the effects of pre existing normal faults on thrust ramp development an example from the northern apennines italy the umbria marche foreland fold and thrust belt in the northern apennines of italy provides excellent evidence to test the hypothesis of synsedimentary structural control on thrust ramp development this orogenic belt consists of platform and pelagic carbonates late triassic to miocene in age whose deposition was controlled by significant synsedimentary extension normal faulting mainly active from jurassic through late cretaceous paleogene time resulted in significant lateral thickness variability within the related stratigraphic sequences by late miocene time the sedimentary cover was detached from the underlying basement and was deformed by east verging folds and west dipping thrusts two restored balanced cross sections through the southernmost part of the belt show a coincidence between the early synsedimentary normal faults and the late thrust fault ramps these evidences suggest that synsedimentary tectonic structures such as faults and the related lithological lateral changes can be regarded as mechanically important controlling factors in the process of thrust ramp development during positive tectonic inversion processes

extension in compressional orogenic belts the eastern alps extension in the eastern alps encompasses the following 1 extension accompanying simple shear crustal stacking subhorizontal stretching occurs along flat lying foliations and regionally consistent stretching lineations with associated high strains on a local scale stretching results from extrusion of ductile sediments squeezed between rigid basement blocks basement blocks during thrust sheet imbrication the overall geometric effect is crustal thickening 2 classical extensional tectonics extension first occurs as a consequence of accretion and underthrusting of continental and oceanic material and represents gravitational adjustment of an unstable orogenic wedge extension then occurs as a consequence of terminal continental collision and is accommodated by lateral extrusion of crustal of crustal blocks the overall geometric effect is crustal thinning extension is generally oriented subparallel to the strike of the orogen

surge zones in the moine thrust zone of nw scotland abstract the caledonian thrust zones of assynt show several examples of large fault bounded structures surge zones up to 8 km 2 in extent which have moved further than adjacent rocks extensional faults can be traced into strike slip faults and then to contractional imbricate faults there are also zones of extensional and contractional flow as shown by strained bioturbation marks in the cambrian pipe rock several other low angle extensional fault zones have been recognized along the length of the moine thrust zone notably in the kinlochewe district recognition of these extensional faults and local surge zones has solved several local problems such as the lack of continuity of the glencoul thrust and the out of sequence character of some of the large low angle faults though the thrust propagation direction was generally from east to west in the transport direction several of the eastern faults have been reactivated later and locally cut down as extensional faults the so called moine thrust shows extensional fault movement at several localities along its length the extensional structures and the surge zones suggest that body forces have been important in driving the faults rather than just a push from the rear the moines and moine thrust zone were presumably driven to the wnw by gravity spreading and thinning of the main scottish caledonides

from mesozoic rifting to apennine orogeny the gran sasso range italy the apennines are a low temperature accretionary prism generated by the west directed subduction of the adriatic ionian plate whose structural origin is still to be fully understood the highest and best exposed seg ment of the apennines the gran sasso range is here documented to unravel the tectonic history of the northern tip of gondwana it is located along a ne verging salient of thrust sheets decoupling the sedimentary cover of the subducting adriatic lithosphere field mapping and structural analysis along the e w trending left lateral transpressive segment of the salient highlight the interplay of the inherited mesozoic passive margin stratigraph ic and tectonic framework with the neogene contraction the rheological differences between the massive carbonate platform and the well bedded turbiditic and pelagic limestones determined along strike undulations of the thrusts geometries and fold styles during shortening heterogeneities are due to inherited syn and post rift mesozoic tectonics the gran sasso overturned anticline shows a backlimb anomalously tilted toward the foreland and we infer this dip as being related to a deeper back thrust of a triangle zone the pinching out of the foredeep sequence on the growth anticline forelimb dates the contractional phases of the region to the late messinian from the late pliocene to present the area has been uplifted and extended about 2 km by oblique normal faults cross cutting the accretionary prism some of them are seismically active as shown by the 2009 mw 6 3 l aquila earthquake

