at least 472 ka and 433 ka in two different ways in the sw presently offshore as a main volcano and in the ne unconformably over the sub aerial remnants of the serra das fontes complex as secondary volcanic edifices 3 the baia do filipe complex was affected by a major flank collapse towards the sw again removing most of the edifice 4 the remnants of the baia do filipe complex were covered in unconformity by the serra dormida complex between ca 330 and 300 ka which in turn was unconformably covered by the younger basaltic cover complex between ca 300 ka and 214 ka these two units were affected by a third major sector collapse that removed the whole western flank the summit and part of the eastern flank of the serra dormida and basaltic cover complexes 5 despite the relatively young age of graciosa the collapse scars are not well preserved and not active anymore 6 a central type volcano has been growing since at least 60 ka at the southeastern end of the island inside the scar left by a fourth sector collapse towards the se which affected most previous complexes contemporaneously parasitic strombolian cones formed all over the island despite the location of graciosa inside the active terceira rift the new data indicate that the evolution of the island has been driven by a competition between volcano growth and repeated destruction by catastrophic sector collapses rather than by slow incremental faulting associated with the tectonics of the rift

neotectonic intersection of the aegean and cyprus tectonic arcs extensional and strike slip faulting in the isparta angle sw turkey abstract the isparta angle and adjacent antalya bay areas constitute an important segment of the eastern mediterranean region located at the intersection of the southward convex aegean and cyprus arcs some recent tectonic maps show the isparta angle as a nw se compressional lineament extending eastwards into the kyrenia range of northern cyprus however fault data from the onshore isparta angle together with offshore shallow seismic reflection data show that the present morpho tectonic setting is dominated by extension the last phase of compression to affect the area studied in the late miocene was accompanied by regional nappe emplacement lycian nappes onshore fault planes measured from fault zones bounding both the limbs and the core of the isparta angle are oriented predominantly ne sw nw se and n s superimposed slickenfibres show that reverse faults were succeeded in turn by right lateral faults then by normal faults the fault phases are dated by stratigraphical and geomorphological evidence reverse faults date from the late miocene or earlier compressional deformation whereas the right lateral faults mainly developed during latest miocene early pliocene normal faulting dominated from the late pliocene recent an interpretation of shallow seismic reflection data shows that antalya bay is characterised by a nw se trending asymmetrical graben system that has continued to be active during the late miocene early pliocene right lateral strike slip resulted from shear along the eastern termination of a zone of extension and rotation that characterises the western aegean this shear was focused in a n s direction by inherited zones of structural weakness in the basement antalya complex the switch to ne sw extension in the late pliocene quaternary relates to a regional change in stress direction throughout the aegean region and was accompanied by strong uplift of the bey da lari region of the taurus mountains bordering the isparta angle in the west the isparta angle is the link between a the extensional province of western turkey bounded to the south by the actively subducting hellenic arc and b the uplifted anatolian plateau bounded to the south by the cyprus subduction zone understanding the miocene to recent tectonic development helps elucidate the kinematics of the region the new structural data presented lend no support for recent suggestions that the isparta angle and antalya bay represent parts of a regional compressional zone related to plate collision

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

continental rifting and metamorphic core complex formation ahead of the woodlark spreading ridge d entrecasteaux islands papua new guinea 1 we evaluate the role of a metamorphic core complex mcc on normanby island in the woodlark rift located 1 km thickness of blueschist derived mylonites formed in a midcrustal shear zone during the pliocene at 400 500 c this top to the north zone appears to have reactivated the gently dipping base of the papuan ophiolite papuan ultramafic body pub and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the mcc and rapid subsidence of the woodlark rise mylonites in the mcc s lower plate have been exhumed along a detachment as a result of 50 km of slip at rates of 12 mm yr the inactive back tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the plio pleistocene plate motion a second se vergent detachment has been established on the opposite flank of this rolling hinge style mcc probably since 0 8 at depth and provide a sufficient mechanism for activating low angle normal faults in the rift mcc inception was not localized to the tip of the woodlark mor instead extreme crustal thinning near the mcc preconditioned later continental breakup the lower crust appears to be weak thickening beneath unloaded footwalls to uplift mccs above sea level and flowing laterally to even out regional crustal thickness contrasts on a 1 6 m y timescale deep seated transforms separate rheologically distinct domains in which extension has been localized along the weak pub to cause mcc formation vs those in which slip is distributed across an imbricate zone of more uniform strength normal faults the trobriand fault connects in the eastern woodlark rift to the owen stanley fault in the papuan ranges which is probably moving at nearly the full plate velocity

