metamorphism melting and extension age constraints from the high himalayan slab of southeast zanskar and northwest lahaul abstract we present evidence for two distinct stages of tertiary metamorphism m1 and m2 in the high himalayan slab of southeast zanskar and northwest lahaul as well as evidence for an older pre himalayan metamorphism pre m1 the m1 was a regional barrovian type event related to crustal shortening and thickening of the indian plate margin while m2 was associated with crustal melting and the emplacement of the gumburanjon leucogranite into the zanskar shear zone at the top of the slab u pb dating of metamorphic and magmatic accessory phases constrains the timing of m1 between 30 and 37 ma and the crystallization and emplacement age of the gumburanjon leucogranite at 21 22 ma inherited accessory phases in metamorphic and magmatic samples suggest that the protoliths of the slab are at least lower paleozoic in age and that they experienced a major pre m1 thermal perturbation at ca 450 500 ma whether this was associated with a regional barrovian type metamorphism or whether it was a thermal event rel

inverted metamorphism in the intracontinental ductile shear zone during himalayan collision tectonics collision tectonics in the himalaya ca 50 ma remobilized the proterozoic basement and its cover of the indian plate into the higher himalayan crystalline belt this belt displays an inverted metamorphism progressing from a garnet to a sillimanite k feldspar zone toward higher topographic and structural levels because previously hypothesized mechanisms are not sufficient to explain inverted metamorphism we propose that ductile shear displacements along ubiquitous closely spaced s c shear planes may have caused metamorphic inversion across a 15 20 km thick southwest vergent higher himalayan shear zone

tectonic implications and morphology of trapezoidal mica grains from the sutlej section of the higher himalayan shear zone indian himalaya abstractmicroscopic trapezoidal micas are reported from the sutlej section of the higher himalayan shear zone india and are attributed to an up dip top to southwest sense of brittle shear along c planes of preexisting ductile shear dipping northeast the longest margins of these trapezoids dip northeast and are oblique to either the c planes or their enveloping planes intragrain slips along cleavages aided their thrusting while the surrounding competent quartzofeldspathic minerals resisted it the cleavages of the trapezoids are parallel to their longest margins these cleavages the longest margins the overall asymmetry of trapezoid aggregates and the trails emanating from their corners are reliable indicators of brittle shear sense trails broken from the corners of developing trapezoids define both the p and the y shear planes subcircular stacks and triangular mineral grains are also possible variations of trapezoids some of the trapezoidal micas were products of ductile brittle deformation va

climatic control on rapid exhumation along the southern himalayan front abstract along the southern himalayan front shf areas with concentrated precipitation coincide with rapid exhumation as indicated by young mineral cooling ages twenty new young 80 of the annual precipitation aft cooling ages reveal a coincidence between rapid erosion and exhumation that is focused in a 50 70 km wide sector of the himalaya rather than encompassing the entire orogen assuming simplified constant exhumation rates the rocks of two age vs elevation transects were exhumed at 1 4 0 2 and 1 1 0 4 mm a with an average cooling rate of 40 50 c ma during pliocene quarternary time following other recently published hypotheses regarding the relation between tectonics and climate in the himalaya we suggest that this concentrated loss of material was accommodated by motion along a back stepping thrust to the south and a normal fault zone to the north as part of an extruding wedge climatically controlled erosional processes focus on this wedge and suggest that climatically controlled surface processes determine tectonic deformation in the internal part of the himalaya

viscosity of himalayan leucogranites implications for mechanisms of granitic magma ascent the viscosity of the gangotri himalayan leucogranite has been experimentally determined between 80 o and 1100oc 300 and 800 mpa for meltwater contents of 3 98 and 6 66 wt the melt viscosity is independent of pressure and shows an arrhenian behavior relative to temperature within the range of conditions investigated we present an empirical relation that can be used to determine leucogranite magma viscosities knowing their meltwater content and temperature this relation together with phase equilibria experiments constrain the viscosity of the itimalayan leucogranit s to be around 104 5 pas during their eraplacement these viscosities and the widths of dikes belonging to the feeder system are consistent with the theoretical relationship relating these two parameters and show that the precursor magma of the leucogranite was at near liquidus conditions when emplaced within host rocks with preintmsion temperatures around 350oc calculated terminal asa nt rates for the magma in the dikes are around 1 m s magma cmber assembly time is on ths basis estimated to be less than 100 years for a volume of 150 icm3 in addition the dynamical regime of the magma flow in the dikes was essentially laminar thus allowing preservation of any chemical heterogeneity acquired in the source these results constrain the viscosity of melts f rmed during the first steps of crustal anatexis those involving muscovite breakdown to be alsoround 104 5 pas thus compaction may not be the only mechanism of melt segregation in partially melted crustal rocks in view of the very short timescale inferred for magma ascent and emplacement

geochronologic and thermobarometric constraints on the evolution of the main central thrust central nepal himalaya the main central thrust mct juxtaposes the high grade greater himalayan crystallines over the lower grade lesser himalaya formation an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust garnet bearing assemblages sampled along the marysandi river and darondi khola in the annapurna region of central nepal show striking differences in garnet zoning of mn ca mg and fe above and below the mct thermobarometry of mct footwall rocks yields apparent inverted temperature and pressure gradients of 18 c km 1 and 0 06 km mpa 1 respectively pressure temperature p t paths calculated for upper lesser himalaya samples that preserve prograde compositions show evidence of decompression during heating whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature in situ i e analyzed in thin section ion microprobe ages of monazites from rocks immediately beneath the greater himalayan crystallines yield ages from 18 to 22 ma whereas late miocene and pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence a lesser himalayan sample collected near the garnet isograd along the marysandi river transect contains 3 3 0 1 ma monazite ages p 0 72 gpa t 535 c this remarkably young age suggests that this portion of the mct shear zone accommodated a minimum of 30 km of slip over the last 3 ma i e a slip rate of 10 mm yr 1 and thus could account for nearly half of the convergence across the himalaya in this period the distribution of ages and p t histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the mct formed by the transposition of right way up metamorphic sequences during late miocene pliocene shearing

growth of lithosphere scale fault system in ne tibet numerical modeling constrained by high resolution seismic reflection data abstract the east kunlun fault is an important strike slip shear zone to understand continental deformation in ne tibet the recent high resolution deep seismic reflection profiling across the kunlun fault reveals a thrust fault system and two decollements in the crust along with another thrust fault system which reaches to the depth of upper mantle and cuts off the moho the mechanism of the growth of these shear zones at lithosphere scale which reveals the type of deep deformation is investigated using the finite element method with an elastic plastic constitutive relationship the results show that the thrust fault system in the crust may be explained by the conjugated plastic deformation belts under compression from the indian plate the pre existing fault in the depth may develop to cut the moho toward two new directions rather than along the original direction the vertical and lateral heterogeneity of material frictional property and geometry of the models all affect the feature of the fault growth the growth of the thrust fault system on both sides of the kunlun fault is only located in the crust it means that the kunlun fault does not reach to the moho depth

heterogeneous sub simple deformation in the gol e gohar shear zone zagros sw iran insights from microstructural and crystal fabric analyses petrofabrics and microstructural characteristics of the quartz rich mylonites were investigated in the nw se trending gol e gohar shear zone with length of ca 25 km within the zagros transpression zone sw iran the semi quantitative vorticity analysis using quartz c axis fabric rxz method and rigid porphyroclast geometry rigid grain net method reveals that mean kinematic vorticity number wm increases towards the baghat thrust fault displaying the occurrence of spatial and temporal variations of flow during progressive deformation the wm values show pure shear dominated flow progressively was changed to a simple shear dominated flow toward to the late stages of ductile deformation implying that the accelerating flow path was accompanied with progressive cooling and strain localization at the lower parts of the zone the measured vertical thinning dip parallel elongation suggests that thrust emplacement and metamorphic rock extrusion likely occurred by a combination of simple shear with a pure shear component of deformation i e general shear the estimated deformation temperatures based on the quartz c axis fabric patterns and their opening angles highlight greenschist to amphibolite facies conditions

non ellipsoidal inclusions as geological strain markers and competence indicators abstract geological objects that do not deform homogeneously with their matrix can be considered as inclusions with viscosity contrast such inclusions are generally treated as initially spherical or ellipsoidal theory shows that ellipsoidal inclusions deform homogeneously so they maintain an ellipsoidal shape regardless of the viscosity difference however non ellipsoidal inclusions deform inhomogeneously so will become irregular in shape geological objects such as porphyroblasts porphyroclasts and sedimentary clasts are likely to be of this kind with initially rectilinear prismatic or superelliptical section shapes we present two dimensional finite element models of deformed square inclusions in pure shear parallel or diagonal to the square as a preliminary investigation of the deformation of non ellipsoidal inclusions with viscosity contrast competent inclusions develop marked barrel shapes with horn like corners as described for natural ductile boudins or slightly wavy rhombs incompetent inclusions develop dumb bell or bone shapes with a surprising degree of bulging of the shortened edges or rhomb to sheath shapes the results lead to speculation for inclusions in the circle to square shape range and for asymmetric orientations anticipated shapes range from asymmetric barrels lemons or flags for competent inclusions to ribbon or fish shapes for incompetent inclusions we conclude that shapes of inclusions and clasts provide an important new type of strain marker and competence criterion

