the january 1998 earthquake swarm at axial volcano juan de fuca ridge hydroacoustic evidence of seafloor volcanic activity on january 25 1998 at 11 33z an intense swarm of earthquakes began within the summit caldera of axial volcano central juan de fuca ridge jdfr the earthquake swarm was detected using the u s navy hydrophones in the northeast pacific ocean the earthquake swarm lasted 11 days and produced 8247 detected 1037 located earthquakes during the first two days of the swarm earthquake activity migrated from the summit caldera 50 km along the south rift zone of axial volcano at rates of 0 92 0 13 and 0 23 0 09 m s 1 earthquake epicenter migration is characteristic of a lateral magma dike injection the largest three swarm earthquakes occurred within the caldera their timing and mechanisms are consistent with adjustment of the caldera floor as magma is removed from beneath the summit earthquakes with short t wave rise times shallow focal depths cluster at two spots along the summit and south rift zone and are considered possible seafloor eruption sites in situ ground deformation and hydrothermal plume monitors and later shipboard observations confirmed the occurrence of a seafloor volcanic eruption at the volcano s summit

local seismicity following the 1998 eruption of axial volcano a local network of seven ocean bottom hydrophones was deployed on axial volcano 14 days after the onset of seismic activity from the january 1998 diking eruption episode eighty one locatable microearthquakes were observed during a 166 day deployment the first group of events observed immediately after the eruption cluster around the rim of the central caldera these events are believed to be aftershocks from the major caldera collapse at the beginning of the diking event the second group of events observed 2 5 months after the eruption are primarily located on a line striking 125 from the caldera onto the southeast shoulder of the volcano with a few events extending into helium basin to the north these events are interpreted to result from horizontal shear associated with the coaxial and vance ridge segment offset or extension across the juan de fuca pacific plate boundary underlying the eastern flank of axial volcano

longitudinal strike slip faults in oceanic rifting a mesostructural study from western to southeastern iceland abstract mesostructural analysis carried out in several localities of western southern central and southeastern iceland shows an unexpectedly frequent occurrence of strike slip faults parallel to or lying at relatively small angles with the axis of rifting some faults can be interpreted in terms of shear parallel to the rifts whereas others form conjugate systems referable to rift parallel compression axial shear can be explained by the accommodation of differences in the directions of tectonic extension between different segments of the axial rift system and subordinately by lateral rift parallel displacements between the major lithospheric plates axial compression may be attributable to rift oblique transform shear or to mantle transport parallel to the spreading axis compensating local deficiencies in mantle upwelling the data regard 1974 fault surfaces mainly small scale

single and joint fault plane solutions for microearthquakes in south iceland abstract we obtain fault plane solutions for a swarm of twelve events and two single events ranging in magnitude between 0 3 and 3 1 the events were recorded by a local network in south iceland consisting of eight three component stations with approximately 30 km between the stations for events larger than m l 1 the solutions are well constrained the non uniqueness of the fault plane solutions increases with diminishing magnitude to enhance the uniqueness of the fault plane solutions for the earthquake swarm we search for mechanisms with a common fault plane but possibly varying slip directions in that plane data for all the events are consistent with faulting on a common plane relative location of the events shows that they all occurred less than 11 m from a single plane fault plane solutions for all the analysed events of the swarm as well as the two single events are consistent with strike slip dominated faulting on faults striking roughly north south the results are in agreement with the strikes of mapped surface fractures and focal mechanisms of larger earthquakes in the area this indicates that the microseismicity is related to the same pattern of regional deformation as the major earthquakes in south iceland

fault kinematics at the hengill triple junction sw iceland derived from surface fracture pattern abstract the hengill triple junction in iceland is formed by the intersection of the western volcanic zone wvz reykjanes peninsula oblique rift rpor and a transform zone the south iceland seismic zone sisz the 19 13 km2 study area thus contains extensional structures related to volcanic systems of the two rift zones hengill and hromundartindur as well as strike slip structures characteristic of the transform zones high definition aerial photographs satellite images and digital elevation models are used supported by field studies to accurately map and classify tectonic features in an effort to separate n s trending strike slip faults from the fissure swarms of the volcanic systems a portion of the southern hengill fissure swarm lies within the area it trends ne sw and exhibits extensional features characteristic of spreading ridges i e normal faults open fractures and eruptive fissures it is clearly delineated by surface fractures and is traced a length of 28 km southwest from the central volcano terminating before reaching the coastline the fissure swarm is 2 3 5 km wide in the study area widening southwards elevation profiles across the fissure swarm suggest a spreading rate comparable to the southern wvz which spreads at 4 5 mm year a portion of the nesjahraun eruptive fissure is anomalously smooth and may represent secondary rifting east west shearing in the rpor and sisz is accomplished by many smaller and less conspicuous northerly oriented strike slip faults in a bookshelf faulting manner these parallel faults are overprinted on the volcano tectonic structures of the rpor evidence of 11 northerly striking strike slip faults is found within the study area the zone of bookshelf faulting is thus shown to be continuous across the triple junction area from the sisz to the rpor finally several structures eruptive fissures exposed dikes and faults strike near perpendicular to the fissure swarm and may be the result of changes in the direction of maximum stress caused by rifting events however their tectonic significance remains uncertain

evolution of strike slip duplexes and wrench related folding in the central part of al jabal al akhdar ne libya abstractthe deformation in the central part of al jabal al akhdar ne libya is dominated by e w right lateral strike slip fault zones that form a conjugate system with the n s left lateral strike slip faulting the dextral wrench dominated transpression is responsible for the formation of strike slip duplexes en echelon folds and thrusts that deform the cretaceous eocene sedimentary sequences the strike slip duplex is formed by the progressive linkage of horsetail like structures at the western tip of the al bayda strike slip fault that joined splays coming from other dextral strike slip faults the master linear fault branches into second order dextral or sinistral faults that regroup into a linear single fault faults started from arrays of en echelon extensional extensional shear fractures that then coalesced into throughgoing strike slip faults shortening was accompanied by significant n s striking left lateral strike slip displacements with the development of ne to ese and sw plunging major f

an example of three dimensional analysis of thrust related tectonites abstract a spaced pressure solution cleavage related to alpine thrusts in mallorca balearic isles is deformed by several sets of shear planes and minor folds all these structures formed synchronously related to the thrust sheet movement shear plane sets show different orientations irrespective of the cleavage disposition slip along these shear planes shows that deformation did not result in plane strain minor fold axes show a wide range of orientations neither folds nor shear planes enable us to directly deduce the sense of thrust motion indicating that these criteria must be used with care previous strain characterization is required for definite conclusions to be reached

