tertiary extensional faulting and evolving ductile brittle transition zones in the northern snake range and vicinity new insights from seismic data new geologic mapping and high resolution seismic data from east central nevada shed light on the evolution of the northern snake range decollement nsrd and its relation to the present basins and ranges we suggest that the nsrd originated in oligocene time as a flat 20 km wide 7 km deep zone of decoupling between localized ductile stretching below and more widely distributed high angle normal faulting above younger east dipping normal faults cut the western part of the nsrd forming the tilted fault blocks and highly asymmetric grabens that constitute the duck creek range duck creek valley and schell creek range spring valley and folded the nsrd to its present antiformal geometry these youngest faults flatten into pervasive subhorizontal reflectors at a depth of 10 to 15 km that we interpret as the youngest ductile brittle transition

fluid inclusion evidence for minimum 11 km vertical offset on the wasatch fault utah the footwall of the wasatch fault in the corner creek area near salt lake city is hydrothermally altered and deformed quartz monzonite of the oligocene little cottonwood stock secondary fluid inclusions are associated with hydrothermal alteration minerals and structural deformation thermometric measurements of fluid inclusion characteristics indicate entrapment of a co2 h2o nacl fluid at minimum temperatures of 223 353 c and minimum fluid pressures of 900 2800 bar the 2800 bar fluid pressure is near lithostatic pressure at a depth of 11 km the minimum displacement of the fault that is required for exhumation of the observed alteration and fluid inclusions and more than three times greater than most previous estimates the large displacement estimate is supported by the occurrence of pyrophyllite in shales in the footwall by the petrology of metamorphosed shale surrounding an unroofed pluton in the footwall and by geologic reconstruction of the eroded footwall

seismic imaging of the solfatara crater campi flegrei caldera southern italy new evidence of the fluids migration pathways in the shallow structures abstract seismic imaging is used in this work to reconstruct the shallow structure of the solfatara crater a surface marker of deep magmatic activity inside campi flegrei caldera southern italy we analyze the 1d orthogonal seismic arrays deployed in nne ssw directions for 400 m through the solfatara crater during the recent active experiment ricen repeated induced earthquake and noise performed in the framework of the eu project medsuv seismic data have been edited in the pre stack stage by removing the noisy traces low frequency noise and reduce the ground roll phases then a velocity analysis has been carried out starting from the velocity spectrum to obtain a rms velocity model which has been followed by nmo residual statics and dmo corrections we used the post stack kirchhoff migration technique before the extraction of the energy root mean square envelope and sweetness attributes defining the highest amplitude zones the time gain attribute in order to interpret the deep reflectors and the variance attribute to define the faults discontinuities and chaotic zones amplitude versus offset avo analysis has been performed to identify the gas saturated zone using partial angle stacks which is a benchmark to verify the presence of gas the main contribution of this work is to provide a more detailed and improved seismic reflection image of the shallow depth of the solfatara crater the reliability of our velocity model has been assessed through the use of the vp velocity model previously obtained by the non linear bayesian technique this provides a final stack showing that the shallower minor faults and the extension of the major faults represent the main pathways for the uprising fluids we present the visual evidence for the fluids trapped in the shallower part of the crater at depths between 20 and 50 m below the surface of the crater

uplift of deep crust during orogenic extensional collapse a model based on field studies in the sogn sunnfjord region of western norway the western gneiss region wgr in norway experienced high pressure metamorphism during silurian devonian continent continent collision the eclogite bearing lower crust is separated from the middle and upper crust by major detachment zones formed during extensional collapse of the orogen formation of the devonian basins is related to the extension the footwall of the detachment zones comprises three structural and metamorphic zones the upper zone zone 1 is characterized by penetrative homogeneous down to the west simple shear developed under retrograde greenschist facies metamorphism zone 2 suffered inhomogeneous simple shear of the same polarity petrography and mineral chemistry data from the lower zone zone 3 show a record of initial eclogite facies metamorphism at 600 c and 16 kbar which was decompressed almost isothermally to amphibolite facies conditions at 550 c and 10 12 kbar both the eclogite and amphibolite facies metamorphism developed in a regime of pure shear with vertical shortening the rapid decompression records an approach of approximately 20 km to the surface related to uplift that was probably the result of the removal of a thickened thermal boundary layer in the mantle lithosphere the pure shear regime which developed initially in the lower crust was truncated by zones of simple shear as the lower crust was uplifted to middle and upper crustal levels extension by simple shear in the upper crust was rooted in the lower crust where extension occurred by pure shear the shear zones in zones 1 and 2 did not penetrate the pure shear regime of the lower crust a considerable amount of tectonic stripping of the orogenic welt predates deposition of the devonian sedimentary basins

tectonic denudation of the darwin metamorphic core complex in the andes of tierra del fuego southernmost chile implications for cordilleran orogenesis cordillera darwin in the southernmost chilean andes is a tectonically denuded metamorphic core complex similar to those widely developed in the north american cordillera detailed comparison can be drawn with the shuswap metamorphic core complex in british columbia and washington analysis of the geotectonic history of the darwin complex reinforces the view that the presence of tectonically thickened continental crust is critical for development of most if not all known core complexes and also suggests that inversion of a marginal basin during a compressional regime is one way of initiating such a crustal welt the cordillera darwin complex is unique in the andes and antarctandes this indicates that although low angle normal faulting occurs at high levels in convergent plate settings such as the central andes and the himalayas extensional denudation of deep crustal levels in active andean type orogens may require special tectonic circumstances uplift of the darwin complex began at about 70 ma in a localized extensional setting within the developing transform zone between the south american and antarctic plates this strike slip setting is analogous to some metamorphic core complexes in north america and possibly to a recently identified core complex in new zealand it is likely to be a common one for core complexes in convergent orogens

