the bristol mendip foreland thrust belt the thrusts and associated fold structures of the bristol mendip area are interpreted in terms of a thin skinned piggy back foreland thrust belt of variscan age with dominant northward transport strike normal balanced sections and compatible strike parallel sections have been constructed to illustrate the 3 d geometry of the linked thrust system north south shortening of c 20 km 40 is calculated from deformed and restored strike normal sections a thick and extensive thrust wedge of pre old red sandstone basement in the west of the area is responsible for culminations expressed at the surface as the mendip periclines uplifted culminations permit erosion to lower stratigraphic levels in the west and south of the area strike parallel sections and a hanging wall cut off map indicate that the majority of thrusts in the area are mainly oblique ramps rather than simple frontal and lateral ramps or flats the en echelon pattern of the mendip periclines may be related to the oblique ne climb of two of these thrusts the angle of frontal ramp climb through the dinantian stratigraphy is generally low and this explains the presence of numerous klippen of lower on upper carboniferous rocks whilst suggesting that certain thrust sheets were much more extensive than present outcrops indicate early pre thrusting movement on basement lineaments in the variscan foreland created uplifts angular unconformities and a predominance of n s structures in the north of the area

thrust fault growth and segment linkage in the active ostler fault zone new zealand abstract thrust faulting is a fundamental mode of crustal deformation yet many of the key geometrical attributes of thrust faults and the controls on fault rupture growth and linkage remain poorly resolved numerous surface rupturing thrust faults cut through upper quaternary glacial outwash terraces within the ostler fault zone an active thrust system in the southern alps new zealand we use these deformed marker surfaces to define the three dimensional deformation field associated with their surface expression and to map displacement and length on 40 fault segments displacement transfer across two fault segment arrays occurs in distinctly different styles in one displacement is transferred between en echelon fault segments to produce a smooth linear displacement gradient in the other large scale folding and a population of small faults transfer displacement between two non overlapping fault segments with a residual displacement minimum within the transfer zone size distribution of fault segment length and maximum displacement follow a power law scaling relationship maximum displacement d max scales linearly with and represents 1 of segment trace length l d max l data from the segments of the ostler fault zone display similar scaling to a global fault dataset this similarity is surprising given that the observed displacements along the ostler fault represent only 20 ky of slip on fault segments that are likely to be many times older

active thrusting and folding along the northern tien shan and late cenozoic rotation of the tarim relative to dzungaria and kazakhstan we have studied geometries and rates of late cenozoic thrust faulting and folding along the northern piedmont of the tien shan mountain belt west of urumqi where the m 8 3 manas earthquake occurred on december 23 1906 the northern range of the tien shan rising above 5000 m overthrusts a flexural foredeep filled with up to 11 000 m of sediment of the dzungarian basement our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front within a 200 km long zone of neogene quaternary anticlines fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range in all the valleys the scarps offset vertically the highest terrace surface by the same amount 10 2 0 7 m inferring an early holocene age 10 2 kyr for this terrace which is continuous with the largest recent fans of the piedmont yields a rate of vertical throw of 1 0 0 3mm yr on the main active thrust at the surface a quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5 5 2 5 m 2 kyr a rather fresh surface scarp 0 8 0 15 m high that is unlikely to result from shallow earthquakes with 6 m 7 in the last 230 years is visible at the extremities of the main fold zone we associate this scarp with the 1906 manas earthquake and infer that a structure comprising a deep basement ramp under the range gently dipping flats in the foreland and shallow ramps responsible for the formation of the active fault propagation anticlines could have been activated by that earthquake if so the return period of a 1906 type event would be 850 380 years the small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth 2 3 is taken up by incremental folding near the surface comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising quaternary ranges such as the san gabriel mountains in california or the mont blanc aar massifs in the alps we estimate the finite cenozoic shortening of the folded dzungarian sediments to be of the order of 30 km and the cenozoic shortening rate to have been 3 1 5 mm yr assuming comparable shortening along the tarim piedmont and minor additional active thrusting within the mountain belt we infer the rate of shortening across the tien shan to be at least 6 3 mm yr at the longitude of manas 85 5 e a total shortening of 125 30 km is estimated from crustal thickening assuming local airy isostatic equilibrium under the same assumption serial n s sections imply that cenozoic shortening across the belt increases westwards to 203 50 km at the longitude of kashgar 76 e as reflected by the westward increase of the width of the belt this strain gradient implies a clockwise rotation of tarim relative to dzungaria and kazakhstan of 7 2 5 around a pole located near the eastern extremity of the tien shan west of hami 96 e 43 5 n comparable to that revealed by paleomagnetism between tarim and dzungaria 8 6 8 7 a 6 mm yr rate of shortening at the longitude of manas would imply a rate of rotation of 0 45 m y and would be consistent with a shortening rate of 12 mm yr north of kashgar taking such values to be representative of late cenozoic rates would place the onset of reactivation of the tien shan by the india asia collision in the early to middle miocene 16 22 9 m y in accord with the existence of particularly thick late neogene and quaternary deposits such reactivation would thus have started much later than the collision roughly at the time of the great mid miocene changes in tectonic regimes denudation and sedimentation rates observed in southeast asia the himalayas and the bay of bengal and of the correlative rapid change in seawater sr isotopic ratio 20 to 15 ma like these other changes the rise of the tien shan might be a distant consequence of the end of indochina s escape

foredeeps versus subduction zones the geologic characteristics of foredeeps and accretionary wedges suggest that these features are distinguishable on the basis of the direction of the associated subduction east north east dipping subduction related accretionary wedges show high relief and broad outcrops of metamorphic rocks they are associated with shallow foredeeps with low subsidence rates in contrast west dipping subduction related accretionary wedges show low relief and involve mainly sedimentary cover the related foredeeps are deep and have high subsidence rates this differentiation is useful both for oceanic and continental subductions e g eastern vs western pacific subductions or east dipping alpine vs west dipping apenninec subductions in a cross section of the alps the ratio of the area of the orogen to the area of the foredeeps is at least 2 1 whereas this ratio is 0 22 1 for the apennines these ratios explain why foredeeps related to east or northeast dipping subduction are quickly filled and are bypassed by clastic rocks whereas foredeeps related to west dipping subduction maintain a deep water environment longer these differences support the presence of an eastward asthenospheric counterflow relative to the westward drift of the lithosphere detected in the hot spot reference frame even in the mediterranean where no hot spots are present in this interpretation the apennines foredeep was caused by the eastward push of the mantle acting on the subducted slab whereas the foredeeps in the alps were caused by the load of the thrust sheets and the downward component of movement of the upper adriatic plate these forces contrast with the upward component of the eastward mantle flow

coseismic displacements and holocene slip rates for two active thrust faults at the mountain front of the andean precordillera 33 s 1 during the last few hundred years several destructive earthquakes occurred along the eastern margin of the andean precordillera where gps data reveal a shortening rate of 4 5 mm a we use fault scarp profiles and age determinations of deformed terraces t1 t4 to infer coseismic displacements and quantify slip rates for the penas and cal thrust faults near mendoza city scarps on the lowest terrace level t1 reveal vertical offsets of 0 8 1 0 m for both faults which are interpreted as coseismic displacements during the last earthquake together with the fault dip these offsets indicate that both faults are capable of producing magnitude mw 6 9 earthquakes which is corroborated by a magnitude ms 7 0 event on the cal fault that destroyed mendoza in 1861 at the penas thrust fault terrace t2 has an age of 3 3 ka and is offset by 1 9 m whereas the 12 ka old terrace t3 is displaced by 11 m combined with the fault dip of 25 the age and offset of terrace t3 define a shortening rate of 2 0 mm a on the penas fault i e about half of the present day shortening at the eastern margin of the precordillera at the cal fault terraces t2 to t4 have ages of 0 8 ka osl 3 9 ka 14c and 12 ka 10be and are vertically offset by 2 6 3 6 and 7 0 m respectively which implies that slip on the fault has recently accelerated hence the cal fault poses a serious seismic hazard to the one million inhabitants of mendoza