the role of ductility contrast and basement architecture in the structural evolution of the crystal creek block mount isa inlier nw queensland australia abstract the leichhardt river fault trough of the mount isa inlier comprises fault blocks which exhibit different local deformational histories and structural orientations many structural patterns in the stratigraphically highest cover sequences do not occur in adjacent fault blocks that expose older and mechanically stronger rocks within the mount isa group sediments of the crystal creek block for example tight e trending upright folds appear to be at odds with the surrounding north south regional trends in this paper it is proposed that an inherited extensional fault geometry played a significant role in controlling the structural patterns developed during later shortening through the integration of detailed surface mapping and forward magnetic modelling an inverted half graben was identified beneath the crystal creek block exposing the underlying origin of local structural complexities structural inversion and buttressing against pre existing faults simultaneously removed extensional displacement across pre rift rocks and induced folds and faults in overlying sag phase sediments this led to the development of two structural levels characterized by different deformational styles and apparently distinct deformational histories

syn contractional overprinting between extension and shortening along the montagna dei fiori fault during plio pleistocene antiformal stacking at the central apennines thrust wedge toe

anatomy of late orogenic extension the northern apennines case abstract late orogenic extensional structures overprint the compressional structures of the northern apennines fold and thrust belt the tyrrhenian sea is the result of major continental extension and rifting which started in the thickened zone of the apulian corso sardinian collision it is believed to have commenced as early as in the late oligocene or early miocene in the corsica basin extension post dates the collision by approximately 20 ma the extensional stress field observed in the internal part of the northern apennines co exists with compression in the foreland part of the belt field borehole and refraction and reflection data were used to investigate the geometry of extension and how it affected the compressional abric in the northern apennines analysis of the collected data has revealed a complex geometry for the extension across the belt a geological cross section from the island of elba to the west of the italian peninsula to ancona in the east as well as a geodynamic model for the evolution of the northern apennines is presented and discussed from the early miocene stretching of the collision thickened crust created a n s rift which evolved into the present northern tyrrhenian sea typically two sets of extensional faults a low angle e dipping and a w dipping one are observed across the chain usually the w dipping faults have similar attitudes to those of the older reverse faults reactivating some of the thrusts as normal faults the e dipping normal faults however cut down section through the pre existing thrusts and in the internal part of the belt accommodate most of the displacement associated with extension a change in fault polarity occurs in the tuscan region and major faults dip towards the west a type subduction of the apulian crust under the corso sardinian block occurred after collision leading to the delamination of the apulian lithospheric mantle away from the crust collision induced delamination of the mantle lithosphere away from the zone of collision resulted in mechanical thinning of the lithosphere beneath the orogenic belt this process induced late orogenic crustal stretching in the internal zone of the northern apennines as the convergence between the apulian and corso sardinian blocks continued the extensional and compressional fronts accompanied by the delamination migrated from the hinterland towards the foreland of the orogen

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

the recognition of reactivation during continental deformation reactivation involves the accommodation of geologically separable displacement events intervals 1 ma along pre existing structures the definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings in neotectonic environments reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime this mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors four sets of criteria may be used to recognize reactivation in the geological record stratigraphic structural geochronological and neotectonic some structural criteria may not be reliable if used in isolation to identify reactivated structures much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend dip or three dimensional shape of structures numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long term and may be invoked to explain reactivation the collage of fault bounded blocks forming most continents therefore carries a long term architecture of inheritance which can explain much of the observed complexity of continental deformation zones

