relationship between non cylindrical fold geometry and the shear direction in monoclinic and triclinic shear zones we investigate the relationship between non cylindrical fold geometry and the shear direction of the hosting high strain zone by numerical modeling and show that apical axes of non cylindrical folds may develop into directions highly oblique to the shear direction if the zone has a pure shear component the common practice of using well developed sheath folds as indicators for the shear direction is not reliable and the use of immature non cylindrical or sheath folds appears more reliable hinge lines of mature sheath folds approach parallelism with the fabric attractor to which all material lines rotate which can have variable angles with respect to the shear direction in thinning zones the fabric attractor is the direction of the maximum principal strain rate of the pure shear component and is parallel to the shear zone boundary in thickening zones it lies somewhere in the quadrant between the direction of the maximum principal strain rate of the pure shear component perpendicular to the shear zone boundary and the simple shear direction and is generally oblique to the shear zone boundary the exact location depends on the flow geometry of the shear zone 2007 elsevier ltd all rights reserved

magnetic susceptibility fabrics in slates structural mineralogical and lithological influences abstract a single penetrative tectonic deformation of archean greywackes accompanied by low grade metamorphism generated low field magnetic susceptibility fabrics that are coaxial with the rock s schistosity although there are multiple sources of magnetic susceptibility pre metamorphic magnetite and the metamorphic minerals pyrrhotite biotite and chlorite variety thuringite daphnite aphrosiderite are responsible for the anisotropy of susceptibility these minerals grew with preferred crystallographic orientations influenced by the prevailing stress regime on its own rotation cannot be invoked to explain the alignment of the metamorphic minerals there are varying proportions of at least two of these minerals in most specimens for 153 specimens the degree of anisotropy increases as the mean susceptibility increases over the range from 100 to 700 in units of 10 6 s i theory shows that this is to be expected either due to increasing the traces of strongly oriented ferrimagnetics or due to increasing the proportion of chlorite in the latter case this simultaneously increased its preferred orientation by closer packing stress influenced crystallization of chlorite rapidly achieves saturation alignment and a limiting magnetic anisotropy although strain may continue to increase samples taken through individual graded beds show that the susceptibility ellipsoid varies from oblate in slaty rocks to a neutral shape in sandier rocks however its orientation is always structurally controlled with the minimum susceptibility axis perpendicular to schistosity

volume loss fluid flow and state of strain in extensional mylonites from the central mojave desert california abstract compositional changes in mylonitic rocks from the central mojave metamorphic core complex california indicate that large volume loss 20 70 attended formation of mylonite along the waterman hills detachment fault strong silica depletion and significant mobility of normally immobile elements imply large fluid rock ratios during mylonitization possibly as a result of multiple pass hydrothermal convection connecting near surface and mid crustal alteration regimes dissolution of quartz during mylonitization indicates that the fluid was undersaturated with respect to silica and therefore was probably not magmatic or connate large volume loss in a shear zone should result in apparent flattening strains yet mylonites throughout the complex including the mylonites that record large volume loss exhibit constrictional strains this inconsistency raises questions about the applicability of the shear zone model to cordilleran metamorphic core complexes

tertiary extension and contraction of lower plate rocks in the central mojave metamorphic core complex southern california several authors recently have proposed the rolling hinge model for the formation of extensional detachment faults in metamorphic core complexes in this model isostatic uplift of the denuded footwall of a major normal fault causes the near surface part of the fault to rotate from a higher to a lower dip for the rolling hinge model to be acceptable the footwalls of detachment faults must have undergone the strain pattern dictated by the rolling hinge in this paper the structural development of the footwall of the central mojave metamorphic core complex is used as a field test tertiary structures in the lower plate of the core complex record ne directed ductile extensional shear followed by cataclastic ne side up shear with a significant component of contraction along the previous extension direction this strain history agrees with the rolling hinge model and particularly with a version of the model in which footwall uplift occurs by a combination of elastic flexure and flexurally driven upper crustal failure similar late stage layer parallel contraction appears to be present in some other tectonically denuded terranes of the basin and range suggesting that the flexural failure rolling hinge model for extensional terranes may be widely applicable

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

folding of mylonitic zones in cordilleran metamorphic core complexes evidence from near the mylonitic front metamorphic core complexes of north america are asymmetric when viewed perpendicular to the direction of tectonic transport on one side mylonitic fabrics and the overlying detachment fault dip gently in the direction of upper plate transport and the mylonitic rocks have a down dip sense of shear on the opposite side the mylonitic zone rolls over to a dip opposed to the direction of upper plate transport diverges downward from the detachment fault and dies out upward across a mylonitic front new kinematic data show that the backdipping mylonitic zone in most complexes is a downfolded continuation of the main mylonitic zone folding of the mylonitic zone was accompanied by the formation of antithetic shear zones and occurred during the late stages of mylonitization

large magnitude miocene extension in the central mojave desert implications for paleozoic to tertiary paleogeography and tectonics this is the published version copyright 1990 american geophysical union all rights reserved

progressive deformation structures associated with ductile thrusts in the moine nappe sutherland n scotland in the deeper parts of mountain belts polyphase structural sequences observed at outcrop scale can arise due either to overprinting of regionally distinct deformation phases e g reworking changes in orogenic boundary conditions or to localized controls that bring about transient changes in the patterns of ductile flow these are unlikely to be mutually exclusive processes and examples from scotland demonstrate that once regionally separate events are delimited using radiometric evidence it is possible to isolate complex deformation sequences arising due to local controls in the western moine nappe of sutherland the dominant structures were formed during caledonian ductile thrusting towards the wnw whilst earlier precambrian phases are relatively minor in importance two groups of caledonian folds and fabrics are recognized in many exposures main phase d2 structures which are broadly contemporaneous with ductile thrust fabrics and later secondary phase f3 folds the latter can be divided into two geometric groups sheath fold types which formed initially as wnw overturned buckles subsequently modified by ductile shearing and asymmetric types which are commonly open folds apparently formed with axes close or sub parallel to the thrust transport direction secondary structures show a close spatial association with high strain zones along ductile thrusts and can be shown to have formed during the later stages of thrusting in certain critical exposures i propose that they may form due to strain perturbations resulting from variations in the relative rates of ductile flow within the mylonites where differential shearing occurs due to flow normal perturbations wrench related asymmetric fold types may form in contrast secondary sheath fold structures may result from localized compression phases caused by flow parallel perturbations local flow perturbation models may be appropriate in situations where the distribution of later structures is apparently related to spatial variations in strain intensity and or there is a strong geometric and kinematic similarity between sets of folds and fabrics if such features are not observed a regional polyphase sequence is more likely and it is therefore necessary to obtain radiometric dates or some other independent source of data to demonstrate a time separation