crustal structure of the azores archipelago from rayleigh wave ellipticity data determining the crustal structure of ocean island volcanoes is important to understand the formation and tectonic evolution of the oceanic lithosphere and tectonic swells in marine settings and to assess seismic hazard in the islands the azores archipelago is located near a triple junction system and is possibly under the influence of a mantle plume being at the locus of a wide range of geodynamic processes however its crustal structure is still poorly constrained and debated due to the limited seismic coverage of the region and the peculiar linear geometry of the islands to address these limitations in this study we invert teleseismic rayleigh wave ellipticity measurements for 1 d shear wave speed vs crustal models of the azores archipelago moreover we test the reliability of these new models by using them in independent moment tensor inversions of local seismic data and demonstrate that our models improve the waveform fit compared to previous models we find that data from the westernmost seismic stations used in this study require a shallower moho depth 10 km than data from stations in the eastern part of the archipelago 13 16 km this apparent increase in the moho depth with increasing distance from the mid atlantic ridge mar is expected however the rate at which moho deepens away from the mar is greater than that predicted from a half space cooling model suggesting that local tectonic perturbations have modified crustal structure the 1 d vs models obtained beneath the westernmost seismic stations also show higher wave speeds than for the easternmost stations which correlates well with the ages of the islands except santa maria island we interpret the relatively low vs profile found beneath santa maria island as resulting from underplating which agrees with previous geological studies of the island compared to a recent receiver function study of the region the shallow structure top 2 km in our models shows lower shear wave speed which may have important implications for future hazard studies of the region more generally the new seismic crustal models we present in this study will be useful to better understand the tectonics seismicity moment tensors and strong ground motions in the region

the azores triple junction evolution since 10 ma from an aeromagnetic survey of the mid atlantic ridge abstract in the past two decades several models have been presented to describe the evolution and the present structure of the azores triple junction these models were mainly based on morphological analysis of sea bottom topography sparse magnetic profiling sidescan sonar surveying over the plateaus and global plate kinematic considerations for the north atlantic in this paper we follow a different approach from a detailed aeromagnetic survey covering both sides of the mid atlantic ridge between 37 n and 40 30 n the magnetic anomalies up to anomaly 5 are accurately identified allowing careful modelling of the kinematics of this region for the past 10 ma and thus establishing a coherent framework for the design of geophysical models for the azores triple junction the analysis of magnetic anomalies and the use of fourier domain inversion techniques show that the ridge is made up of six segments each one varying in length from 50 to 60 km the more continuous section of the ridge can be defined from the first four northern segments although the north azores fracture zone right offsets the ridge at 39 30 n 29 40 w the fifth and sixth segments are respectively right offset by the acor fracture zone at 38 23 n 30 15 w and by the princess alice fracture zone at 38 00 n 30 50 w anomaly identifications using a two dimensional model and plate tectonics reconstruction techniques allowed the calculation of rotation pole parameters the results thus obtained reveal that at least between anomalies 5 and 3 10 3 85 ma the azores displayed an independent motion relative to the neighbouring plates and after anomaly 2a 2 45 ma the azores moved attached to the eurasian plate the triple junction azores north america africa or eurasia north america africa moved northward from 38 00 n 30 50 w to 38 20 n 30 15 w between 4 and 3a and probably to 38 50 n 30 00 w at anomaly 2a time a detailed reconstruction model is presented