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

collapse basin a new type of extensional sedimentary basin from the devonian of norway pure shear and simple shear models provide interpretations of sedimentary basins formed by extension of crust and lithosphere of normal thickness devonian basins of western norway developed during collapse of the thickened caledonian crust and return to a crust and lithosphere of normal thickness because extension did not thin the lithosphere to less than its normal thickness no thermal subsidence phase followed the initial subsidence

mesozoic and cenozoic structural evolution of north oman new insights from high quality 3d seismic from the lekhwair area abstract this paper highlights the role of triassic jurassic extension and late cretaceous compression in the mesozoic cenozoic alpine structuring of north oman the syn post mesozoic regional structural evolution is usually documented as a succession of two stages of deformation the alpine 1 phase late cretaceous in age occurred in association with two ophiolite obduction stages semail and masirah ophiolites it was characterised by strike slip to extensional deformation in the north oman foreland basin sub surface the alpine 2 phase miocene in age was related to the continental collision responsible for both the zagros orogen and the uplift of the oman mountains the alpine 2 deformation was transpressional to compressional observation and interpretation of good quality 3d seismic in the lekhwair high area enabled the distinction of two earlier phases early mesozoic extension occurred concomitantly with the regional triassic to jurassic rifting developing jurassic age normal faults late cretaceous compression occurred prior to the main alpine 1 phase and triggered the inversion of jurassic seated normal faults as well as the initiation of compressional folds in the cretaceous overburden these early phases have been ignored or overlooked as part of the north oman history although they are at the origin of structures hosting major local and regional hydrocarbon accumulations

subduction and eduction of continental crust major mechanisms during continent continent collision and orogenic extensional collapse a model based on the south norwegian caledonides during continental collision in the middle silurian the thickness of the lithosphere under the caledonides of s norway was doubled by subduction of the western margin of baltica including the western gneiss region under laurentia crustal rocks of the baltic plate reached sub moho depths of near 100 km or more as inferred from the presence of coesite in eclogites isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage the subducted lithosphere experienced vertical constrictional strains as a result of slab pull by its heavy and cold root eduction of the deeply buried crustal material was initiated by decoupling of the thermal boundary layer in the subducted lithosphere isostatic rebound resulted in very rapid uplift 1 2 mm yr 1 and the deep crust was exhumed mainly by tectonic extensional stripping over a period of 30 40 myr the eduction was probably related to a rolling hinge footwall uplift mechanism and the early high pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels the model explains the present day normal crustal thickness under the exhumed deep rocks without necessarily invoking large scale lateral flow of material in the lower crust or igneous underplating

scandinavia s north atlantic passive margin 1 the tectonics of the eastern passive margin of the north atlantic are reexamined the scandinavian north atlantic passive margin includes not only the offshore exploration and basin domain but also large portions of the onshore domains of the scandinavian caledonides combined information from structural geology potential field data regional geology basin development and geomorphology made it possible to propose a new definition of the passive margin the rift shoulder is formed by linked faults defining the present rift flank these faults are the extensional structures located furthest to the east onshore norway sweden that can be linked to the rifting that led to the formation of the north atlantic this innermost boundary fault system ibf is formed by a set of normal west dipping crustal faults it extends over a distance exceeding 2000 km from the north sea across the caledonian mountain belt to the barents sea the passive margin width between the continent ocean boundary and the ibf ranges from 550 km in the south to over 700 km in mid norway to 165 km north of lofoten rifting and faulting on the ibf started in permo carboniferous and a succession of rift phases eventually culminated in continental breakup and the formation of the north atlantic basin development between permian and cretaceous was toward the future breakaway fault but faulting was also active on land from mesozoic to present onshore offshore crustal scale cross sections show the geometry of the passive margin the dip orientation of the major faults and the changes in crustal thickness

palaeomagnetism magnetic fabrics and the structural style of the hornelen old red sandstone western norway the old red sandstone of the hornelen district western norway forms part of a major fold terrane and the principal magnetization is associated with the late devonian to early carboniferous svalbardian solundian orogeny the present fault margins of the hornelen massif claimed by various authors to represent subordinate devonian splay faults associated with megashearing in the north atlantic or lateral ramps in a regional extensional tectonic setting are shown to be post devonian structures and the present outline of the hornelen massif is a mesozoic jurassic graben

extensional duplex in the purcell mountains of southeastern british columbia an extensional duplex consisting of fault bounded blocks horses located between low angle normal faults is exposed in proterozoic strata in the purcell mountains of british columbia canada this is one of the rust documented extensional duplexes and it is geometrically and kinematically analogous to duplexes developed in contractional and strike slip fault systems the duplex formed within an extensional fault with a ramp and flat geometry when horses were sliced from the ramp and transported within the fault system

mesozoic reactivation of variscan thrusting in the bristol channel area uk new seismic reflection data from the bristol channel area provide direct evidence of a major variscan thrust in the palaeozoic basement beneath the mesozoic bristol channel syncline the thrust has an average southerly dip of about 25 and it rises into the major fault the central bristol channel fault zone cbcfz at the northern margin of the bristol channel syncline the cbcfz is attributed to reactivation in extension of the variscan thrust probably during the early cretaceous there is no evidence for major synsedimentary movement on the cbcfz and the bristol channel syncline does not represent a rollover structure section restoration suggests that the cbcfz has cut up through a previously tilted sequence of triassic and jurassic strata pervasive nnw sse cross faults are interpreted as original variscan transfer faults that have been reactivated in the mesozoic and tertiary

strain migration during multiphase extension observations from the northern north sea many rifts develop through multiphase extension it can be difficult however to determine how strain is distributed during reactivation because structural and stratigraphic evidence associated with earlier rifting is often deeply buried using 2 d and 3 d seismic reflection and borehole data from the northern north sea we examine the style magnitude and timing of reactivation of a preexisting permian triassic rift phase 1 fault array during a subsequent period of middle jurassic to early cretaceous rift phase 2 extension we show that rift phase 2 led to the formation of new n s striking faults close to the north viking graben but did not initially reactivate preexisting rift phase 1 structures on the horda platform we suggest that at the beginning of rift phase 2 strain was focused in a zone of thermally weakened lithosphere associated with the middle jurassic north sea thermal dome rather than reactivating extant faults diachronous reactivation of the permian triassic fault network eventually occurred with those faults located closer to the middle jurassic to early cretaceous rift axis reactivating earlier than those toward the eastern margin this diachroneity may have been related to flexural down bending as strain became focused within the north viking graben and or the shifting of the locus of rifting from the north sea to the proto north atlantic our study shows that the geometry and evolution of multiphase rifts is not only controlled by the orientation of the underlying fault network but also by the thermal and rheological evolution of the lithosphere and variations in the regional stress field

low angle extensional faulting reactivated mylonites and seismic reflection geometry of the newark basin margin in eastern pennsylvania low angle 25 to 35 dips have been determined for the border fault of the newark basin near riegelsville pennsylvania on the basis of a vibroseis profile and two continuously cored drill holes across faults at the basin margin a group of moderately strong planar reflections in a zone 0 5 km thick in gneiss and carbonate rocks of the footwall block coincides with the updip projection of imbricate fault slices and mylonites associated with the musconetcong thrust system of drake et al 1967 contrasts in acoustic impedance among mylonitic dolostone and mylonitic gneiss and their protoliths determined from measurements on samples from a third cored hole are sufficiently large to account for reflections seen in the footwall block analysis of drill core and surface outcrops supports the conclusion that low angle extensional faulting in the early mesozoic was localized by reactivation of paleozoic imbricate thrust faults in the basement rocks extension in the northwest southeast quadrant was approximately perpendicular to the strike of the ancient thrust faults in eastern pennsylvania the data presented here are the most explicit three dimensional information obtained thus far in the eastern united states in support of the concept of fault reactivation in controlling formation of early mesozoic extensional basins