improved constraints on seismic source parameters of the 2011 tohoku earthquake from grace gravity and gravity gradient changes a new approach of using only the north component of gravity change from gravity recovery and climate experiment grace data reveals that a substantially higher spatial resolution of the observed seismic deformation following the 2011 tohoku earthquake is achievable at 333 km or longer here we show that grace observed north component of gravity change 17 6 1 1 gal and the corresponding gravity gradient change e g txz at 1 25 0 09 meotvos agree well with seismic gps model predictions localized slepian spectrum analysis further confirms that the grace gravity and gravity gradient changes agree well with seismic model spectra and have powers up to the limit of the grace solution complete to spherical harmonic degree 60 using the gravity observations for the fault parameter inversion via simulated annealing algorithm we show that the estimated slip orientation and centroid location are different from gps seismic solutions and potentially due to the additional offshore constraint from grace data

experimental folding and boudinage under pure constrictional conditions abstract constrictional folds are characterized by true fold axis parallel extension if the rock volume does not vary during deformation studies of such folds in experiments using plasticine layers of different apparent viscosity and power law exponent clearly indicate that fold axis parallel stretch may be accompanied by plastic elongation as well as boudinage of the competent layer characteristic aspects of the experimentally folded competent layers are 1 coeval development of folds and boudins 2 layer thickness not changing during deformation 3 layer parallel shortening in sections perpendicular to the fold stretching axis 4 enlargement of the initial thickness of the competent layer results in increasing fold wavelength and decreasing number of boudins the ratio of dominant wavelength to layer thickness of the constrictional folds can be described mathematically approximately by the equation developed for plane strain folding of power law materials

characteristics and dynamic origin of the large scale jiaoluotage ductile compressional zone in the eastern tianshan mountains china abstract the jiaoluotage ductile compressional zone jdcz in the eastern tianshan mountains china developed in volcanic and sedimentary rocks that were deposited in a carboniferous back arc basin situated between the jungar plate and middle tianshan terrane the ductile compressional zone 500 km long and 20 50 km wide consists of a number of east trending vertical foliation zones that display an augen geometry in plan view and a fan like geometry in transverse profiles ductile compressional deformation has produced pervasive foliation radial stretching lineations contemporaneous folds and various macroscopic to microscopic symmetric structures within the jdcz rotations of most clasts and porphyroclasts are bidirectional the preferred orientation patterns of the optical axes of calcite and quartz display axial or orthorhombic symmetry three kinds of strain patterns have been determined in the jdcz apparent flattening plane strain and apparent constriction the overall strain character of the jdcz is cream cake style the volcanic and sedimentary rocks within the jdcz were metamorphosed into greenschist facies with temperature of 400 600 c and pressure of 300 400 mpa development of the jdcz is interpreted as due to n s coaxial compression caused by the collision between the tarim plate and the middle tianshan arc jiaoluotage basin jungar plate system during the permian between 270 and 290 ma

state of stress and modern deformation of the northern basin and range province constraints on the current stress regime of the actively extending northern basin and range province are provided by deformation data focal mechanisms and fault slip studies hydraulic fracturing in situ stress measurements borehole elongation breakouts analyses and alignment of young volcanic vents the integrated data indicate significant variations both in principal stress orientations and magnitudes an approximately e w least principal stress direction appears to characterize both the eastern and western margins of the basin and range province whereas in the active interior parts of the province extension occurs in response to a least principal stress oriented nw to n60 w the contrast in stress orientations between the province boundaries and in the interior suggests that along the margins the least principal stress direction may be locally controlled by the generally northerly trending profound lithospheric discontinuities associated with these margins active deformation along the southeastern and western province margins is characterized by a combination of strike slip and normal faulting focal mechanisms along northeastern province margin wasatch front and in central nevada indicate a combination of normal and oblique normal faulting temporal regional and depth dependent variations in the relative magnitudes of the vertical and maximum horizontal stresses can explain much of the observed variations in deformation styles however some depth variation in faulting style inferred from focal mechanisms may be apparent and simply a function of the attitude of fault planes being reactivated evidence for significant temporal variation or multiple cycles of variation in relative stress magnitude comes from the sierran front basin and range boundary region where recent earthquakes are predominantly strike slip whereas the profound relative vertical relief across the sierra frontal fault zone in the last 9 10 m y implies a normal faulting stress regime using the best data on stress orientation relative stress magnitudes are constrained from slip vectors of major earthquakes and young fault displacements analysis of well constrained slip vectors in the owens valley california area indicate that large temporal variations in the magnitude of the approximately n s oriented maximum horizontal stress are required to explain dominantly dip slip and strike slip offsets on subparallel faults similar faulting relations are observed throughout much of the boundary zone between the basin and range sierra nevada including the walker lane belt along the eastern province margin in the wasatch front area in utah available data suggest that the maximum and minimum horizontal stresses may be approximately equal at depths of 4 5 km earthquake focal mechanisms in this area suggest more variability in relative magnitude of the two horizontal stresses with depth furthermore superimposed sets of young fault striae along a segment of the wasatch fault also indicate temporal variations of relative stress magnitudes sources of regional and temporal variations in the stress field may be linked to variable shear tractions applied to the base of the brittle crust related to intrusion thermally induced flow and the influence of the san andreas plate boundary although difficult to date accurately the fault slip data suggest that the temporal variations in relative magnitudes stress may occur on the time scale of both a single major earthquake cycle 1000 5000 years and multiple earthquake cycles 10 000 years

formation and suppression of strike slip fault systems strike slip faults are a defining feature of plate tectonics yet many aspects of their development and evolution remain unresolved for intact materials and or regions a standard sequence of shear development is predicted from physical models and field studies commencing with the formation of riedel shears and culminating with the development of a throughgoing fault however for materials and or regions that contain crustal heterogeneities normal and or thrust faults joints etc that predate shear deformation kinematic evolution of strike slip faulting is poorly constrained we present a new plane stress finite strain physical analog model developed to investigate primary deformation zone evolution in simple shear pure strike slip fault systems in which faults or joints are present before shear initiation experimental results suggest that preexisting mechanical discontinuities faults and or joints have a marked effect on the geometry of such systems causing deflection lateral distribution and suppression of shears a lower limit is placed on shear offset necessary to produce a throughgoing fault in systems containing preexisting structures fault zone development observed in these experiments provides new insight for kinematic interpretation of structural data from strike slip fault zones on earth venus and other terrestrial bodies