possible thermal buffering by crustal anatexis in collisional orogens thermobarometric evidence from the nepalese himalaya one dimensional thermal models suggest that large thrust faults may exert first order control on the thermal structure of collisional orogenic belts and an increasing body of petrologic data may be interpreted as consistent with these models we present new thermobarometric data from the central nepalese himalaya which imply that large sections of the himalayan orogen may have been at roughly uniform temperatures during active thrusting a phenomenon that is not predicted by existing models we attribute this phenomenon to thermal buffering as a consequence of widespread in situ anatexis if this process is common in the intermediate and lower crust of active orogens and if it is localized spatially one dimensional thermal models may produce an extremely oversimplified picture of heat transfer processes during collisional orogenesis

u pb zircon geochronology of major lithologic units in the eastern himalaya implications for the origin and assembly of himalayan rocks models for the origin and deformation of himalayan rocks are dependent upon geometric and age relationships between major units we present field mapping and u pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern himalaya revealing that 1 the south tibet detachment along the bhutan china border is a top to the north ductile shear zone 2 late triassic and early cretaceous sedimentary samples from the northern indian margin show a similar age range of detrital zircons from ca 3500 ma to ca 200 ma but the late triassic rocks are distinguished by a significant age cluster between ca 280 and ca 220 ma and a well defined age peak at ca 570 ma 3 an augen gneiss in the south tibet detachment shear zone in southeast tibet has a cambrian ordovician crystallization age 4 main central thrust hanging wall paragneiss and footwall quartzites from the far western arunachal himalaya share similar provenance and late proterozoic maximum depositional ages and 5 main central thrust footwall metagraywacke from the central western arunachal himalaya has a paleoproterozoic maximum depositional age indicated by a single prominent age peak of ca 1780 ma recent work in the eastern himalaya demonstrates that in the early middle miocene the himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south a proposed subsequent middle miocene brittle low angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here the ca 280 220 ma detrital zircons of the late triassic strata are derived from an arc developed along the northern margin of the lhasa terrane detritus from this arc was deposited on the northern margin of india during india lhasa rifting along strike heterogeneity in main central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western arunachal nonetheless the late proterozoic rocks in the main central thrust hanging wall and footwall in far western arunachal can be correlated to each other and to previously analyzed rocks in the south tibet detachment hanging wall to the west and in the indian craton to the south these findings are synthesized in a reconstruction showing late triassic india lhasa rifting and cenozoic eastern himalayan construction via in situ thrusting of basement and cover sequences along the north indian margin

flow controlled inverted metamorphism in shear zones abstract numerous processes and models have been developed in order to explain inverted metamorphism in shear zones this article demonstrates by means of continuum mechanics that the orientation of a sample profile across a shear zone before and after deformation can be used to quantify the conditions under which inverted metamorphism is likely to have developed a thrusting shear zone deforming under ideal simple shear will develop an inverted metamorphic zonation if the angle between the thrust and isotherms is greater than 5 10 even at only moderate strains if the flow is general shear this angle is considerably larger i e 60 for a kinematic vorticity number of w k 0 5 additionally the stretch of the sample profile across the shear zone which is not influenced by the flow type bears important implications for the interpretation of petrological geochronological and structural data although this kinematic model is clearly an end member model neglecting dissipative processes it can easily account for the structural and petrological data of the inverted metamorphism observed throughout the higher himalayan crystalline wedge in the sutlej valley india

higher himalayan shear zone sutlej section structural geology and extrusion mechanism by various combinations of simple shear pure shear and channel flow in shifting modes the higher himalayan shear zone hhsz in the sutlej section reveals 1 top to sw ductile shearing 2 top to ne ductile shearing in the upper and the lower strands of the south tibetan detachment system stdsu stdsl and 3 top to sw brittle shearing corroborated by trapezoid shaped minerals in micro scale in the proposed extrusion model of the hhsz the e1 phase during 25 19 ma is marked by simple shearing of the upper sub channel defined by the upper strand of the main central thrust mctu and the top of stdsu as the lower and the upper boundaries respectively subsequently the e2a pulse during 15 14 ma was characterized by simple shear pure shear and channel flow of the entire hhsz finally the e2b pulse during 14 12 ma observed simple shearing and channel flow of the lower sub channel defined by the lower strand of the main central thrust mctl and the top of the stdsl as the lower and the upper boundaries respectively the model explains the constraints of thicknesses of the stdsu and the stdsl along with spatially variable extrusion rate and the inverted metamorphism of the hhsz the model predicts 1 shear strain after ductile extrusion to be maximum at the boundaries of the hhsz which crudely matches with the existing data the other speculations that cannot be checked are 2 uniform shear strain from the mctu to the top of the hhsz in the e1 phase 3 fastest rates of extrusion of the lower boundaries of the stdsu and the stdsl during the e2a and e2b pulses respectively and 4 variable thickness of the stdsl and rare absence of the stdsu non parabolic shear fabrics of the hhsz possibly indicate heterogeneous strain the top to sw brittle shearing around 12 ma augmented the ductile extruded rocks to arrive a shallower depth the brittle ductile extension leading to boudinage possibly did not enhance the extrusion

relationship between surface atmospheric convergence over indian ocean and indian rainfall atmospheric convergence regions over the tropical indian ocean have been mapped for the first time using 31 years of vector wind data the convergence fields reveal that the energetic summer monsoon winds enhance convergence in the central arabian sea and in the eastern bay of bengal when compared with weak winter monsoon winds the summer monsoon is also effective in spatial migration of convergence regions in both basins area integrated convergence time series reveals an annual cycle with high amplitude during summer monsoon which occurs in phase with indian rainfall the study explores the prospects of using the convergence as one of the predictors of indian rainfall

higher himalayan shear zone zanskar indian himalaya microstructural studies and extrusion mechanism by a combination of simple shear and channel flow thin section studies of the zanskar shear zone zsz rocks reveal a top to sw and subsequent primary and secondary top to ne ductile shearing brittle ductile and brittle extensions top to sw brittle shear steep normal faulting and fracturing in the proposed two phase model of ductile extrusion of the higher himalayan shear zone hhsz a top to sw simple shearing during 22 18 ma was followed by a combination of top to sw simple shear and channel flow at 18 16 ma the second phase simulates a thin zsz characterized by a top to ne shearing the channel flow component ceased around 16 ma the extruding hhsz entered the brittle regime but the top to sw shearing continued until perturbed by faults and fractures variation in the extrusion parameters led to variable thickness of the zsz shear strain after the extrusion is presumably maximum at the boundaries of the hhsz and falls towards the base of the zsz which crudely matches with the existing data the other predictions 1 spatially uniform shear strain after the first stage 2 fastest extrusion rate at the base of the zsz and 3 a lack of continuation of the zsz along the himalayan trend are not possible to validate due to paucity of suitable data non parabolic shear fabrics of the zsz indicate their heterogeneous deformation

field documentation and genesis of back structures in ductile and brittle regimes from the foreland part of a collisional orogen examples from the darjeeling sikkim lesser himalaya india in the foreland side of an orogen thrust related structures develop that verge towards the foreland fore thrusts although much less abundant structures with opposite vergence i e towards hinterland named back structures also exist we report back structures exposure scale back thrusts and associated folds from the darjeeling sikkim lesser himalaya india these structures occur mainly in three back structure zones bsz a b and c back structures probably originated in the ductile regime and continued in the brittle domain tectonic setting of the study area such as critical taper duplexing etc indicates a good potential for the presence of back structures in this region along with the previous work on back structures in collisional orogens himalaya in particular these structures seem to be ubiquitous the correlation of field observations with tectonics indicate that mechanisms e g critical taper sub surface barriers and passive roof duplex played roles in forming back structures in this part of the sikkim lesser himalaya