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

extension in the central metasedimentary belt of the ontario grenville timing and tectonic significance a laterally extensive zone of southeast dipping marble mylonites in the central metasedimentary belt cmb of the southern ontario grenville the bancroft shear zone records southeast directed extension and a displacement of about 10 12 km calcite graphite carbon isotope thermometry indicates that the maximum temperature at which shearing occurred was ca 450 c in contrast carbon isotope fractionations 13 c cc gr within the marble protolith consistently give temperatures of 650 700 c the bancroft shear zone postdates 1060 ma and older high grade shear zones in the central gneiss belt and the cmb that are associated with northwest directed thrusting comparison of the temperature data from the retrograde marble mylonites with cooling curves for the cmb gives a late grenvillian age range of 935 1010 ma for the extensional event the geometric relation between thrusting and normal faulting in the cmb is comparable to that in parts of the himalayas and southern tibet and similarly we interpret late normal faulting in the cmb to be a result of gravitational collapse of a thrust thickened crust in contrast to the himalaya tibet region however accretion in the grenville becomes older in the direction of regional thrusting

planar high angle faulting in the basin and range geodetic analysis of the 1983 borah peak idaho earthquake geodetic elevation changes record the broad scale deformation associated with the m 7 0 october 28 1983 borah peak idaho earthquake on the lost river fault the crest of the lost river range rose 0 2 m and adjacent thousand springs valley subsided 1 0 m in relation to reference points 45 km from the main shock epicenter the deformation was modeled by dislocations in an elastic half space a planar fault with uniform dip slip of 2 05 0 10 m dipping 47 o 2osw and extending to a vertical depth of 13 3 1 2 km fits the geodetic data best and is also consistent with the main shock hypocenter and fault plane solution the geodetic moment is 2 6 0 5 x 1019 n m 2 6 0 5 x 1026 dyn cm and the estimated static stress drop is 2 9 0 4 mpa 29 4 bars tests for coseismic slip on listric faults which flatten with depth and on detachments horizontal faults or shear zones showed fits to the geodetic data that are inferior to those for planar high angle faults no detectable coseismic slip occurred on the mesozoic white knob thrust fault although the low angle thrust sheet intersects the south end of lost river fault near the 1983 mainshock epicenter if the high angle lost river fault abuts a flat lying detachment fault or shear zone such a structure must lie at depths of 12 km near the brittle ductile transition where stick slip behavior gives way to creep the depth and geometry of faulting at borah peak is similar to that inferred from seismic and geodetic evidence for the 1954 m 7 2 fairview peak nevada and the 1959 m 7 3 hebgen lake montana events suggesting that if detachments are active at these localities they are deep and most likely slip by creep

the geometry of listric growth faults in the devonian basins of sunnfjord w norway thick devonian clastic sequences accumulated in fault bounded basins in w norway at the close of the caledonian orogeny and mark the beginning of a phase of crustal extension traditionally the eastern boundary fault to the devonian basins has been regarded as a thrust but cross sections based on the use of a branch line reveal it to be an extension fault there is clear omission of structural section across the boundary and by matching up metamorphic rocks in the hanging and foot walls of the boundary fault over 40 km of extensional dip slip can be measured at the n end of the fault the boundary fault is a flat lying listric fault with rollover anticlines in the hanging wall the fault also cuts up and down section along strike to form lateral ramps parallel to the movement direction these ramps appear as wrench faults in outcrop the ramp flat geometry of the fault may have been inherited from a precursor caledonian thrust fault which reversed its sense of slip in early middle devonian times to form the basins over 25 km of onlapping sediment accumulated in the largest basin during faulting in 14 ma with an average slip rate on the fault of 3 mm yr 1

the origin and evolution of complex transfer zones graben shifts in conjugate fault systems around the funan field pattani basin gulf of thailand abstract changes in dip province associated with conjugate fault sets produce complex transfer zones locally known as graben shifts between sets of convergent conjugate faults in the pattani basin 3d seismic data over a well developed graben shift geometry in the funan field area of the pattani basin has revealed details of the transfer zone development in map view the graben shift is seen as west dipping faults forming a wedge shaped incursion into a zone of east dipping faults the wedge boundaries trend ne sw and nw se at a high angle to the n s striking normal faults within the transfer zones faults only slightly overlap with their neighbours with individual faults tending to widen upwards the narrow regions of overlap indicate displacement transfer must occur between more widely separated faults not neighbouring faults the initial oligocene early miocene syn rift fault pattern appears to have been strongly influenced by pre existing basement trends as indicated by 1 the restriction of west dipping secondary faults to the present day area of the graben shift 2 the line of the ne sw boundary to the graben shift coinciding with fault segment linkage geometries in two major syn rift faults and 3 curvature of minor fault tips into the transfer zone the subsequent conjugate fault system developed during thermal subsidence shows that the dip asymmetry and map boundaries of the fault dip panels are strongly influenced by the syn rift fault geometry there is no indication of active strike slip faulting affecting the graben shift geometry fault displacement diagrams commonly show linkage between deeper and shallower displacement maxima indicating early late oligocene early miocene minor west dipping normal faults influenced the location and width of higher middle late miocene conjugate faults hence two stages of structural inheritance led to the middle late miocene graben shift geometry