using along strike variation in strain and kinematics to define the movement direction of curved transpressional shear zones an example from northwestern superior province manitoba a review of current understanding of structures in shear zones reveals that correlating the orientation of stretching lineations with movement direction once a common practice based on the simple shear model is not generally applicable to transpressional zones it is difficult to determine the movement direction of a transpressional zone based on internal structures we show that the movement direction can be better defined by considering the variation in strain geometry and kinematics of differently oriented and kinematically related shear zones or different segments of a curved shear zone application of this method to a shear zone in the northwestern superior province of manitoba shows that the movement direction there is approximately north south in contrast to a commonly held view for similarly oriented shear zones in the superior province that the movement direction is approximately east west

low temperature thermochronology of northern baja california mexico decoupled slip exhumation gradients and delayed onset of oblique rifting across the gulf of california 1 the northern gulf extensional province displays key structural relationships that characterize the magnitude direction and timing of neogene rift related transtension during the opening of the gulf of california apatite fission track and u th he thermochronology from the sierra san felipe document moderate cooling 4 c myr 7 c myr during the early paleogene associated with progressive unroofing caused by erosional downwearing of the ancestral peninsular ranges beginning at 45 35 ma a period of tectonic quiescence with low cooling rates 1 c myr marks the development of a regional oligocene miocene peneplain rift related exhumation began at 9 7 ma and attains 2 5 km in the hinges of two antiformal megamullions decoupling between exhumation and finite displacement in certain fault segments is explained by vertical deflections associated with extension perpendicular folding of the fault surfaces the main faults of the detachment system were active contemporaneously thus forming a mechanically linked array of large displacement normal faults in the hanging wall of the main gulf escarpment the late miocene onset of transtension in the sierra san felipe suggests that widespread deformation may only have localized in the gulf extensional province between 9 and 7 ma some 3 5 ma after a major plate reorganization associated with cessation of subduction in the trench to the west between 12 ma and 9 7 ma plate boundary shearing was likely distributed between the continental borderland west of baja california and the southern basin and range province in mexico

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

transect across the northwestern grenville orogen georgian bay ontario polystage convergence and extension in the lower orogenic crust the grenville orogenic cycle between 1190 and 980 ma involved accretion of magmatic arcs and or continental terranes to the laurentian craton a transect across the western central gneiss belt georgian bay ontario which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20 30 km offers some insights on the thermal and mechanical behavior of the lower crust during the development of the grenville orogen prior to grenvillian metamorphism this part of laurentia consisted largely of meso proterozoic 1450 ma granitoid orthogneisses granulites and subordinate mafic and supracrustal rocks grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed 1160 ma parry sound domain over the craton sometime between 1120 ma and 1080 ma this stage of transport was followed by out of sequence thrusting and further convergence along successively deeper foreland propagating ductile thrust zones a major episode of extension at 1020 ma resulted in southeast directed transport of allochthonous rocks along the midcrustal shawanaga shear zone the final stage of convergence involved deformation and metamorphism in the grenville front tectonic zone at 1000 980 ma peak metamorphism along most of the transect at 1065 1045 ma followed initial transport of allochthonous rocks over the craton by 15 35 m y regional cooling which postdated peak metamorphism by 70 m y was probably delayed by the combined effects of late stage extension and convergence transport of allochthons at least 100 km over the craton was accomplished along a weak migmatitic decollement further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement extensional deformation was associated with distributed ductile flow the formation of regional transverse folds with axes parallel to the stretching direction and reactivation of the allochthon parautochthon thrust boundary as an extensional decollement the extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the central gneiss belt

heterogeneous constrictional deformation in a ductile shear zone resulting from the transposition of a lineation parallel fold abstract we use new micro structural petrofabric strain and vorticity data to analyze the deformation path in a mesoscopic quartz mylonite zone the mylonite zone resulted from the complete transposition of a stretching lineation parallel isoclinal fold symmetric cleft girdle quartz c axis fabrics were recorded in the middle domain which occupies the inner limbs of the precursor isoclinal fold while asymmetric cleft and crossed girdle fabrics were observed in the upper and lower domains that represent the outer limbs constrictional strain with increasing k values towards the middle domain is inferred from petrofabric and 3d strain data oblique grain shape fabrics yield vorticity estimates of 0 72 0 90 in the zone however in the middle domain pure shear dominated deformation is suggested by orthorhombic crystallographic fabrics strain rate is constant throughout the zone a strain decrease towards the zone center implies that deformation ceased earlier in the middle domain the data indicates that fold transposition and subsequent mylonitization started as pure shear dominated constrictional deformation and progressively changed to simple shear dominated plane strain during this flow path the asymmetric quartz c axis fabrics likely developed by depopulation of cleft girdle maxima rather than from the synthetic rotation of fabric maxima itself

paleomagnetism geochronology and possible tectonic rotation of the middle miocene barstow formation mojave desert southern california the barstow formation of the central mojave desert in the mud hills consists of 965 m of epiclastic and volcanic sediments deposited in an extensional rift this widespread unit is of importance because its mammalian faunas characterize the barstovian land mammal age previous isotopic ages on interbedded tuffs have been used to calibrate middle miocene time and it was deposited in a tectonically active region volcanic ash samples were collected from the barstow formation for isotopic dating using conventional 40 k 40 ar and 40 ar 39 ar single crystal laser fusion methods multiple analyses on minerals from 5 superposed tuffs yielded the following mean 40 k 40 ar ages rak tuff at 16 3 0 3 m y oreodont tuff at 15 9 0 2 m y dated tuff at 14 8 0 2 m y hemicyon tuff at 14 0 0 1 m y and lapilli tuff at 13 4 0 2 m y similar mean ages were also obtained on the following tuffs using single crystal 40 ar 39 ar laser fusion methods oreodont tuff at 15 9 0 06 m y dated tuff at 14 8 0 06 m y and hemicyon tuff at 14 0 0 09 m y in order to develop a local magnetostratigraphy 272 paleomagnetic sites were collected from 7 important sections within the mud hills saturation isothermal remanence experiments suggested the presence of either hematite or magnetite as carriers of remanence after either stepwise alternating field or thermal demagnetization characteristic directions were usually isolated between 30 and 50 mt or 300 and 500 c positive reversal and fold tests indicate a stable remanence acquired prior to folding there are 21 magnetic polarity zones interpreted from the continuous part of the barstow formation in the study area by using the 5 isotopic ages determined from interbedded tuffs the local composite polarity pattern is correlated to the top of chron c5d through the bottom of c5ab on the magnetic polarity time scale or an age range from 17 6 to 13 4 m y the boundary between hemingfordian and barstovian land mammal ages herein occurs 30 m below the oreodont tuff 15 8 0 2 m y within c5b and is taken at about 16 0 m y ago the barstow formational mean declination is 347 1 inc 47 5 95 6 51 d 7 97 suggesting negligible or possibly slight counterclockwise rotation r 4 1 r 10 5 for this part of the mojave block during the past 18 m y a 40 ar 39 ar mean age of 19 3 0 02 m y for the red tuff which is near the local base of the barstow formation and unconformably underlying our composite section constrains a minimum age for the beginning of local basin filling with principally epiclastic sediments