variscan inversion within the cheshire basin england carboniferous evolution north of the variscan front abstract a comprehensive seismic reflection and borehole database over the whole of the cheshire basin has allowed examination of the carboniferous succession beneath a thick permo triassic cover construction of a seismic sequence stratigraphy involving the recognition of unconformity bounded packages has allowed the examination of the control by tectonics of sedimentation within the pre permo triassic of the cheshire basin the carboniferous has been divided into two unconformity bound megasequences i e dinantian ms10 0 and namurian westphalian ms20 0 these are deformed by a hitherto unknown series of variscan structures interpretation of these structures has allowed assessment of the variscan orogeny in this area of the variscan foreland considerable variscan inversion in the area exists 100 150 km north of the variscan fold thrust front and is suggested to illustrate the effects of predominantly e w directed compression especially to the north of the northern apex of the adjacent midlands microcraton compression occurred during two late carboniferous pulses of thrusting

gps estimate of relative motion between the caribbean and south american plates and geologic implications for trinidad and venezuela global positioning system gps data from eight sites on the caribbean plate and five sites on the south american plate were inverted to derive an angular velocity vector describing present day relative plate motion both the caribbean and south american velocity data fit rigid plate models to within 1 2 mm yr the gps velocity uncertainty the caribbean plate moves approximately due east relative to south america at a rate of 20 mm yr along most of the plate boundary significantly faster than the nuvel 1a model prediction but with similar azimuth pure wrenching is concentrated along the approximately east striking seismic el pilar fault in venezuela in contrast transpression occurs along the 068 trending central range warm springs fault in trinidad which is aseismic possibly locked and oblique to local plate motion

subduction kinematics and dynamic constraints the kinematics of subduction zones shows a variety of settings that can provide clues for dynamic understandings two reference frames are used here to describe the simple 2d kinematics of subduction zones in the first the upper plate is assumed fixed whereas in the second frame upper and lower plates move relative to the mantle relative to a fixed point in the upper plate u the transient subduction hinge h can converge diverge or be stationary similarly the lower plate l can converge diverge or be stationary the subduction rate vs is given by the velocity of the hinge h minus the velocity of the lower plate l vs vh vl the subduction rate 1 increases when h diverges and 2 decreases when h converges combining the different movements at least 14 kinematic settings can be distinguished along the subduction zones variable settings can coexist even along a single subduction zone as shown for the 5 different cases occurring along the apennines subduction zone apart from few exceptions the subduction hinge converges toward the upper plate more frequently along e or ne directed subduction zone whereas it mainly diverges from the upper plate along w directed subduction zones accompanying backarc extension before collision orogen growth occurs mostly at the expenses of the upper plate shortening along e ne directed subduction zones whereas the accretionary prism of w directed subduction zones increases at the expenses of the shallow layers of the lower plate the convergence shortening ratio is 1 along e or ne directed subduction zones whereas it is 1 along accretionary prisms of w directed subduction zones backarc spreading forms in two settings along the w directed subduction zones it is determined by the hinge divergence relative to the upper plate minus the volume of the accretionary prism or in case of scarce or no accretion minus the volume of the asthenospheric intrusion at the subduction hinge since the volume of the accretionary prism is proportional to the depth of the decollement plane the backarc rifting is inversely proportional to the depth of the decollement on the other hand along e or ne directed subduction zones few backarc basins form e g aegean andaman and can be explained by the velocity gradient within the hangingwall lithosphere separated into two plates when referring to the mantle the kinematics of subduction zones can be computed either in the deep or in the shallow hotspot reference frames the subduction hinge is mostly stationary being the slab anchored to the mantle along w directed subduction zones whereas it moves w or sw ward along e or ne directed subduction zones surprisingly along e or ne directed subduction zones the slab moves out of the mantle i e the slab slips relative to the mantle opposite to the subduction direction kinematically this subduction occurs because the upper plate overrides the lower plate pushing it down into the mantle as an example the hellenic slab moves out relative to the mantle i e sw ward opposite to its subduction direction both in the deep and shallow hotspot reference frames in the shallow hotspot reference frame upper and lower plates move westward relative to the mantle along all subduction zones this kinematic observation casts serious doubts on the slab negative buoyancy as the primary driving mechanism of subduction and plate motions w directed subduction zones rather provide about 2 3 times larger volumes of lithosphere re entering into the mantle and the slab is pushed down this opposite behavior is consistent with the down dip extension seismicity along e ne directed subduction zones and the frequent down dip compression along the w directed subduction zones subduction kinematics shows that plate velocity is not dictated by the rate of subduction along the w directed subduction zones the rate of subduction is rather controlled i by the hinge migration due to the slab interaction with the easterly trending horizontal mantle wind along the global tectonic mainstream ii by the far field plate velocities and iii by the value of negative buoyancy of the slab relative to the country mantle alternatively e ne nne directed subduction zones have rates of sinking chiefly determined i by the far field velocity of plates and ii by the value of negative buoyancy of the slab relative to the country mantle along this type of subduction the subduction hinge generally advances e ne ward toward the upper plate decreasing the subduction rate but it moves w sw ward relative to the mantle all this indicates that subduction zones have different origin as a function of their geographic polarity and the subduction process is more a passive feature rather than being the driving mechanism of plate motions a rotational component combined with mantle density and viscosity anisotropies seems more plausible for generating the global tuning in the asymmetry of subduction zones

quaternary tectonic evolution of the pamir tian shan convergence zone northwest china the pamir tian shan collision zone in the western tarim basin northwest china formed from rapid and ongoing convergence in response to the indo eurasian collision the arid landscape preserves suites of fluvial terraces crossing structures active since the late neogene that create fault and fold scarps recording quaternary deformation using geologic and geomorphic mapping dgps surveys of deformed terraces and osl dating we create a synthesis of the active structures that delineate the timing rate and migration of quaternary deformation during ongoing convergence new deformation rates on eight faults and folds when combined with previous studies highlight the spatial and temporal patterns of deformation within the pamir tian shan convergence zone during the quaternary terraces spanning 130 to 8 ka record deformation rates between 0 1 and 5 6 mm yr on individual structures in the westernmost tarim basin where the pamir and tian shan are already juxtaposed the fastest rates occur on actively deforming structures at the interface of the pamir tian shan orogens farther east as the separation between the pamir tian shan orogens increases the deformation has not been concentrated on a single structure but rather has been concurrently distributed across a zone of faults and folds in the kashi atushi fold and thrust belt and along the ne pamir margin where shortening rates vary on individual structures during the quaternary although numerous structures accommodate the shortening and the locus of deformation shifts during the quaternary the total shortening across the western tarim basin has remained steady and approximately matches the current geodetic rate of 6 9 mm yr

tectonic erosion of the peruvian forearc lima basin by subduction and nazca ridge collision 1 subsidence of lima basin part of the peruvian forearc is controlled by tectonic erosion by the subducting nazca plate multichannel seismic reflection data coupled with age and paleowater depth constraints derived from ocean drilling program odp coring now allow the rates of erosion to be reconstructed through time in trenchward locations the forearc has experienced limited recent relative uplift 700 850 m likely due to preferential basal erosion under the center of lima basin long term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench since 47 ma eocene up to 148 km of the plate margin have been lost at an average rate of up to 3 1 km myr 1 appoximately 110 km of that total appears to be lost since 11 ma implying much faster average rates of trench retreat 10 km myr 1 since collision of the nazca ridge with the lima basin at 11 ma although there is no clear subsidence event at odp site 679 during the time at which nazca ridge was subducting beneath this part of the forearc 4 11 ma the more trenchward odp sites 682 and 688 show significant deepening after 11 ma indicating that subduction of the ridge accelerates tectonic erosion long term rates of crustal erosion in the region of lima basin are greater than estimates of regional arc magmatic productivity implying that such margins are net sinks of continental crust

the structure of southwest taiwan the development of a fold and thrust belt on a margins outer shelf and slope d b j a m and c b acknowledge funding provided by the spanish ministerio de economia y competividad grant cgl2013 43877 p h k c acknowledges funding by most 104 2628 m 008 005 my3