the age and tectonic significance of dolerite dykes in western norway coast parallel dykes in sw norway primarily of permo triassic age have been linked regionally to the early tectonic evolution of the norwegian continental shelf we demonstrate from palaeomagnetic data mean declination 206 1 inclination 30 1 a 95 11 8 that dolerite dykes in the coastal sunnfjord region of western norway and immediately west of the devonian basins are also of permian c 250 270 ma age and not lower or pre devonian as previously advocated the sunnfjord dykes appear to be contemporaneous with dykes from sw norway at sotra 262 6 ma and the oldest dykes from sunnhordland 260 280 ma and geochemical data attest to a transition from sub alkaline to alkaline magmatism at the dawn of the mesozoic the sunnfjord dykes are not simple records of e w extension and magma intrusion but instead represent significant mid late permian time markers within a complex zone of fault activation and rejuvenation late mesozoic cenozoic magnetic overprinting mean declination 348 6 inclination 68 9 a 95 12 and metamorphic alteration documented by these dykes are directly dependent upon proximity to major e w brittle faults south of the hornelen devonian basin hence some motion and related fluid activity do post date dyke intrusion the e w high angle normal or oblique slip faults can be regionally traced offshore to the oygarden fault zone onshore these faults truncate the hornelen low angle detachment which in turn cuts folded d evonian strata these observations along with evidence for permian and late jurassic cretaceous extension from the nearby dalsfjord region demonstrate important reactivation of a late to post caledonian detachment and high angle fault system in western norway

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

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

mineralogical and microfabric characteristics of magnetite in the wuyang precambrian bifs southern north china craton implications for genesis and depositional processes of the associated bifs abstract precambrian banded iron formations bifs are widely distributed in the north china craton ncc among them the wuyang bifs located in the southern margin of ncc occur in the late archaean tieshanmiao formation and can be subdivided in two different sub types i quartz magnetite bifs qmb consisting of magnetite fine microcrystalline quartz and minor calcite and ii pyroxene magnetite bifs pmb composed of pyroxene fine microcrystalline quartz and subordinate feldspars both sub types display apparent discrepancies in terms of petrography and mineral composition as shown in electron backscattered diffraction ebsd images and micrographs magnetite grains from the qmb range in size from tens up to hundreds of m whereas magnetite crystals from the pmb can be up to a few tens of m across the x ray diffraction xrd structural data indicate that magnetite from both bif sub types is equiaxed cubic and was generated by sedimentary metamorphic processes the cell parameters of magnetite in the qmb are a b c 8 396 a and z 8 which deviate slightly from these of magnetite in the pmb a b c 8 394 a and z 8 the analytical results of raman spectroscopy analysis revealed micro structural signatures of both magnetite raman shifts near 552 cm 1 and 673 cm 1 and hematite raman shifts near 227 cm 1 295 cm 1 and 413 cm 1 in magnetite from both qmb and pmb the crystallinity degree is similar for magnetite micro structures but varies significantly for hematite micro structures oxygen fugacity fo2 conditions fluctuated during the recrystallization of magnetite in the qmb whereas no evident variation of fo2 occurred during the formation of magnetite in the pmb analytical results of laser ablation inductively coupled plasma mass spectrometry la icp ms show that the si al and mg abundances are higher in magnetite from the qmb whereas the ti and mn contents are more elevated in magnetite from the pmb magnetite composition also denotes that both bif sub types are sedimentary metamorphic origin whereas the deposition of pmb was also affected by volcanic activities overall data indicate that the differences in the depositional environment of each bif sub type are due to the involvement of volcanic eruption processes in the genesis of the pmb thus this paper indicated that the qmb was deposited by chemical deposition at the long term interval of volcanic eruptions and the pmb were the product of chemical deposition affected by the volcanic eruption

exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone 1 the nordfjord sogn detachment zone nsdz is widely cited as one of the primary structures responsible for the exhumation of norwegian ultra high pressure uhp rocks here we review data from the considerable volume of research describing this shear zone and compile a strikeparallel cross section along the nsdz from the solund basin in the south to the soroyane uhp domain in the north this cross section highlights several previously unrecognized patterns revealing a shear zone with top to the west asymmetric fabrics that 1 initiated at amphibolite facies 2 overprints metamorphic breaks and tectonostratigraphic contacts and 3 has a gradational continuum of muscovite cooling ages these patterns constrain the kinematic evolution of the nsdz and suggest a new three step model for the exhumation of norwegian u hp rocks the initial stages of exhumation were characterized by the rise of crustal rocks from u hp depths to the base of the crust by buoyancydriven mechanisms not specified in this paper mantle exhumation was followed by top to thewest normal sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation in the final stages of crustal exhumation top w brittleductile detachments soled into and partially excised this ductile shear zone dropping the devonian basins into contact with rocks of varying tectonostratigraphic levels this new interpretation of the nsdz is significant as it accounts for the extreme crustal excision observed in western norway using three sequentially overprinting structures active at different stages of uhp rock exhumation citation johnston s m b r hacker and t b andersen 2007 exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone tectonics 26 tc5001

extensional tectonics in the death valley area california transport of the panamint range structural block 80 km northwestward in the hypothesis presented here the panamint range is a structural block that was detached during late cenozoic time from underlying rocks that now form the black mountains and was transported tectonically an estimated 80 km to the northwest transport occurred along a single low angle westward dipping detachment fault or perhaps along a system of such faults the estimate of 80 km of transport distance is based on the apparent right lateral offset of late precambrian and paleozoic facies and thickness trends along the furnace creek fault zone that bounds the detached block on the northeast the death valley turtleback surfaces are considered to be gigantic mullions related to the detachment surface