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

retroward extrusion of high pressure rocks an example from the hellenides pelion blueschist nappe nw aegean current tectonic models interpret the hellenides as a unidirectional sw vergent orogenic belt new micro structural and amphibole chemistry analyses show instead that the exhumation of the pelion blueschist nappe pbn of the internal hellenides was achieved by retroward ene directed ductile extrusion which opposes the principal proward orogenic vergence retroward extrusion occurred between two sub parallel major ductile shear zones basal thrust and upper detachment with opposite shear senses which operated simultaneously under blueschist to greenschist facies conditions during the late eocene oligocene because the pbn is tectonically sandwiched between pelagonian basement rocks we suggest that the pbn protolith was accumulated during the late cretaceous in an incipient backarc basin developed above the ne dipping subduction of the pindos ocean beneath the pelagonian microcontinent subsequent apulian pelagonian collision triggered basin inversion and the proward dipping intracontinental subduction that caused the early middle eocene blueschist facies metamorphism followed by the retroward extrusion of the pbn this article is protected by copyright all rights reserved

magnitude and significance of miocene crustal extension in the central mojave desert california the newly recognized waterman hills detachment fault whdf of the central mojave desert california is significant because it provides the first unambiguous evidence for large scale core complex style crustal extension in the central mojave desert and because it has significantly rearranged the pre miocene paleogeography of the mojave desert the whdf places steeply dipping to overturned miocene volcanic and sedimentary rocks upon mylonitic pre tertiary basement the mylonites which apparently formed during extension are predominantly l tectonites which manifest top to northeast shear the whdf dips to the northeast beneath dominofaulted ranges of the central mojave desert and detachment faults of the colorado river trough forming an imbricated early miocene system of detachment faults extension continued in the colorado river trough after extension had ceased in the central mojave desert tentative correlations of mesozoic intrusions suggest about 40 km of slip across the whdf which carries eugeoclinal paleozoic rocks in its hanging wall and cratonal miogeoclinal paleozoic rocks in its footwall restoration of 40 km of slip 1 removes a prominent kink in the boundary between eugeoclinal and cratonal miogeoclinal facies 2 aligns cratonal miogeoclinal strata near victorville more closely with the late paleozoic continental margin farther north 3 places cratonal miogeoclinal rocks structurally beneath eugeoclinal rocks implying that the facies were stacked by thrusting and 4 straightens the western margin of the late jurassic independence dike swarm

complex strain patterns developed at the frontal and lateral tips to shear zones and thrust zones abstract many of the complex strain patterns seen in shear zones and thrust zones such as variable fabric orientations refolded folds and fabrics together with folds with hinges almost parallel to the main transport direction can be explained in terms of differential movement within the shear zones these strains are developed at the frontal and lateral tips of the zones as they propagate examples are taken from the moine thrust zone of scotland which show variations in strains particularly at the lateral tips the form of differential movement described here may lead to complex strain paths and non plane strain ellipsoids and the spatial variations in finite strain may be used to delineate zones of extensional and compressional flow and differential movement in the shear zones or thrusts

orogen scale l tectonite domain in the tongbai orogenic belt central china geological setting and origin abstract l tectonite is well developed and widely distributed in the tongbai orogenic belt in central china the orogenic belt as a whole has an antiformal geometry and the hinge of the antiform is subhorizontal and trends nw se the l tectonite occurs in the core of the antiform in a zone that is 10 30 km wide and over 100 km long lineations in the l tectonite are sub horizontal parallel to the hinge of the antiform sheath folds are also well developed associated with the l tectonite with the hinges parallel to the lineations migmatite occurs in the core and structurally below the l tectonite and has a gradational relationship with the l tectonite the domain of l tectonite is bounded by three ductile shear zones on the north at the top and on the south respectively well developed shear sense indicators indicate that the southern the overlying and the northern shear zones have a dextral top to nw and sinistral sense of shear respectively these geometrical and kinematic data indicate that the three shear zones are likely part of a single shear zone that wraps around the l tectonite domain the l tectonite zone in the core moves southeast relative to the hanging wall the development of the tectonite is interpreted to be a result of this special geometry and kinematics and reflects a post collisional orogen parallel extension synchronous with migmatization and the continuing convergence between the yangtze block and the north china block in the early cretaceous

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

folding in regions of extension summary many extension zones have been subjected to folding and shortening in a direction perpendicular to the stretching such deformation can be accounted for by the extension of a thin superficial elastic layer overlying a substrate that has small elastic moduli or that deforms in a viscous regime laboratory experiments are used to document the wavelength and amplitude of the folds for a range of geometrical configurations folding is observed even for very small amounts of extension less than 1 per cent with characteristics that are consistent with finite amplitude scaling laws because of the intrinsically 3 d nature of the deformation field the size of the region affected by folding and the direction of the fold axes depend on the orientation of the extension with respect to the rigid blocks that bound the deforming region for regions of extension where the elastic thickness is about 10 km as in the basin and range province for example it is predicted that folding occurs with wavelengths in a 20 40 km range such that it induces little deformation in the lower crust and maintains a flat moho discontinuity these predictions are consistent with the observations the characteristics of faulting that is associated with such deformation are discussed

constriction structures related to viscous collision southern prince charles mountains antarctica macroscopic structures are investigated in a zone of highly contorted migmatites from the southern prince charles mountains antarctica here l tectonite fabrics rods mullions boudin pods elongate enclaves and fold hinges are persistent linear features all plunging gently to the northeast in contrast amoeboid folds ptygmatic folds and folded boudins with different orientations are the characteristic structures in transverse sections perpendicular to the lineation no consistent shear sense is recognised in any dimension together with strain and shape analysis these observations strongly suggest that the deformation pattern is one of folding and stretching by constriction previous timing constraints indicate that this deformation overlapped with the waning stages of anatexis during decompression at approximately 510 ma up to 30 million years after initial orogeny at 540 ma the zone affected by constriction is several kilometres wide and has a contorted flower like shape confined between two broad domal antiforms in this context the constricted zone is interpreted as a relatively late tectonic feature that could have formed via deep seated viscous collision in response to orogenic collapse and doming

