strain softening along the mct zone from the sikkim himalaya relative roles of quartz and micas abstract in the darjeeling sikkim himalaya two distinct faults form the main central thrust mct the structurally higher mct1 and the lower mct2 each has accommodated translation greater than 100 km the lower mct2 places greater himalayan amphibolite grade paro lingtse gneiss over lesser himalayan greenschist grade daling metapelites the mct2 is folded by the underlying lesser himalayan duplex and is exposed at different structural positions of the fold at pelling the mct2 zone is exposed as a 373 m thick nw dipping fault zone that exposes 19 m of hanging wall mylonitized lingtse gneiss the lingtse protolith shows evidence of amphibolite grade plastic deformation features in quartz and feldspar within the hanging wall mylonite zone hwmz quartz and feldspar have undergone grain size reduction by different deformation mechanisms and feldspars are sericitized suggesting the presence of fluids during deformation we estimate a temperature of 300 c within the fault zone during fluid assisted retrogression and deformation reaction softening of feldspars produced a large proportion of intrinsically weak matrix this in combination with development of a strong foliation defined by parallel mica grains resulted in strain softening along the mct2 zone and concentrated the deformation along a thin zone or zones

structural relationships along a greenstone shallow water shelf contact belingwe greenstone belt zimbabwe the belingwe greenstone belt zimbabwe contains a thick tholeiitic basalt and komatiite sequence structurally above shallow water shelf rocks that in turn rest nonconformably over an older gneiss terrain the nature of the contact between the basalt komatiite sequence and underlying shallow water shelf rocks represents one of the most critical relationships upon which general models for the tectonic setting of greenstone belts and the petrotectonic environment of komatiites are based this contact is a 20 to 200 m thick high strain zone characterized by a wide variety of shear zone tectonites derived from both the shelf and overlying lava succession in the ultramafic part of the high strain zone tectonites include ductile ultramafic mylonites ultramafic ultramylonites ultramafic phyllonites and serpentinite schists enclosing less deformed ultramafic serpentinite blocks in the part of the shear zone derived from shallow water sedimentary rocks shear zone tectonites include chert c s mylonite phyllonite and iron rich argillaceous shear bands both parts of the shear zone are cut by brittle fault gouge fabrics we suggest that the tholeiitic basalt and komatiite sequence is allochthonous with respect to the shallow water shelf and refer to it as the mberengwa allochthon contrary to prior suggestions there is no evidence that the tholeiitic basalt and komatiite succession of the belingwe greenstone belt sits conformably over the shallow water shelf rocks documentation of the shear zone at the base of the mberengwa allochthon lends support to the idea that the allochthon represents a fragment of an off axis oceanic plateau structurally emplaced over a shallow water shelf during a convergent plate interaction prior to 2 6 ga

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

structural geometry microstructural and strain analyses of l tectonites from paleoproterozoic orthogneiss insights into local transport parallel constrictional strain in the sikkim himalayan fold thrust belt paleoproterozoic lingtse orthogneiss shows three dominant structural associations in the sikkim himalayan fold thrust belt ftb a these rocks locally define the pelling thrust pt zone that lies in the footwall of the main central thrust mct b they occur as linear almost n s trending imbricates within the pt sheet and c they form a n s trending 13 5 km long 2 km wide discontinuous linear mylonitic klippen within the daling formation of the lower lesser himalayan sequence lhs that lie east of the n s trending teesta valley the klippe records n s trending strong l tectonites that are spatially associated with ls and sl tectonites the stretching lineations are oriented along the transport direction these rocks are exposed along three mylonite zones from north to south these are the phodong mylonite zone the tumin eingee mylonite zone and the rumtek mylonite zone all the three mylonite zones are overall folded along gently plunging transport parallel n s trending synform with the strongest l fabric developed along the hinge zone and its dominance decreases progressively toward the eastern and western structural contacts of the klippen the highest constrictional strain is recorded in phodong and tumin eingee mylonite zones and decreases south toward rumtek mylonite zone the long axes x of the quartz strain ellipsoids show n s trend and gentle plunge that agrees with the stretching lineations observed in the field additionally the xy plane of the generated ellipsoids overall agree with the observed variation of the mylonitic foliation planes in the field from all the three locations microstructural analysis from the mylonite klippe suggests that quartz and feldspar have undergone 35 and 76 grain size reduction with respect to the protolith dislocation creep and microfracturing are the dominant deformation mechanisms in quartz and feldspar respectively the grain size reduction is more prominent near the eastern and the western boundaries as they mark structural contacts between the l tectonites and the daling phyllites strain is partitioned between quartz and feldspar in the mylonite zones with quartz recording constrictional strain while feldspar records flattening strain we explain this variation due to their different deformation mechanisms shear sense analysis of feldspar porphyroclasts suggest dominantly transport parallel flow along with a component of transport perpendicular flow in the mylonite zones existing models on mechanism of l tectonite formation and transport parallel folds do not completely explain our observations we propose the possible existence of a lateral ramp beneath the pt sheet to explain the generation of l tectonites and transport parallel fold in the study area