implications of structural inheritance in oblique rift zones for basin compartmentalization nkhata basin malawi rift ears abstract the cenozoic east african rift system ears is an exceptional example of active continental extension providing opportunities for furthering our understanding of hydrocarbon plays within rifts it is divided into structurally distinct western and eastern branches the western branch comprises deep rift basins separated by transfer zones commonly localised onto pre existing structures offering good regional scale hydrocarbon traps at a basin scale local discrete inherited structures might also play an important role on fault localisation and hydrocarbon distribution here we consider the evolution of the central basin of the malawi rift in particular the influence of pre existing structural fabrics integrating basin scale multichannel 2d and high resolution seismic datasets we constrain the border mlowe nkhata fault system mnf to the west of the basin and smaller mbamba fault mf to the east and document their evolution intra basin structures define a series of horsts which initiated as convergent transfers along the basin axis the horsts are offset along a ne sw striking transfer fault parallel to and along strike of the onshore karoo permo triassic ruhuhu graben discrete pre existing structures probably determined its location and oriented obliquely to the extension orientation it accommodated predominantly strike slip deformation with more slowly accrued dip slip to the north of this transfer fault the overall basin architecture is asymmetric thickening to the west throughout while to the south an initially symmetric graben architecture became increasingly asymmetric in sediment distribution as strain localised onto the western mnf the presence of the axial horst increasingly focussed sediment supply to the west as the transfer fault increased its displacement so this axial supply was interrupted effectively starving the south east while ponding sediments between the western horst margin and the transfer fault this asymmetric bathymetry and partitioned sedimentation continues to the present day overprinting the early basin symmetry and configuration sediments deposited earlier become increasingly dissected and fault juxtapositions changed at a small 10 100 m scale the observed influence of basin scale transfer faults on sediment dispersal and fault compartmentalization due to pre existing structures oblique to the extension orientation is relevant to analogous exploration settings

structural controls on the continent ocean transition in the northern gulf of california in the gulf of california the pacific north america plate boundary changes character from an oceanic type spreading center and transform fault system to the south to a region of diffuse continental deformation to the north the presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric heat flow gravity or seismic data which indicate significant differences north and south of latitude 30 n we suggest instead that north of 30 n a continent ocean transition begins which we name the wagner transition zone wtz diffuse deformation characterizes the wtz where slip occurs along reactivated north to nnw striking normal faults developed during late miocene or pliocene ene directed extension transtensional deformation varies from ene directed extension along dip slip faults in the west to dextral shear along the coast to dextral oblique slip along inferred north to nnw striking faults submerged in the northern gulf by accounting for rotational and extensional plate motion deformation in northeastern baja california vector constraints require that submerged structures accommodate 30 mm yr of slip in a direction slightly clockwise of the relative plate motion direction the juxtaposition of the discrete spreading center system in the central gulf with the diffuse wtz appears to have been a stable configuration since 4 6 ma perhaps controlling the evolution of spreading center jumps between upper and lower tiburon and delfin basins due to the juxtaposition of kinematically partitioned structural domains different histories of prerift extension and subduction related arc magmatism along the length of the gulf partly related to the migration of the rivera triple junction may explain the location of the continent ocean transition

structural evolution of the teletsk graben russian altai abstract lake teletskoye in the northeastern part of the altai mountain range has attracted the attention of geo scientists for a long time because it fills an impressive tectonic depression the lake is 77 km long and 4 km wide and it has a maximum water depth of 325 m the vertical offset of the basement surface is up to 3000 m a multidisciplinary study of the teletsk graben was carried out during the last few years including satellite image and air photo analysis bathymetric structural and geomorphological mapping high resolution seismic profiling and seismic refraction the structural study revealed that reactivation of preexisting weak basement zones is important in controlling the basin formation these zones separate different tectonic terranes at the contact of which the teletsk graben developed this study identifies the significance of the basin in the regional neotectonic context it shows that the major vertical movements are restricted to the basin itself but do not characterize the whole region outside of the basin recent tectonic structures have the same pattern as adjacent areas of northeast altai and west sayan quaternary glaciations have had no major influence on the basin formation two stages of faulting are identified first transpressive movements restricted to discrete reactivated fault zones controlled the opening of the basin in the second stage normal faulting is dominant and is responsible for the modern basin outline an echo sounding survey led to the recognition of several morphological characteristics of the lake bottom in the southern part the uppermost sediments seem slightly disturbed whereas further north transverse ridges and slope breaks are increasingly common the deepest part of the lake is located in a highly disturbed zone of normal fault bounded blocks the structural difference between the southern and northern subbasins is supported by the interpretation of a deep seismic refraction profile which indicates a substantial increase of basement isochores in the area where the reactivated teletsk paleozoic shear zone crosses the lake correlation of high resolution seismic profiles suggests that the teletsk graben started to evolve during the pleistocene and that its present shape was formed in two stages the first stage was responsible for the opening of the southern basin it probably started in the middle pleistocene a second kinematic stage induced by a sinistral reactivation of the ne striking west sayan fault initiated the opening of the different segments of the northern subbasin due to opposite movements between the reactivated teletsk and west sayan faults this second stage was active after the end of late pleistocene glaciations and during the holocene the recent lateral extension and the related n s trending normal faults result from a change in tectonic regime with related extensional movements along the main reactivated fault zones these recent movements result in the lateral escape of the lake borders and the collapse of the area between them