the permian to jurassic stratigraphy and structural evolution of the central cheshire basin new seismic reflection data reveal details of the permo triassic basin fill in the central cheshire basin the seismic stratigraphy indicates that the permian and triassic succession is far thicker than previously estimated seismic data and backstripping of the preserved permian to early jurassic sedimentary succession suggest that at least two phases of fault controlled subsidence occurred in early permian and early triassic times an intra triassic unconformity is seen on the seismic data near the top of the sherwood sandstone group this may equate with the hardegsen unconformity it is possible that this unconformity was generated by syn extensional regional uplift accompanying lithospheric thinning the gross morphology and location of the faults on the eastern margin of the cheshire basin are interpreted to be at least locally controlled by extensional re activation of a westwards dipping reverse fault possibly an extension of the pontesford linley lineament

mechanical stratigraphy and the transition from trishear to kink band fault propagation fold forms above blind basement thrust faults a discrete element study abstract we employ a discrete element technique to investigate the influence of mechanical stratigraphy on the transition from trishear to kink band fault propagation fold forms above a blind basement thrust fault we find that a classic trishear fold consisting of a broad upwards widening monocline with limb dips increasing towards the fault tip occurs where the cover stratigraphy is weak and has no strong mechanical layering in contrast we find that marked bedding parallel mechanical layering in the cover promotes much narrower kink like fold geometries with more constant limb widths and dips we discuss the modelling results in terms of their implications for structural interpretation and the application of kinematic models of fault related folding in contractional settings

co seismic ruptures of the 12 may 2008 ms 8 0 wenchuan earthquake sichuan east west crustal shortening on oblique parallel thrusts along the eastern edge of tibet abstract the m s 8 0 wenchuan earthquake which devastated the mountainous western rim of the sichuan basin in central china produced a surface rupture over 200 km long with oblique thrust dextral slip and maximum scarp heights of 10 m it thus ranks as one of the world s largest continental mega thrust events in the last 150 yrs field investigation shows clear surface breaks along two of the main branches of the ne trending longmen shan thrust fault system the principal rupture on the nw dipping beichuan fault displays nearly equal amounts of thrust and right lateral slip basin ward of this rupture another continuous surface break is observed for over 70 km on the parallel more shallowly nw dipping pengguan fault slip on this latter fault was pure thrusting with a maximum scarp height of 3 5 m this is one of the very few reported instances of crustal scale co seismic slip partitioning on parallel thrusts this out of sequence event with distributed surface breaks on crustal mega thrusts highlights regional ew directed present day crustal shortening oblique to the longmen shan margin of tibet the long rupture and large offsets with strong horizontal shortening that characterize the wenchuan earthquake herald a re evaluation of tectonic models anticipating little or no active shortening of the upper crust along this edge of the plateau and require a re assessment of seismic hazard along potentially under rated active faults across the densely populated western sichuan basin and mountains

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

seismic structure and the active hellenic subduction in the ionian islands in the region of the ionian islands of western greece the active margin of the hellenic domain passes from oceanic subduction in the south to continental collision in the north linked by the right lateral cephalonia transform fault a slightly landward dipping interface revealed at 13 km depth by a single previous line in the channel between cephalonia and zante has been suggested as the interplate subduction boundary new marine multichannel reflection profiles and obs refraction and wide angle reflection data confirm the reflector as a regional feature these data evidence its extension to the south where large low angle thrust earthquakes occur offshore to zante the new profiles establish a coincidence between the focal depths of these large subduction events and the imaged bright reflective level confirming its tentative interpretation as the interplate boundary which generally appears with a positive reflection polarity in this context the ionian islands outcrop corresponds to a shallowing of the interplate boudary from south to north in the south offshore zante the interplate boundary comprises a stratified zone that may be considered as the sedimentary cover of the ionian basin oceanic like crust which forms the lower plate here the shallower position and single cycle reflection character of the interplate further north suggest that the lower plate could there be the apulian paleomargin to that basin

numerical modeling of trishear fault propagation folding in contrast to kink band migration modeling methods trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges modeled fold hinges tighten and converge downward within a triangular zone of distributed deformation which is focused on the fault tip such features have been reported from field studies and are also seen in analogue models of compressional deformation however apart from its initial application to laramide folds little quantitative work has been undertaken on trishear fault propagation folding in other settings in addition no study has been undertaken into the growth strata which might be associated with such structures this paper uses an equivalent velocity description of the geometric model of trishear together with models of erosion and sedimentation to investigate trishear fault propagation folding of both pregrowth and growth strata the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement the models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata when total slip on a structure is high the model predicts overturned pregrowth and growth strata during the initial stages of deformation beds in the forelimb thicken but later thin when they become steep or overturned the effect of variations in fault propagation to slip ratios on two dimensional finite strain in the models is assessed by the use of initially circular strain markers high fault propagation to slip p s ratios lead to narrow zones of high finite strain while lower p s ratios lead to more ductile deformation and broader zones of high strain in all cases hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault modeled structures are compared with natural examples

crustal structure beneath southern england from deep seismic reflection profiles seismic reflection events from recently acquired profiles in southern england are interpreted in terms of stratigraphic and structural interfaces between rock units in particular evidence is brought forward that a series of reflection events dipping between 22 and 27 with azimuth averaging 160 represents 2 major thrust zones occurring in the variscan floor beneath mesozoic cover a geological model based upon borehole information and regional geological considerations is presented the tentative recognition of crystalline basement overlain and overstepped by late proterozoic rocks of variable thickness suggests that basement in this area was unstable from late precambrian times the mesozoic structural evolution can be related to tectonic re activation of major concealed thrust zones in the palaeozoic 9basement9

application of trishear fault propagation folding to active reverse faults examples from the dalong fault gansu province nw china abstract determining accurate fault slip rates at 1 ka to 1 ma timescales requires well constrained palinspastic reconstructions of dateable geomorphic and or geologic markers although general kinematic models have been developed to simultaneously reconstruct both bedrock e g bedding and fault attitudes and neotectonic markers e g strath terraces along active strike slip and thrust faults it is not clear if these models can also account for deformation along steeply dipping 45 reverse faults to address this problem we have investigated the active 50 dipping dalong reverse fault system this 40 km long fault system forms part of the aksai restraining stepover along the active left slip altyn tagh fault in northwestern china our geometric and kinematic analyses show that conventional fault bend fold models cannot satisfy the steeply dipping fault geometry we observe in the bedrock record likewise standard fault propagation fold models fail to match our measurements of a set of fluvial terraces however by expanding the trishear model of fault propagation folding to track both bedrock and neotectonic markers we are able to match both sets of records in particular we have developed trishear kinematic models for two sites liuchenzi and qingyazi using the numerical modeling program fault fold v 5 0 this work indicates that an important implication of active trishear fault propagation folding is that terrace deformation extends for over 1 km on either side of the fault trace thus to accurately measure the total magnitude of vertical separation between matching terraces in the hanging wall and footwall terrace profiles across active reverse faults must extend 1 2 km on either side of this zone of deformation

mechanical models of trishear like folds previous workers have formulated velocity descriptions of the trishear kinematic model of fault propagation folds which are inherently non unique we present two complete mechanical models of fault related folding and assess the validity of the assumptions used in the assignment of velocities in the trishear description and to eliminate the untenable situation of an infinite number of possible solutions for velocity fields the mechanical model of forced folding forced fold based on viscous folding theory is used to derive the velocity fields in an anisotropic sedimentary cover overlying faulted and displaced rigid basement blocks the solution of displacements around a stress free fault in an elastic body is used to model fault arrest folds that form around a fault imbedded in a deformed medium the results indicate that the velocity fields assumed in the trishear model more closely resemble the velocities derived in the mechanical forced fold model than the mechanical model of fault arrest folding the forced fold model produces a triangular region of concentrated deformation similar to the trishear region assumed in the kinematic models while the deformation produced by the fault arrest model is not concentrated within a triangular zone