baltica a synopsis of vendian permian palaeomagnetic data and their palaeotectonic implications torsvlk t h smethurst m a van der voo r trench a abrahamsen n and halvorsen e 1992 baltlca a synopsis of vendlan permmn palaeomagnetic data and their palaeotectonlc implications earth sci rev 33 133 152 in light of recent additions to the palaeozoic palaeo magnetlc data base particularly for the ordovlclan era a revised apparent polar wander apw path for baltica has been constructed following a rigorous synthesis of all late precambrlan perm lan data the apw path is characterized by two prominent loops firstly a late precambrlan camb rlan loop probably relating to a rifting event and secondly a younger loop relating to a mid silurian scandlan colhsion event these features imply major change in plate tectonic reconfiguratlon baltlca probably represented an individual continental unit m early palaeozoic times and was positioned m high southerly latitudes in an inverted geographic orientation in such a reconstruction baltlca was separated from the northern margin of gondwana by the tornqulst sea and from laurentla by the iapetus ocean the tornqulst zone is thus interpreted as a passive or dextral transform margin during the early palaeozoic while undergoing counter clockwise rotations up to 1 6 ma baltica drifted northward through most ol the palaeozoic except for a short period of southerly movement in late silurian early devonian times after colhslon with laurentla rapid movements in latitude up to 9 cm yr are noted m late precambrlan earl y palaeozoic times and slgmficant decrease in velocities throughout palaeozoic time probably reflect the progressive amalgamation of a larger continent by early devonian euramerlca and permian pangea times the tornqulst sea had a principal component of palaeo east west orientation hence it is difficult to be precise in the timing of when micro continents such as eastern avalonla and the european masslfs ultimately collided along the southwestern margin of baltica these micro continents are considered to have been peripheral to gondwana m high southerly latitudes during the early ordovlclan eastern avalonla clearly had rifted off gondwana by llanvlrn llanded o times and may have collided with baltlca during late ordovlclan times although the present available sdunan palaeomagnetlc data from eastern avalonia may suggest colhslon in late silurian times across the lapetus facing margin of baltica laurentla was s tuated in equatorml to southerly latitudes during most of the lower palaeozoic these continents collided in mid silurian times l e a first collision between southwestern norway and greenland scotla nd which gave rise to the early scandlan orogeny 425 ma in southwestern norway possible followed by a later but less dramatic scandlan event in northern norway at around 410 ma since baltlca was geographically reverted m early palaeozoic times the colhsional margin could not have been a margin that once rifted off laurentm as assumed in a number of plate tectonic models

detached strata in a tertiary low angle normal fault terrane southeastern california a sedimentary record of unroofing breaching and continued slip miocene sedimentary strata exposed in the eastern chemehuevi mountains southeastern california record development of an evolving low angle normal fault system the sequence includes more than 1 km of conglomerate and sandstone with rare interbedded monolithologic breccia and volcanic flows clasts of peach springs tuff in basal units indicate that this succession is younger than 18 ma these rocks have been displaced by a regionally extensive low angle normal fault the chemehuevi detachment and are folded and faulted structural reconstructions and the character of associated fault rocks indicate that the chemeheuvi fault was initiated at a low angle and that the footwall was progressively unloaded through thinning and displacement of its cover during extensional deformation the syntectonic sedimentary rocks described here provide evidence that movement continued on the gently dipping 15 sup 0 fault even after part of the fault was breached and the footwall eroded the conglomerates and sandstones were deposited by stream flow and debris flow on alluvial fans synsedimentary faulting is suggested by angular discordance below one monolithologic breccia bed and by local coarsening upward sequences clast types reveal progressive unroofing of hanging wall rocks to exposer the chemehuevi fault zone from which chloritic brecciated granite clasts were derived clasts weremore then derived from both the hanging wall and the footwall footwall debris being dominant high in the section distinctive clasts show that late displacement on this evolving fault system was on the order of 1 to 5 km less

cenozoic and mesozoic structure of the eastern basin and range province utah from cocorp seismic reflection data cocorp seismic reflection data collected from the eastern basin and range in west central utah provide information on cenozoic extensional tectonics mesozoic thrusting and their interrelationships those data show a series of remarkably continuous low angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15 20 km over that distance none of these events are significantly cut by any high angle normal faults a major detachment beneath the sevier desert can be traced from a surface zone of normal faulting to a depth of 12 15 km with a regional apparent westward dip of 12 tentative correlation of upper and lower plate events suggests 30 60 km of extensional displacement on this detachment whether this structure is a reactivated mesozoic thrust is uncertain west steepening splays off the end of the detachment reach depths of 20 km and may represent a major mesozoic ramp or zones of distributed ductile shearing during extension some events are interpreted to be mesozoic thrusts of which at least one beneath the house range has been reactivated during the cenozoic the snake range decollement dips gently east and has a sense of cenozoic displacement opposite to that of other cenozoic detachments farther east deep events are most numerous beneath the east side of the sevier desert where they occur to depths of 30 km at the top of or perhaps partly within the anomalously low velocity upper mantle

the role of fault reactivation and growth in the uplift of western fennoscandia new structural data published aft data and topographic data suggest that the latest cretaceous cenozoic uplift of western norway was associated with normal reactivation of the more trondelag fault complex reactivation focused along the base of today9s topographic rise with maximum displacements in the order of 2 3 km structural and aft data indicate that reactivation occurred along a displacement gradient with least normal displacement in the ne and increasing displacements towards the sw reactivation commenced in connection with the triassic to earliest cretaceous phases of rifting on the norwegian margin and probably continued through most of the tertiary the late cretaceous to cenozoic topographic development of the scandes mountains followed structurally induced templates such as those described from other areas of active normal faulting a fault model for the cenozoic uplift of the norwegian mainland provides a framework for provenance erosion and transport of sediments eventually deposited in the offshore post rift basins in the latest cretaceous palaeogene and neogene the asymmetric topographic profile of fennoscandia is reflected in the aft data and suggests that lithospheric flexure places a first order control on the shape of the scandes mountains and the central fennoscandian craton

application of spectral decomposition and seismic attributes to understand the structure and distribution of sand reservoirs within tertiary rift basins of the gulf of thailand the tertiary rift basins in the gulf of thailand are the major hydrocarbon producing areas of thailand the reservoirs in these basins are predominantly lower to middle miocene fluvial channels and overbank sands and these reservoirs are segmented due to post depositional faulting racey 2011 there are small closely spaced sealing faults which compartmentalize the sand reservoirs and are not easy to map on conventional full spectrum seismic data this fault system is oblique to the regional north south extensional fault system and is related to graben shifts transfer zones associated with changes in dip of graben provinces and zig zag map pattern of the regional north south faults kornsawan and morley 2002 most wells until now have been drilled on structures along the faults and it is common to encounter different sand facies with different fluid contents in closely spaced wells at the same depth intervals because of these structural and stratigraphic complexities it is required to develop geoph

structural evolution of the whipple and south mountains shear zones southwestern united states the whipple and south mountains of the southwestern united states have undergone a strikingly similar sequence of deformations in both ranges gently dipping mylonitic fabrics have been overprinted by successively more brittle structures associated with a low angle detachment fault kinematic indicators reveal that the mylonitic rocks brittle structures and detachment faults are kinematically coordinated and were all formed by top to the northeast shear the structural evolution of both areas can be explained in terms of major shallow dipping shear zones that accommodated tertiary crustal extension we suggest that detachment faults and associated zones of brecciation cataclasis and seismic slip were originally continuous downdip along the low angle shear zones into mylonitic gneisses formed below or near the ductile brittle transition as the mylonites were drawn out from beneath the brittlely extending upper plate they were progressively uplifted above the ductile brittle transition and were overprinted by successively more brittle structures