the relationship between seismic deformation and deep seated gravitational movements during the 1997 umbria marche central italy earthquakes abstract this paper re evaluates the origin of some peculiar patterns of ground deformation in the central apennines observed by space geodetic techniques during the two earthquakes of the colfiorito seismic sequence on september 26th 1997 the surface displacement field due to the fault dislocation as modelled with the classic okada elastic formulations shows some areas with high residuals which cannot be attributed to non simulated model complexities the residuals were investigated using geomorphological analysis recognising the geologic evidence of deep seated gravitational slope deformations dsgsd of the block slide type the shape and direction of the co seismic ground displacement observed in these areas are correlated with the expected pattern of movement produced by the reactivation of the identified dsgsd at least a few centimetres of negative line of sight ground displacement was determined for the costa picchio mt pennino and mt prefoglio areas a considerable horizontal component of movement in the costa picchio dsgsd is evident from a qualitative analysis of ascending and descending interferograms the timing of the geodetic data indicates that the ground movement occurred during the seismic shaking and that it did not progress appreciably during the following months this work has verified the seismic triggering of dsgsd previously hypothesized by many researchers a further implication is that in the assessment of dsgsd hazard seismic input needs to be considered as an important cause of accelerated deformation

tertiary extensional tectonics in tuscany northern apennines italy abstract two extensional events have been recognized in the tertiary evolution of tuscany the first event occurred in the period between the late oligocene and late tortonian and caused a stretching of 120 or greater the second event occurred between the late tortonian and quaternary periods and caused a stretching not greater than 10 the first event is characterized by the exhumation of middle crust rocks and therefore by the development of core complexes the most significant structures of the first event are low angle normal faults which characterize the upper plate of the core complexes and ductile shear zones which characterize the lower plate the low angle normal faults tend to die out in the late triassic evaporite level which constitutes the main detachment of the core complexes the main structures of the second extensional event are high angle normal faults which dissect all the previous tectonic features these faults border tectonic depressions which were the site of clastic sedimentation from late tortonian to quaternary time these normal faults may tend to flatten in a seismic lozenge shaped band explained as an active extensional shear zone which is located at the brittle ductile boundary a similar explanation is given for the detachment located in the late triassic evaporite level during the first extensional event while the first extensional event is in the framework of the northern apennine post collisional evolution the second extensional event because it affects an already stretched crust could be attributed to the southern tyrrhenian opening which began during late tortonian

normal faults in thrust sheets pre orogenic extension post orogenic extension or both abstract in fold and thrust belts that experienced both pre orogenic and post orogenic extension it may be difficult to establish whether observed normal faults pre dated post dated or were synchronous with thrusting geometrical structural patterns may be insufficient to constrain the relative chronology of extensional and contractional deformations the systematic use of kinematic criteria makes it possible to unequivocally define the timing relationships of reverse and normal fault development and hence to correctly unravel complex structural evolutions kinematic analysis in the southernmost umbria marche apennines of italy where both normal and thrust faults are present revealed a history of repeated tectonic inversion characterised by two distinct stages of extension separated by an episode of folding and thrusting structural overprinting relationships observed at thrust normal fault intersections were useful for i removing sequentially younger deformations and hence ii separating and quantifying the effects of orogenic contraction from those of both pre orogenic and post orogenic extension

the influence of intrabasinal faults on the development of a linked thrust system the mesozoic and cenozoic rocks exposed in the arve valley region of the external french alps are used to assess the role of early intrabasinal faults on later thrust fault evolution the early intrabasinal faults produced at some time from latest upper cretaceous to tertiary strike parallel or subparallel to later neo alpine thrusts where early faults dip away from the thrusts they are generally cut through and occasionally are overturned during this process in one example extreme overturning has allowed partial reactivation early faults dipping in the same direction as thrusts may a be reactivated b initiate ramping of the thrust ahead of the preexisting fault c be cut through by the thrust d cause pinning of the thrust at the footwall of the fault and folding against the fault as displacement continues buttressing from this work it is evident that intrabasinal faults exerted a major influence on the distribution of mechanical heterogeneities these heterogeneities include variations of stratigraphic thickness and type across faults fault related unconformities and the presence of the fault itself during the period of contraction such features strongly controlled the development of the stress field produced ahead of an advancing thrust and hence influenced the position of thrust fault propagation within the stratigraphy