continental crust crustal underplating and low q upper mantle beneath an oceanic island arc a detailed structural model of the crust subducting slab and underlying upper mantle across the northern izu ogasawara bonin island arc system is derived from a marine seismic reflection and ocean bottom seismographic refraction survey and subsequent forward modeling combined with tomographic inversion the model indicates that the crust is thickest beneath the presently active rift zone and a granitic crust may have formed in the mid crust a highly attenuative mantle that is one with low quality q seems to be confined mainly beneath the presently active rift zone in contrast high p wave velocity persists in the lower crust between the forearc and eastern margin of the back arc basin suggesting a large scale magma input responsible for the arc formation

decollement depth versus accretionary prism dimension in the apennines and the barbados 1 along representative cross sections of the apennines and the northern barbados accretionary prisms we measured the area the decollement depth the angle a of the upper envelope and the angle b of the dip of the regional monocline the continental sections of the apennines accretionary prism have a deeper decollement than the oceanic sections of the northern barbados 6 10 km depth and 1 km depth respectively because the sediment pile is thinner on the incoming barbados plate and its denser oceanic structure is more easily subducted considering the frontal 50 km the apennines have an average cross sectional area of 500 km 2 and the northern barbados ridge of 100 km 2 the total area is a function of the depth of the decollement plane therefore at a given amount of subduction the deeper the decollement depth is the bigger the area of the wedge will result assuming negligible compaction and erosion as a consequence the larger area volume and higher elevation of the apennines with respect to the barbados is determined by the apenninic deeper decollement despite these differences the geometry of both decollements is in some cases comparable in particular close to the boundary between the crystalline crust and the sediment pile where the main density and strength contrasts are concentrated variations in depth of the decollement occur moving along strike in both accretionary prisms the geometry of the prisms is further controlled by the different values of a and b their sum and the distance of the accretionary prism relative to the subduction hinge index terms 8099 structural geology general or miscellaneous 8150 tectonophysics evolution of the earth plate boundary general 3040 8122 tectonophysics dynamics gravity and tectonics keywords decollement depth accretionary prism apennines barbados citation bigi s f lenci c doglioni j c moore e carminati and d scrocca decollement depth versus

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

holocene shortening rates of an andean front thrust southern precordillera argentina abstract a significant part of the quaternary shortening between the chilean trench and the relative stable interior of the south american plate at the pampean flat slab 27 33 s has been accommodated at the eastern foothills of the andes and mainly within a narrow neotectonic belt along the eastern side of the argentine precordillera tectonic geodesy results point out that this area is being shortened at a 2 4 mm a rate whereas shortening rates estimated over longer time periods 1 20 ma suggest values ranging from 1 mm a to 16 mm a geomorphic and geologic evidence indicate that the east directed las higueras thrust system is one of the main structures that has accommodated quaternary deformation at this section of the andean orogenic front 32 05 32 35 s an outcrop exhibiting the thrust propagation into fluvial sediments allows the holocene shortening rates at the northern end of this structure to be estimated based on retrodeformation of holocene strata and radiocarbon dating of two charcoal bearing beds estimated shortening rates yielded mean values of 1 90 0 28 mm a for the last 4495 143 cal yr bp and 1 53 0 26 for the last 8245 48 cal yr bp these results pose some uncertainties due to the incompleteness of the exposed deformation at the hanging wall however they correspond to a key timescale which helps to bridge the gap between rates derived from the short term gps data and the long term permanent deformation rates obtained through geologic studies although the estimated rates suggest that slip on the thrust could have accelerated during the last 4 ka more data are necessary to reliably address this key issue

sks splitting measurements in the apenninic tyrrhenian domain italy and their relation with lithospheric subduction and mantle convection 1 we present a compilation of singular shear wave splitting measurements in the italian region we used the method of silver and chan 1991 to determine the splitting parameters of fast direction and delay time from broadband sks waveforms the stations analyzed were mainly deployed in temporary experiments a few are permanent mednet broadband stations the splitting parameters reveal the presence of strong seismic anisotropy in the mantle beneath italy the fast polarization directions define two anisotropic domains a tyrrhenian domain with e w prevalent fast directions and an apennines adriatic domain with nw se to nnw sse prevalent fast directions the stations along the boundary between these two main domains show fast polarization directions that vary for earthquakes coming from different back azimuths this variability is a sign of a complex upper mantle structure also shown by seismic tomography but allows us to constrain the depth of the anisotropic layer in the 100 250 km range at least in the boundary zone between the two domains data at station vsl sardinia suggest the presence of depth dependent anisotropic structure where the shallower signature is consistent with the tyrrhenian domain results and the deeper one is possibly related to the presence of a subhorizontal slab laying on the 670 km discontinuity we argue that the complex pattern of fast directions is strictly related to the upper mantle structure and is caused by asthenospheric flows induced by slab rollback and contemporary deformation of the overriding tyrrhenian plate

quaternary active thrusts scarps tested as deformation markers by trishear models in the southern precordillera of argentina abstract scarps related to quaternary active thrust splays propagating in alluvial bajadas of the southern precordillera of argentina 32 20 were studied through detailed topographic surveys in order to understand the reliability of these landforms as deformation markers at an outcrop scale geometric parameters of causative structures were used to constrain balanced cross sections through trishear modelling for shortening estimation and also with the aim of comparing the predicted geometry of these models with the scarp profile shortening estimation through retro deformation of scarps contours was also conducted as an alternative approach for calculating deformation affecting these alluvial surfaces results indicate that fold geometries predicted by trishear modelling do not fit with the present scarp shapes suggesting modifications and slope angle decrease of the front limb in order to match the modelled folds with the scarp profiles it is necessary to place constraints to fault parameters such as ramp angle and tip point position not compatible with field observations trishear models predict shortening values 8 to 15 times larger than those obtained through line retrodeformation of scarp profiles therefore scarp shapes at a mesoscopic scale in the study area are not considered a reliable deformation marker for shortening estimation of propagating thrusts through an unconsolidated cover

inversion tectonics of the variscan foreland of the british isles late westphalian inversion structures on the variscan foreland display wide variations in orientation that are the product of the underlying basement grain the orientation of the major basement lineaments in relation to the direction of maximum shortening also determined the severity of deformation of the inverted basins ne sw trending faults were oriented roughly perpendicular to the nw se to nnw sse direction of maximum shortening determined from thrust transport directions in the variscan orogen to the south as a consequence ne sw trending basins in the variscan foreland have been strongly inverted the silesian post rift fill has commonly been expelled from the basins and the syn rift sediments have been deformed by chevron folds dublin and bowland basins in contrast the n s and nw se trending faults were oriented more obliquely to the direction of maximum shortening and as a consequence display a significant component of oblique slip this oblique slip component is manifested as en echelon periclines and flower structures in general the n s and nw se trending basins were less strongly inverted and the silesian post rift fill has been retained

geomorphology kinematic history and earthquake behavior of the active kuwana wedge thrust anticline central japan 1 we combine surface mapping of fault and fold scarps that deform late quaternary alluvial strata with interpretation of a high resolution seismic reflection profile to develop a kinematic model and determine fault slip rates for an active blind wedge thrust system that underlies kuwana anticline in central japan surface fold scarps on kuwana anticline are closely correlated with narrow fold limbs and angular hinges on the seismic profile that suggest at least 1 3 km of fault slip completely consumed by folding in the upper 4 km of the crust the close coincidence and kinematic link between folded terraces and the underlying thrust geometry indicate that kuwana anticline has accommodated slip at an average rate of 2 2 0 5 mm yr on a 27 west dipping thrust fault since early middle pleistocene time in contrast to classical fault bend folds the fault slip budget in the stacked wedge thrusts also indicates that 1 the fault tip propagated upward at a low rate relative to the accrual of fault slip and 2 fault slip is partly absorbed by numerous bedding plane flexural slip faults above the tips of wedge thrusts an historic earthquake that occurred on the kuwana blind thrust system possibly in a d 1586 is shown to have produced coseismic surface deformation above the doubly vergent wedge tip structural analyses of kuwana anticline coupled with tectonic geomorphology at 10 3 10 5 years timescales illustrate the significance of active folds as indicators of slip on underlying blind thrust faults and thus their otherwise inaccessible seismic hazards

