strike slip faulting and nested block rotations structural evidence from the cordillera de domeyko northern chile abstract based on a structural analysis tectonic rotations around vertical axes have been found affecting the sedimentary sequence of the purilactis group from the cordillera de domeyko of northern chile the affected rocks form the northern part of a pre existing syncline that was offset by two sets of strike slip faults during the upper eocene incaic orogeny uplift and shortening in the hangingwall of two main bounding thrusts initiated the strike slip faults which divided the purilactis group into several fault blocks further contraction of the enclosed block faulted area led to domino style anticlockwise rotation of the central main blocks and the bounding block faults with rotations up to 40 additionally eastward displacement of these blocks was accompanied by local indentation of the adjacent smaller blocks to the east giving rise to opposite rotation directions the block rotations may have been accommodated by a decollement located in the mudstone and evaporite of the basal part of the sequence tonel formation based on a reconstruction of the initial rock distribution different fault models are discussed and a model resembling pure shear deformation is proposed to explain the initiation of strike slip faults the occurrence of block rotations and the distortion of the block faulted domain

sequence stratigraphy of the palaeozoic welsh basin the timing and extent of unconformities in the welsh basin are investigated using rock preservation curves derived from outcrop stratigraphic logs four basin wide unconformities occur focussed in late precambrian late tremadoc pusgillian early ashgill and mid devonian times these bound three megasequences equating with newly defined lithostratigraphic units the dyfed gwynedd and powys supergroups less extensive unconformities bound 18 component sequences the majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea level change the megasequence boundaries are attributed to late precambrian to early cambrian onset of rifting to form a passive margin tremadoc onset of subduction with intra arc then back arc extension late caradoc end to subduction and late early devonian collisional deformation the megasequences and controlling events can be tentatively matched with other basins on the avalonian margin more generally this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

structure and geodynamic evolution of the central bransfield basin nw antarctica from seismic reflection data abstract the bransfield basin is a young active rift basin located between the northern margin of the antarctic peninsula and the south shetland islands margin deception and bridgeman islands divide the bransfield basin in three subbasins the western central and eastern specific morpho tectonic features and sediment fill differentiate each subbasin the structure and geodynamic evolution of the central bransfield basin which is in a stage of incipient seafloor spreading have been investigated in detail from a dense grid of single channel seismic reflection data the central bransfield basin is dominated by two families of normal faults which are oriented northeast and northwest the ne trending faults define three graben systems that are roughly parallel to the basin axis in an across basin direction the mean trend of this family of faults ranges from n71 the graben system nearest to the antarctic peninsula over n64 the intermediate graben system to n53 the graben system nearest to the south shetland islands the nw trending family of faults is responsible for the deepening of the basin from southwest to northeast both families of faults define the overall central bransfield basin structure resulting in a complex division of the basin floor additionally tens of volcanic edifices are located on the basin floor the larger ones being associated to the nw trending faults interaction of tectonics and sedimentation give place to the differentiation of three tectonostratigraphic units tu1 tu2 and tu3 from oldest to youngest tu1 occupies the southernmost graben system and it is affected by the ne trending bounding normal faults tu2 extends further northwestwards than tu1 and essentially fills the intermediate graben system tu3 represents a further extension of the sediment infill over most of the central bransfield basin and marks the initiation of the infill of the northernmost graben system faults bounding this graben also determine the straight and abrupt morphology of the south shetland islands margin the observed arrangement of the graben systems and the filling tectonostratigraphic units reveal a migration of extensional tectonics and associated depocentres from the antarctic peninsula margin to the south shetland island margin the left lateral rotation from n71 to n53 in the mean trend of the three successive graben systems could have been produced by the oblique subduction of the phoenix plate and the effect of the sinistral strike slip movement of the south scotia ridge 200 km northeastwards of the central bransfield basin

structural styles of paleozoic intracratonic fault reactivation a case study of the grays point fault zone in southeastern missouri usa abstract a pronounced subparallel set of northeast striking faults occurs in southeastern missouri but little is known about these faults because of poor exposure the commerce fault system is the southernmost exposed fault system in this set and has an ancestry related to reelfoot rift extension recent published work indicates that this fault system has a long history of reactivation the northeast striking grays point fault zone is a segment of the commerce fault system and is well exposed along the southeast rim of an inactive quarry our mapping shows that the grays point fault zone also has a complex history of polyphase reactivation involving three periods of paleozoic reactivation that occurred in late ordovician devonian and post mississippian each period is characterized by divergent right lateral oblique slip faulting petrographic examination of sidewall rip out clasts in calcite filled faults associated with the grays point fault zone supports a minimum of three periods of right lateral oblique slip the reported observations imply that a genetic link exists between intracratonic fault reactivation and strain produced by paleozoic orogenies affecting the eastern margin of laurentia north america interpretations of this link indicate that right lateral oblique slip has occurred on all of the northeast striking faults in southeastern missouri as a result of strain influenced by the convergence directions of the different paleozoic orogenies