analogue modelling of syntectonic leucosomes in migmatitic schists migmatites from the cap de creus tectonometamorphic belt display a wide variety of structures from those formed when the leucosomes were melt bearing to those developed during solid state deformation the observed field structures have been modelled by means of analogue experiments the materials used in the models are layered plasticine as a schist analogue and chocolate as analogue of the crystallizing leucosome a model for the development of syntectonic migmatites is proposed in which initial melt bearing patches preferentially formed within fertile pelitic layers progressively evolve towards lens shaped veins furthermore heterogeneous deformation of anisotropic metasediments facilitates formation of extensional sites for further melt accumulation and transport melt crystallization implies a rapid increase in effective viscosity of leucosomes producing a reversal in competence contrast with respect to the enclosing schists during the whole process deformation localizes around crystallizing veins giving rise to different and contrasting structures for melt bearing and for solid state stages

crustal deformation and strain analysis in nepal from gps time series measurement and modeling by arma method high precision global positioning system gps measurements from 2002 to 2017 in nepal has been analyzed and modeled with autoregressive moving average arma method the measured velocities of the gps sites in itrf 2014 indicate 6 12 mm year n s convergence inside the nepal between india and tibet this convergence occurs in the southern and western border of nepal but it is negligible to the eastern border of nepal arma modeled result of india fixed velocities is able to successfully model within 0 5 mm year the current measured velocities in nepal synthetic or modeled velocities using arma method and measured velocity have difference 2 mm year at some sites this indicates the possibility of very small level of a ductile deformation in the present best fit arma model the modeled velocities 8 10 mm year convergence taken up from northern to southern boundaries of nepal indicates slip along active main himalayan thrust mht which is the cause of change in tectonic strain and may be able to trigger earthquakes inside the country dislocation modeling of active mht is also carried out in the study using gps vertical displacement the geometric model of the mht is characterized by a rectangular plane with dip angle of 9 5 and the reverse kinematics with a slip of 19 mm year the thrust is located at a depth of 20 km in addition the regional deformation of the nepal has been checked by baseline measurement and coordinate stability of gps sites for the different zones the baseline divergence and convergence of nearest gps sites have been tested using triangular sites the summary of regional deformation implies n s convergence only and rules out any type of e w convergence finally the investigation of observed and modeled strain confirms that there is no significant strain accumulation due to localized regional deformation or earthquakes specific to the active dislocations and faults this suggests that the convergence from northern to southern boundaries of nepal is associated with the slip along the mht only

deformation and kinematic evolution of the subsurface structures zagros foreland fold and thrust belt northern dezful embayment iran the dezful embayment is located in the foreland part of the zagros fold and thrust belt structural style of folding and thrusting vary in the dezful embayment in this study balanced cross sections and subsurface data including 2d seismic profiles and wells data decoded structural style of the subsurface structures in the northern dezful embayment presence of the multiple decollement horizons is the main controlling factor of the structural style in this area the subsurface anticlines have been formed between two main decollement horizons which include the miocene gachsaran formation as upper decollement and permian dashtak evaporites and lower cretaceous garau shales as the middle decollement horizons geometry of the subsurface anticlines differs much vertically and horizontally growth strata indicate folding is started in middle miocene time in this region anticlines formed as open wide and disharmonic structures active processes in the evolution of anticlines are limb rotation and hinge migration which was resulted in increase of inhomogeneous shortening rate more shortening rate indicates more structural relief in anticlines these anticlines are formed as a detachment folds in initiation and then during their evolution converted to fault propagation fold and fault bend fold final geometric shape of these anticlines depends on the geometry of thrusts propagation that formed in the forelimb

superimposed fabrics due to reversal of shear sense an example from the bergen arc shear zone western norway abstract a profile through the upper margin of a large scale ductile shear zone the bergen arc shear zone western norway has been studied in detail within the shear zone asymmetric structures indicate dextral or top to nw movement which is interpreted as representing late or post caledonian extensional tectonics the hanging wall of the shear zone is characterised by kinematic indicators such as shear bands associated with curved or inclined foliation and asymmetric boudins indicating sinistral or top to se movements these are interpreted as related to thrusting during the caledonian orogeny the studied profile shows the progressive overprinting of the sinistral structures by the later dextral shear this first led to a decrease in the obtuse angle between shear bands and the curved inclined foliation subsequently backfolding of asymmetric boudins in quartz veins occurred and a composite cleavage developed from shear bands and associated curved inclined foliation dextral asymmetric extensional shear bands were developed at a later stage when the folded quartz veins had attained tight to isoclinal geometries

partial melt in the upper middle crust of the northwest himalaya revealed by rayleigh wave dispersion article i nfo seismic shear wave velocities are sensitive to the partial melts that should be present in the himalayan orogen if low viscosity channel flow is active at the present day we analyzed regional earthquakes in the western himalaya and tibet recorded on 16 broadband seismometers deployed across the nw indian himalaya from the indian platform to the karakoram range we used a multiple filter technique to calculate the group velocity dispersion of fundamental mode rayleigh waves and then inverted the dispersion records to obtain separate one dimensional shear wave velocity models for five geologic provinces the tibetan plateau ladakh arc complex indus tsangpo suture zone tethyan himalaya and himalayan thrust belt our velocity models show a low velocity layer lvl with 7 17 velocity reduction centered at 30 km depth and apparently continuous from the tethyan himalaya to the tibetan plateau this lvl shows good spatial correspondence with observations of low resistivity from magnetotelluric studies along the same profile of the possible explanations for low velocity and low resistivity in the mid crust only the presence of melts or aqueous fluids or both satisfactorily explains both sets of observations elevated heat flow observed in the nw himalaya implies that if aqueous fluids are present in the mid crust then the mid crust is well above its solidus comparison of our results with laboratory measurements and theoretical models suggests 3 7 melt is present in a channel in the upper middle crust of the nw himalaya at the present day and the physical conditions to enable active channel flow may be present

near n s paleo extension in the western deccan region india does it link strike slip tectonics with india seychelles rifting this is the first detailed report and analyses of deformation from the w part of the deccan large igneous province dlip maharashtra india this deformation related to the india seychelles rifting during late cretaceous early paleocene was studied and the paleostress tensors were deduced near n s trending shear zones lineaments and faults were already reported without significant detail an e w extension was envisaged by the previous workers to explain the india seychelles rift at 64 ma the direction of extension however does not match with their n s brittle shear zones and also those faults sub vertical ne sw nw se and few n s we report and emphasize in this work slickenside bearing fault planes brittle shear zones and extension fractures in meso scale enabled us to estimate the paleostress tensors directions and relative magnitudes the field study was complemented by remote sensing lineament analyses to map dykes and shear zones dykes emplaced along pre existing n s to ne sw nw se shears fractures this information was used to derive regional paleostress trends a nw se ne sw minimum compressive stress in the oldest kalsubai subgroup and a n s direction for the younger lonavala wai and salsette subgroups were deciphered thus a nw ne to n s extension is put forward that refutes the popular view of e w india seychelles extension paleostress analyses indicate that this is an oblique rifted margin field criteria suggest only ne sw and nw se with some n s strike slip faults brittle shear zones we refer this deformation zone as the western deccan strike slip zone wdsz the observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies these geophysical findings too indicate oblique rifting in this part of the w indian passive margin we argue that the seychelles microcontinent separated from india only after much of the dlip erupted further studies of magma rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required

duplex structures in the lewis thrust sheet crowsnest pass rocky mountains alberta canada abstract the lewis thrust sheet of the southern canadian rocky mountains contains many spectacular examples of small scale duplex structures this paper presents the results of a detailed analysis of such structures found in the mississippian carbonates of the banff formation at crowsnest pass southwestern alberta foreland dipping hinterland dipping and antiformal stacked duplexes are found in the hangingwall of the lewis thrust out of sequence thrusts back thrusts and folds that push out of the plane of the cross section termed lateral lobes give rise to complex internal geometries dominant slip vectors are towards 080 090 but the complex fault geometries have generated significant variations in slip away from this direction the duplex structures occur as discrete thrust fault bounded packages with each package having different slip vectors the panels above and below the duplex structures show consistent slip vectors towards 080 090 whereas the duplexes exhibit a wide scatter of slip vectors from 350 160 the stacking of duplexes with many horses can be likened to the stacking of many inverted soup bowls herein termed turtle back structures and will involve a wide scatter of slip directions particularly if the horses are of limited lateral extent such a stacking mechanism involving out of section movement invalidates the assumption of two dimensional plane strain in the plane of the cross section that contains the regional tectonic transport direction correctly balanced cross sections cannot be constructed through such stacked duplex structures as described in this paper