the tectono magnetic signature of the old red sandstone and pre devonian strata in the hasteinen area western norway and implications for the later stages of the caledonian orogeny the structural fabrics recorded from devonian rocks and substrate in the hasteinen area western norway show near e w trends in the devonian rocks these fabrics are a consequence of post depositional regional folding at a considerable depth presumably attended by the aquisition of an thermoviscous tvrm or thermochemical tcrm remanence group a post or possibly late syntectonic group a remanences declination 210 inclination 18 95 9 constitute the principal remanence in both devonian rocks and their substrate whereas a younger group b magnetization declination 047 inclination 59 95 7 relates to early mesozoic shearing the palaeomagnetic group a and structural data from the hasteinen and kvamshesten old red sandstone massifs favor a regional compressive n s shortening event svalbardian solundian which probably occurred during latest devonian times some recent extensional basin models appealing to listric normal faulting to account for both the sedimentation and the subsequent deformation of the western norwegian devonian rocks are rejected on the basis of inadequate structural information

heterogeneous stretching simple shear and basin development abstract the models of basin development which involve either homogeneous stretching of the whole lithosphere or displacement on a lithospheric scale shear zone are but two end members of a range of possible extensional models the homogeneous extension model thins the lower lithosphere beneath the thinned upper crust and superimposes a thermal subsidence basin on the earlier fault bounded basin the shear zone model offsets the zone of lower lithospheric stretching and thermal subsidence it is more likely that the zones of upper and lower lithospheric stretching will be heterogeneous and patchy but will often overlap in plan view this will produce localised uplift and subsequent thermal subsidence within the faulted basin and may explain many of the anomalies between the various stretching estimates made using different structural stratigraphic and geophysical techniques the model which combines heterogeneous lithospheric stretching and associated simple shear may explain 1 variations in dip of the major detachment zones in the large basins 2 variations in types of strain on or beneath the detachment zones 3 regional uplift of part of a basin to erode the earlier fault blocks and 4 the development of volcanism in basins with only low values of upper crustal extension the zone of stretched lower crust and lithospheric mantle may lie beneath the centre of the zone of upper crustal stretching or to one side it may be linked to the upper crustal zone by faults which dip consistently in one direction or by extensional systems which change their dominant dip direction with depth pulling out the mid crust as one or more wedges possible examples are discussed from the basin and range province and northwest european continental shelf

implications of post thrusting extension and underplating for p t t paths in granulite terranes a grenville example mineral zoning studies in several granulite terranes have yielded nearly isobaric retrograde pressure temperature p t paths with significant path dispersion explanations include magmatic underplating incomplete isostatic rebound following thrusting and cooling after tectonic denudation associated with crustal extension one dimensional thermal models help to interpret the p t t time paths generated by such processes basaltic underplating yields p t paths in good agreement with data for the grenville province of ontario but yields t t paths that disagree strongly with thermochronometry in addition seismic heat flow and gravity data from the grenville province are not consistent with a mafic lower crust calculations based on simple thrust models agree with the t t data but yield isothermal rather than isobaric initial p t paths and consistently underestimate the pressure at which peak temperature is achieved the only model we found that fits both the p t and t t data incorporates a post thrusting episode of tectonic denudation near the peak of metamorphism this model also suggests that the temperature dependence of diffusion in garnet may explain the difference between measured isobaric and isothermal paths in granulites and amphibolites

from orogen to passive margin constraints from fission track and u th he analyses on mesozoic uplift and fault reactivation in sw norway the post caledonian tectonic history and landscape evolution of southwestern norway are poorly understood primarily owing to the lack of onshore post devonian sediments to bridge this knowledge gap low temperature thermochronological techniques were applied to investigate vertical movements in the upper crust new apatite fission track and apatite and zircon u th he analyses on samples from southwestern norway yielded permian to jurassic triassic to cretac eous and carboniferous to triassic ages respectively thermal history modelling indicates rela tively high cooling rates 2 3 8 cm a 21 throughout permian to early jurassic times since the jurassic samples from coastal areas have remained close to the surface and were reheated to 30 50 8c during sedimentary burial in the cretaceous inland samples experienced lesser amounts of permo triassic exhumation continued to cool slowly 1 8 cm a 21 throughout the jurassic cretaceous and did not reach the surface until the cenozoic both fission track and u th he ages are offset across faults highlighting the importance of fault activity throughout the mesozoic in combination with previously published results the new data suggest that the geo morphological evolution of southwestern norway is closely connected to rift and post rift tec tonics related to north sea and north atlantic rifting the topographic relief was most likely repeatedly rejuvenated during periods of tectonic activity

a paleomagnetic pole position of late jurassic early cretaceous dolerites from hinlopenstretet svalbard and its tectonic implications abstract paleomagnetic examination of late jurassic early cretaceous dolerites hinlopenstretet reveal a mean paleofield direction d 181 i 71 95 4 2 k 117 9 17 sites 96 samples which is argued to represent a deuteric magnetization formed at the time of cooling the resultant paleopole 66 n 200 e dp 7 dm 8 accords with paleomagnetic poles of similar age for north america after taking into account post jurassic continental separation the result from the present study is interpreted to represent a late jurassic early cretaceous paleopole for eurasia the similarity of the paleopoles from cratonic north america and eurasia in its pre drift configuration at about 145 ma makes it reasonable to conclude that relative displacement due to extension between eurasia and north america prior to the onset of sea floor spreading must have taken place after 145 ma and before the start of sea floor creation in the cretaceous

overview of the cocorp 40 n transect western united states the fabric of an orogenic belt the cocorp 40 n transect of the cordillera of the western united states crosses tectonic features ranging in age from proterozoic to recent and provides an acoustic cross section of a complex orogen affected by extension compression magmatism and terrane accretion the key features of the transect centered on the basin and range province include 1 asymmetric seismic fabrics in the basin and range including west dipping reflections in the eastern part of the province and predominantly subhorizontal ones in the west 2 a pronounced reflection moho at 30 2 km and locally as deep as 34 km in the basin and range with no clear sub mono reflections and 3 complex dipping reflections and diffractions locally as deep as 48 km in the colorado plateau and sierra nevada the eastern part of the transect shot above known and inferred precambrian crystalline basement probably records features related to the entire history of the orogen locally perhaps as old as 1800 ma in this region major paleotectonic features probably controlled subsequent structural development in title western half of the transect however most reflectors are probably no older than mesozoic within the basin and range province there appears to be a strong cenozoic overprint that is characterized by asymmetric half grabens low angle normal faults and a pervasive subhorizontal system of reflections in the lower crust no one model of intracontinental extension is universally applicable processes that produce or are accompanied by thermal anomalies magmatism enhanced ductility and extension appear to be essential in developing a highly layered lower crust