the driving mechanism of plate tectonics relation to age of the lithosphere at trenches sea floor age depth and age trench depth relations suggest that the oceanic lithosphere continues to thicken and subside with age beyond 80 ma if so slab pull fs and ridge push fr forces can be calculated from the age of the sea floor at trenches t and if the shear stress acting on the base of the plate increases with absolute velocity a the approximate force balance equation is dva2 svat32 rt where d s and r are constants related to drag slab pull and ridge push respectively the relation between velocity and age is va2 s vat12 r we have tested this model by linear regression using rate and age data for 15 trenches va mm a 2 9 20 32 vat12 26 250 the strength of this correlation r 0 98 is strong evidence in favor of the validity of this simple model these results suggest that ridge push is just sufficient to overcome drag at the base of the plate and does not contribute significantly to the motions of oceanic plates though the value of r is consistent with the motions of plates not attached to subducted slabs

a reinterpretation of the concealed variscan structure beneath southern england by section balancing recently chadwick et al 1983 presented an interpretation of the concealed variscan structure beneath southern england based on deep seismic reflection profiles we re evaluate the status of reflection events identified along some of the profiles and draw attention to major uncertainties in the interpretation presented the geological section of chadwick et al is admissible in that it portrays similar structures to those seen to the west in the mendips however the section is not a valid balanced section as it cannot be restored to a pre thrusting stratigraphy using the published seismic reflection data and borehole information we present a new balanced cross section of the variscan structures for which a total tectonic shortening of about 20 is calculated

uplift and late orogenic deformation of the central european variscan belt as revealed by sediment provenance and structural record in the carboniferous foreland basin of western poland the carboniferous foreland basin of western poland contains a coherent succession of late visean through westphalian turbidites derived from a uniform group of sources located within a continental magmatic arc detrital zircon geochronology indicates that two main crustal components were present in the source area of namurian a sediments they represent late devonian and early carboniferous ages respectively the detritus from westphalian d beds is much more diversified and contains admixture of late carboniferous zircons suggesting rapid unroofing of variscan igneous intrusions in the hinterland between namurian a and westphalian d times tectonic repetitions of tens of metres thick fault bounded stratigraphic intervals recorded in several wells provide evidence for compressional regime that occurred in the sw part of the carboniferous basin not earlier than during the westphalian c and produced nw se trending folds concordant with the structural grain of the adjacent ne part of the bohemian massif

viscosity of oceanic asthenosphere inferred from remote triggering of earthquakes a sequence of large interplate earthquakes from 1952 to 1965 along the aleutian arc and kurile kamchatka trench released accumulated stresses along nearly the entire northern portion of the pacific plate boundary the postseismic stress evolution across the northern pacific and arctic basins calculated from a viscoelastic coupling model with an asthenospheric viscosity of 5 x 10 17 pascal seconds is consistent with triggering of oceanic intraplate earthquakes temporal patterns in seismicity at remote plate boundaries and space based geodetic measurements of anomalous velocity over an area 7000 by 7000 kilometers square during the 30 year period after the sequence

new gps constraints on the africa eurasia plate boundary zone in southern italy 1 southern italy is a key area for understanding the tectonic processes in the africa eurasia collision zone we analyze new gps measurements carried out between 1994 and 2001 the results are presented in terms of time series trajectories and velocities of crustal motion as well as a geodetic strain rate field while central italy corsica sardinia and the tyrrhenian sea move like the eurasian plate the overall motion of the sicily rift zone region matches african plate motion unexpected are the north northwest directed motions of northeastern sicily and the eolian islands most striking on the deformation field are a north south oriented compression along the northern sicilian coast compression between apulia and northwestern greece as well as extension in the sicily rift zone and the interior of sicily

timing and nature of alluvial fan and strath terrace formation in the eastern precordillera of argentina abstract sixty eight 10 be terrestrial cosmogenic nuclide tcn surface exposure ages are presented to define the timing of alluvial fan and strath terrace formation in the hyper arid san juan region of the argentine precordillera this region is tectonically active and numerous fault scarps traverse quaternary landforms the three study sites marquesado strath complex loma negra alluvial fan and carpinteria strath complex reveal a history of alluvial fan and strath terrace development over the past 225 ka the marquesado complex q3 m surface dates to 17 3 ka whereas the loma negra q1 ln q2 ln q3 ln q4 ln and q5 ln surfaces date to 24 3 ka 48 2 ka 65 13 ka 105 21 ka and 181 29 ka respectively the carpinteria complex comprises eight surfaces that have been dated and include the q1 c 23 3 ka q2 c 5 5 ka q3a c 25 12 ka q3b c 29 15 ka q4 c 61 12 ka q5 c 98 18 ka q6 c 93 18 ka and q7 c 212 37 ka 10 be tcn depth profile data for the loma negra alluvial fan complex and carpinteria strath terrace complex as well as osl ages on some carpinteria deposits aid in refining surface ages for comparison with local and global climate proxies and additionally offer insights into inheritance and erosion rate values for tcns 10 10 4 10 be atoms g of sio 2 and 5 m ma 1 respectively comparison with other alluvial fan studies in the region show that less dynamic and older preserved surfaces occur in the carpinteria and loma negra areas with only younger alluvial fan surfaces preserved both to the north and south these data in combination with that of other studies illustrate broad regional agreement between alluvial fan and strath terrace ages which suggests that climate is the dominant forcing agent in the timing of terrace formation in this region

modern sand from obducted ophiolite belts sultanate of oman and united arab emirates abstract oman and the united arab emirates host the largest ophiolites on earth and are characterized by arid tropical climate and desert conditions they thus provide an unexcelled natural laboratory in which to study sediments derived from allochthonous oceanic lithosphere and to define an actualistic petrogenetic model for obduction orogen provenance spectacularly exposed in se arabia are two distinct obducted ophiolite sequences sama il and masirah that contrast widely in formation setting spreading rate detachment level thermal state during emplacement and relief masirah beach sand chiefly consists of basaltic diabase to metadiabase plagioclase gabbroic and dunite grains from progressively deeper seated levels of the multilayered oceanic crust dense minerals include diopsidic clinopyroxene and hornblende from gabbros epidote and tremolite actinolite from altered dikes and augite from basaltic lavas in contrast sand from the sama il ophiolite includes cellular serpentinite grains and en

end carboniferous fold thrust structures oxfordshire uk implications for the structural evolution of the late variscan foreland of south central england interpretation of commercial seismic reflection data from the west of banbury south central england has provided information on the late carboniferous structural evolution of the oxfordshire syncline end carboniferous structures are preserved in the footwall of the main basin bounding faults of the permo triassic worcester graben restoration of major pre triassic normal faults has allowed reconstruction of earlier compressional structures in contrast to the wnw ese trend of structures to the south in the variscan foreland fold thrust belt major faults and folds in oxfordshire are approximately n s trending the structures seen to deform westphalian d stephanian aged sequences on the seismic profiles are a combination of steep 30 40 dip fault bounded uplifts and less dominant thin skinned tectonics apparently showing transport to the east in the south some thin skinned structures are present and in the north cover sequences are uplifted in a narrow zone of deformation on reactivated basement faults the amount of shortening also changes along strike from c 1600 m in the south to c 950 m in the north reactivation of major basement faults with differential basement shortening accounts for the main structures localized detachment in the young and weak sediments close to the palaeo land surface explains the thin skinned structures seen on some of the seismic lines these end carboniferous structures in oxfordshire and structures in the malvern abberley hills represent the structural limits of a large n s striking intra stephanian uplift in the late variscan foreland it is suggested that the structures formed during a discrete phase of intra stephanian e w directed regional shortening prior to the final phase of movement in the variscan belt