upper crustal shortening and forward modeling of the himalayan thrust belt along the budhi gandaki river central nepal a balanced cross section along the budhi gandaki river in central nepal between the main central thrust including displacement on that fault and the main frontal thrust reveals a minimum total shortening of 400 km minimum displacement on major orogen scale structures include 116 km on the main central thrust 110 km on the ramgarh thrust 95 km on the trishuli thrust and 56 km in the lesser himalayan duplex the balanced cross section was also incrementally forward modeled assuming a generally forward breaking sequence of thrusting where early faults and hanging wall structures are passively carried from the hinterland toward the foreland the approximate correspondence of the forward modeled result to observe present day geometries suggest that the section interpretation is viable and admissible in the balanced cross section the trishuli thrust is the roof thrust for the lesser himalayan duplex the forward model and reconstruction emphasize that the lesser himalayan duplex grew by incorporating rock from the footwall and transferring it to the hanging wall along the main himalayan thrust as the duplex developed the lesser himalayan ramp migrated southward the movement of lesser himalayan thrust sheets over the ramp pushed the lesser himalayan rock and the overburdens of the greater and tibetan himalayan rock toward the erosional surface this vertical structural movement caused by footwall collapse and duplexing in combination with erosion exhumed the lesser himalaya

review of flanking structures in meso and micro scales a variety of host fabric elements he cut by cross cutting elements ce in rocks defines flanking structures fs on mesoscopic and microscopic scales there has been renewed interest in studying and classifying the fs for their morphologies useful as shear sense indicators and geneses existing non genetic morphologic parameters for the fs are reviewed and two new classification schemes are presented one of these is based on the nature of the ce and whether he penetrates it the other scheme takes account of all the potential combinations of drag no drag and slip no slip of the he deciphering the shear sense of the rock body from fs is complicated because the angular relationship between the ce and the primary shear planes might be opposite to what is found between s and c ductile shear fabrics further single ces can curve and several similar fs occur in reverse forms as with mineral fish the shape asymmetries of microscopic ces indicate the shear sense conjugate fs with non parallel ces with interfering perturbation fields around the ces are more reliable shear sense indicators than fs with single ce during low but increasing bulk strains fs may evolve from one type to another e g from a to s type at high strain fs can resemble intrafolial or sheath fold whether the drag is normal or reverse depends fundamentally on the initial angle between the he and the ce and the relative magnitudes of throw and vertical separation

thermal constraints on the emplacement rate of a large intrusive complex the manaslu leucogranite nepal himalaya the emplacement of the manaslu leucogranite body nepal himalaya has been modelled as the accretion of successive sills the leucogranite is characterized by isotopic heterogeneities suggesting limited magma convection and by a thin 100 m upper thermal aureole these characteristics were used to constrain the maximum magma emplacement rate models were tested with sills injected regularly over the whole duration of emplacement and with two emplacement sequences separated by a repose period additionally the hypothesis of a tectonic top contact with unroofing limiting heat transfer during magma emplacement was evaluated in this latter case the upper limit for the emplacement rate was estimated at 3 4 mm year or 1 5 myr for 5 km of granite geological and thermobarometric data however argue against a major role of fault activity in magma cooling during the leucogranite emplacement the best model in agreement with available geochronological data suggests an emplacement rate of 1 mm year for a relatively shallow level of emplacement granite top at 10 km uninterrupted by a long repose period the thermal aureole temperature and thickness and the isotopic heterogeneities within the leucogranite can be explained by the accretion of 20 60 m thick sills intruded every 20 000 60 000 years over a period of 5 myr under such conditions the thermal effects of granite intrusion on the underlying rocks appear limited and cannot be invoked as a cause for the formation of migmatites

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

pre miocene deformation of the himalayan superstructure hidden valley central nepal abstract throughout the himalaya the tethyan sedimentary sequence forms the detached carapace or superstructure to exhumed mid crustal rocks in central nepal low metamorphic grade cambrian to jurassic rocks of this carapace preserve five phases of deformation structurally dominated by second phase d 2 north verging back folds that oppose the vergence of the orogen the folds are overprinted and cross cut by the c 22 ma south tibetan detachment system and the manaslu leucogranite new structural mapping in hidden valley provides further constraints on the character of d 2 megascopically large asymmetric north verging folds define d 2 microscopically it is characterized by an axial planar cleavage balanced cross sections and bed length restoration of f 2 folds indicate a minimum of 32 38 shortening and 180 thickening during d 2 these data indicate that north verging folds played a significant role in pre miocene crustal thickening of the himalayan superstructure the formation of these folds is compatible with wedge extrusion or channel flow of the underlying mid crustal rocks whereby their geometry reflects early coupling between the upper and middle crust

zanskar shear zone northeast southwest extension within the higher himalayas ladakh india a complete geologic section through the northwestern himalayas is well exposed in ladakh india the collision between the indian shield and the tibetan platform caused large scale intracrustal thrusting and piling up of the himalayan nappes this compressional event was accompanied by prograde regional metamorphism the metamorphism ranges from low grade anchimetamorphism in the indus suture zone in the northeast to upper amphibolite grade in the higher himalaya tectonic unit in the southwest in the zanskar area a well exposed and morphologically prominent shear zone 2 25 to 6 75 km wide can be followed from the northwest to the southeast for at least 80 km in contrast to the main compressional tectonics this shear zone indicates an extensional event within the higher himalaya tectonic unit the zanskar shear zone involves granitic rocks leucogranites of miocene age and various metasediments and in general separates the late precambrian early cambrian sedimentary sequence phe formation from the underlying crystalline basement zanskar crystalline unit a normal sense of shear has been determined by using asymmetric macroscopic and microscopic structures such as pegmatite boudins feldspar augen and crystallographic fabrics the metamorphic isograds are very close within this shear zone as a result of shear deformation a gradual transition from upper amphibolite to lower greenschist facies occurs within 200 m movement in this zone occurred late in the metamorphic history under greenschist or lower metamorphic conditions the minimum horizontal extension and vertical displacement are on the order of 16 and 19 km respectively

first evidence for high grade himalayan age synconvergent extension recognised within the western syntaxis nanga parbat pakistan we present evidence for horizons of medium to high grade 400 600 c deformation accompanying normal sense displacement in high strain himalayan gneisses and schists from the southeastern nanga parbat massif the western syntaxis in the pakistan himalaya in their present orientation the broadly n s trending steeply dipping gneisses show microstructural and outcrop scale evidence for dextral and sinistral shear in discrete layers we interpret these as horizons of thrust and normal motion within the footwall of the original main mantle thrust once neogene antiformal folding is removed this is the first report of significant normal motion in the main mantle thrust footwall in the nanga parbat syntaxis and may indicate that synconvergent extension in the himalaya extended to the western syntaxis this episode of extension could correspond to a period of similar normal motion in the central himalaya or represent a separate event earlier in the orogeny

microstructures and strain variation evidence of multiple splays in the north almora thrust zone kumaun lesser himalaya uttarakhand india abstract the north almora thrust zone natz marks the boundary of the almora crystalline complex acc against the lesser himalayan sedimentary sequence lhs in the north its southern counterpart the south almora thrust sat is a sharply marked contact between the acc and the lhs in the south published studies argue various contradictory emplacement modes of the north almora thrust recent studies have implied splays of smaller back thrusts in the natz the present study investigates meso and microstructures and strain distribution in the natz and compares it with strain distribution across the sat in the natz field evidence reveals repeated sequence of 10 500 m thick slices of proto to ultra mylonite thrust over the lesser himalayan rautgara quartzite in accordance with the field evidence the strain analysis reveals effects of splays of smaller thrust in the natz the study therefore argues that contrary to popular nomenclature the northern contact of the acc with the lhs is not a single thrust plane but a thrust zone marked by numerous thrust splays

the influence of strain localisation on the rotation behaviour of rigid objects in experimental shear zones abstract mica fish and tourmaline fish from natural mylonites were analysed in thin section to determine their orientation distribution they are oriented with their long axes tilted with respect to the mylonitic foliation and fish with a small aspect ratio exhibit a slightly larger angle than fish with a large aspect ratio this orientation seems to be a stable orientation for the mica and tourmaline fish analogue experiments with two rheologically different matrix materials were performed to explain the data one material was pdms a linear viscous polymer the other was tapioca pearls a granular material with low cohesion and mohr coulomb type behaviour in contrast to a fairly homogeneous strain distribution in pdms distinct small scale shear bands developed in tapioca pearls during deformation experiments modelled different vorticity numbers and parallelogram shaped rigid objects with different aspect ratios were used rotation rates of objects in a viscous matrix are very similar to analytical solutions for ellipses in viscous flow but stable orientations differ from data of natural examples in all experiments with a mohr coulomb matrix elongated objects had a stable orientation due to small scale strain localisation we therefore suggest that small scale strain localisation mm that might be hidden by ongoing deformation and recrystallisation processes is an important characteristic of the rheology of mylonites