multiple scales of mechanical stratification and decollement fold kinematics sierra madre oriental foreland northeast mexico regional to outcrop scale stratigraphic relationships within mesozoic shallow marine carbonates and clastics at sierra del fraile anticlinorium in northeast mexico play an important role in controlling the deformation distribution strain history and kinematics during folding structural analyses of meso and microscopic deformation features from all stratigraphic and structural positions within the anticlinorium allow for the delineation of five regional scale lithotectonic units kilometer to decameter scale stacking patterns of the preorogenic succession and the meter scale stacking patterns of individual shallowing upward carbonate cycles are shown to influence deformation kinematics in predictable ways mesoscopic faults and cleavage patterns along with penetrative strain from anisotropy of magnetic susceptibility ams fabrics constrain fold kinematics as progressive limb rotation about pinned anticlinal and synclinal hinges with constant limb length and a mobile basal decollement horizon within the evaporites of the minas viejas formation that core the anticlinorium ams fabrics are shown to be a useful proxy for nascent penetrative strain orientations in sedimentary rocks macroscopic differences in strain suggest strong lithologic facies control responding to differences in bed thickness grain size and carbonate content at the regional scale variation in deformation between the lithotectonic units identifies decollement horizons coincident with lithotectonic boundaries in the minas viejas upper taraises and parras formations

seismotectonic zoning in east central italy deduced from an analysis of the neogene to present deformations and related stress fields this paper concerns the space time evolution of the late miocene to present contractional and extensional deformations within east central italy with an analysis of the pattern and orientation of the related stress field an insight into the cover basement relations and a discussion of the seismotectonic implications the pattern and orientation of the deformation and stress fields have remained unchanged since late miocene time and have been always characterized by the coexistence and parallelism in contiguous areas in a given time of shortening to the east and elongation to the west with time during a multistage deformation history both the contraction and the extension fields have moved eastward basement involved tectonics with a west dipping multiple detachment system would have allowed the progressive eastward shift of crustal slices corresponding at the surface to well defined tectonic domains with consequent crustal thinning in the tuscan zone and crustal thickening beneath the apennine mountain chain by comparing macroseismic epicenters hypocenters and focal mechanisms with crustal structure and independently known active faults it appears evident that the pattern frequency and distribution of the crustal seismicity are tectonically controlled and that a contraction extension pair is active also at the present time in fact along the adriatic coastline contractional deformation prevails with involvement of pleistocene layers whereas beneath the apennine range extensional deformation prevails both t axes in the inner domain and p axes in the outer domain are oriented southwest northeast or west south west east northeast and this coaxiality reflects the situation depicted by the geologic stress field on this basis we have produced a seismotectonic zoning of east central italy with three distinct domains intramountain seismic zone isz foothills seismic zone fsz and coastal seismic zone csz a plausible explanation for the genesis of the active seismic deformations of east central italy is that the locally intense extensional upper crustal seismicity of the intramountain seismic zone is confined to the hanging wall of a major west dipping active extensional discontinuity whereas the less frequent contractional seismic activity of the coastal seismic zone is within the hanging wall of an outer major active west dipping contractional discontinuity an area of overlap between these two zones is represented by the foothills seismic zone where superposed epicentral patterns from different surfaces of weakness at different depths are the predictable result of the asymmetry of the deformation