push ups fracture patterns and palaeoseismology of the leirubakki fault south iceland abstract field measurements and gps mapping were conducted along the leirubakki fault a 010 trending right lateral strike slip fault located in the holocene lava flows of the south iceland seismic zone because of poor exposures south and north of the holocene lava flow dissected by the fault its length can only be traced for 7 5 km the exposed surface rupture contains fractures that range in size over four orders of a magnitude from the main fault through fault segments 1000 2000 m long fracture arrays 100 250 m long to individual fractures 8 125 m long detailed measurements were made of 63 push ups located at junctions between individual fractures and between fracture arrays and ranging in maximum height from 0 35 to 4 35 m using the push ups the maximum right lateral displacement on the leirubakki fault is estimated at 2 67 m using empirical relationships established globally it is concluded that for a maximum displacement of 2 67 m the true rupture length of the leirubakki fault is around 50 km and that it gave rise to an earthquake of a moment magnitude 7 1

tectonic adaptation of the external zones around the curved core of an orogen the gibraltar arc abstract there is no real curvature in the external zones of the arc of gibraltar these external zones are made up by the juxtaposition of segments with nnw sse or e w trending structures juxtaposition does not result from late tectonics nor from a paleogeographic layout but only from miocene tectonics during and after the late burdigalian relative motion between europe and africa was essentially a n s convergence which accounts for the e w structural trends the nnw sse trends are the result of offset of the two main plates along sinistral strike slip fault zones that cross cut the previously mobile block of the internal zones and that shifted southward through time to explain the sharp juxtaposition of different trending structures two tectonic stages must be considered in each area of the external rif during the first one the rocks behaved plastically while during the second they could only move as a block the internal zones alboran block slid independently between europe and africa only until early burdigalian after this time they were locked in the main plates and broken together with them to allow further displacements

transform zone migration implications of bookshelf faulting at oceanic and icelandic propagating ridges at a propagating ridge a migrating transform zone mtz offsets the propagating and retreating spreading axes we examine several kinematic models of mtz deformation to determine which models can generate observed seafloor fabric orientations vertical tectonics and faulting patterns within the 95 5 w galapagos propagating ridge pr system models that involve transform parallel simple shear within the mtz can fit observed seafloor fabric rotation patterns within the 95 5 w mtz however these models predict pervasive transform parallel strike slip faults within the mtz that are not observed and do not predict the dramatic shear concurrent deepening and post shear uplift that is seen in this propagating ridge system since we only measure the amount of structural rotation of the abyssal hill fabric within the mtz and do not measure extensional or compressional deformation perpendicular or parallel to abyssal hills this structural information can only determine the rotational component of the finite strain tensor thus this simple shear family of kinematic deformation models is actually only a small subset of mtz deformation patterns that can produce the same finite rotation patterns but with different amounts of shortening and or extension parallel or normal to abyssal hills there is strong seismic and surface fault break evidence in iceland that the south iceland seismic zone mtz propagates along an array of en echelon transform perpendicular strike slip faults a likely reason for this is that if deformation were accommodated through transform parallel strike slip faults then migration of the transform zone would require the creation of new transform parallel strike slip faults for each increment of transform migration instead if transform migration can be accommodated through slip on preexisting faults then transform migration would only require the incremental growth of and slip along preexisting faults for each increment of transform zone migration we examine the kinematic consequences of finite mtz deformation within this generalized bookshelf faulting deformation scenario we find that our kinematic realization of generalized bookshelf faulting predicts structural rotation and vertical tectonic patterns that are consistent with observations at the 95 5 w galapagos pr system vertical tectonics are an almost inevitable by product of bookshelf shearing on dipping faults because of the geometric strain incompatibility associated with this mode of deformation thus local extension within the abyssal hills of de steiguer deep may occur because these abyssal hills do not join a spreading axis where the geometric strain incompatibilities associated with bookshelf slip can be accommodated we conclude that generalized bookshelf faulting is a promising candidate for the dominant deformation process within a mtz since it can explain not only the structural rotations observed at the 95 5 w propagating ridge but can also relate these rotations to the uplift history recorded there

from extension to shortening tectonic inversion distributed in time and space in the alboran sea western mediterranean 2d seismic reflection data tied to biostratigraphical and log information from wells in the central and south eastern alboran sea have allowed us to constrain the spatial and temporal distribution of rifting and inversion normal faults tilted basement blocks and growth wedges reveal a thinned continental crust that formed in response to nw se extension to the east a secondary sw ne trend of extension affects the transitional crust adjacent to the oceanic algerian basin the maximum thickness of syn rift sediments is 3 5 km and the oldest recorded deposits are serravallian the wnw ese yusuf fault formed a buttress separating and accommodating variable extension between two different tectonic domains the thinned continental crust of alboran and the oceanic spreading of the algerian basin late tortonian to present day nw se africa eurasia plate convergence drove shortening and reactivation of some of the earlier extensional structures as reverse and strike slip faults forming complex compartmentalised sub basins tectonic inversion coexisted with the formation of new faults and folds inversion was partial along the habibas basin and al idrisi fault but complete along the alboran ridge where some sw ne trending faults were perpendicular to the recent nw se plate convergence and were reactivated as thrusts the wnw ese yusuf fault is oblique to the convergence vector and therefore reactivation is mainly expressed as transpressional deformation volcanic rocks intruded along the alboran ridge and yusuf faults during the latest stages of extension formed rheological anisotropies that localised the later inversion

faulting associated with historical and recent earthquakes in the eastern mediterranean region summary this paper summarizes evidence for surface faulting in historical and recent earthquakes in the eastern mediterranean region and in the middle east such information is particularly important for studies of active tectonics and for palaeoseismology we have found 78 cases of faulting pre 1900 and 72 post 1900 some of which show that faults that have apparently been inactive this century had already ruptured before 1900 for some cases faulting could not have been predicted from 20th century activity and in others it could have been expected but has not been observed during the instrumental period the data are suycient to allow the derivation of relationships between magnitude and rupture length

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

a structural and palaeomagnetic study of a section through the eastern subbetic southern spain a well exposed 2 km section through the eastern subbetic immediately to the south of the crevillente wrench fault zone can be divided into an upper thrust sheet of triassic rocks a middle thrust sheet of cenomanian turonian basinal sediments and a lower thrust sheet of jurassic limestones unconformably overlain by upper cretaceous palaeocene sediments structural relationships suggest that the middle sheet overthrust the lower sheet and both units were subsequently folded and imbricated this sequence was then overthrust by the upper sheet incorporating a footwall duplex of cenomanian turonian sediments these structures can be related to the complex deformation associated with a single overthrusting event the dominant thrust transport direction is to the east strongly oblique to the overall trend of the orogen however a palaeomagnetic study isolates stable components which indicate no cumulative vertical axis rotation of the thrust sheets the section is cut by strike slip faults whose displacements appear to be relatively minor the triassic rocks of the upper thrust sheet form a highly disrupted thrust sheet extending over much of the subbetic rather than being local to the crevillente fault zone this section through the subbetic can be interpreted as the result of a major east directed thrust of basinal sediments of the median subbetic over marginal shelf facies of the internal subbetic strongly oblique to the general ese trend of the orogen the shortened sequence is cut by a later set of strike slip faults which are probably related to movement on the crevillente fault