kinematics of strike slip faulting builth inlier mid wales abstract strike slip faulting in the builth ordovician inlier is demonstrated by large scale maps of the llanelwedd quarries near builth wells and by fault plane and slickenline data in the main quarry steep nnw striking strike slip faults dominate the structure together with significant strike slip displacement on the w dipping bedding surfaces and bedding parallel faults a zone of steep n striking extensional dip slip faults links two of the strike slip strands and there is a weaker e striking set of strike slip faults when the four fault sets are rotated so that the regional bedding is horizontal three become vertical and one horizontal probably their attitude during active life in late ordovician to early silurian time they formed a linked fault system capable of accommodating three dimensional bulk strain the fault flats have the same kinematic role in a strike slip system as lateral ramps or transfer faults in dip slip systems in the nearby gelli cadwgan quarry strike slip faults are again dominant but strike e or ese this heterogeneity of fault pattern in the southern builth inlier resolves into more homogeneous domains with areas of 0 1 to 0 5 km 2 separated by e to ne striking dextral strike slip faults domainal structure an important general feature of strike slip tectonics may be present on a variety of scales

aftershocks of the campania lucania italy earthquake of 23 november 1980 abstract the result of a study of 1130 aftershocks of the 1980 campania lucania earthquake are presented the data were collected primarily with a dense array of portable smoked drum seismographs deployed in the epicentral region a few days after the main shock tests performed on subsets of the data to establish the location accuracy are described in detail and epicentral maps and cross sections are presented and discussed one hundred and sixty well constrained fault plane solutions using upward traveling rays are presented the distribution of aftershocks forms an elongate cloud which does not define a plane this is not because the events are poorly located a subset of events located with epicentral errors of less than 1 km and depth errors less than 2 km forms a similar cloud to the complete aftershock set the aftershock mechanisms are very varied some are similar to the main event but many differ profoundly these include events on shallow or vertical planes and thrust events the latter include some events which require contraction in the direction in which the main event indicates extension the aftershocks of all mechanisms are intermixed in space in such a way that no simple system of faults can explain them a model to explain the aftershocks and the lack of surface ruptures in the main event is offered it is proposed that the main shock cuts between two aseismic detachment surfaces and that the aftershocks are a consequence of the complicated deformation that such a geometry demands whatever interpretation is adopted the results are a challenge to the commonly accepted view that aftershocks even approximately define simple fault planes or that aftershock mechanisms replicate the mechanism of the main shock defects in interpretations of the main shock and aftershocks process arise largely from a lack of information about the geometry of structures at depth in the epicentral region suitable deep reflection profiles could help to resolve this problem

oversized u shaped canyon development at the edge of rotating blocks due to wrench faulting on margins of the dead sea graben strike slip fault systems on the western margin of the dead sea active pull apart basin central part of the dead sea transform have been mapped in detail in the vicinity of the ya elim valley in the southeastern judean desert compressional tensional and strike slip shearing features are described and the geomorphology of the valley which changes over a 6 km long segment from a mature meandering valley to a deep and narrow canyon and then to an oversized very deep and very wide u shaped canyon is elucidated using profiles and superimposed cross sections a kinematic model for the formation of the ya elim valley is proposed reflecting dextral shearing on the margins of counterclockwise rotating blocks in the same area subrecent recent spreading of tensional fractures has occurred separating sections of blocks which protrude eastward into the deepest on land depression the large width of shear zones sub perpendicular to the transform may relate to repeated reversals in the sense of rotation of second order rigid block domains caused by rotations of neighbouring first order domains due to earthquake ruptures on individual parallel faults of the transform

amplification of strong ground motions at heathcote valley during the 2010 2011 canterbury earthquakes the role of 2d nonlinear site response