elastic stress relaxation a mechanism for opposite sense of secondary faulting with respect to a major fault abstract the motion along a major fault perturbs the local stress field and produces numerous secondary fractures among these the secondary fractures that have the opposite sense of motion with respect to the main fault are particularly intriguing for example the right lateral reverse motion of the ms 7 1 loma prieta earthquake of 18 october 1989 produced a series of left lateral normal fractures located in a narrow zone about 1 km in widt subparallel to the trace of the san andreas fault through a rigorous inversion of geodetic data associated with the earthquake that allowed the slip to vary on the rupture surface we studied the origin of the left lateral normal fractures by comparing the location orientation and slip sense of the observed fractures with the perturbed elastic stress field calculated from the estimated slip distribution of the earthquake the perturbed state of stress at the earth s surface is complex and the greatest principal stress changes from horizontal to vertical the area of left lateral tensile state of stress coincides with that of the observed left lateral normal fractures the magnitudes of the induced fault normal tensile stresses of 60 120 bars and left lateral fault parallel shear stresses of 0 4 bars in the area are large enough to produce the ground failures thus the observed left lateral normal fractures adjacent to the san andreas fault are interpreted to be related to coseismic elastic stress relaxation the same mechanism can also be used to explain the normal faulting in the upthrown blocks of the thrust faults which produced the 1980 el asnam algeria and the 1968 tokachi oki japan earthquakes

the influence of pre existing thrust faults on normal fault geometry in nature and in experiments relations between normal faults and pre existing thrust faults are classically described in terms of three basic situations normal faults can cross cut thrust faults they can branch out from thrust faults at depth on a de collement level or they can entirely reactivate thrust planes the mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the brittle rocks and silicone putty an interlayered de collement the models underwent compression producing thrust faults with variable dips followed by extension three possible ways of interaction are described here a no interaction occurs in the case of low dip thrust faults 32 1 and normal faults are developed independently displaying a listric geometry b branching at depth on the de collement level occurs when dip of the thrust faults reaches 32 1 in this case the dip of the normal faults whose geometry becomes planar decreases with increasing thrust dip we suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre existing fault zone where a drop in the friction coefficient is likely to occur c reactivation occurs in brittle material when dip of the pre existing fault exceeds 41 1

models of strain variation in nappes and thrust sheets a review abstract strain within nappes and thrust sheets can be modelled using simple boundary conditions an original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust deformation gradient tensors are determined for the models and solutions found for the principal strains a two dimensional model considers heterogeneous simple shear combined with a stretch in the direction of shear simple shear zones arise as a special case where 1 recumbent fold nappes characteristic of gravitational collapse can be modelled with 1 and thrust sheets with layer parallel shortening and folding with differential transport of nappes gives rise to additional wrench type shears solutions for the principal strains in this type of model are found and used to interpret side wall ramps and steep zones parallel to nappe movements by including a stretch in the transport direction these differential transport models produce prolate strains 1 and oblate strains finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history these predict possible crenulation of early thrust fabrics at ramps

comparison between reprocessed seismic profiles seismologic and geologic data a case study of the colfiorito earthquake area seismic reflection data are used to reconstruct the subsurface geologic structures below the umbria marche region in italy a highly seismogenic area with a recent history of seismic activity the 1997 1998 colfiorito sequence we reprocess three vibroseis seismic profiles acquired in the early 1980s for hydrocarbon exploration whose stacked sections were optimized for relatively deep oil targets on the reprocessed seismic profile closest to the epicentral area we construct the main reflectors to a depth of about 4 s two way time and compare this interpretation with the available hypocenters of the 1997 earthquakes the improvements in visualizing the shallow and deep reflections provide a better correlation between the reflectors and the observed surface structures as well as a better delineation of the basement rock geometry we find that part of the colfiorito sequence is localized around some of the reflectors in the reflection profile which we interpret as related to the active normal faults