evolution of a passive crustal scale detachment syros aegean region insights from structural and petrofabric analyses in the hanging wall abstract new detailed micro structural investigations quartz petrofabric analyses and geological structural mapping in southeast syros cycladic massif aegean region allow us to place new constraints on the tectonic evolution of the uppermost unit which occupies the immediate hanging wall of the crustal scale vari detachment we show that the eocene oligocene deformation history in the hanging wall of this detachment is associated with sw directed ductile shearing this history includes an early distributed constrictional deformation expressed by transport parallel upright folds l tectonites and cleft girdles quartz c axis fabrics that were formed at temperatures 500 c ductile deformation progressively localized at the bottom of the uppermost unit leading to the formation of a greenschist facies mylonitic zone under plane strain conditions inferred from type i cross girdles quartz c axis fabrics the ongoing mylonitization was also associated with temporally increasing pure shear component of deformation coupled with cooling from 500 c to 400 c we suggest that the vari detachment represents a passive normal sense roof fault resulted from the ne directed ductile extrusion of the blueschist unit footwall at middle eocene oligocene times in the middle miocene the activation of the brittle ssw directed late vari detachment enhanced the brittle exhumation of both the blueschist unit and the vari detachment

neogene structural evolution of the sierra san felipe baja california evidence for proto gulf transtension in the gulf extensional province abstract the sierra san felipe located in the gulf extensional province of northeastern baja california experienced a complex deformation history of integrated normal and strike slip faulting block rotations and extension parallel folding as a result of neogene transtensional plate margin shear between the pacific and north american plates the eastern range front of the sierra san felipe is defined by three left stepping en echelon detachment faults that are linked by dextral and sinistral transfer faults and accommodation zones the las cuevitas santa rosa and huatamote detachments comprise ne to se dipping moderate to low angle normal faults that accommodate between 1 5 and 9 km of broadly e to se directed extension and show strong along strike displacement gradients hanging wall half grabens are infilled with northwest tilted miocene pliocene volcanic and sedimentary rocks that have been deposited nonconformably onto the batholithic basement stratigraphic relationships indicate that faulting on the las cuevitas and santa rosa detachment faults initiated synchronously as a kinematically linked fault system before 7 ma 9 8 ma based on the timing of footwall exhumation during the so called proto gulf phase of rifting paleostress calculations suggest a transtensional stress regime with ne to se directed extension and permutating vertical to n s trending subhorizontal contraction fault kinematics and paleostress orientations of the fault array do not vary through time but reflect the spatial distribution of fault planes with respect to a transtensional stress regime that lasted throughout the entire slip history of the detachments our data indicate that clockwise vertical axis block rotations and constrictional folding were an integral part of the deformation history in the sierra san felipe since rifting began in the late miocene and may have played an important role in facilitating the transfer of deformation between the main gulf escarpment and offshore faults in the gulf of california these observations support the hypothesis that middle miocene to present oblique divergent plate motion was accommodated by a single phase of integrated transtensional shearing in the gulf extensional province

the mojave extensional belt of southern california it is proposed that the regional pattern of early miocene low and high angle normal faulting and dike swarm emplacement in the western and central mojave desert is the result of regional extension developed within a roughly east trending zone named here the mojave extensional belt this zone of extension although now disrupted by late cenozoic right slip faulting can be traced from the intersection of the san andreas and garlock faults to the eastern mojave desert near bristol lake and the granite mountains fault restoration of post oligocene movement along the san andreas fault system places the now buried western part of the mojave extensional belt opposite a similar age extended area of west central california surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the mojave extensional belt was low angle normal sense simple shear detachment faulting strain within the mojave extensional belt is partitioned between four domains edwards waterman daggett and bullion terranes that each consist of one or more half grabens each half graben is composed of tilted normal fault bounded blocks that lie above a rooted low angle brittle ductile normal sense shear zone tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping 80 planar to slightly curviplanar normal faults at least one generation of high angle faults postdates the formation of the detachments differential extension between the domains was accommodated by strike slip faults transfer zones similarly the lack of early miocene extensional elements north of the western segment of the garlock fault suggests that the garlock may have been active in the early miocene and served as the northern transfer boundary of the mojave extensional belt the mojave extensional belt initially began to open about 22 m y ago with the major phase of extension occurring between 22 and 20 ma local high angle normal faults cut older detachments and were active between 20 and 17 ma kinematic indicators suggest that the western part of the mojave extensional belt opened in a ne sw direction whereas the eastern portion extended in an ene wsw direction present day reference frame rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the mojave extensional belt was originally oriented n s and was rotated as much as 50 clockwise between 20 and 18 ma early miocene

when do dragfolds not develop into sheath folds in shear zones abstract using a unified model for high strain zones rotation paths of material lines in different flow types are investigated and presented as stereographic projections a material fabric element such as a mineral lineation or a fold axis can be plotted on such stereograms and the rotation path revealed using this method we examine the rotation of folds and the development of sheath folds in general three dimensional zonal deformation we show that the evolution of dragfold geometry in a high strain zone depends on the flow type and the initial perturbation of the fold axis in simple shear zones and thickening zones monoclinic or triclinic dragfolds will evolve into sheath folds in monoclinic thinning zones with biaxially stretching boundaries if the shear direction is parallel to the maximum stretching direction of the boundary sheath folds will develop if the shear direction is parallel to the minimum stretching direction of the boundary dragfolds become more cylindrical as deformation advances in thinning zones with boundaries equally stretched in all directions the likelihood of sheath fold development depends on the ratio of the shear strain rate to the thinning rate in triclinic thinning zones fold evolution depends on the details of the flow and the value of initial fold axis perturbation

mechanisms for the formation of domal and basinal detachment faults a three dimensional analysis detachment faults i e regional low angle normal faults in metamorphic core complexes of the north american cordillera commonly exhibit doubly plunging antiformal domal and synformal basinal geometries these geometries have been previously attributed to the superposition of two processes 1 the antiforms and synforms with axes parallel to the extension direction originated as primary fault undulations or corrugations developed coevally with fault slip and 2 the antiforms and synforms with axes perpendicular to the extension direction were produced by isostatic uplift due to tectonic denudation however the coaxial relationship between undulations of detachment faults and the sedimentary beds and metamorphic foliations in both the upper and lower plates of some detachment fault systems in the u s cordillera suggests that the synforms and antiforms with axes parallel to the extension direction may have formed as folds and that the domal and basinal geometry of detachment faults may have formed synchronously by a single process a three dimensional elastic thin plate model is developed to investigate possible mechanisms for the formation of domal and basinal detachment faults this model explores the interactions among the vertical horizontal and basal shearing forces during the formation of warped detachment faults the model considers the role of compression perpendicular to the extension direction and stress reduction parallel to the extension direction the results of the model suggest that the stress reduction itself in the extension direction is insufficient to cause buckling of a thin elastic crust two mechanisms are most likely to explain the formation of domal and basinal geometries of detachment faults 1 an upward pushing by undulatory crustal roots or buoyant synextensional plutons beneath the extensional belt and 2 buckling caused by a compression perpendicular to the extension direction both mechanisms are consistent with the geologic constraints during the development of cordilleran core complexes in addition to isostatic uplift due to tectonic denudation upward warping of detachment faults can also be caused by buckling basal shearing and buoyant forces