first field identification of the cuonadong dome in southern tibet implications for ew extension of the north himalayan gneiss dome the cuonadong dome exposes in east southern margin of the north himalayan gneiss domes nhgd which is reported first time in this study the cuonadong dome is located at the southern part of the zhaxikang ore concentration area which is divided into three tectono lithostratigraphic units by two curved faults around the dome geometry from upper to lower or from outer to inner the upper unit middle unit and lower unit and the outer fault is nading fault while the inner fault is jisong fault the cuonadong dome is a magmatic orthogneiss and leucogranite mantled by orthogneiss and metasedimentary rocks which in turn are overlain by jurassic metasedimentary and sedimentary rocks the grades of metamorphism and structural deformation increase towards the core which is correspondence with the ridang formation low metamorphic schist tourmaline granitic biotite gneiss garnet mica gneiss and mylonitic quartz mica gneiss the cuonadong dome preserves evidences for four major deformational events firstly top to s thrust d1 early approximately n s extensional deformation d2 main approximately e w extensional deformation d3 and late collapse structural deformation d4 around the core of the cuonadong dome which are consistent to three groups lineation approximately n s trending lineation including l1 and l2 e w trending l3 and l4 with plunging towards outside of the dome respectively the formation of the cuonadong dome was probably resulted from the main e w extensional deformation which is a result of eastward flow of middle or lower crust from beneath tibet accommodated by northward oblique underthrusting of indian crust beneath tibet the establishment of the cuonadong dome enhanced the e w extension of the nhgd which is further divided into two structural dome zones according to the different extensional directions approximately n s extensional north himalayan gneiss domes ns nhgd and e w extensional north himalayan gneiss domes ew nhgd the ns nhgd developed by a dominantly n s contraction and locally extensional regime and keep a close relationship to the south tibetan detachment system whereas the ew nhgd formed by an e w extensional deformation along the north south trending rifts

tectonic evolution of the himalayas and tibet shackleton r m dewey j f and windley b f editors 1988 philosophical transactions of the royal society of london series a 325 pp price 65

deformational and strain patterns of an intracontinental collision ductile shear zone an example from the higher garhwal himalaya the higher himalayan metamorphic belt of garhwal appears to have evolved in a major 15 20 km wide ne dipping ductile shear zone of the overthrust type due to intracontinental crustal shortening during cenozoic continental collision within the indian plate out of the four major distinct deformation phases the most widespread d2 deformation phase is marked by a prominent s2 foliation axial planar to the reclined and recumbent f2 folds a coaxial ne plunging l2 stretching lineation and syntectonic growth of index metamorphic minerals these deformational structures are developed during the sw directed d2 ductile shear deformation irrespective of the orientation of the s2 foliation and thrust zones regional strain patterns in quartzite of the lesser himalayan garhwal group beneath the main central thrust mct and the central crystalline zone using rf data on quartz clasts and aggregates reveal that the maximum values of finite strain es are attained within narrow mylonitic shear zones within the broad ductile shear zone during its progressive translation towards higher structural levels in late d2 and d3 deformation phases the shortening was accommodated by development of thrust sheets with the mct and jutogh thrust surfaces coinciding with zones of mylonite and maximum strain at higher levels ductile shear zones became compressional brittle ductile zones which resulted in the development of sw verging f3a and f3b folds during the southward migration of the thrust sheets in the lesser himalaya

mineral fish their morphological classification usefulness as shear sense indicators and genesis mineral fish are sheared and commonly asymmetric mineral grains or clusters of grains this work reports 11 sub types of mineral fish showing a top to se sense of ductile shearing in the karakoram metamorphic complex kmc the mineral fish are of three broad geometries sigmoid lenticular and parallelogram reliable senses of shear are indicated by the overall asymmetry and inclination of mineral fish on the other hand the true shear sense is not always indicated by either the orientations of their cleavage planes or those of the individual grains in composite mineral fish the ranges of local orientations of single sigmoid mineral fish that include the lower values 23 in the kmc indicate their extensive ductile shearing the studied mineral fish were products of a range of deformation mechanisms including homogeneous deformation simple shear intra granular slip crystal plastic deformation fracturing and synthetic shearing additionally some examples might have undergone duplex slips and a few nucleated and grew either prior to or during the top to se shearing

high precision geothermobarometry across the high himalayan metamorphic sequence langtang valley nepal one of the long recognized features of himalayan geology is the apparent inversion of metamorphic sequences as evidenced in both metamorphic parageneses and thermobarometric data with the aid of an extended thermobarometric dataset from the langtang valley section of the higher himalayan crystallines it can be demonstrated that the relatively large uncertainties associated with traditional thermobarometric techniques severely limit the tectonic interpretation of metamorphic gradients across the himalayas we apply the recently developed dpt approach which significantly improves the precision to which pressure and temperature diverences between samples may be calculated high precision thermo barometric data reveal an isothermal rather than inverted temperature array at langtang while the pressure data suggest significant structural complexity with the higher himalayan crystallines in the langtang section comprising two distinct possibly duplicated sequences each having experienced con siderable structural attenuation following metamorphism

viscous dissipation pattern in incompressible newtonian simple shear zones an analytical model an analytical model of shear heating in an inclined simple shear zone with newtonian rheology under a reverse shear sense and an upward resultant pressure gradient is presented neglecting a number of secondary factors the shear heat is expressed as a function of the total slip rates at the boundaries pressure gradient dip and thickness of the shear zone and density viscosity and thermal conductivity of the rock a quartic temperature profile develops with a point of maximum temperature near the bottom part of the shear zone in general the profile is parabolic if pressure gradient vanishes leading to a couette flow the profile attains a bell shape if there is no slip at the boundaries i e a true pouseille flow the present model of shear heating is more applicable in subduction channels and some extruding salt diapirs where the rheology is newtonian viscous and pressure gradient drives extrusion

micromechanisms of shear zone propagation at the brittle viscous transition our investigation of progressively strained rock samples from the margins of greenschist facies shear zones utilizes a space for time approach to reveal how the mylonitic overprint of metapsammitic and pelitic host rocks at the cap de creus involved brittle fracturing we present a set of microscale observations indicating that microfractures formed immediately prior to or coevally with a fine grained mylonite microfracturing dominated early stages of strain localization on the scale of the shear zones on the microscale centimeter long fractures facilitated strain softening by allowing enhanced fluid access thereby accelerating the dynamic recrystallization of quartz and a metamorphic reaction of biotite as these two processes produce a polyphase matrix of small dislocation poor grains that eventually form an interconnected rheologically weak phase fractures become inactive this represents a strain dependent brittle viscous transition we outline this transition in a conceptual model for the rheological evolution of mid crustal shear zones

controls on cu mineralization in central part of the kerman porphyry copper belt se iran constraints from structural and spatial pattern analysis abstract the sarduiyeh area in the central part of the kerman porphyry copper belt kpcb se iran contains numerous cu deposits and prospects this area is bound by e w trending major faults which show the northern and southern borders of fault duplex minor faults first second or third orders were identified in three trends of nw n s and ne analysis of the fault kinematics data revealed an extensional fault duplex pattern for this area in this fault zone cu deposits are associated with faults that have a special trend this proposed kinematic model for evolution of sarduiyeh extensional fault duplex emphasizes that the main cu deposits in the sarduiyeh area are located within the dilatational zones related to nw se trending normal faults in this study the spatial pattern analysis conducted by fry fractal fracture density and buffer zones methods support the conceptual model of district scale mechanism of structural controls on final emplacement of cu deposits this investigation of sarduiyeh cu mineralization revealed that the nw se trending faults are effective in localization of cu mineralization in the central part of kpcb

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

subsurface profiling of granite pluton using microtremor method southern aravalli gujarat india we report using the microtremor method a subsurface granitic pluton underneath the narukot dome and in its western extension along a wnw profile in proximity of eastern fringe of cambay rift india the dome and its extension is a part of the champaner group of rocks belonging to the mesoproterozoic aravalli supergroup the present finding elucidates development of an asymmetric double plunge along narukot dome microtremor measurements at 32 sites were carried out along the axial trace n95 of the dome fourier amplitude spectral studies were applied to obtain the ratio between the horizontal and vertical components of persisting rayleigh waves as local ambient noise fundamental resonant frequencies with amplitude 1 sigma for each site are considered to distinguish rheological boundary two distinct rheological boundaries are identified based on frequency ranges determined in the terrain 1 0 2219 10 364 hz recorded at 31 stations identified as the champaner metasediment and granite boundary and 2 10 902 27 1119 hz recorded at 22 stations identified as the phyllite and quartzite boundary the proposed equation describing frequency depth relationship between granite and overlaying regolith matches with those already published in the literature the morphology of granite pluton highlights the rootless character of champaner group showing sharp discordance with granitic pluton the findings of manifestation of pluton at a shallower depth imply a steep easterly plunge within the champaner metasediments whereas signature of pluton at a deeper level implies a gentle westerly plunge the present method enables to assess how granite emplacement influences the surface structure