reply to the comment by quartau et al on construction and destruction of a volcanic island developed inside an oceanic rift graciosa island terceira rift azores j volcanol geotherm res 284 32 45 by sibrant et al 2014 abstract in sibrant et al 2014 we reconstructed the evolution of graciosa island azores we extensively discussed the nature and the meaning of the destruction episodes either tectonics or gravitational and concluded that the island has evolved through major landslides in their comment quartau et al 2015 conclude that 1 sibrant et al 2014 is based almost solely on subaerial observations which is false because we used the bathymetric data available to us figs 3 and 4 in sibrant et al 2014 2 the published multibeam sonar data around graciosa reveals that their proposed successive phases of destruction of the volcanic edifices composing the island by massive landslides is incompatible with the high resolution bathymetry first saying that the data were published is misleading because only two images are now provided in quartau et al 2015 most of the high resolution data used by quartau et al 2015 are not published and they still do not release the data for us to analyse and use in this reply second the high resolution bathymetric maps are not incompatible with our model for instance mounds on the eastern submarine slope may actually be landslide blocks and the platform developed to the s may correspond to flank collapses of the successive volcanoes blanketed more recently by the young basaltic cover 3 the interpretation of collapse structures appears to have originated partly from a misreading of the volcano stratigraphy and tectonic structures we certainly did not misread the volcanic stratigraphy and tectonic structures in graciosa in great contrast to quartau et al 2015 we a used major unconformities to establish the volcano stratigraphy b calibrated this stratigraphy with high precision k ar dating c made careful measurement of lava flow attitudes to infer the pre collapse position of the main edifices and d did not use hypothetical tectonic faults as quartau et al 2015 did to draw an evolutionary cartoon of graciosa the lava flows in the southern cliffs of graciosa dip inland as recognised by gaspar 1996 fig 3b in great contrast to the claims of quartau et al 2015 and therefore do not support successive volcanoes coinciding with the centre of the island 4 from unreleased high resolution bathymetric data quartau et al 2015 consider that no large landslide s occurred in graciosa and propose a new model first their geological model is not new as it reproduces a cartoon in gaspar 1996 fig 3a second quartau et al 2015 have not reported any major faults exposed at the surface as expected for an island scale graben but their summary cartoon of graciosa evolution is based on major faults third the new model of quartau et al their fig 8 and our fig 4 misrepresents the initial model of gaspar 1996 figs 4 1 and 4 2 as the whole sw flank of the island has been arbitrarily removed on their drawing see our fig 3 in contrast sibrant et al 2014 propose that the sw flank has been effectively removed but by landslides therefore their new model for the evolution of graciosa is based on misleading hypotheses rather than convincing arguments in conclusion there is no need to change our proposed new stratigraphy and evolution of the island based on quartau et al s claims because these are not supported by actual geological data well established and calibrated stratigraphy geometry of lava flows and geometry and kinematics of faults given the absence of sound geological data to support the evolutionary cartoon proposed by quartau et al 2015 we conclude that it is mostly flawed and therefore mere speculation