brecciation related argon redistribution in alkali feldspars an in naturo crushing study abstract alkali feldspar thermochronologic modeling with the 40ar 39ar method has generated marked advances in knowledge of the mechanisms for argon diffusion in feldspars while the goal in many cases has been to extrapolate the observed and modeled argon behavior in feldspars to natural geological settings scientific debate surrounding the true feasibility of such extrapolations and indeed the validity of thermochronologic modeling in itself have provided much impetus to improve laboratory techniques to test and increase basic understanding of argon diffusion two cornerstones for the debate over the feasibility of alkali feldspar thermochronology for modeling natural geologic processes have been 1 is volume diffusion the main mechanism for argon movement in feldspars and 2 if volume diffusion is a viable mechanism does argon then reside in discrete domains within the feldspar lattice we describe a study of alkali feldspars from a profile through a well controlled brittle fault zone in western norway the feldspars document argon loss during deformation and strongly suggest the existence of argon domains within the feldspars at least during laboratory step heating the progressive change in the character of argon diffusion is recognizable in the logr ro diffusion data from the feldspars and is mimicked by physical changes observed optically in the feldspars through progressive degrees of brittle deformation modeling results indicate a reduction in size of the biggest domains and the appearance of smaller domains during the strongest stages of deformation whether or not this reveals the existence and the transformation of the domain structure in naturo is difficult to prove from our data alone but interestingly this behaviour corresponds directly to the physical optical appearance of more intense crack networks and subgrains in progressively more brecciated feldspars because the thermochronologic histories derived from modeling the feldspar data conform very well to the known tectonic history of the area the feldspars appear to have successfully retained physical optical and isotopic records of episodic tectonic processes operating from ductile through low temperature brittle regimes in rocks with a caledonian history overprinted by several later younger geologic events however because the cold brecciation is the last tectonothermal event recorded by these rocks it is impossible to truly test for the existence of diffusion domains in naturo argon loss appears to have been effective only in the most highly brecciated deformed samples where the combination of the connected crack network increased fluid flow and higher temperatures enhanced diffusion via fast diffusion pathways and thus volume diffusion from the lattice only minor argon loss occurred in zones of lower brittle strain although some development of cracks and brittle features is evident independent of the existence of diffusion domains this study highlights the possible pitfalls when cooling histories are deduced from brecciated feldspars in which age and diffusion charateristics have been decoupled while the geochronological memory has survived and is identical to that of nonbrecciated feldspars suggesting no loss and minor effects of deformation the diffusion characteristics have been completely transposed by brecciation and the appearance of new domains modeling feldspars with these latter characteristics would effectively utilise a new feldspar diffusion structure with an old relict age memory

numerical simulation of rifting in the northern viking graben the mutual effect of modelling parameters numerous basin modelling studies have been performed on the viking graben in the northern north sea during the past decades in order to understand the driving mechanisms for basin evolution and palaeo temperature estimations in such modelling it is important to include lithospheric flexure the values derived for the lithospheric strength by these studies vary considerably i e up to a factor of 30 in this study which is based on new interpretation of a regional transect we show that both the estimated value of the effective elastic thickness and the derived b profile are dependent on the assumed value of the depth of necking the use of models that implicitly set the level of necking at a depth of 0 km generally leads to an underestimation of the lithospheric strength and an overestimation of the thinning factors in the northern viking graben a necking depth at intermediate crustal levels gives results comparable to the observations extension by faulting is modelled to be a significant factor in conclusion rifting in the northern viking graben can be explained with various models of effective elastic thicknesses eet varying from 1 km for a zero necking depth to the depth of the 450 jc isotherm for an intermediate level of necking it is also shown that the development of the basin during the post rift phase cannot be explained by pure shear simple shear extension two mechanisms are proposed here to explain the post rift subsidence pattern namely intra plate stress and phase boundary migration the two extreme models for eet mentioned above 1 km for a zero necking depth to the depth of the 450 jc isotherm for an intermediate level of necking give very different responses to compressional stress the latter gives basically no response for realistic intra plate stress d 2004 elsevier b v all rights reserved

quantitative interpretation of seismic attributes for reservoir characterization of early middle miocene syn and post rift successions songkhla basin gulf of thailand abstract the major hydrocarbon reservoirs in the gulf of thailand including the songkhla basin are deposited throughout the syn and post rift section late eocene recent with the main reservoirs occurring in the early miocene to middle miocene section these reservoirs are predominantly fluvial channels with complex lateral and vertical geometries and distributions particularly as point bars overbank deposits and sand or mud filled channels this study characterizes the reservoir architecture and depositional environment of songkhla basin from late syn rift to post rift during the miocene by integrating the seismic attributes rms similarity and spectral decomposition well log interpretations and well biostratigraphic data two distinctive channel characteristics and patterns were interpreted a n s oriented meandering channel complexes aligned sub parallel to the western boundary faults developed during the late syn rift upper early miocene b ne sw oriented long narrow fluvial channels with tidal and marine influence developed in the post rift phase i middle miocene these channel orientations reflect changes in the basin architecture passing from the late syn rift stage to the post rift syn rift channel orientation was controlled by topography created by motion on the western boundary fault system while during the post rift stage channel orientations were affected by mild inversion on the boundary fault system and thermal subsidence which resulted in an eastward shift in the basin depocentre towards the flexural margin

the tectonic implications of solundian upper devonian magnetization of the devonian rocks of kvamshesten western norway the allochthonous old red sandstone of kvamshesten western norway records polyphase orogenic deformation and palaeomagnetic results from both the devonian sediments and mylonites associated with the basal thrust define a syn to post tectonic magnetization withd 218 i 3 and 95 9 7 the corresponding pole position lat 21 s long 324 e suggests a late devonian early carboniferous magnetization age solundian and probably dates the time of thrust movements