tectonic significance of the south iceland seismic transform zone the subaerial expression of the mid atlantic ridge on iceland comprises two overlapping spreading axes referred to as the west volcanic zone wvz and the east volcanic zone evz respectively the way the spreading rate is distributed on both volcanic zones has an important impact on the stress pattern in the overlap area our field data from the area trace the evolution of the stress direction as recorded by slip motion on fault planes we found four different strike slip stress phases an early n s compression phase a preceded a ne sw compression phase b this phase was followed by a se nw compression phase c however we cannot date an e w compression phase d relative to the other phases numerical modeling based on the assumption that the wvz has been permanently active during the last 3 myr and that the evz is propagating southward confirms that stress directions have rotated clockwise by more than 140 these results fit perfectly with our field analysis and we propose that phase a corresponds to initial evz ridge tip propagation phase b to emplacement of the evz southern tip near torfajokull and phase c to an extreme southern location near the surtsey islands of the evz southern tip phase d could correspond to an intermediate stage between phases b and c we suggest that this sequence of tectonics recorded in a regional overlapping ridge tip setting is directly analogous to smaller scale and more common phenomena at second order ridge discontinuities throughout the global mid ocean ridge system

timing and kinematics of deformation in the malaguide complex internal zone of the betic cordillera southeast spain the betic cordillera of southern spain forms the westernmost of the mediterranean alpine orogenic belts and is divided traditionally into internal and external zones in the external zone fold and thrust belt unmetamorphosed cover rocks of the iberian margin were deformed from the early miocene onward in contrast internal zone rocks are dominantly metamorphic and are generally assumed to have been deformed and metamorphosed in an alpine event resulting from late cretaceous palaeogene convergence between africa and europe in the sierra espuna at the northern edge of the internal zone in the eastern betics the uniquely preserved permo triassic to miocene stratigraphy of the unmetamorphosed malaguide complex offers a rare opportunity to date the internal zone deformation kinematic data thrust geometries and structural facing of folds suggest that the imbricate thrust stack of malaguide rocks formed as a result of nnw directed motion within present geographical coordinates a fossilized front to this thrust stack preserved beneath a thick sequence of oligocene conglomerates indicates that the timing of the thrust deformation is late eocene in age deformation propagated into the foreland basin in the late oligocene with renewed thrusting subsequently the whole stack has been folded into a n nw vergent regional recumbent fold the espuna fold during the early miocene

riedel shear structures in the co seismic surface rupture zone produced by the 2001 mw 7 8 kunlun earthquake northern tibetan plateau abstract we present a case study of riedel shear structures related to the co seismic surface ruptures produced during the 2001 mw 7 8 kunlun earthquake in northern tibet which are related to strike slip movement along the kunlun fault field investigations and interpretations of high resolution remote sensing images show that the 2001 co seismic surface ruptures striking wnw ese are mainly characterized by riedel shear structures including t fractures and r y and p shears a left lateral shear sense is indicated to assess the riedel shear fabrics quantitatively we measured the azimuth of 19 455 co seismic surface rupture strands the width of rupture zones at 474 profiles and 336 fold axes of mole track structures using 1 m resolution ikonos and 0 61 m resolution quickbird images acquired soon after the earthquake the analytical results show that i the co seismic surface ruptures are generally concentrated in up to five subparallel sub rupture zones with individual sub rupture zones varying in width from 3 to 350 m generally 1 2 km iii t fractures are mainly developed within the alluvial deposits at counterclockwise angles of 15 40 relative to the general trend of the rupture zone iv p shears are also mainly found in the alluvial deposits but at counterclockwise to clockwise angles of 5 10 relative to the general trend and v y and r shears are developed within both the alluvial deposits and basement rocks mainly representing the reactivation of pre existing fault traces the results demonstrate that the co seismic riedel shear structures are primarily controlled by the local geology during surface rupture formation consistent with the idea that riedel shear structures are common fault patterns within strike slip shear zones and that their development is related to the early stages of fault evolution

ocean ridge discontinuities in iceland abstract the tjornes fracture zone in north iceland and the south iceland seismic zone are ocean ridge discontinuities that are partly exposed on land these are zones of high seismicity but contrasting style of crustal deformation detailed field studies and numerical modelling suggest that the tjornes fracture zone has all the characteristics of an oceanic transform fault is 7 9 ma old and developed along the zone of maximum shear stress between the nearby tips of the rift zones of the kolbeinsey ridge and north iceland conversely the results indicate that the south iceland seismic zone developed within an area of shear stress concentration between the overlapping rift zones in south iceland the development of the south iceland seismic zone is a consequence of the propagation of the eastern rift zone to the southwest during the past 3 ma

unidirectional slip vectors in the western betic cordillera implications for the formation of the gibraltar arc we have investigated the kinematics of deformation in a sector of the gibraltar arc in sw spain by synthesizing lineation data from major faults the variation in structural trends and palaeomagnetically determined vertical axis rotations the kinematic data show that transport directions throughout formation of the arc were consistently wnw regardless of structural trend which varies between 050 and 350 in the study area the data suggest that this part of the gibraltar arc can be explained simply by oblique convergence with a curved indenter taken together with other published data it appears likely that the variation in thrust transport direction within the betic cordillera as a whole may be considerably less than that in the moroccan part of the arc unidirectional convergence combined with the presence of palaeomagnetic rotations implies either eycient overprinting of old rotated lineations and or a convergence rate that is much greater than the rotation rate also required is a low degree of partitioning in the thrust wedge the latter probably increases east of the study area where the structural trend becomes more oblique to the transport direction and major strike slip faults occur

east west thrusting and anomalous magnetic declinations in the sierra gorda betic cordillera southern spain abstract structural and palaeomagnetic studies in the sierra gorda sierra de loja located in the external zone of the betic cordillera indicate that westward directed thrusting is not associated with significant rotations about a vertical axis detailed mapping and slip vector analysis show that the sierra gorda is a thrust complex composed of three thrust sheets the uppermost thrust places early jurassic pelagic carbonates on top of jurassic to oligocene sediments that form a large doubly plunging footwall syncline the eastern limb of this syncline has been overturned and is tectonically thinned as a result of the overthrusting palaeomagnetic results from mesozoic and tertiary sediments both within and around the perimeter of the sierra gorda indicate that 1 the average remanence vector of the seven late jurassic localities sampled within the sierra gorda has a direction d 328 and i 38 that is not significantly different from the expected declination for the upper jurassic of stable iberia and 2 there is no significant difference between the remanences in the two upper thrust sheets indicating that differential rotation did not occur during the initiation and displacement on the thrusts in contrast the one late jurassic site that was sampled to the west of the sierra gorda is rotated like the rest of the subbetics 60 clockwise of the reference direction the unrotated directions obtained in the sierra suggest either that it has rotated in a clockwise sense concordant with the rest of the subbetic zone and has then been backrotated or that it has never rotated relative to stable iberia in the latter simpler hypothesis the unrotated declinations may be explained in terms of orthogonal convergence along an irregular continental margin