repeated folding during late holocene earthquakes on the la cal thrust fault near mendoza city argentina in 1861 one of the most destructive earthquakes in the history of argentina destroyed the city of mendoza currently 1 million inhabitants the magnitude m s 7 0 earthquake is inferred to have occurred on the 31 km long la cal thrust fault which extends from mendoza to the north where it offsets an alluvial fan and small inset terraces along a well preserved fault scarp a trench excavated on a terrace that is vertically offset by 2 5 m exposes two main stratigraphic units separated by an erosional unconformity the coarse grained upper unit is deformed by three east vergent folds f 1 f 3 retrodeformation of these folds yields total displacements of 2 0 m 2 4 m and 0 5 m on the underlying fault splays respectively the displacement of 2 0 m recorded by fold f 1 is interpreted as the result of the fault rupture that caused the 1861 earthquake f 2 and f 3 were presumably generated during the penultimate event with an inferred magnitude of m w 7 0 although formation during two distinct ruptures cannot be excluded finite element modeling shows that coseismic folding above the tip of a blind thrust fault is a physically plausible mechanism to generate these folds a published luminescence age of 770 76 years which is interpreted to date the formation of the deformed terrace indicates that the two or possibly three scarp forming events occurred during the last 800 years the fine grained sediments below the erosional unconformity that contain evidence for at least one older earthquake are dated at 12 kyr our results indicate that elastic strain energy which is accumulating at the front of the precordillera today as shown by global positioning system gps data was repeatedly released during earthquakes on the la cal fault in the past hence the la cal thrust fault poses a serious threat to the city of mendoza

coseismic fault related fold model growth structure and the historic multisegment blind thrust earthquake on the basement involved yoro thrust central japan 1 we use high resolution seismic reflection profiles boring transects and mapping of fold scarps that deform late quaternary and holocene sediments to define the kinematic evolution subsurface geometry coseismic behavior and fault slip rates for an active basement involved blind thrust system in central japan coseismic fold scarps on the yoro basement involved fold are defined by narrow fold limbs and angular hinges on seismic profiles suggesting that at least 3 9 km of fault slip is consumed by wedge thrust folding in the upper 10 km of the crust the close coincidence and kinematic link between folded horizons and the underlying thrust geometry indicate that the yoro basement involved fold has accommodated slip at an average rate of 3 2 0 1 mm yr on a shallowly west dipping thrust fault since early pleistocene time past large magnitude earthquakes including an historic m 7 7 event in a d 1586 that occurred on the yoro blind thrust are shown to have produced discrete folding by curved hinge kink band migration above the eastward propagating tip of the wedge thrust coseismic fold scarps formed during the a d 1586 earthquake can be traced along the en echelon active folds that extend for at least 60 km in spite of different styles of folding along the apparently hard linked nobi ise blind thrust system we thus emphasize the importance of this multisegment earthquake rupture across these structures and the potential risk for similar future events in en echelon active fold and thrust belts

structural complexities in a foreland thrust belt inherited from the shelf slope transition insights from the alishan area of taiwan the alishan area of taiwan spans the transition from the platform with full thickness of the eurasian continental margin in the north to the thinning crust of its slope in the south this part of the foreland thrust and fold belt includes important along strike changes in structure stratigraphy and seismic velocities in this paper we present the results of new geological mapping from which we build geological cross sections both across and along the regional structural trend fault contour stratigraphic cutoff and branch line maps provide 3 d consistency between the cross sections minimum shortening is estimated to be 15 km with displacement overall to the northwest a p wave velocity model helps constrain the structure at depth by providing insight into the possible rock units that are present there p wave velocities of 5 2 km s point toward the presence of basement rocks in the shallow subsurface throughout much of the southeastern part of the area forming a basement culmination the changes in strike of thrusts and fold axial traces the changing elevation of thrusts and stratigraphic contacts and the growing importance of middle miocene sediments that take place from north to south are interpreted to be associated with a roughly northeast striking lateral structure coincident with the northern flank of this basement culmination these transverse structures appear to be associated with the inversion of eocene and miocene age extensional faults along what was the shelf slope transition in the early oligocene uplifting the margin sediments and their higher p wave velocity basement during pliocene pleistocene thrusting

deep structure of the southern apennines italy thin skinned or thick skinned 1 the deep structure of the southern apennines sa accretionary wedge is still debated since industrial seismic reflection and well data provide reliable constraints only to a depth of about 10 km as a consequence two directly linked questions regard 1 the shortening in the accretionary prism particularly within the buried apulian thrust units and 2 the degree of involvement of the lower plate basement i e the apulian crystalline basement to address these issues we have constructed a regional section along a recently released deep seismic reflection profile crop 04 which intersects the entire sa the resulting cross section adequately constrained to a depth of about 15 km has been framed in a geodynamic scenario characterized by the eastward roll back of the westward subducting apulo adriatic lithosphere on the basis of this section we speculate on the deep structure building both thin and thick skinned thrust models a cross check of these end members models against documented tectonic geophysical and geochemical features shows that the thin skinned model is generally more consistent with the available data the development of basement slices with thicknesses of tens of kilometers is unlikely while it remains possible that the apulian basement could have been involved with its upper few kilometers in the thin skinned model the total shortening of the allochthonous units i e apennine and apulian carbonate platforms and lagonegro basin is estimated to be greater than 280 300 km some 90 km of shortening can be attributed to the apulian thrust units

systematic variation in the depths of slabs beneath arc volcanoes summary the depths to the tops of the zones of intermediate depth seismicity beneath arc volcanoes are determined using the hypocentral locations of engdahl et al these depths are constant to within a few kilometres within individual arc segments but differ by tens of kilometres from one arc segment to another the range in depths is from 65 km to 130 km inconsistent with the common belief that the volcanoes directly overlie the places where the slabs reach a critical depth that is roughly constant for all arcs the depth to the top of the intermediate depth seismicity beneath volcanoes correlates neither with age of the descending ocean floor nor with the thermal parameter of the slab this depth does however exhibit an inverse correlation with the descent speed of the subducting plate which is the controlling factor both for the thermal structure of the wedge of mantle above the slab and for the temperature at the top of the slab we interpret this result as indicating that the location of arc volcanoes is controlled by a process that depends critically upon the temperature at the top of the slab or in the wedge of mantle immediately below the volcanic arc

palaeo stress fields in the variscan foreland during the carboniferous from microstructural analysis in the british isles abstract north of the variscan front the carboniferous series of the british isles is affected by mainly n s trending folds which are almost perpendicular to this e w oriented front flat and unconformable overlying permian strata demonstrate a variscan age for the deformation a microtectonic survey made in dinantian and to a lesser extent namurian calcareous layers has revealed two successive events according to chronological criteria the first event maximum stress e w trending led to the formation of n s folds with local reorientation in the vicinity of major pre existing faults this compression may have been active since the namurian it pre dates a second compressional event orientated n s which is related to the final emplacement of the variscan nappes

recognition of paleoearthquakes on the puente hills blind thrust fault california borehole data from young sediments folded above the puente hills blind thrust fault beneath los angeles reveal that the folding extends to the surface as a discrete zone 145 meters wide buried fold scarps within an upward narrowing zone of deformation which extends from the upward termination of the thrust ramp at 3 kilometers depth to the surface document the occurrence of at least four large moment magnitude 7 2 to 7 5 earthquakes on this fault during the past 11 000 years future events of this type pose a seismic hazard to metropolitan los angeles moreover the methods developed in this study can be used to refine seismic hazard assessments of blind thrusts in other metropolitan regions

the control of basin evolution on patterns of sedimentation and diagenesis an example from the mississippian great orme north wales the mississippian north wales platform is located on the margins of the east irish sea basin and has been little studied over the last 30 years the exposed visean limestones provide new insights into the deposition porosity evolution distribution of dolomitization and pb zn and cu mineralization on the north wales carbonate platform this is of relevance to the characterization of fault related dolomite hydrocarbon reservoirs and age equivalent mississippi valley type mineral deposits in particular the study demonstrates the intimate relationship between sedimentation basin scale tectonism and post depositional fluid flux depositional cyclicity is marked with metre scale upward shallowing cycles in which pervasive marine and meteoric calcite cements occlude matrix porosity and syndepositional fractures consequently subsequent burial diagenetic replacive dolomitization is matrix selective and cements are primarily restricted to fractures seven phases of dolomite are defined based on texture and cathodoluminescence petrography with phases d1 d3 as the most volumetrically significant dolomite phases d0 d2 are matrix replacive cross cutting stratigraphy and locally fingering along beds for several metres dolomite phases d3 d7 are hosted by faults and fractures and also line vugs evidence of telogenesis is recorded where burial diagenetic products are post dated by calcite cements precipitated from meteoric fluids dolomitization probably occurred during the mississippian and continued into the pennsylvanian pb zn mineralization is also interpreted to have occurred during the pennsylvanian associated with variscan tectonism overall the north wales platform displays a more complex paragenesis than age equivalent platforms in the pennine basin owing to multiple phases of burial and exhumation the study demonstrates the importance of linking burial history to detailed field and petrographical data to understand and predict the spatial and temporal controls on diagenetic processes and products within syn and post rift sequences