timing quantification and tectonic modelling of pliocene quaternary movements in the nw himalaya evidence from fission track dating abstract variable exhumation rates deduced from the pliocene quaternary ft zircon apatite ages from the himalayan metamorphic belt hmb of the nw himalaya along the sutlej valley in himachal pradesh have been modelled in the tectonic framework of fast exhumed lesser himalayan windows which caused lateral extensional sliding of the metamorphic nappe cover along the well known main central thrust mct and differential movements along thrust zones as well in the northern belt of the higher himalayan crystallines hhc two distinct clusters of the ft apatite ages have been deciphered apatite ages having a weighted mean of 4 9 0 2 ma 1 in basal parts on the hanging wall of the mct and 1 49 0 07 ma 1 in the hanging wall of a newly recognized ne dipping chaura thrust further north fast exhumation of the chaura thrust hanging wall has been inferred at a rate of 4 82 0 55 mm yr from the zircon apatite cogenetic pairs during 1 54 ma and 0 97 ma and 2 01 0 35 mm yr since 1 49 ma in comparison its foot wall has been exhumed at a much slower rate of 0 61 0 10 mm yr since 4 9 ma the overlying vaikrita thrust zone rocks reveal an exhumation rate of 1 98 0 34 mm yr from 2 70 0 40 ma to 1 31 0 22 ma and 2 29 0 66 mm yr since 1 31 0 22 ma using these data a vertical displacement of ca 2 08 0 68 km has been calculated along the chaura thrust between 4 9 and 1 50 ma on an average rate of 0 6 mm yr it is of the order of 1 18 km from 2 70 ma to 1 54 ma along the vaikrita thrust and 0 78 mm yr from 1 31 ma to 0 97 ma and has behaved as an extensional normal fault during these periods tectonic modelling of the exhumation rates in the nw himalaya reveals fastest uplifting himalayan domes and windows like the nanga parbat in pakistan suru and chisoti domes in zanskar and kishwar kulu rampur window axis in se kashmir and himachal pradesh during pliocene quaternary these windows appear to have caused lateral extensional sliding of the himalayan metamorphic nappes in the lower parts the middle parts of the hhc belt have witnessed both overthrusting and extensional faulting due to complex and variable exhumation patterns within the hanging and foot walls of the mct and vaikrita thrust along the sutlej valley thus causing movement of upthrust crustal wedge between the extensional ones thus ft zircon apatite ages provide evidence for the presence of a number of crustal wedges having distinct tectonothermal history within the hhc

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

analog modeling of one way gravitational spreading of hot orogens a case study from the svecofennian orogen fennoscandian shield abstract geodynamic models offer insight into deformation styles of syn and post collisional extensional processes such models often ignore the effect of lateral viscosity contrast between accreted terranes and inherited tectonic boundaries this paper presents a set of crustal scale analog experiments of one way post collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechanically contrasting blocks pre existing cuts with varying dips and three layers with contrasting rheologies two blocks represent strong mature and weak juvenile crustal parts both blocks have three horizontal crustal layers brittle upper weak partially molten middle and dry strong lower layer cuts represent pre existing crustal scale shear zones and tectonic boundaries the experiments indicated that tectonic boundaries may facilitate exhumation and increase extensional strain when the boundaries dip opposite to the flow direction the tectonic boundaries may also split and divide the crust into separate blocks with independent deformation signatures shortening elongation or rotation the tectonic boundaries rotated along vertical axes toward the flow direction the modeling results suggest that in areas with inherited tectonic boundaries and compositional differences the amount of extension is unevenly distributed between the different parts during the widespread unilateral gravitational spreading and that the flow has different effects on the contrasting crustal units in both lateral and vertical directions to validate the modeling the results are compared to geophysical and geological data from the paleoproterozoic svecofennian orogen in the fennoscandian shield the comparison suggests that the orogen underwent 50 extension and was thinned by maximum of 20 via westward gravitational spreading this spreading rotated the crustal scale blocks shear zones and tectonic contacts it also brought exhumed high grade middle crustal blocks into contact with subsided low grade upper crustal blocks we suggest that results of the analog models can be used as proxies in both ancient and modern hot accreted orogens which have undergone post convergence continental mid crustal weakening and have comprised of juxtaposed terranes with varying rheological compositions

pressure temperature and time constraints on himalayan metamorphism from eastern kashmir and western zanskar himalayan metamorphism in the kashmir and zanskar sector of the high himalaya resulted from thrust and fold related crustal thickening within the indian plate following the collision of india and asia at c 50 ma interbanded metapelites marbles calcareous schists amphibolites and quartzites represent metamorphosed equivalents of the palaeozoic lower mesozoic continental margin rocks granitic rocks include pre collision k feldspar megacrystic and biotite granites and post collision two mica garnet tourmaline granites average pressure temperature conditions of equilibration using the self consistent thermodynamic data set of holland powell 1990 are presented across a 45 km traverse of the eastern kashmir western zanskar high himalaya kyanite is the stable aluminosilicate phase across 35 km outcrop width in the middle of the slab complex microstructures indicate that prograde metamorphism up to kyanite grade and fabric development in the upper structural nappes is early and unrelated to the main central thrust diachronous metamorphism propagated southwards with the overall structural evolution early isograds and thrust fold structures were carried passively in the hanging wall of the main central thrust peak metamorphism based on 40 ar 39 ar hornblende ages occurred pre 30 7 2 0 ma at the top of the slab and in the middle of the slab was pre 22 1 0 ma probably 25 28 ma the lower structural levels in zanskar record peak metamorphic conditions around 700 750 c and 8 kbar reflecting depths of burial of 28 30 km a new younger schistosity which is not present in the higher structural levels was developed under kyanite grade conditions the regional distribution of high temperatures recorded by thermo barometry does not support the concept of additional heat being supplied by frictional heating along the main central thrust or magmatic heat resulting from anatexis exhumation of the himalayan metamorphic rocks was achieved by three major processes erosion as a result of crustal thickening uplift of the rocks along the hangingwall of the main central thrust above a major frontal ramp in the kishtwar window area and extensional unroofing along the footwall of a large scale ne dipping normal fault at upper crustal levels probably synchronous with compression at depth

米国南西部 コロラド川伸張地域 black山脈アコモデーション帯の背斜部分における中新世伸張と伸張しゅう曲運動 原標題は英語

shear zones between rock units with no relative movement abstract shear zones are normally viewed as relatively narrow deformation zones that accommodate relative displacement between two blocks that have moved past each other in opposite directions this study reports localized zones of shear between adjacent blocks that have not moved past each other such deformation zones which we call wakes form due to the movement of exotic blocks within a viscous medium denser blocks sinking within a salt structure the paths between separated boudins melt in partially molten surroundings melt movement during migmatisation or solid blocks sinking through a partially molten magma body stoping from the fluid dynamics perspective these shear zones can be regarded as low reynolds number deformation zones within the wake of a body moving through a viscous medium while compact moving bodies aspect ratio 1 1 1 generate axial symmetric cone like shear zones or wakes elongated bodies vertical plates or horizontal rod like bodies produce tabular shear zones or wakes unlike conventional shear zones across which shear indicators usually display consistent symmetries shear indicators on either side of the shear zone or wake reported here show reverse kinematics thus profiles exhibit shear zones with opposed senses of movement across their center lines or planes we have used field observations and results from analytical and numerical models to suggest that examples of wakes are the transit paths that develop where denser blocks sink within salt structures bodies of melt rise through migmatites between boudins separated by progressive extension and perhaps where slabs of subducted oceanic lithosphere delaminate from the continental crust and sink into the asthenosphere we also argue that such shear zones may be more common than they have been given credit for and may be responsible for some reverse kinematics reported in shear zones

comment on coexistent pre existing extensional and subsequent compressional tectonic deformation in the kashmir basin nw himalaya by akhtar et al 2017 article in press

eocene partial melting recorded in peritectic garnets from kyanite gneiss greater himalayan sequence central nepal the himalayan mountain belt is characterized by the impressive continuity over 2500 km of tectonic units thrusts and normal faults as well as large volumes of high grade metamorphic rocks and granite exposed at the surface visona et al 2012 although there are many studies on metamorphism melt generation and deformation concentrated in the same time span as activity on the mct and std 23 17 ma godin et al 2006 much fewer data on deformation metamorphism melt generation and geochronology in the greater himalayan sequence ghs is available for the large time span 30 m y between collision at 55 ma and mct std activities carosi et al 2010 2013 montomoli et al 2013 melt generation and granite emplacement have been mainly related to the exhumation stage of the ghs during early miocene harris and massey 1994 very few evidences of partial melting during prograde metamorphism have been reported until now in the southern part of the belt godin et al 2001 imayama et al 2012 the ghs of eastern nepal and sikkim india shows the occurrence of melting at the bottom of the ghs barun gneiss at nearly 33 27 ma imayama et al 2012 ferrero et al 2012 rubatto et al 2012 anatectic melt inclusions nanogranites and nanotonalites have been found in garnet of kyanite gneiss at the bottom of the greater himalayan sequence fig 1 along kali gandaki valley central nepal nearly 1 km structurally above the main central thrust in situ u th pb dating of monazite included in garnets in the same structural positions as melt inclusions allowed to constrain partial melting starting at 36 41 ma carosi et al in press