interaction of permo triassic and jurassic extensional fault blocks during the development of the northern north sea the main structural elements resulting from late permian early triassic extension have been resolved across the total width of the northern north sea the northern segment of the basin north of 61 n bounded to the west by the hutton fault and to the east by the norwegian mainland was occupied by four westerly tilted half grabens 25 35 km in width prior to jurassic extension the central segment of the basin 60 20 61 n exhibits a full graben beneath the jurassic horda platform flanked by half grabens tilted towards the basin margins the change in permo triassic fault block configuration defining structural domains is largely constrained by a basement discontinuity which runs diagonally ne sw across the basin maximum jurassic extension is represented by several centres of subsidence aligned nne ssw across a n s oriented and broader permo triassic basin jurassic extension was concentrated within the viking graben proper south of 60 n which in this area is represented by single westerly tilted en echelon half graben segments 20 25 km wide the area affected by jurassic extension becomes progressively wider to the north again governed by the basement discontinuity eventually encompassing the total width of the basin c 200 km north of 61 n the central segment of the northern north sea displays the greatest contrast in permo triassic and jurassic structural styles and basin geometry jurassic stretching and faulting was negligible to the east horda platform precisely the area which had been the site of maximum permo triassic stretching and fault activity it has generally been assumed that jurassic fault configuration along the eastern margin of the central segment of the viking graben to a large extent reflects the reactivation of major permo triassic faults this study demonstrates the interference of permo triassic and jurassic basin geometries in this area due to major faults of the two generations dipping in opposite directions as a consequence the e tilted oseberg fault block of jurassic origin and c 20km in width is underlain by a wider c 35 km and w tilted permo triassic fault block

the tectonic significance of pre scandian 40ar 39ar phengite cooling ages in the caledonides of western norway pre silurian continental margin deposits in western norway non conformably overlying allochthonous continental orthogneisses retain ordovician 40 ar 39 ar cooling ages for phengites implying either rapid cooling immediately after a late ordovician orogenic event or less likely a slow cooling following an early ordovician or older orogeny the dalsfjord suite hoyvik group basement cover pair are probably a lateral equivalent to late proterozoic sandstones sparagmites covering the jotun nappe gneisses of the middle allochthon in central south norway the hoyvik group underwent polyphase deformation greenschist facies metamorphism t max 40 ar loss probably occurred during subsequent scandian deformation and late to post orogenic extension the hoyvik group rocks were unroofed before the wenlock time 423 428 ma and cooled through the temperature for argon retention in phengite at c 447 4 ma indicating a maximum cooling rate between 14 and 22c ma 1 through ashgill and llandovery times before being subjected to low grade metamorphism during the scandian orogeny rapid pre scandian cooling combined with peak metamorphic conditions of 450c or less may indicate that the dalsfjord hoyvik basement cover pair were avected by an orogenic event during the late ordovician caradoc time the data also suggest that the caledonian margin of baltica may have experienced a more protracted tectonism during the caledonian cycle than previously models focusing on early caledonian and tremadoc or older ophiolite obduction and the scandian continental collision between baltica and laurentia

kinematics of the hoybakken detachment zone and the more trondelag fault complex central norway the tectonic disintegration of the caledonian orogen through combined extension contraction and strike slip was characterized by spatial and temporal strain partitioning through a period of at least 30 ma early to mid devonian exhumation of the central norway basement window was associated with retrograde top to the sw extensional shearing in the hoybakken detachment zone sinistral shearing along the more trondelag fault complex and formation of extension parallel folds progressive exhumation led to increasing strain localization and to the transition from ductile to brittle deformation in the interval between c 370 and 320 ma the hoybakken detachment fault cut previously folded detachment mylonites capturing mylonites in its hanging wall 40 ar 39 ar mica and k feldspar ages indicate a late devonian or younger age for the uppermost parts of the adjacent old red basin gentle folding of this stratigraphic level attests to the continuation of shortening and orogen oblique extension into late devonian carboniferous time shortening was intensified along strands of the more trondelag fault complex as shown by mutually cross cutting reverse and strike slip faults flower structures may be particularly common in constrictional strain systems where strike slip faults develop parallel to the principal elongation trend but normal to the principal axis of shortening

palaeomagnetic dating of fault rocks evidence for permian and mesozoic movements and brittle deformation along the extensional dalsfjord fault western norway summary palaeomagnetic and structural studies of the dalsfjord nappe western norway show that the basal ow angle detachment dalsfjord fault is a long lived fault zone and that the most important phase of faulting was of devonian extension probably nucleated on an earlier silurian scandian thrust fault rocks produced during subsequent movements indicate that the dalsfjord fault underwent periods of brittle low angle extensional reactivation during the permian 250 260 ma and upper jurassic lower cretaceous c 150 ma corresponding with stages of major extensional movements on the continental shelf palaeomagnetic studies may be of great importance for dating faults and major movement stages in long lived fault systems the particular importance of the results is that they show that low angle normal faults can operate in a brittle upper crustal regime

kinematics of post caledonian polyphase brittle faulting in the sunnfjord region western norway abstract structural studies in the sunnfjord region of western norway focused on faulting episodes that post date the major period of early devonian extension clarify the style and prolonged complex evolution of brittle faulting major brittle structures in the region can generally be divided into 1 e w faults and 2 n s fault joint zones with subordinate ne sw and nw se structures in close association with these large scale features four distinct groups populations of mesoscopic faults can be distinguished with basis in common characteristics spatial distribution orientation sense of slip and cross cutting relationships two reference areas the eastern parts of the kvamshesten and hornelen devonian basins feature clear cross cutting relationships of major faults and folds such relations and kinematic patterns of the distinct populations of mesoscopic faults and their relationship to dated fault rocks and dykes form the basis for a four stage history of post early devonian brittle faulting stage 1 is characterised by n s shortening expressed by thrusting and folding related to either late devonian carboniferous transpression or compression stage 2 is characterised by n s shortening and e w extension on conjugate strike slip faults and bisecting normal faults and is probably related to the regional e w folding during progressive shortening stage 3 is characterised by approximate e w extension and is manifested as low angle silicified fault breccias of possible permian age that mainly bound the eastern margin of the devonian basins stage 4 features n s extension on steep e w striking normal faults that possibly relates to a regional late jurassic early cretaceous extensional phase significant throw 500 1000 m on some of the faults emphasise the importance of late devonian and younger faulting indicating that these deformation episodes had a regional extent and therefore also occurred in the neighbouring offshore areas the established four stage fault evolution is examined in the light of known episodes of rejuvenation of faults on the nearby shelf and along the more trondelag fault zone emphasising the importance of movement on master faults with respect to local strain conditions in the sunnfjord region