comment on construction and destruction of a volcanic island developed inside an oceanic rift graciosa island terceira rift azores by sibrant et al 2014 and proposal of a new model for graciosa s geological evolution abstract volcanoes rising above sea level within extensional oceanic plate boundaries provide accessible locations with which to study the effects of plate tectonic and volcanic processes of such areas however relying solely on subaerial observations can lead to biased interpretations reconciling the information provided by multibeam echo sounders on the submarine parts of volcanic islands with geology and geomorphology observable above sea level can potentially provide more robust interpretations in this comment of the study of sibrant et al 2014 which is based almost solely on subaerial observations we show how the published multibeam sonar data around graciosa reveals that their proposed successive phases of destruction of the volcanic edifices composing the island by massive landslides is incompatible with the high resolution bathymetry the data reveal no large scale debris avalanche deposits or characteristic flank collapse scars where sibrant et al 2014 propose these landslides to have occurred instead the data show volcanic constructional areas some of which have simply been eroded by wave abrasion the interpretation of collapse structures appears to have originated partly from a misreading of the volcano stratigraphy and tectonic structures overall wave erosion coupled with subaerial erosion and tectonic activity can more easily explain the onshore observations of sibrant et al 2014 providing a less catastrophic explanation for the evolution of graciosa island

new k ar ages chemical analyses and magnetic data of rocks from the islands of santa maria azores porto santo and madeira madeira archipelago and gran canaria canary islands k ar determinations major and trace element chemical analyses and magnetic data are reported for rocks from santa maria azores madeira and porto santo and gran canaria based on these data the age of the basement of santa maria is believed to have formed between about 5 2 and 4 6 m y the unconformably overlying pillow complex interbedded with fossiliferous calcarenites about 3 8 to 3 3 m y with the capping subaerial basalt being part of the same magmatic phase the major erosional phase levelling the basement is thus approximately synchronous with the major pliocene regression r2 on gran canaria lietz andschmincke 1975 possibly indicating a widespread eustatic event the upper part of the submarine partly fossiliferous series of porto santo was dated as ca 12 to 13 m y and the quaternary age for the major basalt formation in eastern madeira watkins andabdel monem 1971 is confirmed the ages of several formations on gran canaria were slightly revised

deformation in a hyperslow oceanic rift insights from the tectonics of the sao miguel island terceira rift azores the evolution of hyperslow oceanic rifts like the terceira rift tr in the azores is still poorly understood here we examine the distribution of strain and magmatism in the portion of the tr making up the nubia eurasia plate boundary we use sao miguel island because it stretches most of the tr width which allows to investigate the tr s architecture and shedding light on tr s age and mode of deformation from topography and structural analysis and new measurements of 380 faults and dikes we show that 1 sao miguel has two main structural directions n150 and n110 mostly concentrated in the eastern part of the island as an onshore continuation of the faults observed offshore in the ne n110 faults and sw n140 tr walls 2 a new n50 n80 fault system is identified in sao miguel 3 fault and dike geometries indicate that eastern sao miguel comprises the tr s northern boundary and the lack of major faults in central and western sao miguel indicates that rifting is mostly concentrated at master faults bounding the tr based on tr s geometry structural observations and plate kinematics we estimate that the tr initiated between 1 4 and 2 7 ma ago and that there is no appreciable seafloor spreading associated with rifting based on plate kinematics on the new structural data and on sao miguel s structural and volcanic trends we propose that the eastern two thirds of sao miguel lie along a main tr related transform fault striking n70 n80 which connects two widely separated n130 n150 tr trending segments

oblique slip and the geometry of normal fault linkage mechanics and a case study from the basin and range in oregon abstract map patterns of normal fault linkages near summer lake oregon show a systematic relationship between echelon step sense oblique slip sense and the position of linking faults where the step sense is the same as the sense of oblique slip e g left step and left oblique slip the faults are linked in the lower part of their relay ramp where the step sense and slip sense are opposite e g left step and right oblique slip the faults are linked in the upper part of the ramp a boundary element code is used to calculate the stress field around echelon normal faults during oblique slip and the model results reveal a relationship similar to the field observations if step sense and oblique slip sense are the same there is a greater potential for deformation ahead of the tip of the front fault and in the lower part of the ramp if step sense and oblique slip sense are opposite there is a greater potential for deformation ahead of the tip of the rear fault and in the upper part of the ramp the field model comparison confirms that oblique slip modifies the mechanical interaction among fault segments and thus influences fault growth and the geometry of fault linkage