synorogenic extension quantitative constraints on the age and displacement of the zanskar shear zone northwest himalaya structural thermobarometric and geochronological data place limits on the age and tectonic displacement along the zanskar shear zone a major north dipping synorogenic extensional structure separating the high grade metamorphic sequence of the high himalayan crystalline sequence from the overlying low grade sedimentary rocks of the tethyan himalaya a complete barrovian metamorphic succession from kyanite to biotite zone mineral assemblages occurs within the 1 km thick zanskar shear zone thermobarometric data indicate a difference in equilibration depths of 12 3 km between the lower kyanite zone and the garnet zone which is interpreted as a minimum estimate for the finite vertical displacement accommodated by the zanskar shear zone for the present day dip of the structure 20 a simple geometrical model shows that a net slip of 35 9 km is required to regroup these samples to the same structural level because the kyanite to garnet zone rocks represent only part of the zanskar shear zone and because its original dip may have been less than the present day dip these estimates for the finite displacement represent minimum values field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the zanskar shear zone occurred as a continuous process starting at the barrovian metamorphic peak and lasting throughout the subsequent extension induced exhumation geochronological dating of various leucogranitic plutons and dikes in the zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19 8 ma and that extension most likely initiated shortly before 22 2 ma

setting new constrains on the age of crustal scale extensional shear zone vivero fault implications for the evolution of variscan orogeny in the iberian massif the vivero fault is crustal scale extensional shear zone parallel to the variscan orogen in the iberian massif belt with an associated dip slip movement toward the hinterland to constrain the timing of the extension accommodated by this structure we performed zircon u pb la icp ms geochronology in several deformed plutons some of them emplaced syntectonically the different crystallization ages obtained indicate that the fault was active at least between 303 2 and 287 3 ma implying a minimum tectonic activity of 16 5 ma along the fault the onset of the faulting is established to have occurred later than 314 2 ma the geochronological data confirm that the vivero fault postdates the main variscan deformation events in the nw of the iberian massif and that the extension direction of the late carboniferous early permian crustal scale extensional shear zones along the ibero armorican arc was consistently perpendicular to the general arcuate trend of the belt in sw europe

impact of the ocean carbonate chemistry on living foraminiferal shell weight comment on carbonate ion concentration in glacial age deep waters of the caribbean sea by w s broecker and e clark broecker and clark 2002 use the size normalized weight of planktic foraminifera to estimate the carbonate ion concentration co3 of atlantic glacial upper deep water this method was introduced by lohmann 1995 and is based on the fact that within a defined size fraction dissolution decreases shell weight in proportion to the degree of undersaturation with respect to caco3 based on the following assumptions 1 the thickness of the foraminiferal shell wall does not depend on growth conditions 2 the saturation co3 for calcite co3 sat increases by 20 mol kg 1 km 1 broecker and clark 2001 3 the weight loss slope is universal and ca 0 3 g mol kg 1 1 broecker and clark 2001 4 the offset between the bottom water and pore water co3 was the same during glacial time as during the holocene broecker and clark 2002 calculate that the co3 of atlantic glacial upper deep water was 14 mol kg 1 higher than during the holocene although they recognize that some of their assumptions are not strictly valid they do not assess the impact of those assumptions on their co3 estimate here we comment on several of those assumptions and attempt to quantify their impact on broecker and clark s 2002 calculations

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

structural insights into the early stages of exhumation along an orogen scale detachment the south tibetan detachment system dzakaa chu section eastern himalaya abstract structural transects through the south tibetan detachment system stds in the dzakaa chu valley tibet reveal a 1000 m thick low angle

channel flow and the himalayan tibetan orogen a critical review the movement of a low viscosity crustal layer in response to topographic loading provides a potential mechanism for 1 eastward flow of the asian lower crust causing the peripheral growth of the tibetan plateau and 2 southward flow of the indian middle crust to be extruded along the himalayan topographic front thermomechanical models for channel flow link such extrusion to focused orographic precipitation at the surface isotopic constraints on the timing of fault movement anatexis and thermobarometric evolution of the exhumed garnet to sillimanite grade metasedimentary rocks support mid crustal channel flow during the early to mid miocene exhumed metamorphic assemblages suggest that the dominant mechanism of the viscosity reduction that is a requirement for channel flow was melt weakening along the upper surface defined by the south tibetan detachment system and strain softening along the base bounded by the main central thrust neotectonic extrusion bounded by brittle quaternary faults south of the main central thrust is positively correlated with the spatial distribution of precipitation across a north south transect suggesting climate tectonic linkage over a million year time scale a proposed orogen wide eastward increase in extrusion rate over 20 ma reflects current precipitation patterns but climate tectonic linkage over this time scale remains equivocal

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

vorticity analysis in the zagros orogen shiraz area iran quantitative vorticity analyses in orogenic belts are essential for studying the kinematics of deformation and can be performed using a range of methods the combination of microstructural analysis for vorticity with other methods creates a more rigorous analysis in order to determine the degree of non coaxiality and spatial pattern of vorticity during deformation in the zagros orogenic belt a study area containing the boundary of the zagros folded belt and the zagros fold and thrust belt is selected the study area is situated in the shiraz region of e zagros in iran the kinematic vorticity analysis is carried out using 4 methods based on 1 the degree of asymmetry of the calcite c axis fabric 2 the assumption that the orientation of the long axes of calcite within an oblique stylolite foliation delineates the direction of the instantaneous stretching axis 3 the assumption that the tension gash tips determine the direction of the instantaneous stretching axis and 4 stylolite teeth determine the direction of the instantaneous stretching axis c axis data from calcite give a kinematic vorticity number between 0 68 and 0 83 and the orientation of the long axes of calcite grains yields a range between 0 5 and 0 84 stylolites provide a kinematic vorticity number between 0 5 and 0 79 and tension gashes provide a kinematic vorticity number between 0 56 and 0 81 this range of vorticity numbers confirms the contributions of both simple 33 59 and pure shear 41 67 twining of calcite also reveals that the last stage of deformation occurred at a temperature of 170 200 c spatial analysis reveals an increase in the simple shear component from the sw of the zagros folded belt to the ne of the zagros fold and thrust belt

ductile shear as a cause of inverted metamorphism example from the nepal himalaya in many mountain belts metamorphic sequences occur in an inverted arrangement such that rocks of higher metamorphic grade overlie rocks of lower metamorphic grade in some locations such as the main central thrust mct zone of the himalaya the metamorphic inversion is coincident with the zone of ductile thrust deformation although many explanations have been suggested for the metamorphic inversion of the himalaya none are consistent with all of the observed features a new model based on observations from the eastern nepal himalaya consists of inversion of the metamorphic sequence due to ductile shearing of a pre existing zone of right way up metamorphism this simple structural explanation creates an inverted metamorphic sequence consistent with observations of 1 a zone of distributed deformation 2 metamorphic textures overprinted by deformation and 3 metamorphic isograds sub parallel to shear fabric these observations are not all well accounted for by previously proposed models in this st

mechanical modeling and insar measurements of mount sedom uplift dead sea basin implications for effective viscosity of rock salt we present a mechanical model for the growth of an emerging salt diapir in a tectonically active basin the analytical model is applied to and serves to constrain the effective viscosity of rock salt and strain rates during diapirism of the wall shaped mount sedom rock salt diapir dead sea basin the model is based on one dimensional flow of newtonian viscous fluid salt in a vertical channel that has been driven by the load of the overburden and affected by shear along the channel walls because the poiseuille channel flow profile is parabolic and the couette shear flow profile is linear a one dimensional model provides three sets of predicted profiles topography uplift rate and shear strain the present topography of mount sedom represents the shape of the sedom diapir and hence the effective viscosity of rock salt can be constrained by a model that best fits the present topography of the mountain the resulting sedom rock salt viscosity is determined to be between 2 and 3 1018 pa s and the associated strain rate is between 5 and 6 10 13 s 1 geological structures indicate strain rates of 9 10 13 s 1 and 3 10 14 s 1 during the holocene emerging stage and at the plio pleistocene pre emergent stage of the sedom diapir respectively the uplift history of mount sedom predicted by the model and the current topography are compared to interferometric synthetic aperture radar insar measurements of salt uplift the maximum uplift rates of mount sedom are 8 3 and 5 5 mm yr for its northern and southern parts respectively the insar uplift profiles resemble topographic profiles obtained along the same traverses implying that the uplift history during the last 14 000 years is stable steep uplift gradients observed by insar along the western margin of the diapir are higher than predicted by modeling of newtonian viscous flow this could imply that flow of power law viscous fluid may be more suitable than that of newtonian viscous fluid for the sedom rock salt at high strain rates above 8 10 13 s 1