wrenching in the external zone of the betic cordilleras southern spain abstract regional structural trends in the caravaca huescar area forming the central part of the external zone in the betic cordilleras indicate that deformation of the area results from right lateral wrenching subparallel to the paleogeographic zonation the structural trends do not fit the nappe interpretation generally used elsewhere in the external zone arguments for wrenching in the area are the structural inverse symmetry with respect to a central transcurrent fault zone the crevillente fault zone the distributional pattern of stratigraphic anomalies and types of deformation and the outcrop pattern of the paleogeographic zones it is argued that certain tectonic units in the area formerly interpreted as klippes in a nappe configuration form centers of flower structures and are vertically squeezed out blocks a theoretical model is sketched illustrating the evolution of a wrench zone under the stratigraphic conditions of the studied area and resulting in its present characteristics many of the features of the caravaca huescar area are also present elsewhere in the external zone and support the idea that wrenching was a major deformational mechanism it is therefore suggested that the external zone should in general not be described in terms of a nappe structure as has been done so far but in terms of a strike slip orogen or a wrenched continental margin

kinematics of a twisted core complex oblique axis rotation in an extended terrane betic cordillera southern spain 1 the sierra de las estancias is a metamorphic core complex that developed during the late oligocene to early miocene extensional collapse of the betic cordillera structural and paleomagnetic analysis shows that the northern half of the complex has been strongly modified by rotation about an inclined axis requiring reassessment of the kinematics of the extensional phase of deformation the extensional tectonic event produced two major detachment faults associated mylonitic foliation and lineation and pervasive sets of shear bands and brittle normal faults with a top to ne shear sense an older foliation was largely transposed and reoriented during this event the regional foliation now defines a broad antiform with a steep north dipping limb and the trend of shear related lineations swings from nne to ene from the southern gently dipping limb into the north dipping limb the remanent magnetization in a suite of near vertical early miocene mafic dikes intruded into the north dipping limb has a declination of 243 and an inclination of 27 assuming that the dikes were originally vertical a rotation of approximately 50 about an inclined ssw plunging axis preserves the near vertical orientation of the dikes and brings the remanence into agreement with the expected orientation for an early miocene age correction for this rotation brings the foliation and lineation in the surrounding schists into orientations subparallel to those on the present southern limb of the antiform and the dikes into a vertical e w orientation the corrected orientation of the shear related lineation nne normal faults and dikes suggests that the original direction of extension during the late orogenic extensional event in the internal betic cordillera may therefore have been around nne ssw rather than e w as currently accepted the observed net tectonic rotation can be decomposed into a series of geologically plausible rotations involving domino style rotation due to slip on the late brittle normal faults tilting about the regional fold axis related to thrust ramping onto the iberian margin during miocene collision and a 30 clockwise rotation about a vertical axis related to oblique convergence with the iberian margin

volcanic action at axial seamount axial seamount is an underwater volcano that lies on a seafloor spreading centre off the west coast of southern canada it is being intensively studied both through continuous monitoring by instruments at or near the site and through ship borne event response visits by scientists the most recent burst of volcanic activity at axial seamount happened early in 1998 and eight papers now describe the scientific findings

distinguishing lateral folds in thrust systems examples from corbieres sw france and betic cordilleras se spain abstract folds with axes almost parallel to the mean tectonic transport direction are frequent in fold thrust belts particularly along the borders of arcuate belts such structures have been frequently explained in terms of superimposed tectonic events but in some cases they are most realistically understood in terms of lateral folds inherited from lateral footwall ramps and initiated subparallel to the tectonic transport direction in order to distinguish frontal and lateral folds we have established criteria based on field examples from two regions where folds occur parallel to the regional transport direction the betic cordilleras southern spain and the corbieres thrust front northeast pyrenees france the main differences between frontal and lateral folds concern the geometry the evolution of the structures and the modes of strain accommodation in the roof strata enveloping imbricate stacks recalling that above a duplex or even on individual horse the roof layers are thinned along sections normal to transport we examine the precise location where the extension is expressed illustrated by examples from the two regions this leads us to address some tectonic problems of these erogenic zones such as the thrust sequence and development of thrust related cleavage

anomalous helium and heat signatures associated with the 1998 axial volcano event juan de fuca ridge water column plumes sampled before and after the 1998 eruptive event on axial volcano juan de fuca ridge showed a complex pattern in he and heat very different from that observed during previous magma injection events plumes detected in 1997 before the event had relatively high he heat consistent with the volatile rich character of the known hydrothermal vents within the summit caldera a response cruise in february 1998 immediately after the event failed to detect any event plumes but instead found an intense steady state type plume with high temperature signals moderate he enrichments and low he heat ratios this plume was present over the caldera and extended a considerable distance off axis in contrast all of the plumes detected during follow up cruises in july september 1998 had moderate temperature signals high he concentrations and elevated he heat ratios comparison with previous events suggests that the low he heat plume sampled in february was either the remnant of an event plume which had already migrated off axis or an event plume like discharge which was smeared out by strong currents it is likely that the high he heat plumes observed in july september were derived from the injected dike or from the new lava flow

1998 eruption of axial volcano multibeam anomalies and sea floor observations lava flows erupted during the january february 1998 seismic swarm at axial volcano on the juan de fuca ridge have been identified by differencing of pre and post event multibeam bathymetric surveys and by seafloor observations a sheet flow more than 3 km in length and 500 800 m wide erupted from the uppermost south rift is the site of a robust hydrothermal system 1998 lavas occur over about 9 km of the upper south rift zone or about 20 of the along axis length of the seismicity event 50 km the estimated volume of lava erupted is 18 76 106 m for the extrusion and 100 106 m for the intrusion the total volume is consistent with the volume from modeling of seafloor strain measurements recorded during the event

dynamics of volcanic systems in iceland example of tectonism and volcanism at juxtaposed hot spot and mid ocean ridge systems volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate pull as the plates are pulled apart associated with the mid ocean ridges and the magma dynamics of the iceland mantle plume most systems are 40 150 km long 5 20 km wide and develop a central volcano they supply magma to all eruptions in iceland data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment as a result the geometry of the plume is better constrained and the crust previously considered thin 10 km is now modeled as thick 20 40 km depending on the location of the volcanic systems their activity either decreases or increases faulting in the two main seismic zones from this we can infer that emplacement of the feeder dike to the largest historical eruption in iceland that of laki in 1783 increased shear stress in the south iceland seismic zone and almost certainly triggered the largest m 7 1 in 1784 historical earthquake in ic