using multi band insar data for detecting local deformation phenomena induced by the 2016 2017 central italy seismic sequence abstract in this work we exploit x and c band insar data for detecting local deformation phenomena induced by the 2016 2017 central italy seismic sequence our goal is to highlight the usefulness of multi band insar analysis for hazard assessing and rapid mapping purposes when in situ investigations are difficult or dangerous to be performed indeed local seismic induced effects such as landslides avalanches subsidence etc could severely impact the environment and the population in the surrounding of areas hit by earthquakes we focused on four areas named monte vettore podalla bolognola and cicconi where insar outcomes revealed how the main seismic events of the sequence activated several landslides and secondary faults interested by deformation of 2 3 cm along the satellite line of sight los the use of multi band insar data allows the observation of multi scale deformation phenomena with both different spatial resolution and coverage highlighting the limits and constraints of different sar sensors moreover it ensures the crosschecking of displacement patterns retrieved through different insar products especially when no ground truth or in situ ancillary data are available for validation purposes as a result the retrieved insar information can support the scientific community and the institutions in the management of crisis emergencies

mesozoic crustal thickening of the longmenshan belt ne tibet china by imbrication of basement slices insights from structural analysis petrofabric and magnetic fabric studies and gravity modeling this work first presents field structural analysis anisotropy of magnetic susceptibility ams measurements and kinematic and microstructural studies on the neoproterozoic pengguan complex located in the middle segment of the longmenshan thrust belt lmtb ne tibet these investigations indicate that the pengguan complex is a heterogeneous unit with a ductilely deformed nw domain and an undeformed se domain rather than a single homogeneous body as previously thought the nw part of the pengguan complex is constrained by top to the nw shearing along its nw boundary and top to these shearing along its se boundary where it imbricates and overrides the se domain two orogen perpendicular gravity models not only support the imbricated shape of the pengguan complex but also reveal an imbrication of high density material hidden below the paleozoic rocks on the west of the lmtb regionally this suggests a basement slice imbricated structure that developed along the margin of the yangtze block as shown by the regional gravity anomaly map together with the published nearby seismic profile and the distribution of orogen parallel neoproterozoic complexes integrating the previously published ages of the nw normal faulting and of the se directed thrusting the locally fast exhumation rate and the lithological characteristics of the sediments in the lmtb front we interpret the basement slice imbricated structure as the result of southeastward thrusting of the basement slices during the late jurassic early cretaceous this architecture makes a significant contribution to the crustal thickening of the lmtb during the mesozoic and therefore the cenozoic thickening of the longmenshan belt might be less important than often suggested

development of normal faults during emplacement of a thrust sheet an example from the lewis allochthon glacier national park montana u s a abstract geologic mapping in southern glacier national park montana reveals the presence of widespread e dipping normal faults within the basal portion of the lewis allochthon the displacement along the normal faults increases downward from less than 1 m at the highest exposure 200 300 m above the lewis thrust to a maximum of 200 m near or at the lewis thrust the normal faults are located below discrete bedding parallel shear zones associated with mesoscopic structures characterized by ne or sw trending striations on bedding surfaces and asymmetric e verging folds these shear zones lie directly below the e directed brave dog fault a major bedding subparallel fault within the lewis allochthon the shear zones are interpreted to have formed during the development of the brave dog fault striations on the brave dog fault normal faults and shear surfaces in the shear zones are consistent with the transport direction of the lewis thrust the kinematic compatibility of the normal faults with the lewis thrust the concentration of the normal faults along the basal part of the lewis plate and the increase in displacement along them toward the lewis thrust all suggest that their development was synkinematic with eastward emplacement of the lewis allochthon the normal faults may have formed as riedel shears r that accommodated a bulk simple shear strain within the thrust plate between the simultaneously moving subhorizontal brave dog and lewis faults