active fragmentation of adria the north african promontory central mediterranean orogen global positioning system gps velocities indicate that adria no longer behaves as a rigid tectonic indenter into southern europe and is divided into northwestern and southeastern velocity domains differential motions are recognized in a velocity field determined from the peri tyrrhenian geodetic array ptga and international gps service igs sites in the circum tyrrhenian region of the central mediterranean and published gps velocities from the eastern adriatic coast in a fixed eurasian reference frame ptga and igs site velocities in sicily and southern italy are as much as 10 mm yr in a northward direction similar to gps velocities along the eastern coast of the adriatic sea in contrast velocities in northern italy are small or statistically insignificant and similar to velocities in sardinia and corsica outboard of western adria the tectonic boundary dividing adria is seismically active and passes around the southern and eastern margins of the tyrrhenian basin crosses central italy extends into the adriatic sea and follows the western margin of the dinaride tectonic belt north to the gulf of venice the eastern margin of adria is approximately located and follows the axis of the central dinaric alps southeast to the hellenic arc southeastern adria has a velocity related to northward motion of africa whereas northwestern adria has negligible differential motion in the eurasian frame and now is part of the alpine collage of southern europe

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

transient and permanent deformation of central japan estimated by gps 2 strain partitioning and arc arc collision abstract deformation of central japan can be divided into two main components elastic deformation due to the interseismic locking of subduction thrusts and deformation associated with intra arc tectonics in a companion paper henry et al earth planet sci lett 2000 this issue we estimated an elastic deformation field based on the modeling of global positioning system gps strain rates in this study we show that the corrected field gps minus predicted elastic deformation is consistent with geological and seismological observations and should represent long term plastic deformation we use this corrected gps deformation field to describe the kinematics of partitioning of the philippine sea plate amurian plate amu convergence in relation with the izu bonin arc collision about one third of the relative philippine sea amur motion is transferred to the central japan crust north of the collision front this transfer induces high strain rates in southern kanto and in the northern japanese alps the latter region corresponds to the eastern boundary of the amu we propose that the independent motion of central japan results from the delamination and the underthrusting of the izu bonin arc lower crust and the upward bent oceanic slab

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

global azimuthal seismic anisotropy and the unique plate motion deformation of australia differences in the thickness of the high velocity lid underlying continents as imaged by seismic tomography have fuelled a long debate on the origin of the roots of continents1 2 3 4 5 some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth with horizontally polarized shear waves travelling faster than vertically polarized ones2 this azimuthally averaged anisotropy could arise from present day deformation at the base of the plate as has been found for shallower depths beneath ocean basins6 such deformation would also produce significant azimuthal variation owing to the preferred alignment of highly anisotropic minerals7 here we report global observations of surface wave azimuthal anisotropy which indicate that only the continental portion of the australian plate displays significant azimuthal anisotropy and strong correlation with present day plate motion in the depth range 175 300 km beneath other continents azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans we infer that the fast moving australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents

representative styles of deformation along the chelungpu fault from the 1999 chi chi taiwan earthquake geomorphic characteristics and responses of man made structures the chi chi earthquake provides dramatic evidence of the damaging effects of surface ground deformation to buildings lifelines and other facilities much of the building damage is associated with surface faulting and folding along the chelungpu thrust fault our detailed surveying at representative sites along the fault shows that the rupture commonly is a relatively simple 1 to 4 m high scarp with minor hanging wall deformation and localized but severe uplift folding and graben formation along the scarp crest for individual scarps the width of deformation is about 10 to 20 times the net vertical displacement distributed secondary faulting and folding on the hanging wall occurred as much as 350 m from the primary fault near the northern end of the rupture growth of a pre existing 1 km wide late quaternary anticline produced severe ground rupture along multiple thrusts and backthrusts but only minor tilting between fault strands the pattern of building damage coincides with the pattern of geologic deformation with severe damage along large fault scarps and lesser but still significant damage attributable to distributed secondary surface deformation on the hanging wall rupture related building damage on the footwall occurred next to the prerupture fault trace where the hanging wall bulldozed onto the footwall the width of this damage zone is related to the local horizontal shortening along the fault and generally is less than 10 m building zonation along reverse faults should account for this pattern of surface deformation in addition buildings with massive foundations locally influenced the style and location of near surface deformation producing variations in fault strike or accentuated secondary deformation on the hanging wall manuscript received 15 january 2001

inference of trishear faulting processes from deformed pregrowth and growth strata near surface strata deformed by propagation of a fault situated underneath these strata hold explicit records of fault activities the delicate deformed strata exposed in investigation trenches for paleoseismologic study conform to fault propagation folds growing above a blind thrust growth strata deposited subsequently across the coseismic fold scarps are preserved by onlap and unconformities information from these sedimentary and folding processes was used in conjunction with inverse modeling using the trishear model technique proposed by allmendinger allmendinger r w 1998 inverse and forward numerical modeling of trishear fault propagation folds tectonics 17 640e656 in estimating the times and amounts of slips this methodology becomes necessary especially when the extent of a deformed profile is insufficiently exposed to observation or the location of the fault tip is invisible the proposed method can deal with growth strata that can be selectively removed during fold restoration the interpretations on the deformation history obtained by inverse analysis and trench investigation were then compared the combination of these two approaches seemed to enhance and complement each other and allows a better and more complete understanding of the trishear faulting processes and is useful in seismic hazard assessment 2007 published by elsevier ltd

convergence vs retreat in southern tyrrhenian sea insights from kinematics 1 the tyrrhenian sea is an extensional basin opened by trench retreat and back arc extension during subduction of the calabrian slab in the last 10 12 my subduction is still active beneath the semost part of the tyrrhenian sea as testified by seismicity down to 500 km depth by analyzing seismicity and geodetic data together with recent tomographic images we define the present day situation an evident n s compressional regime prevails in the tyrrhenian region west of the aeolian archipelago while east of them a nnw sse extension is documented by focal mechanisms and gps data with a much smaller strain rate with respect to the past the transition between these two domains is accommodated by a n s discontinuity zone which runs from aeolian islands to mt etna with an extensional to strike slip deformation

the adriatic region an independent microplate within the africa eurasia collision zone 1 we use gps measurements and block modeling to investigate the present day deformation of the adriatic region whose kinematics within the nubia eurasia plate boundary zone is not well constrained and remains controversial block modeling allows us to compute rigid plate angular velocities while accounting for elastic strain accumulation along block bounding faults results suggest that the adriatic is a microplate adria and that the southern boundary with the nubia plate and the aegean domain may be located along the apulia escarpment and the kefallinia fault geodetic data alone cannot discriminate between a single block ap or a two blocks gdap description of adria but the gdap model predicts boundary slip rates that are in better agreement with observations from previous studies