behaviour of a rupture of the 21 june 2000 earthquake in south iceland as revealed in an asphalted car park abstract a major earthquake m 6 6 occurred on 21 june 2000 in south iceland this paper presents an unusual example of left lateral strike slip displacement recorded in a newly asphalted car park surface through a mechanically consistent pattern of open fissures and pressure ridges resulting from simple shear and rotation measurement of these features allows accurate reconstruction of the local deformation the behaviour of the asphalt layer resembles that of analogue physical models especially in terms of rotations induced by shear deformation it is finally shown that through a wide range of scales some basic patterns associating rotation and opposite senses of strike slip exist in the south iceland seismic zone

the 1912 iceland earthquake rupture growth and development of a nascent transform system we have mapped in detail surface ruptures of the 1912 magnitude 7 0 strike slip earthquake in south iceland this earthquake ruptured fresh basalt flows that had covered the pre existing fault the observed style of surface fracturing closely matches both theoretical predictions of the first stages of shear fracture development and microscopic scale observations from laboratory experiments the shear offset distributed across the zone of surface fractures produced by this earthquake is right lateral and is in the range of 1 to 3 m total mapped rupture length is 9 km but total rupture length is probably at least 20 km this interplate earthquake had an exceptionally high ratio of slip to fault length and by inference stress drop the north south trending rupture of the 1912 earthquake is part of the bookshelf faulting in the east west trending south iceland seismic zone we ascribe the bookshelf faulting in the south iceland seismic zone to a combination of the early development stage of the transform and regional strength anisotropy of the crust

evidence for internal zone unroofing from foreland basin sediments betic cordillera se spain tertiary sediments at the boundary of the internal and external zones in the eastern betic cordillera record evidence of compressive deformation in the internal zone and its subsequent exhumation sediments on the north side of the sierra espun associated with the structurally highest unit of the internal zone the malaguide complex are interpreted as a foreland or piggyback basin which formed in response to the deforming internal zone basin evolution has been reconstructed using an integrated analysis of sedimentary facies petrography heavy minerals and structural data the basin fill contains eocene shallow marine carbonate shelf ramp sediments early oligocene fan conglomerates and coralgal reefs transitional to probable slope deposits of the late oligocene and a late oligocene to mid miocene deep water sequence with pelagic faunas the provenance analyses illustrate an unroofing history of the internal zone orogenic edifice locally uplifted mesozoic to eocene carbonates at the thrust front were the source of lower oligocene conglomerates clastic detritus supplied to the late oligocene to aquitanian basinal sediments was mainly derived from the permo triassic and palaeozoic basement of the malaguide complex the presence of small amounts of first cycle metamorphic minerals indicates the start of exhumation of the deeper metamorphic alpujarride complex from the burdigalian onwards a metamorphic terrane dominated the source area of the deep water sediments and by the langhian high pressure minerals from the deepest parts of the internal zone orogenic edifice appear in the sedimentary sequence rapid exhumation recorded in the early miocene sediments may be the result of extensional exhumation of the internal zone metamorphic complexes

co seismic mole track structures produced by the 2001 ms 8 1 central kunlun earthquake china extensive co seismic mole track structures have been observed along a 400 km long surface rupture zone associated with the 2001 ms 8 1 central kunlun earthquake north tibet the mole track structures appear in an angular ridge pattern which resembles an angular triangle shaped frame and in a bulge pattern forming small flat hillocks the mole tracks are developed within alluvial fans river channels and bajada and form a linked array of structures along the rupture zone the angular ridge structures resulted from flexural slip folding and faulting of frozen alluvial deposits and surface ice whereas bulge type structures developed by squeezing up of unconsolidated to weakly consolidated alluvial sediments the mole tracks generally formed within the co seismic rupture zone due to contraction in the right bend and right stepover areas between sinistral shear faults and are controlled by the pre existing fault and fold structures along the left lateral strike slip kunlun fault

the malaguide alpujarride boundary a major extensional contact in the internal zone of the eastern betic cordillera se spain abstract the internal zone of the betic cordillera in southern spain was formed by thrusting and horizontal ductile shortening in the late cretaceous palaeogene but was subjected to a major phase of vertical thinning during the neogene the contact between the malaguide the structurally highest thrust complex and the underlying alpujarride complex is an example of several major tectonic contacts across which the stratigraphy has been repeated by thrusting but the metamorphic grade increases abruptly downwards detailed work in the eastern betic cordillera documents the relationships across this contact in the sierra espuna petrographic and illite crystallinity studies show that the highest imbricate thrusts of malaguide rocks have only been diagenetically altered whereas the deeper parts of the thrust stack exhibit anchizone facies metamorphism in the northern sierra de las estancias apatite fission track studies confirm that there too malaguide rocks have never been above 200 c in both areas the underlying alpujarride rocks exhibit epizone or greenschist facies metamorphism and a major fault zone with both brittle and ductile structures separates the two complexes the observed rapid increase in metamorphic grade over very small structural thicknesses suggests that part of the original metamorphic zonation has been excised across the contact thus in its present form we interpret the malaguide alpujarride boundary as an extensional contact the extensional motion took place in early middle miocene times as constrained by fission track cooling ages the fault zone at the contact is sub horizontal in the sierra espuna but in the sierra de las estancias it is mainly sub vertical kinematic data from a 5 20 m thick calcmylonite layer within the fault zone in the sierra de las estancias corrected for later tilting indicates that the extensional movement was originally directed with the top to the east northeast the interpretation of the malaguide alpujarride boundary as an extensional contact is similar to recent interpretations of the nevado filabride alpujarride contact at deeper structural levels in the central and eastern betics the identification of another major extension fault system confirms that miocene extension was widespread at all structural levels during the early middle miocene the sierra espuna and the northern margin of the sierra de las estancias lay on the boundary between the region undergoing horizontal extension in the internal zones and the alboran sea to the south and the region undergoing horizontal shortening in the external zones to the north as a result the detailed history of the alpujarride malaguide contact is complicated extensional structures cross cut structures related to earlier alpine shortening and were then affected by further shortening as convergence with the external zones continued

evidence for deformation associated with the 1998 eruption of axial volcano juan de fuca ridge from acoustic extensometer measurements acoustic extensometer instruments capable of making precise daily measurements of horizontal distance were deployed across the north rift zone of axial volcano in june 1996 and were in place when a submarine eruption began on axial s south rift zone in january 1998 the instruments recorded a gradual 9 cm extension over a 405 m baseline leading up to the eruption and then an abrupt 4 cm contraction at the time of the eruption an elastic point source deformation model shows that deflation of axial s summit can explain both the 4 cm distance decrease at the extensometer array and a 3 2 m subsidence measured by another instrument in the caldera if the pressure source is located at a depth of 3 8 km below the center of the caldera the 9 cm distance increase may represent pre eruption spreading across the rift zone