the volcanotectonic evolution of flores island azores portugal abstract from the study and interpretation of the volcanic products and structures of flores island we infer that its volcanic history was dominated by two major periods 1 proto insular volcanism which includes all the submarine and emergent activities and 2 insular volcanism consisting exclusively of subaerial eruptions the first period includes two phases 1 the oldest 2 2 to 1 5 ma of shallow submarine volcanism 2 the youngest 1 0 to 0 75 ma includes emergent volcanism throughout the second period three volcanic stages are recognized 1 the first one 0 7 to 0 5 ma includes the most voluminous volcanism balanced between effusive and explosive events 2 an intermediate stage 0 4 to 0 2 ma that involves a larger number of small scale feeder centres with effusive eruptions prevailing 3 the third stage is the latest volcanic activity of the island 0 003 ma with strombolian and subsequent phreatic and phreatomagmatic activity centred at four volcanic vents from 1 0 ma to the present the volcanotectonic development of flores island also reflects the operation of two major tectonic processes 1 a marked volcanotectonic uplift during the first stage 1 0 to 0 55 ma and 2 subsidence particularly intensive in the island s central area which led to the subsequent formation of two and perhaps three large calderas during the period 0 55 to 0 4 ma

chemical molecular and isotopic differentiation of organic facies in the tertiary lacustrine duaringa oil shale deposit queensland australia abstract total organic matter extractable organic matter bitumen and pyrolysates released on low temperature sealed tube pyrolysis of kerogen were analysed chemically and isotopically based on bulk bitumen and biomarker parameters the sediments are immature to a depth of approx 1300 m within the terrestrial and lacustrine depositional environments of the tertiary duaringa basin various organic facies have been classified according to inorganic input and maceral abundances of organic matter derived from land plant and algal sources the latter derived mainly from pediastrum and with a varying botryococcus content the lacustrine upper lamosite is chemically and isotopically distinct from the lower lamosite due to a changing algal community relative contributions from allochthonous land plants and for the lower lamosite a slightly more saline and restricted lake environment the lower lamosite is associated with the highest concentrations of c 28 c 30 4 methylsteranes and 13 c depletion in toc in the upper lamosite there is a strong positive correlation between botryococcus content and 13 c enrichment the organically lean claystones have sources nearly identical to the lamosites but the organic matter is diluted due to rapid influx of siliciclastics the terrestrial facies the cannel coal and carbonaceous shales are mainly distinguished from the lacustrine facies by higher relative proportions of c 29 steranes c 30 4 methylsteranes and 13 c values for the pyrolysed hydrocarbons intermediate between the two lamosite facies the carbonaceous oil shale has a composite geochemical signature intermediate between the two end members lamosite and cannel coal oleanane and ring a degraded triterpanes derived from lupane arborane and oleanane skeletons are distributed throughout all organic facies representing both autochthonous and allochthonous higher plant inputs higher concentrations of oleanane and des a lupane are associated with the terrestrial facies hopanes dominate over steranes in all organic facies and their concentration ranges are independent of facies for the lamosites the aliphatic biopolymer algaenan is implicated as the dominant source for n alkanes from analysis of their yields homologue abundances and individual 13 c values in the terrestrial facies resinites are paraffinic and are a major source for the n alkanes released on pyrolysis compound specific analysis of extractable and pyrolysate hydrocarbons allowed deconvolution of different source inputs to free lipids and kerogen for the lamosites low molecular weight n alkanes in the bitumen are derived mainly from a pediastrum source while botryococcus is associated with waxy n alkanes however the situation is reversed in the kerogen pediastrum algeanan has a longer average alkyl chain length than botryococcus algaenan the magnitude of the isotopic differences between single n alkanes released at different temperatures is useful in determining multiplicity of sources