structural trends in central britain from images of gravity and aeromagnetic fields digital image processing techniques have been used to analyse the regional gravity and aeromagnetic datasets in central britain colour and shaded relief images have been generated which convey information on both anomaly amplitude as colour and anomaly gradient as relief and highlight structural trends lineaments and textural contrasts not easily discernible on standard contour maps we have identified some of the more important features and have tried to relate these to the evolution of the crust in each region on a broad scale the images emphasize caledonoid ene trends to the north of the solway line and both tornquist se and caledonoid ne to ene trends to the south the pattern of magnetic anomalies over central england seems to define the extent of the shallow late precambrian early palaeozoic basement of the midlands microcraton and delineates crustal elements and characteristic trends within it magnetic lineaments and the grain of gravity anomalies trending in northeasterly and northwesterly directions on either side of the microcraton seem to relate to structures which originated during the evolution of the welsh and eastern english caledonides respectively where lower palaeozoic and precambrian rocks form the concealed basement as in most of england the gravity lineaments tend to reflect the influence of basement fractures on the subsequent pattern of sedimentation whereas many of the features on the magnetic images are more directly related to structures within the basement itself

mechanical models of fault propagation folds and comparison to the trishear kinematic model abstract faults propagating through the earth generate a wave of deformation ahead of their tip lines we have modeled this process to understand the relationship between fold geometry and fault propagation using finite element modeling fem we investigate the response of incompressible frictionless and frictional materials and a compressible frictional material with associated flow to vertical and dipping faults whose tip lines propagate at rates 3 3 5 times their slip rates the fold geometries finite strain and velocity fields in models with incompressible materials are very similar to those produced by the trishear kinematic model even though the latter uses a purely ad hoc linear velocity field furthermore when the trishear grid search is applied to the final geometry of the mechanical folds the best fit kinematic models have approximately the same propagation to slip ratio as was used in the fem experiments however when the compressible frictional material is used the mechanical models exhibit a main triangular shear band in front of the tip line and a conjugate shear band in the fold backlimb both migrating with the propagating tip line the conjugate shear band antithetic to the fault produces a gentle anticlinal back limb even though there is not a bend in the fault

quantitative analysis of movement along an earthquake thrust scarp a case study of a vertical exposure of the 1999 surface rupture of the chelungpu fault at wufeng western taiwan a vertical exposure across the principal thrust scarp of the 1999 mw 7 6 earthquake allows quantification offault slip the exposure is located on the active chelungpu fault near wufeng along the range front of the fold and thrust belt in western taiwan the 1999 surface ruptures at the wufeng site are characterized by a west facing 2 to 3 m high principal thrust scarp and an east facing lesser backthrust scarp we mapped a 15 mlong 5 m deep exposure across the principal thrust scarp and characterized complex deformation structures which include a main basal thrust fault a wedge thrust and a pop up anticlinal fold with two secondary opposing thrust faults the vertical displacement across the principal thrust scarp is measured directly from the offsets of the same sedimentary horizons between the hangingwall and the footwall the average vertical displacement is 2 2 0 1 m and the maximum displacement is 2 5 m at the crest of the small pop up fold horizontal displacement estimates were determined using line and area balancing methods with line length methods we estimated a horizontal displacement of 3 3 0 3 m across the principal scarp for four sedimentary horizons for area balancing first we selected three horizontal soil sand deposits with a total thickness of about 0 5 m the estimate yields an average horizontal displacement of 4 8 1 0 m using these individual and relatively thin stratigraphic layers yielded significant standard deviations in displacement estimates as a result of thickness variations second we used the 3 mthick overbank soil sand and the lower part offluvial pebble cobble to calculate a horizontal displacement of 2 6 0 2 m with the area balancing technique according to the geometry of the dip angle 35 408 of the basal thrust the line length measurement and the 3 m thick package area balancing both providedreasonable results of horizontal displacement by comparing thedifferent deposits applied to theline and area balancing methods we interpret that decoupling of deformation occurred between the lower fluvial gravels and the upper overbank sand and mud deposits due to lesser confining pressure at the surface additional deformation occurred in the upper 1 2 m thick overbank deposits this additional deformation yielded further vertical uplift of 0 3 0 5 m and horizontal displacement of 0 2 0 8 m around the core of the pop up fold our work suggests that determination of slip across surface thrust ruptures varies as a function of the mechanical behavior of young late quaternary deposits q 2003 elsevier ltd all rights reserved

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

first deep seismic reflection images of the eastern alps reveal giant crustal wedges and transcrustal ramps 1 the alps are considered as a classical example for an orogen created by continental plate collision in this study we present new images obtained from deep seismic reflection profiling in the eastern alps between munich and venice which give rise to examine and revise existing concepts the seismic sections exhibit a prominent bi verging reflection pattern at crustal scale a major ramp like structure outcropping at the inn valley fault can be traced southward over 80 km into the mountain root where relics of the subducted penninic ocean are expected new models of the evolution of the eastern alps show an upper lower crustal decoupling along transcrustal thrust faults with opposite thrust directions of both the european and the adriatic african continents

growth of wheeler ridge anticline california geomorphic evidence for fault bend folding behaviour during earthquakes abstract unique landforms which we term fold growth terraces and alluvial fan ridges are interpreted to be produced by kink band migration of the front limb of wheeler ridge anticline an active fault bend fold in the southern san joaquin valley based on a folding related model these features form by onlap of alluvium across an active axial surface along the tip of a thrust wedge structure alluvial fan ridges record the sense of slip on the underlying blind thrust which is composed of both thrust and left lateral components our model predicts formation of fold growth terraces by upward propagation of an active axial surface through onlapped alluvium during a folding event to produce a prism of translated but otherwise undeformed sediment alternative hypotheses for the terraces include flexural slip faulting solifluction and erosive processes the width of fold limb between adjacent terraces is equivalent to fault displacement at depth which we interpret to be related to slip caused by earthquakes on the underlying blind thrust earthquake magnitudes derived from the terraces are much larger than expected for the length of the active fold and thrust belt and may indicate a kinematic link with the san andreas fault that drives shortening in the region

seismic images of modern convergent margin tectonic structure in 1980 a w bally assembled and edited an innovative three volume picture and work atlas of seismic reflection record sections this compilation of more than 130 excellent seismic section reproductions provided much new data from the earth s subsurface for petroleum geologists academic scientists and students it is the field geologist s counterpart of outcrops and type sections the smaller collection of seismic sections presented in this studies 26 publication was inspired by bally s atlas and follows his general format and philosophy the objective is to provide a reference series of sections for the earth sciences and a vehicle for continuing scientific dialogue relating to modern convergent margins this publication s assembled sections represent a single facet of that dialogue by bringing together a series of exceptional seismic sections from the fronts of presently active convergent margins around the pacific

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

a new variscan deformation map of england and wales collections of bedding orientations from subareas of 550 km 2 are used to define bedding fabrics from the variscan fold belt and foreland in england and wales the bedding fabrics constructed from data taken from published geological maps 1 50 000 scale are analysed in terms of the scheidegger watson orientation tensor the calculated eigenvalues of the orientation tensor define the average structural trends including the best fit fold axis for each subarea the eigenvectors of the tensor allow the original sedimentary fabrics to be distinguished from those modified by deformation the progressive tectonic modification of these fabrics leads to the concept of fabric evolution paths and the extent of the modification is expressed by a deformation parameter although it is emphasised that this parameter is not solely a function of finite strain magnitude its variation is represented on a map which brings out different degrees of variscan tectonic modification of bedding fabrics this map delineates a variscan bedding fabric front corresponding to the northern boundary of a zone of pervasive and strong deformation in the foreland this new analysis allows the variation of variscan trends to be mapped and the significance of these trends to be quantified in terms of deformation magnitude 1999 elsevier science b v all rights reserved

on the post 25 ma geodynamic evolution of the western mediterranean during the neogene and quaternary western mediterranean geodynamics were dominated by the eastward migration of the apenninic arc and associated back arc basins the migration was controlled by retreat of the apenninic slab and was associated with boudinage of the lithosphere in the back arc area palaeo reconstruction of the kinematics of the arc suggests about 775 km of migration from the late oligocene to present along a transect from the gulf of lions to calabria a maximum of 135 km of n s converge occurred between africa and europe during the same time span the western mediterranean was thus mainly shaped by the migration of the slab related to west directed subduction it is hypothesized that minor n s convergence deformed the arc but was not the cause of its formation 1998 elsevier science b v all rights reserved

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses

diachronous variscan late orogenic collapse as a response to multiple detachments a view from the internides in france to the foreland in the irish sea