stress changes induced at neighbouring faults by the june 2000 earthquakes south iceland seismic zone the south iceland seismic zone sisz is an e w trending active transform zone with high seismicity n s trending right lateral strike slip faults accommodate the left lateral transform motion using seismological data recorded from 1991 to 2007 we carried out stress inversion of focal mechanisms of 1340 earthquakes that affected the skard and leirubakki faults eastern sisz not only did the inversion show typical deviations of stress across the faults it also revealed anticlockwise and clockwise rotations of stress axes with time numerical models of the leirubakki fault show that these rotations are consistent with the mechanical effect of a lowered friction coefficient during the post seismic period relative to the pre seismic period the skard fault reveals a more complex behaviour associated with a higher post seismic friction resulting from a higher density of pre existing fracturing and probable stress interaction between faults our results suggest that faults where micro earthquakes occur during neighbouring major seismic events may undergo significant stress changes at the scale of several kilometres and on time scales of several years

origin of the western subbetic arc south spain palaeomagnetic and structural evidence abstract an integrated structural and palaeomagnetic study in late cretaceous eocene rocks of the western subbetic arc in the betic cordillera of southern spain shows that the change in fold trend around the arc is significantly greater than the variation in primary magnetic declination this suggests that the arcuate geometry is not primarily a consequence of differential rotation the area has experienced a regional clockwise rotation of at least 40 which was most probably imposed during oblique wnw directed convergence along the southern iberian margin in early miocene time before the arcuate structure was formed kinematic indicators associated with the arcuate folds and thrust traces arc for the most part consistently wnw directed suggesting that the arc is not primarily a result of variations in the direction of tectonic transport the arcuate structure was probably initiated without significant rotation by the wnw directed indentation of the curved western end of the alboran domain into the iberian margin and was then tightened involving up to about 35 of further rotation in different segments of the arc finally one sector in the arc was rotated anticlockwise by about 40 with respect to the rest of the region reorientating the magnetic declinations the fold trends and the kinematic indicators this rotation was associated with local sinistral shear

paleomagnetic rotations and the kinematics of the gibraltar arc paleomagnetic investigations of a mesozoic limestone sequence around the gibraltar arc show that there have been large systematic rotations about a vertical axis and imply that there must have been a significant component of westward motion within the betic rif orogenic belt rotations of the late jurassic and late cretaceous limestones in the betic cordillera of southern spain are clockwise with the exception of the unrotated sierra gorda whereas the jurassic sites in the rif mountains of morocco are rotated counterclockwise except in the area around tetuan these data are generally consistent with formation of the arc either by a westward moving alboran microplate or by extensional collapse of a collisional mountain chain the former model however fails to recognize the nonrigid nonplate like character of the alboran domain and lacks a definable driving mechanism a model of a collapsing east west elongated ridge that takes into account the obliquity of the convergence along the active margin may provide mechanisms both for the observed rotations and for the large westward component of motion and therefore can explain the geometry of the gibraltar are

repeated palaeoseismic activity of the ventas de zafarraya fault s spain and its relation with the 1884 andalusian earthquake one of the most destructive historical earthquakes m 6 7 in spain occurred in 1884 along the normal ventas de zafarraya fault located in the central betic cordilleras palaeoseismic and radiocarbon data presented in this study are the first to constrain the timing of the pre 1884 fault history in the last 10 ka these data yield a recurrence interval of between 2 and 3 ka for major earthquakes under the assumption of uniform return periods along the normal fault the holocene slip rate is estimated to be in the order of 0 35 0 05 mm year which is significantly higher than the mean slip rate of 0 17 0 03 mm year since the tortonian several of the most important deformations and secondary features such as landslides and liquefaction are related to strong ground motion and document the holocene activity of the ventas de zafarraya fault

surface deformation and tectonic background of the 2019 ms 6 0 changning earthquake sichuan basin sw china abstract the june 17 2019 ms 6 0 changning earthquake at a depth of 3 5 km struck the changning anticline fold system no active faults have been observed in the region some studies have attributed the earthquake to the regional shale gas exploitation following the earthquake we conducted a field survey to investigate co seismic surface deformation in the area and observed many surface fractures and uplifted areas our work indicated that the deformation followed a riedel shear pattern the fractures and uplifted areas represent t fractures and s extrusions respectively the observed riedel shear pattern is indicative of left lateral strike slip motion to ascertain the relationship between the riedel shear pattern and regional tectonics we investigated the trends in layer striations and fault scratches the results indicated that there were two stages in the evolution of the anticline in the first stage the principal compressive stress that dominated the original formation of the anticline trended ne sw in the second stage the regional principal compressive stress shifted to the current e w trend generating oblique motion that included reverse and left lateral components we propose that the tectonic stress field in this region had a strong bearing on the focal mechanism of the mainshock and the co seismic surface deformation both the eastward expansion of the xianshuihe area and southeastward expansion of the longmen shan have affected the current changning anticline fold system

mid crustal detachment in the betic system of southeast spain abstract the tertiary orogenic belts of eastern spain can be interpreted using the principles of thin skinned thrusting with a fundamental detachment at mid crustal level in the eastern betics the compression involves an oblique restacking of a stretched continental margin much of the shortening is transmitted on permian and triassic detachments which emerge as forward and back thrust duplexes an interpretation of the guadarrama range of central spain is presented in which significant shortening is shown on a mid crustal detachment propagating from the southeast it is suggested that this represents the external limit of a compressive system linked to the betics the pyrenees and eastern cantabrians are a separate tertiary compressive system again resulting from restacking of an extensional basin during the collision of iberia and france previous authors have proposed mid crustal detachment in the pyrenees observation of the geological map of the iberian ranges suggests a broadly similar structural origin may apply in which case the iberian ranges may represent the most external zone of a major pyrenean system again linked by a mid crustal detachment thus it is suggested that the whole of eastern spain is made up of two major compressive systems linked at mid crustal level

role of strike slip faults in the betic rifian orogeny abstract a new model for the betic rifian orogeny of the western mediterranean spain and north africa is proposed in which four strike slip faults play an important role the faults are not of the same age two faults the left lateral jebha fault to the south in morocco and principally in the mediterranean sea and the right lateral north betic fault southern spain to the north define the boundaries of the alboran block betic and rifian internal zones final movement along these faults was during the burdigalian time two other faults the left lateral nekor fault north africa to the south of the jebha fault and the right lateral crevillente fault somewhat to the north of the north betic fault define a larger alboran block including part of the betic and rifian external zones that was present during the tortonian the following sequence of events is proposed 1 a during the eocene and oligocene the african and european plates converged in a n s sense causing the breakup and overthrusting of the betic rifian and kabyle internal zones and then the movement towards the wsw of the alboran block by slip along the jebha and north betic faults 2 b by the end of burdigalian time movement along the jebha and north betic faults ceased 3 c with continued n s convergence the nekor and crevillente faults which bound a larger alboran block were formed during the mid and late miocene the arc of gibraltar the zone lying between the four major faults seems to be a result of wsw motion of a crustal block being thrust over external zones the model proposed adds to the earlier idea that tectogenesis proceeds from the interior to the exterior of an erogenic belt in the betic rifian orogeny major strike slip fracture zones shifted to the exterior of the orogenic belt as the orogeny progressed in order to relieve the stress caused by locking of the more internal faults