volcanic history and 40ar 39ar and 14c geochronology of terceira island azores portugal abstract seven new 40ar 39ar and 23 new radiocarbon ages of eruptive units in support of new geologic mapping improve the known chronology of middle to late pleistocene and holocene volcanic activity on the island of terceira azores and define an east to west progression in stratovolcano growth the argon ages indicate that cinco picos volcano the oldest on terceira completed its main subaerial cone building activity by about 370 380 ka collapse of the upper part of the stratovolcanic edifice to form a 7 9 km caldera occurred some time after 370 ka postcaldera eruptions of basalt from cinder cones on and near the caldera floor and trachytic pyroclastic flow and pumice fall deposits from younger volcanoes west of cinco picos have refilled much of the caldera the southern portion of guilherme moniz volcano in the central part of the island began erupting prior to 270 ka and produced trachyte domes flows and minor pyroclastic deposits until at least 111 ka the northern part of guilherme moniz caldera is less well exposed than the southern part but reflects a similar age range the northwest portion of the caldera was formed sometime after 44 ka several well studied ignimbrites that blanket much of the island likely erupted from guilherme moniz volcano the pico alto volcanic center a tightly spaced cluster of trachyte domes and short flows is a younger part of guilherme moniz volcano stratigraphic studies and our new radiocarbon ages suggest that most of the pico alto eruptions occurred during the period from about 9000 to 1000 years bp santa barbara volcano is the youngest stratovolcano on terceira began erupting prior to 29 ka and has been active historically

geometry of the livingstone mountains border fault nyasa malawi rift east africa we use multifold seismic reflection profiles to constrain the geometry and slip sense of a half graben border fault and the extension direction in the adjacent rift basin our data are from the western branch of the east african rift system often cited as an analog of early stage passive continental margin formation in this case study classic tilt block half graben clearly occur along a documented strike slip fault the livingstone mountains border fault bounds the northernmost basin in the nyasa malawi rift and is a series of straight dextral oblique slip fault segments which mimic the characteristics of a parallel underlying prerift ductile shear zone structures adjacent to the border fault show extension slightly oblique to the border fault indicating that the border fault is predominantly a strike slip fault consistent with studies of border fault exposures data indicate overall extension in the adjacent livingstone basin is at a much higher angle to the strike slip border fault consistent with the tilt block half graben architecture this characteristic is similar to basins bounded by weak strike slip faults some aspects of livingstone basin fault geometry resemble pull apart strike slip duplex and extensional imbricate fan fault geometries however the overall inexact match suggests the livingstone basin may be an incipient form of these classic strike slip basin models

extensional deformation and paleomagnetism at the western margin of the gulf extensional province puertecitos volcanic province northeastern baja california mexico during the late miocene east northeast directed extension in the gulf of california extensional province the western rift margin in northeastern baja california mexico was segmented at the northwest striking matomi accommodation zone the accommodation zone passed through the northern puertecitos volcanic province and separated pre 6 ma extension to the north from the unextended region to the south pre 6 ma northeast side down displacement is documented across the accommodation zone which may have undergone dextral oblique slip motion the rift margin migrated westward during the latest miocene or pliocene bypassing accommodation zone structures and incorporating the puertecitos volcanic province into the region of gulf extensional province deformation east northeast to east directed extensional deformation is at least post 6 ma in the northern puertecitos volcanic province and deformation currently continues pliocene changes in the gulf of california spreading center system may have triggered incorporation of the puertecitos volcanic province into the gulf extensional province as recently as 2 3 ma paleomagnetic analyses of 6 3 6 6 ma pyroclastic flow deposits show no consistent evidence for rotational deformation although minor 10 15 clockwise rotation is possible given anomalous declination directions recorded at some sites comparisons with paleosecular variation models and the late miocene paleopole for stable north america imply no statistically significant rotation relative to geomagnetic north although minor 10 clockwise rotation is permissible given uncertainties geologic relationships show that pre 6 ma accommodation zone structures identified in this study did not mark the southern boundary of later rotational deformation documented to the north the boundary of pliocene to holocene rotations may be a broad diffuse zone of extensional shear encompassing the northeastern puertecitos volcanic province which accommodated small rotations