miocene unconformities in the central apennines geodynamic significance and sedimentary basin evolution abstract this paper shows the results obtained from an integrated study geology biostratigraphy and geochemistry carried out on the miocene edimentary deposits in central italy in order to define the timing of the sedimentary basin evolution this paper deals also with the causes of the unconformities recorded in these basins in the miocene deposits of the latina valley and the ernici simbruini mts several unconformities which distinguish different stratigraphic sequences have been recognized d 0 d 1 d 2 d 3 and d 4 for each unconformity a general description together with a geodynamical significance is provided in particular d 0 unconformity appears to be related to a regional tectonic event adria europe collision as a consequence the adria lithosphere folded and the area underwent a regional erosive event d 1 d 2 and d 3 unconformities have had a more local tectonic control since they represent the stratigraphic record of the migration of the apennines thrust belt foredeep system d 1 and d 2 unconformities are related to the late tortonian foredeep stage whereas d 3 is linked to the early messinian piggy back stage moreover the d 4 unconformity which took place during the messinian piggy back stage is strictly linked to the sea level drop of the messinian salinity crisis in this paper the genesis and evolution of a late tortonian foreland basin is also stressed latina valley foredeep basin finally taking into account sequence boundaries nannofossil biostratigraphy and geochemistry isotopic data a comparison with the curve of the 3rd order of the relative coastal onlap haq et al 1988 has been attempted in order to distinguish the unconformities controlled either by tectonic or eustatic processes

late paleogene extensional collapse of the cordilleran foreland fold and thrust belt the cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle eocene to early miocene ca 49 20 ma episode of crustal extension the sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern canada to central utah extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy structural relief and sole faults of preexisting thrust fold structures the sole faults dip 3 6 west above an undeformed precambrian crystalline basement and accommodated tectonic transport of a thick up to 20 km eastward tapering hanging wall during regimes of crustal shortening and extension the chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces dated movement on two extensionally reactivated thrusts the lewis thrust of northwest montana and southeast british columbia and the medicine butte thrust of southwest wyoming and northeast utah suggests that the hiatus between the end of crustal shortening in the early or early middle eocene and the start of extension in the early middle eocene was brief lateral spreading and extensional basin formation in the cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late paleogene cordilleran orogenic wedge is accomplished by applying the extensional wedge coulomb critical taper model the rapid drop in north america pacific plate convergence rate and or steepening of the subducted oceanic slab at ca 50 ma resulted in a large reduction in east west horizontal compressive stress in the cordillera as a result the cordilleran orogenic wedge was left unsupported and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca 20 ma subsequently crustal extension and magmatism during the basin and range event ca 17 0 ma overprinted much of this earlier phase of extension

the effect of shear load on frictional healing in simulated fault gouge we report on frictional strengthening healing in granular quartz gouge as a function of time of true stationary contact to distinguish between the slip dependent ruina 1983 and time dependent dieterich 1979 friction constitutive laws we designed tests similar to conventional slide hold slide shs tests except that shear load was completely removed prior to holds we find large healing values 0 033 0 054 for holds of 101 102 s compared with quasi static shs experiments and our data indicate time dependent weakening in contrast with strengthening observed from shs tests gouge layer compaction increases with increasing hold time comparable to observations from conventional shs tests our data indicate that purely time dependent processes have a minor influence on healing under the conditions studied and or that such effects are efficiently erased by particle rearrangement during removal reapplication of shear load the data are not adequately described by either the time dieterich or slip ruina dependent state evolution laws

the gubbio normal fault central italy geometry displacement distribution and tectonic evolution abstract normal faults within orogenic belts can be pre syn or post orogenic features we studied the gubbio normal fault central italy which is an example of a pre orogenic fault reactivated in a post orogenic stage the gubbio fault is a 22 km long fault bordering a quaternary basin and part of an active faults system in the umbria marche region central italy the interpretation of a set of seismic profiles enables us to reconstruct the fault geometry in detail and to measure displacement and throw distributions along the fault strike seismic data indicate that the gubbio fault represents an example of multiple reactivation at least a portion of the fault was active in the miocene and only a part of the total displacement was achieved in the quaternary the reconstruction of the fault geometry at depth shows that the fault is characterised by listric geometry the fault is also characterised by a bend along strike and structure contours show that this geometry is maintained at depth as the fault is commonly addressed as presently active the maximum fault dimensions are correlated to the maximum expected earthquake and the presence of the fault bend is discussed as a possible barrier to seismic ruptures propagation