structural features of an active strike slip fault on the sliding flank of mt etna italy abstract the strike slip pernicana fault system pfs was activated along the eastern flank of mt etna during an earthquake in september 2002 and one month later during the eruption of the ne rift structural and volcanological data suggest that the pfs was activated as a result of the slide of the ne flank of etna this activation produced surface fracturing on walls and on paved and unpaved roads the segments of the pfs arranged in a right stepping en echelon configuration show a an inverse proportion between length and frequency b fractal behavior over scales of 10 2 10 1 m between their length overstep and overlap c consistent strike with regard to their fault array and d a progressive eastward decrease in the displacement along the smallest faults the consistent geometric and kinematic features of the pfs related to the sector collapse of etna are similar to those of faults in strike slip settings

in situ ground deformation measurements from the summit of axial volcano during the 1998 volcanic episode since september 1987 seafloor instrumentation monitoring ground deformation has been maintained on the summit of axial volcano on the central juan de fuca ridge during a january 1998 volcanic episode pressure records from the central caldera showed a significant increase in pressure indicating caldera subsidence of over 3 m this observed subsidence is interpreted to represent volcanic deflation as magma is erupted and or injected into the volcanic flanks temperature anomalies of nearly 0 45 c were also observed a comparison of the subsidence record with hydroacoustic recordings indicates a continuous volcanic tremor signal coincident with the subsidence event the time coincidence of subsidence of the summit caldera with migration of epicenters into the volcanic rift zones confirms that during the 1998 episode the volcanic flanks were supplied by lateral dike injection from a central magma body rather than locally derived magma sources

the internal external zone boundary in the eastern betic cordillera se spain reply abstract the internal external zone boundary iezb in the betic cordillera of southern spain separates the internal zone which was deformed and in part metamorphosed before the early miocene from the external zone which consists of the cover rocks of the iberian margin shortened to form a thin skinned fold and thrust belt in early to middle miocene times over much of its length the iezb is roughly linear trending approximately 070 it has been referred to as a major dextral strike slip zone and has even been considered as the northern boundary of a westward moving alboran microplate field and kinematic data from the eastern betic cordillera show that the iezb crops out over a 60 km distance as a gently dipping thrust with displacement to the southeast or south southeast oblique to its regional trend there is no evidence of dextral strike slip movement along the boundary new micropalaeontological studies of calcareous nannoplankton indicate that the oligo miocene basin along the boundary was the site of continuous deposition until the beginning of the middle miocene the thrust at the iezb cuts early miocene rocks and is overlapped by the middle miocene thrusting therefore occurred in this time interval the iezb is therefore unlikely to have been the dextral boundary of a westward moving alboran microplate at this time but was more likely to have been the locus of nw directed dextrally oblique convergence

geometry and scale of fault segmentation and deformational bulging along an active oblique slip fault wairarapa fault new zealand the active dextral reverse wairarapa fault in the north island new zealand cuts ca 12 ka post last glacial maximum post lgm fluvial gravels with a highly segmented fault trace this

oyster shells as recorders of short term oscillations of salinity and temperature during deposition of coral bioherms and reefs in the miocene lorca basin se spain abstract we use 18 o and 13 c of oyster shells inferred seawater 18 o w and temperature tolerance ranges of modern zooxanthellate coral communities to constrain and model paleo salinities of marginal marine waters of the miocene lorca basin spain salinities ranged from 22 to 32 psu during growth of coral bioherms and carpets of the soriana and parrilla formations and from 35 to 39 psu during deposition of the hondo formation reef complex the periods with lower modeled salinities correspond with deltaic and alluvial fan sedimentation in the area the modeling is supported by 87 sr 86 sr data and provides new insights into the evolution of the local depositional settings departures of sr isotope values from the global oceanic curve occur within the intervals with the lowest modeled salinities and the non oceanic 87 sr 86 sr preserved in individual oyster shells was probably sourced from uplifted aquitanian burdigalian limestones 87 sr 86 sr values that plot along the global ocean sr isotope curve for the late miocene indicate a tortonian age for the upper part of the soriana formation and the parrilla formation and a messinian age for the hondo formation more generally we demonstrate how the chemical analyses of oyster shells provide additional information regarding short term oscillations of salinity and temperature of seawater beyond what can be obtained by traditional paleoecological methods

geologic evolution of the betic cordilleras in the western mediterranean miocene to the present abstract the betic cordilleras were formed during the eo meso and neo alpine evolution of the western mediterranean area during the neo alpine stage the northward subduction of the african plate took place and the algerian provencal basin opened with the creation of oceanic floor associated space problems produced the expulsion and extensive stretching of the internal zones to the west which in turn disrupted and deformed the external zones of the betic and rif cordilleras these occurrences essentially took place in the burdigalian and continued with diminished intensity into the middle miocene following the suturing of the transcurrent contact between the internal and external zones important n60 90 e and nw se faults occurred the betic cordilleras are characterized as having been deformed by regional horizontal shortening compression oriented wnw ese or nw se which gradually rotated to the nnw sse in the late miocene when its most important intramontane neogene basins were formed the alboran basin with a thinned continental crust was formed as a western prolongation of the opening of the algerian provencal basin with its basic features already apparent in the burdigalian the volcanism present in the southeast of the betic cordilleras and in northeast morocco crossing the alboran basin is mainly late miocene in age it is linked to fracturing and exhibits chemistry related to the different thickness of the crust

the betic segment of the lithospheric trans alboran shear zone during the late miocene abstract the betic sinistral transcurrent shear zone betic cordilleras crosses the southeastern part of spain the study of the geodynamics of this area is approached from tectonic sedimentary magmatic and geophysical data large strike slip movements tectono sedimentary processes and magmatic events are seen at the boundary between two different lithospheric slabs they are located in a thinned compartment with a high geothermal gradient the large scale movements vertical and horizontal and the intensity of magmatic activity are the expression of the mechanical behaviour of this hot and thin crust all the geological events from the late miocene to recent are integrated in a crustal scale cross section which shows that the betic sinistral transcurrent shear zone widely affects the lithosphere it appears that subduction zones are not the only geodynamic context in which calc alkaline magmatism occurs