frictional properties of low angle normal fault gouges and implications for low angle normal fault slip abstract the mechanics of slip on low angle normal faults lanfs remain an enduring problem in structural geology and fault mechanics in most cases new faults should form rather than having slip occur on lanfs assuming values of fault friction consistent with byerlee s law we present results of laboratory measurements on the frictional properties of natural clay rich gouges from low angle normal faults lanf in the american cordillera from the whipple mts detachment the panamint range front detachment and the waterman hills detachment these clay rich gouges are dominated by neoformed clay minerals and are an integral part of fault zones in many lanfs yet their frictional properties under in situ conditions remain relatively unknown we conducted measurements under saturated and controlled pore pressure conditions at effective normal stresses ranging from 20 to 60 mpa corresponding to depths of 0 9 2 9 km on both powdered and intact wafers of fault rock for the whipple mountains detachment friction coefficient varies depending on clast content with values ranging from 0 40 to 0 58 for clast rich material and 0 29 0 30 for clay rich gouge samples from the panamint range front detachment were clay rich and exhibit friction values of 0 28 to 0 38 significantly lower than reported from previous studies on fault gouges tested under room humidity nominally dry conditions including samples from the same exposure samples from the waterman hills detachment are slightly stronger with ranging from 0 38 to 0 43 the neoformed gouge materials from all three localities exhibits velocity strengthening frictional behavior under almost all of the experimental conditions we explored with values of the friction rate parameter a b ranging from 0 001 to 0 025 clast rich samples exhibited frictional healing strength increases with hold time whereas clay rich samples do not our results indicate that where clay rich neoformed gouges are present along lanfs they provide a mechanically viable explanation for slip on faults with dips p f 3 overpressures and or correcting for 5 of footwall tilting furthermore the low rates of frictional strength recovery and velocity strengthening frictional behavior we observe provide an explanation for the lack of observed seismicity on these structures we suggest that lanfs in the upper crust depth

structural and stratigraphic development of the whipple chemehuevi detachment fault system southeastern california implications for the geometrical evolution of domal and basinal low angle normal faults recent debates on the development of detachment faults during continental extension have been centered on whether they were active at low angles and initiated as relatively planar faults in addition intense investigations have concentrated on how the detachment fault geometry evolved in time and space and how the extension was accommodated in their upper and lower plates the superb exposure of the mid tertiary whipple mountains detachment fault system in southeastern california provides an unusual opportunity to address these questions analysis of the stratigraphy and structures of upper plate rock units constrains the three dimensional kinematic evolution of the mid tertiary whipple chemehuevi detachment fault system in southeastern california tertiary volcanic strata in the northwestern whipple mountains and central chemehuevi valley consist of similar rock types that have undergone about 15 extensional strain in contrast a different succession of tertiary volcanic and sedimentary rocks underlies the northeastern whipple mountains along the structural trend of central chemehuevi valley these strata have been extended by 40 differences in magnitudes of extension and mid tertiary stratigraphies between these two areas argue for the presence of a synformal gently east dipping normal fault above the whipple fault in the central chemehuevi valley this fault accommodates 15 km of the total 40 km extension along the whipple chemehuevi detachment fault system in addition tilting patterns of upperplate strata and the presence of an areally extensive syn extension landslide 70 km2 show that 1 the whipple detachment fault was active at low angles 70 km2 from the north side of the whipple mountains its emplacement occurred before the end of detachment faulting

sheath folds as discriminators of bulk strain type abstract nested elliptical closures defining eye folds represent classic y z cross sections through highly curvilinear sheath folds generated during intense ductile deformation in metamorphic rocks systematic analysis of 1425 such eye folds based on our own field observations and examples from the published literature reveals distinct and consistent differences in ellipticites measured from the outer r yz to the inner most r y z elliptical rings of individual sheaths the variation in overall aspect ratios from outer to inner rings is defined as r where r r yz r y z and may display a relative increase or decrease in ellipticity to define cats eye r bulls eye r 1 fold patterns respectively layer thicknesses may also be measured along the y axis parallel to the axial surface t y and at 90 to this along the z axis t z to define the ratio of t yz sheath folds generated during broadly simple shear deformation k 1 display mean r yz 4 61 t yz 3 31 and cats eye folds r 0 69 sheath folds generated during general shear also display cats eye folds with identical mean r 0 69 values but greater thickness variations and elliptical ratios t yz 4 35 r yz 5 76 thus within both simple and general shear dominated deformations the overall variation in layer thickness t yz and ellipticity of eye folds r yz increases with increasing deformation and a greater component of pure shear whilst the r value remains constant and reflects original fold patterns sheath folds formed during constrictional k 1 deformation display markedly lower aspect ratios r yz 2 42 and thickness variations t yz 2 94 together with distinctive bulls eye folds r 1 23 these empirical relationships suggest fundamental and universal constraints on curvilinear fold generation across this broad spectrum of deformation types and allow sheath folds to act as both effective 95 consistent and robust discriminators of bulk strain type

the effect of hydrous mineral content on competitive strain localization mechanisms in felsic granulites abstract this study investigates the grain scale mechanisms that facilitated strain localization in the lower crust within the east athabasca mylonite triangle of northern saskatchewan canada there two distinct lithotectonic units the upper deck and northwestern domain are juxtaposed across a km scale shear zone that has been previously interpreted to record thrust and or normal sense displacement our study reports new quartz c axis opening angle ti in biotite and ti in quartz geothermometry and amphibole plagioclase geothermobarometry within syn kinematic phases indicating that both units were deformed and metamorphosed at upper amphibolite to granulite facies 725 c and 0 8 gpa while sharing similar conditions the dominant mechanisms of deformation across the two units are strongly linked to modal proportion of syn kinematic hydrated mineral phases detailed microstructural and crystallographic preferred orientation cpo analyses reveals that specimens poor in hydrated minerals 10 pressure solution and re precipitation processes enhanced phase mixing and promoted grain boundary sliding resulting in weakly developed or absent quartz cpo these results indicate a strong mineralogical and fluid control on deformation and therefore which strain softening process es may dominate in lower crustal shear zones