a palaeomagnetic study of the builth wells llandrindod wells ordovician inlier wales palaeogeographic and structural implications summary discrepancies exist in the southern british ordovician palaeomagnetic data set with individual studies indicating either high southerly or temperate southerly palaeolatitudes although previous palaeomagnetic investigations from the builth inlier have delineated a three component remanence structure doubts have been raised concerning i the magnetization age of the supposed primary component p ii the structural setting of the sampling area llanelwedd quarry within a zone of strike slip deformation and iii whether detailed step wise demagnetization has fully separated the various magnetization components in addressing these problems we present new palaeomagnetic results as follows i a positive palaeomagnetic conglomerate test establishing a pre late llanvirn age for component p ii an enlarged geographic sampling embracing several tectonic domains indicating that relative rotations linked to strike slip tectonism are minor within the builth sampling area and iii evidence that component p is uncontaminated by overprints at some sites suggesting the resulting palaeolatitude of 35 s to be representative for the builth inlier we also present evidence for local syn volcanic deformation within llanvirn volcanics and for intrusion related deformation of llanvirn shales when the revised llanvirn pole from the inlier 3 s 4 e is combined with other data from southern britain in a statistical spline analysis a mean palaeolatitude of 44 5 s is calculated for builth in llanvirn times a palaeoreconstruction for 468 474 ma c mid llanvirn is presented utilizing this analysis

a block model of distributed deformation by faulting it is clear from palaeomagnetic evidence that large and hitherto unexpected rotations about a vertical axis are common in regions of distributed continental deformation we propose a simple two dimensional model to illustrate how fault movement and block rotation within a zone of distributed deformation may be related to the relative motion of the rigid plates that bound the zone one surprizing feature of this model is that the component of strike slip motion on the faults within the deforming zone is in the opposite sense to the strike slip component across the zone as a whole thus slip vectors within the zone are not the same as that between the bounding rigid plates in contrast to deformation at oceanic plate boundaries examples of active fault geometries similar to those of our model can be seen in greece iran and the western usa

the relationship between strain rates crustal thickening palaeomagnetism finite strain and fault movements within a deforming zone the deformation of continental lithosphere generally does not occur on a single fault but is distributed over a zone up to a few hundred kilometers wide if the deformation rate is constant throughout the zone and if the velocity is continuous everywhere and does not change with time the deformation gradient tensor can be obtained analytically this tensor directly determines the amount of crustal thickening and is related in a complicated manner to the finite strain tensor the declination of remnant magnetization measured within the zone can also be rotated by the deformation because palaeomagnetic measurements are generally made on rigid blocks which have not been deformed there is no simple relationship between such rotations and the finite strain expressions for all these quantities are obtained in terms of the velocity gradients which are taken to be constant the history of western north america is used to illustrate the relationship between the present deformation by strike slip and thrust motions the rotations of the remnant magnetization vectors and the shape of accreted terranes at the surface distributed deformation is taken up by motion on faults which separate the zone into blocks the displacement rate on the faults their strike and the finite rotations of the blocks are all controlled by the velocity gradients analytic expressions relating these quantities are obtained a deforming zone containing normal faults which crosses central greece takes up both strike slip and extension and illustrates the relationship between the structures within and the motion across the zone a similar but more complicated zone in northeastern iran involves both strike slip and shortening

evidence for the importance of small faults on block rotation field and earthquake data are presented which show that small faults can be important during block rotation small here means those faults which are at least an order of magnitude smaller than the largest faults at a particular scale of observation the nw se trending dextral faults of sw england have millimetres to kilometres of displacement the largest of these faults such as the sticklepath lustleigh fault do not appear to cause more than a few degrees of rotation from the general e w strike of beds tens of degrees of rotation are often visible however on faults with millimetres of displacement similarly the apparent rotation of beds in the badajoz cordoba shear zone spain increases as the resolution of the faults is increased the power law scaling relationship of earthquake magnitudes in the san andreas fault zone illustrates that small faults can also be important in regions of active block rotation small faults can allow deformation within rotating blocks and can allow high displacement gradients to occur on the block bounding faults this would reduce the need for void creation which is a requirement of rigid block rotation models a tentative model is presented which incorporates the concept that fault behaviour is fractal emphasising the contribution of small faults it is suggested that future studies of block rotation should rigorously test the contribution of small faults and that particular care is needed when comparing palaeomagnetic data with regional scale structures

an association between organic matter and localised prehnite pumpellyite alteration at builth wells wales u k metcalfe r banks d and bottrell s h 1992 an association between organic matter and localised prehnite pumpellyite alteration at builth wells wales u k chem geol 102 1 21 the builth volcanic group of the builth wells ordovician inlier wales is a sequence of basic intermediate lavas and volcaniclastics which shows a unique association between pyrobitumen and localised domainal calc silicate alteration the pyrobitumens occur in calcite veins and disseminated throughout alteration domains which form haloes to these veins the pyrobitumens were emplaced by ordovician hydrothermal fluids which were also responsible for the domainal alteration combined petrographic gas chromatography and stable isotopic studies show that light organic gases c2 c5 were liberated into the hydrothermal fluids by devolatilisation of liquid petroleum to form the pyrobitumens these light organic gases mixed with methane characterized by 13c 13 o which was already present in the fluids this methane was most likely mantle derived or produced from inorganic synthesis the organic content of the fluids exerted an important control on the inorganic secondary assemblages a similar control of inorganic assemblages by organic matter might possibly occur deep within modern petroleum bearing hydrothermal systems such as the guaymas basin and escanaba trough hydrothermal systems