palaeomagnetic rotations in the eastern betic cordillera southern spain palaeomagnetic declinations from the external zones of the eastern betic cordillera southern spain and an adjacent area of the internal zone indicate variable and locally very large clockwise rotations the rotations occurred after latest oligocene times and probably before the late miocene this overlaps a period of dextrally oblique convergence during the early to middle miocene along the internal external zone boundary and within the external zone rotations in the external zone are recorded by palaeomagnetic results from 27 sites in upper jurassic ammonitico rosso facies limestones from the subbetic zone and one site of a similar age from the prebetic the sites show either two or three component behaviour the low temperature component is coincident with the present field direction the intermediate temperature component was probably acquired during miocene folding the high temperature component passes fold tests miocene age and a conglomerate test eocene early oligocene age palaeomagnetic declinations from individual tectonic blocks in the subbetic are consistent but indicate large differential rotations between blocks these blocks are underlain by low angle thrust faults which probably accommodated much of the rotation the largest rotations occur on relatively small isolated blocks of jurassic carbonates in a highly deformed triassic evaporite sequence the single site in the prebetic has not rotated significantly relative to stable iberia in malaguide rocks of the sierra espun a in the adjacent internal zone stable palaeomagnetic components were measured at one site in upper miocene sedimentary rocks at three sites in upper oligocene lower miocene red marls at one site in oligocene marls at one site in jurassic limestones and at two sites in permo triassic red beds the palaeomagnetic results suggest that about 60 of clockwise rotation occurred in the latest oligocene earliest miocene and a further 140 of clockwise rotation subsequently one site in late miocene sedimentary rocks yields unrotated declinations which if not representing an overprinted direction indicate that the rotation was complete by the end of the miocene

mode of extensional tectonics in the southeastern betics se spain implications for the tectonic evolution of the peri alboran orogenic system the gibraltar arc which closes the westernmost part of the mediterranean basin is a miocene a type subduction arc formed by the continental collision of various pre miocene terranes in the major zone of collision between the iberian and african cratons the hanging wall block known as the alboran domain has undergone more than 300 km migration from a more easterly position where it was the continuation of the alpine cretaceous paleogene orogen contemporaneous with thin skinned thrusting in the footwall the alboran domain underwent two episodes of nearly orthogonal extension in which extensional systems developed with directions of extension varying from a nnw sse system orthogonal to the belt axis in the late burdigalian langhian to a wsw directed orogen parallel one in the serravallian the superposition of these two systems resulted in a chocolate tablet megastructure this extensional pattern is not satisfactorily explained in previously proposed models for the evolution of the arc orthogonal extension is plausible in a process of the gravitational collapse of an overthickened crust nevertheless orogen parallel extension is more difficult to explain in this context we advocate that the wsw directed low angle normal faults formed during large scale extension in connection with important westward arc migration the driving force of extension in a general context of convergence is controversial and varies between a convective removal model and a delamination model constraints on both the timing and the kinematics of extension as presented in this paper seem to support the contribution of both mechanisms convective removal may have started the process but continued n s convergence could have resulted in westward tectonic escape and asymmetric lateral inflow of asthenospheric material accompanying lithospheric delamination

seismotectonics of the central part of the south iceland seismic zone abstract the largest earthquakes in iceland are associated with strike slip faults in the south iceland seismic zone sisz major destructive earthquake sequences occur in this zone at an average interval of 80 years the last two sequences being in 1784 and 1896 the last major single earthquake in the zone was an earthquake of magnitude 7 m in 1912 the seismicity is mostly associated with a conjugate system of nne trending mainly right lateral faults and ene trending mainly left lateral faults but in addition there are many nw trending faults in the sisz particular attention is paid to the comparison between present day fault activity earthquake mechanisms and quaternary faulting fault slip data this comparison reveals a general similarity despite the temporal difference between these subsets independently collected the fault populations are well exposed in the hyaloclastite mountain vordufell near the central part of the south iceland seismic zone where considerable fault slip occurred during the earthquake sequences of 1784 and 1896 the cumulative displacement on most of these faults is 1 6 m in addition to the large scale faults we measured 79 minor faults displacements of the order of centimetres in the vordufell mountain most of the minor faults are left lateral analysis of these minor faults recording palaeostresses in rocks ranging in age from 3 1 to 0 7 ma as well as 50 focal mechanisms from a swarm of present day microearthquakes occurring beneath the vordufell mountain gave consistent results in terms of seismotectonic stress orientations of stress axes and ratios of principal stress differences this shows that the tectonic regime remained unchanged through recent times the results indicate however that there are two contrasting stress regimes in this part of the sisz the primary subset indicates nw se tension and ne sw compression whereas the secondary subset indicates nw se compression and ne sw tension we propose that the primary subset is consistent with the dominating time averaged regional stress field in the sisz this dominating stress field can be largely explained as a consequence of simple plate pull parallel with the spreading vector in south iceland the secondary subset and the associated stress field however may be partly related to stress release and rebound and partly to dike injections in the nearby segments of the rift zone in south iceland

secondary earthquake fractures generated by a strike slip fault in the south iceland seismic zone most large earthquakes in the south iceland seismic zone occur on n s trending dextral strike slip faults the resulting rupture zones display complex en echelon patterns of secondary structures including nne trending arrays of mostly ne trending open fractures and hillocks three spatial scales characterize the surface faulting pattern the length of the main fault mf 104 m the arrays of fractures here interpreted as surface evidence of subordinate faults sf 102 m and the individual open fractures of 10 m in order to improve our understanding of the genetic relationship between the of and the mf we computed the stress field induced by slip on a buried mf using a dislocation model in a layered half space where the fault surface is assumed to be embedded in the basement rock topped by a softer tens of meters thick near surface layer the of were modeled as secondary fractures of either of the following types 1 pure mode i cracks opening in the near surface layer in the direction of the tensile principal stress or 2 mixed mode cracks slipping at depth as shear cracks and opening near the surface due to low confining pressure assuming that both types of secondary fractures occur in the sisz we can explain their first order characteristics by reasoning that they are the end product of a hierarchy of deformation produced by the main fault the coulomb failure function after the earthquake obtained by summing the mf stress change and the lithostatic stress suggests that subordinate faulting sf must be expected to occur in response to the main rupture below the upper soft layer extending to a depth of 100 m the total stress change induced by the mf and the sf is shown to yield quantitative explanations of the spatial extent the double en echelon arrangement and the orientation of surface fractures observed in the field in terms of a simple frictional model of rupture

overlapping rift zone segments and the evolution of the south iceland seismic zone the south iceland seismic zone is a 20 60 km wide north south and up to 70 km long zone of north and north northeast trending holocene arrays of en echelon tension fractures these fracture arrays are related to dextral strike slip faults buried by holocene lava flows in this zone major destructive earthquake sequences occur at intervals of 45 112 years the largest events reaching magnitude 7 ms we propose that this seismic zone is located between overlapping rift zone segments spreading centers where the eastern segment has been propagating to the south during the past 3 ma we made a finite element study of this configuration with the segments modeled as mode i cracks loaded in tension the results suggest that the south iceland seismic zone in general and the north and north northeast trending dextral faults in particular develop in response to the shear stresses generated between the rift zone segments

evolution of migrating transform faults in anisotropic oceanic crust examples from iceland major transform fault zones link extensional segments of the north american eurasian plate boundary as it transects the iceland hotspot changes in plate boundary geometry involving ridge jumps