rapid extension recorded by cooling age patterns and brittle deformation naxos greece the metamorphic core complex exposed as the island of naxos in the aegean sea records an unusually complete sequence of structures developed as a result of continental extension the structures formed during miocene rise and cooling from ductile upper amphibolite facies and anatectic conditions to brittle near surface conditions beneath the naxos detachment fault top to the north ductile fabrics in the footwall which initially developed during amphibolite facies prograde metamorphism were overprinted by a succession of north directed normal sense lower temperature brittle structures as the footwall was tectonically unloaded and unroofed pseudotachylite and cataclasite formation brittle faulting alteration and erosion of the footwall occurred during continued slip and tectonic denudation neogene conglomerate and megabreccia in part derived from exhumation of this footwall lie structurally above the peripheral naxos fault published k ar and 40ar 39ar ages for hornblende white mica and biotite in the footwall decrease northwestward apparent ages 17 50 ma in the southeast correspond to areas of low metamorphic grade where preextension argon was partially retained published ages 16 10 ma in higher grade rocks of the domal core in the north are cooling ages that for each of the three minerals show a component of younging in the nne direction of extension assuming this is the direction of unroofing we interpret the rate of this younging as the fault slip rate as the footwall rocks moved 20 km ssw relative to their hanging wall along the base of the naxos detachment fault the calculated rates of slip average 5 8 mm yr comparable to maximum rates reported in the basin and range province

parallel stretching lineations and fold axes oblique to a shear displacement direction a model and observations abstract spatial variations in shear strain rate are expected in ductile shear zones where the variation is a change in shear strain perpendicular to the displacement direction the effect is to rotate the shear slip planes this is a mechanism for giving a rotation of fold axes into parallelism with the slip and extension direction in a rock if such a variation in shear strain affects rocks with a strong planar anisotropy it is possible to produce a fabric with an apparent stretching lineation parallel to fold axes but both significantly oblique to the slip direction a possible example of this is seen in strongly deformed quartz mica schists from syros greece where a stretching lineation is seen parallel to fold hinges over a range of fold axes orientations of at least 40

magnitude of crustal extension in the southern great basin strike slip faults in the southern great basin separate areas of cenozoic upper crustal extension from relatively stable tectonic blocks linear geologic features offset along the garlock fault las vegas valley shear zone and lake mead fault system allow reconstruction of the southern great basin to a pre extension configuration the sierra nevada mojave desert spring mountains and colorado plateau are treated as stable unextended blocks that have moved relative to each other in response to crustal extension with the spring mountains held fixed to the mojave block our reconstruction indicates a minimum of 65 extension 140 km between the southern sierra nevada and colorado plateau

general shear deformation in the pinaleno mountains metamorphic core complex arizona abstract granitic mylonites from the pinaleno mountains metamorphic core complex in southeastern arizona record general shear deformation the mean kinematic vorticity number w m was estimated using 1 the strain ratio r s and the angle between the long axis of the strain ellipsoid with respect to the high strain zone boundary and 2 the porphyroclast hyperbolic distribution method w m for protomylonites and mylonites ranged from 0 6 to 0 9 ultramylonites record lower w m values 0 1 0 3 suggesting that the incremental vorticity changed during deformation three dimensional strain analysis indicates that deformation in protomylonites and mylonites approximates plane strain and has a monoclinic symmetry the vorticity path followed by rocks tectonically exhumed in the footwall of major extensional fault systems may evolve from a significant pure shear component early in the deformation towards simple shear as overburden load decreases during uplift

tubular folds and sheath folds definitions and conceptual models for their development with examples from the grapesvare area northern sweden abstract two parameters are used in the classification of highly non cylindrical folds the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross section of the structure at the position where the hinge angle was measured if in a non cylindrical fold a cross section can be selected so that the hinge angle w measured where the cross section cuts the cone is less than 90 and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross section the fold is a sheath fold a tight sheath fold with w x y 1 can be termed a tubular fold a tubular fold is thus an extreme type of sheath fold while sheath folds with w 20 but x y 0 25 but tubular folds from the grapesvare area comply to the definition given above and they occur in a structural setting that is consistent with the models described

what caused the denudation of the menderes massif review of crustal evolution lithosphere structure and dynamic topography in southwest turkey the deformation of earth s lithosphere in orogenic belts is largely forced externally by the sinking slab but can also be driven by internal delamination processes caused by mechanical instabiliti

constrictional strain in a non coaxial shear zone implications for fold and rock fabric development central mojave metamorphic core complex california finite strain and fold analyses of footwall mylonites in the central mojave metamorphic core complex cmmcc reveal two distinct deformation paths that generated non coaxial constrictional strain the first path recorded in the waterman hills accomplishes constrictional strain through the formation of l tectonites at the grain scale the second path recorded in the mitchel range involves a combination of plane strain at the grain scale and y axis shortening through syn mylonitic folding the ductile shear zone in the waterman hills is approximately 500 m thick and is developed entirely within a miocene syn kinematic granodiorite strain magnitude decreases structurally downward the mylonites contain abundant ne directed non coaxial microstructures yet are l tectonites rf o analysis of quartz ribbon grain shapes indicate prolate strains k 6 and bulk chemical data suggest that mylonitization was isochemical or involved volume loss therefore ductile shearing in the waterman hills involved true constrictional strain at the grain scale immediately to the southeast in the mitchel range the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre tertiary basement the mylonitic fabric comprises a well developed foliation and ne trending stretching lineation rf o strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite in contrast to the waterman hills finite strains in these l s tectonites approximate plane strain k 1 1 however the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation the coaxial folds range from open to isoclinal we interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished y axis shortening in the shear zone macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone suggesting that y axis shortening continued after the fault zone had reached the brittle regime corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in cordilleran metamorphic core complexes therefore constrictional strain may be common in core complexes strain compatibility with upper plate rocks in core complexes may be maintained by y axis shortening along conjugate strike slip faults a nearly uniaxial stress field 3 2 1 is consistent with most of the structures formed in basin and range extension and could generate the observed constrictional strain

large magnitude continental extension an example from the central mojave metamorphic core complex the central mojave metamorphic core complex is defined by a belt of miocene brittle ductile extension and coeval magmatism the brittle ductile fault zone defines a basin and dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes one fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km the axial surfaces of these folds can be traced from the footwall mylonites through the brittle detachment and into hanging wall strata however folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime possibly after movement across the fault zone ceased mesoscopic fabrics related to the transport parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction we interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large magnitude extension the other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km mesoscopic fabrics related to these folds include northwest striking joints kink bands and en echelon tension gash arrays these fabrics formed after mylonitization and record both layer parallel extension and northeast side up subvertical shear the postmylonitic fabrics are kinematically compatible with rolling hinge style isostatic rebound of the footwall following tectonic denudation the relative timing of extension related magma intrusion and ductile deformation varies through the central mojave metamorphic core complex on the scale of the small mountain ranges that make up the central mojave metamorphic core complex no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension related igneous rocks this suggests that models that invoke a single upper crustal genetic relationship such as magmatism triggering extension or vice versa do not apply to the central mojave metamorphic core complex systematic variation in the relative timing of dike emplacement and mylonitization suggests that at the time of dike emplacement rocks in the mitchel range were below the brittle ductile transition while those in the hinkley hills were above it the hinkley hills and mitchel range are separated by 2 km in the dip direction of the fault zone which suggests that the vertical thickness of the brittle ductile transition probably was between 100 and 950 m