the pontesford lineament welsh borderland the ne trending pontesford lineament centres on the pontesford linley and adjacent faults in the shelve longmynd inlier to the ne it is usually drawn along the se edge of the cheshire basin and to the sw along a variety of lines parallel to or converging with the church stretton fault new mapping and reassessment of published data suggest that sw of the pontesford linley fault the lineament is logically drawn along the clun forest disturbance a post pridoli late silurian fold fault belt then across the nw side of the llandrindod inlier to flank the tywi lineament the lineament was intermittently active at least from mid ordovician through to triassic time with both strike slip and dip slip displacements that involved the upper part of the precambrian basement late ordovician and early silurian strike slip was particularly important the lineament controlled the sites of several silurian sedimentary basins ordovician contrasts across the lineament can be explained either by contemporary fault control or by subsequent large displacements juxtaposing unlike terranes the long history of reactivation and basement control on the pontesford lineament is typical of lineaments throughout wales and the borderland it suggests that both caledonian and variscan structural trends n of the variscan front may be inherited from older pre silurian or even precambrian basement structures

superimposed recent permo carboniferous and ordovician palaeomagnetic remanence in the builth volcanic series wales the builth volcanic series of llanvirnian age in llanelwedd quarries mid wales carries three components of natural remanent magnetisation component p regarded as primary is a thermochemical remanence directed at d 181 7 i 54 5 95 4 4 relative to bedding component s is a secondary component with in situ d 178 7 i 6 7 95 5 4 and is believed to be a low temperature chemical remanence crm of permo carboniferous age component r is directed close to the present geomagnetic field and is believed to be a recent viscous remanence vrm the results are of interest for three reasons first they are an unusually good example of multi component nrm analysis the three components being so clearly discriminated by thermal demagnetisation because they have almost completely separate blocking temperature ranges second they provide evidence of a permo carboniferous event possibly a mild thermal or hydrothermal pulse promoting crm acquisition some 40 km north of the hercynian orogenic front third they illustrate very clearly the importance of detailed demagnetisation this work revises the pole position for these rocks by 10 and removes an obstacle to the palaeomagnetic recognition of the 1000 km wide iapetus ocean cutting britain in ordovician time

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

reply to comment of a trench and t h torsvik on paleomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain

fault related rocks suggestions for terminology many traditional terms for fault related rocks have undergone recent dynamic metamorphism under high pressure discussions by various groups of specialists a generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic engineering and legal purposes such a framework is proposed here focusing on a rate of strain versus rate of recovery diagram and relating this framework to the products of brittle and ductile deformation along faults

deformation processes and weakening mechanisms within the frictional viscous transition zone of major crustal scale faults insights from the great glen fault zone scotland abstract the great glen fault zone ggfz scotland is a typical example of a crustal scale reactivated strike slip fault within the continental crust analysis of intensely strained fault rocks from the core of the ggfz near fort william provides a unique insight into the nature of deformation associated with the main phase of sinistral movements along the fault zone in this region an exhumed sequence of complex mid crustal deformation textures that developed in the region of the frictional viscous transition ca 8 15 km depth is preserved fault rock fabrics vary from mylonitic in quartzites to cataclastic in micaceous shear zones and feldspathic psammites protolith mineralogy exerted a strong control on the initial textural development and distribution of the fault rocks at lower strains crystal plastic deformation occurred in quartz dominated lithologies to produce mylonites simultaneously with widespread fracturing and cataclasis in feldspar and mica dominated rocks at higher strains shearing appears to increasingly localise into interconnected networks of cataclastic shear zones many of which are strongly foliated textures indicative of fluid assisted diffusive mass transfer mechanisms are widespread in such regions and suggest that a hydrous fluid assisted grainsize controlled switch in deformation behaviour followed the brittle comminution of grains the fault zone textural evolution implies that a strain induced fluid assisted shallowing and narrowing of the frictional viscous transition occurred with increasing strain it is proposed that this led to an overall weakening of the fault zone and that equivalent processes may occur along many other long lived crustal scale dislocations