a structural transect in the lower dolpo insights on the tectonic evolution of western nepal abstract we present the results of a structural transect in lower dolpo cross cutting the upper part of the lesser himalaya lh the higher himalayan crystallines hhc and the lower part of the tibetan sedimentary sequence tss the mct zone affects the upper part of the lh as well as the lower part of the hhc and shows a later brittle reactivation mean vorticity in the mct points to non coaxial deformation these data together with available kinematic data along the belt on the south tibetan detachment system stds and in the core of the hhc point to increasing simple shear toward the tectonic boundaries a top to the sw high temperature shear zone toijem shear zone is recognized in the middle part of the hhc at the boundary between units 1 and 2 it developed during the earlier stages of exhumation of the hhc enhancing the decompression of the hanging wall and the emplacement of leucogranite dykes and sills its development could be explained by a change in the velocity profile during the extrusion of the hhc triggered by first order changes in rock types of the tectonic unit the stds is marked by a wide zone of high strain and by a metamorphic jump from amphibolite facies in the carbonate rocks of the upper part of the hhc to greenschist facies marbles in the lower part of the tss the development of a pervasive foliation towards the bottom of the tss indicates increasing strain related to top down to the ne tectonic transport a low p metamorphic event marked by the growth of post d1 biotite porphyroblasts at the base of the tss is related to the conductive heating from the underlying hhc

interaction of metamorphism deformation and exhumation in large convergent orogens coupled thermal mechanical models are used to investigate interactions between metamorphism deformation and exhumation in large convergent orogens and the implications of coupling and feedback between these processes for observed structural and metamorphic styles the models involve subduction of suborogenic mantle lithosphere large amounts of convergence 450 km at 1 cm yr 1 and a slope dependent erosion rate the model crust is layered with respect to thermal and rheological properties the upper crust 0 20 km follows a wet quartzite flow law with heat production of 2 0 w m 3 and the lower crust 20 35 km follows a modified dry diabase flow law with heat production of 0 75 w m 3 after 45 myr the model orogens develop crustal thicknesses of the order of 60 km with lower crustal temperatures in excess of 700 c in some models an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 pa s at 700 c in the upper crust and 900 c in the lower crust in these models a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where t 600 c the channel flow zone is characterised by a reversal in velocity direction on the pro side of the system and is driven by a depth dependent pressure gradient that is facilitated by the development of a temperature dependent low viscosity horizon in the mid crust different exhumation styles produce contrasting effects on models with channel flow zones post convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust related exhumation on the flanks velocities in the pro side channel flow zone are enhanced but the channel itself is not exhumed in contrast exhumation resulting from erosion that is focused on the pro side flank of the plateau leads to ductile extrusion of the channel flow zone the exhumed channel displays apparent normal sense offset at its upper boundary reverse sense offset at its lower boundary and an inverted metamorphic sequence across the zone the different styles of exhumation produce contrasting peak grade profiles across the model surfaces however p t t paths in both cases are loops where pmax precedes tmax typical of regional metamorphism individual paths are not diagnostic of either the thickening or the exhumation mechanism possible natural examples of the channel flow zones produced in these models include the main central thrust zone of the himalayas and the muskoka domain of the western grenville orogen

kinematic evolution of thrusts wedge and erratic line length balancing insights from deformed sandbox models kinematic evolution of fold thrust structures has been investigated by analogue models that include syntectonic sedimentation different decollement dips and basement thicknesses produced different wedge geometries and propagating characteristics a model with one decollement level was characterized by a closely spaced thrust system during early stages of shortening as compared to the late stages the frequency of fault nucleation was rapid during the early stages of deformation conversely the frequency of fault nucleation was low and thrust spacing was significantly wider in a model with two decollement levels individual faults became locked at steep dips and deformation stepped forward as a new fault nucleated in sequence in front of the older locked structure once the thrust system was established up to 27 overall shortening an overlying bed was introduced to simulate syntectonic deformation model sand wedge did not grow self similarly but rather its length and height increased episodically with deformation restoration of deformed models show that layer parallel shortening accommodated for approximately half of the total model shortening across the multilayers calculated error in apparent layer shortening from the restored layers revealed a direct relation with depth of the layers in the models the experimental results are comparable to a natural example from the northern apennines fold and thrust belts

timing of an extensional detachment during convergent orogeny new rb sr geochronological data from the zanskar shear zone northwestern himalaya the zanskar shear zone of northwest india forms the western segment of the south tibetan detachment system a north dipping normal fault and shear zone that unroofed high grade metamorphic rocks during contraction of the himalayan orogen new rb sr mineral ages from the shear zone and its footwall show that ductile deformation was ongoing at 26 ma and continued to 16 ma in some sections the nature of the deformation varies along strike displacement at 25 ma or earlier in the western section did not result in a large thermal offset by contrast the sections to the east display substantial ductile thinning at midcrustal levels that was superseded by brittle detachments as the system continued to exhume rocks of the high himalayan crystalline sequence the implication is that the zanskar shear zone may have had a smaller offset in the west and cut down section to the east the time of the bulk of the deformation of this structure is determined to be similar to that of the equivalent structure in most of the eastern and central himalaya

structure of the main central thrust zone and extrusion of the high himalayan deep crustal wedge kishtwar zanskar himalaya the main central thrust is a crustal scale ductile shear zone between 1 5 and 3 km wide which places the oligocene miocene metamorphic rocks of the high himalayan zone south or sw over the unmetamorphosed or weakly metamorphosed rocks of the lesser himalaya the high strain zone of the main central thrust is coincident with an inverted metamorphic field gradient from biotite to kyanite grade over a structural thickness of 1500 m kyanite grade rocks metamorphosed at 9 5 10 kbar were exhumed from depths of 33 37 km along the main central thrust hanging wall and emplaced over lesser himalayan rocks never buried deeper than 10 12 km exhumation of the deepest buried kyanite grade rocks occurred along the zone above the main central thrust zone approximately 45 km width 28 km structural thickness of the high himalaya exposes sillimanite grade gneisses migmatites and 19 5 21 5 ma old leucogranites formed at pressures between 4 5 and 7 kbar and depths of 16 25 km a ne dipping normal fault ductile shear zone at the top of the slab zanskar shear zone shows condensed but right way up isograds from sillimanite to chlorite grade over 2 400 metres structural thickness 40 ar 39 ar geochronology shows that most of the high himalayan slab had cooled below 350 c by 16 ma supporting models linking the two bounding faults of the high himalaya both kinematically and temporally there is no evidence of melting along the main central thrust zone and the himalayan leucogranites were generated 10 30 km structurally above the main central thrust frictional heating along the main central thrust could not therefore have played any role in generating the leucogranites thus far there is little evidence for late miocene reactivation along the main central thrust as seen elsewhere along the main central thrust in nepal

grain boundary migration microstructures in a naturally deformed quartzite abstract in this study several grain scale microstructures are presented that are thought to demonstrate the migration direction of once mobile grain boundaries in a naturally deformed quartzite an analysis is presented of the sense of migration of the boundaries and the characteristics of the patterns of relative grain growth and shrinkage grain boundary migration seems to be correlated with the relative crystallographic orientations of neighbouring grains for the quartz quartz grain boundaries and the pattern of preferred grain growth is roughly symmetrical about the mica foliation plane

v pull apart microstructures a new shear sense indicator abstract granitoids deformed in a low grade shear zone show conspicuous development of asymmetric pull apart microstructures in feldspars characterized by v shaped gaps between the separated fragments v pull aparts they occur beside active shear planes and result from rigid body rotation of one fragment relative to the other the shear sense in the adjacent flow surface can be easily determined through the asymmetry normally shown by v pull aparts where the fracture wall at a lower angle to the shear plane indicates which fragment has rotated synthetically to the adjacent shearing another diagnostic feature is the foliation deflection that occurs adjacent to the rotated fragment the v shaped gaps are infilled by recrystallized quartz mica aggregates showing quartz c axis patterns indicative of strong participation of crystal plastic processes in these domains evidence of crack seal by precipitation c axes parallel to the direction of separation have also been found the rigid body rotation indicated by these microstructures worked as a mechanism of removal of protruding porphyroclast edges enabling the smoothing of the shear planes rigid body rotation should operate in circumstances where other mechanisms such as dissolution or crystal plasticity did not succeed in eliminating undulations along the shear planes