tectonics of mid tertiary extension along a transect through west central arizona large magnitude miocene extension in west central arizona occurred primarily along three imbricate northeast dipping normal faults the structurally highest of these faults the gently dipping buckskin rawhide detachment fault accommodated approximately 66 km of crustal extension whereas the two structurally lower faults accommodated a total of about 20 km extension due to this large magnitude extension an area at the earth s surface that was 10 to 20 km wide is now over a 100 km wide and crystalline rocks with mid tertiary mylonitic fabrics uncovered by detachment faulting are exposed over roughly 2000 km in the harcuvar metamorphic core complex most of the upper plate of the buckskin rawhide detachment fault was largely undeformed by internal extension only the thin tapered end of the upper plate was highly extended during extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded forming extension parallel folds in the lower plate low flexural strength characterized the lower plate during denudation and a highly mobile low viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere

timing and tectonic setting of tertiary low angle normal faulting and associated magmatism in the southwestern united states major episodes of tertiary low angle normal faulting in the southwestern united states south of latitude 37 n moved northward with time occurring about 25 million years ago in southern arizona 20 million years ago in the mojave desert region and 10 to 15 million years ago in the las vegas area in most areas faulting and associated volcanism occurred when the mendocino triple junction lay to the west this correspondence probably resulted from the unstable configuration of the triple junction east west to northeast southwest horizontal stretching as plates pulled apart at the triple junction prevented development of a hole in the lithosphere at the triple junction the regional strain history predicted by this model is consistent with observed tertiary low angle faults strike slip faults and folds

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

reply to the comment by g capponi et al on subduction polarity reversal at the junction between the western alps and the northern apennines italy by g vignaroli et al tectonophysics 2008 450 34 50

structural significance of l tectonites in the eastern central laramie mountains wyoming abstract the formation of l tectonites is little understood and scarcely studied however it is probably an important part of plastic deformation in the crust to improve our understanding of this strain phenomenon i present a detailed case study of a kilometer wide domain of l tectonites developed in and around the 2 05 ga boy scout camp granodiorite bscg in the laramie mountains wyoming detailed mapping and structural analyses allow for the reconstruction of the structural setting of this domain of apparent constrictional strain elongation lineations plunge moderately to the south southwest and lie parallel with both the local fold hinge lines and regional fold axes whereas poles to foliation generally cluster in the northwest quadrant roughly defining fold axial surfaces map scale folds are west northwest vergent but at the outcrop and thin section scales there is no evidence for a significant component of simple shear reconstruction of the orientation of contacts in and around the bscg i

late cretaceous to early tertiary transtension and strain partitioning in the chugach accretionary complex se alaska abstract shear zones in the late cretaceous sitka graywacke of the chugach accretionary complex in southeast alaska record constrictional finite strains with maximum principal stretches plunging shallowly subparallel to strike of the shear zones macrostructural analysis indicates the finite strain formed during one deformation event microstructural analysis of the shear zones shows that this deformation is ductile promoted mostly through deformation of low strength lithic clasts and pressure solution kinematic indicators from some of the shear zones indicate dominantly dextral motion although multiple scenarios can explain constrictional finite strains in a shear zone these dextral strike slip shear zones must have experienced a component of extension across them in order to generate constrictional finite strains therefore the shear zones are dextral transtensional shear zones an uncommon tectonic regime in an accretionary complex the transtensional shear zones reflect strike slip motion related to partitioning of late cretaceous to early tertiary right oblique convergence between north america and the farallon plate the extensional component that was superposed on the strike slip shear zones to generate transtension resulted from contemporaneous collapse of the forearc following thickening related to underplating

alternating contraction and extension in the hinterlands of orogenic belts an example from the raft river mountains utah combined macroscopic to microscopic structural analyses detailed geologic mapping and thermochronology were used to study the structural evolution of midcrustal rocks of the sevier orogenic belt hinterland in northwestern utah these results when combined with previous studies provide new insight into the structural history of this region and suggest alternating tectonic contraction and extension during mesozoic to early cenozoic time two allochthons form the upper plate of the miocene raft river detachment fault in the eastern raft river mountains the lower allochthon comprises neoproterozoic ordovician and pennsylvanian strata and is bounded below by the raft river detachment fault and above by the middle detachment fault strata within the lower allochthon were dramatically attenuated by two episodes of ductile deformation the first deformation d1 took place at metamorphic temperatures of 500 c and resulted in penetrative fabrics throughout these rocks that record combined flattening and top to northeast shearing strains the second deformation d2 resulted in significant stratigraphic attenuation along discrete top to the west shear zones that are generally parallel to lithologic contacts separates of synkinematic muscovite from the penetrative fabric yield 40ar 39ar cooling ages that indicate that d1 deformation occurred prior to cooling ca 90 ma both fabrics were subsequently folded about d3 kilometer scale recumbent folds the areally extensive middle allochthon composed chiefly of pennsylvanian and permian rocks metamorphosed in the greenschist facies was emplaced d4 along the low angle middle detachment fault this fault cuts across various structural levels of the recumbently folded lower allochthon in its footwall and juxtaposes greenschist facies over amphibolite facies metamorphic rocks the lower and middle allochthon were subsequently deformed d5 into open folds with north trending axes neogene extension d6 produced an 200 m thick top to east ductile shear zone in precambrian rocks and formed the younger raft river detachment fault which forms the present upper contact of the ductile shear zone northeast vergent d1 fabrics probably record shortening deformation on the basis of fabric correlations with the grouse creek and albion mountains deformation kinematics and synkinematic prograde metamorphism d2 attenuation faults have been interpreted to record crustal extension of late cretaceous age if d3 recumbent folds developed during extension then the deformation sequence records early shortening followed by protracted extensional unroofing with variable structural styles the favored alternative is that d3 recumbent folds developed during shortening in this case d1 through d4 record episodic alternations of contraction and extension such alternations are consistent with observations from analog and theoretical models of contractional mountain belts and suggest that the hinterland of the sevier orogenic belt underwent dynamic adjustments in crustal thickness and deformation kinematics in response to changes in the boundary conditions of the orogenic belt