significance of transport parallel strain variations in part of the raft river shear zone raft river mountains utah usa this paper examines part of the raft river shear zone rrsz in northwestern utah that exhibits an extreme transport parallel increase in strain intensity coupled with a transition from flattening to constrictional strain detailed geologic mapping and finite strain quartz c axis fabric and kinematic vorticity analyses demonstrate local necking of the shear zone associated with an increase in transport parallel elongation accommodated by a stretching fault at the base of the shear zone the domain of intense deformation and necking of the shear zone is localized where the basal stretching fault cuts rheologically weak rocks this domain is characterized by strain in the constrictional field caused by transport perpendicular flow into the area of high transport parallel elongation where rocks cut by the stretching fault are rheologically strong the rrsz locally records flattening strain and lower strain intensities limiting the amount of stretching needed at the base of the shear zone in any one direction and or recording transport perpendicular flow into adjacent highly extended domains the rheology of rocks cut by the stretching fault directly controlled the amount and style of zone normal shortening and transport parallel elongation and these observations provide an example of deformation partitioning in a crustal scale structure driven by local rheological transitions

constraints upon water advection in sediments of the mariana trough thermal gradient measurements consolidation tests and pore water compositions from the mariana trough imply that water is moving through the sediments in areas with less than about 100 m of sediment cover the maximum advection rates implied by the thermal measurements and consolidation tests may be as high as 10 5 cm s 1 but are most commonly in the range of 1 to 5 10 6 cm s 1 theoretical calculations of the effect of the highest advection rates upon carbonate dissolution indicate that dissolution may be impeded or enhanced depending upon the direction of flow by a factor of 2 to 5 times the rate for diffusion alone the average percentage of carbonate is consistently higher in two cores from the area with no advection or upward advection than the average percentage of carbonate in three cores from the area with downward advection this increase in average amount of carbonate in cores with upward moving water or no movement cannot be attributed solely to differences in water depth or in amount of terrigenous dilution if the sediment column acts as a passive boundary layer then the water velocities necessary to affect chemical gradients of silica are in the range 10 9 to 10 10 cm s l however if dissolution of silica occurs within the sediment column then the advection velocities needed to affect chemical gradients are at least 3 10 8 cm s l and may be as high as 3 10 6 cm s l this order of magnitude increase in advection velocities when chemical reactions occur within the sediments is probably applicable to other cations in addition to silica if so then the advection velocities needed to affect heat flow 10 8 cm s 1 and pore water chemical gradients are much nearer in magnitude than previously assumed

a new kinematic evolutionary model for the growth of a duplex an example from the rangit duplex sikkim himalaya india abstract the lesser himalayan duplex lhd is a prominent structure through much of the lesser himalayan fold thrust belt in the darjeeling sikkim himalaya a component of the lhd is exposed in the rangit window as the rangit duplex rd the rd consists of ten horses of the upper lesser himalayan sequence gondwana buxa upper daling the duplex varies from hinterland dipping in the north through an antiformal stack in the middle to foreland dipping in the south the ramgarh thrust rt is the roof thrust and based on a balanced cross section the main himalayan sole thrust is the floor thrust at a depth of 10 km and with a dip of 3 5 n retrodeformation suggests that the rd initiated as a foreland dipping duplex with the early ramgarh thrust as the roof thrust and the rt as the floor thrust the rt became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent rt reactivation this kinematic history best explains the large translation of the overlying mct sheets the restoration suggests that rd shortening is 125 km and the original gondwana basin extended 142 km northward of its present northernmost exposures within the window

origin of elevated sediment permeability in a hydrothermal seepage zone eastern flank of the juan de fuca ridge and implications for transport of fluid and heat elevated sediment porosity and permeability may help to focus upward fluid seepage observed over a buried basement high on the eastern flank of the juan de fuca ridge consolidation and permeability tests of fine grained hemipelagic and turbidite sediments from the seepage site and from nearby sites that are not experiencing seepage indicate that sediments at the seepage site primarily hemipelagic have 10 15 higher porosity and 10 higher permeability at a given depth and 10 100 lower hydraulic impedance for the 40 m thick sediment column the correlation of consolidation and permeability properties with sediment type rather than location indicates that the relatively high porosity and permeability do not result from fluid flow but are intrinsic properties of the hemipelagic sediment on the basis of measured sediment properties fluid pressure at the top of basement is 5 kpa greater than hydrostatic a simple circulation model that incorporates this estimate and proximity to the nearest basement outcrops 4 20 km suggests that local basement permeability is between 3 10 13 and 3 10 11 m2 within the range measured in nearby boreholes the measured sediment properties combined with other published data indicate that a few tens of meters of fine grained terrigenous hemipelagic or calcareous marine sediment effectively seal the basement aquifer whereas much thicker siliceous or pelagic sediment may support thermally and chemically significant fluid flow the large contrast in hydraulic impedance among sediment columns of different types could cause regional variation in the evolution of ridge flank hydrothermal systems and in the contribution of seepage to ridge flank fluxes

permeability underpressures and convection in the oceanic crust near the costa rica rift eastern equatorial pacific in situ permeability and pore pressures were measured 200 m deep beneath the top of the oceanic crust at dsdp site 504b these measurements have relevance for the transition from convective to conductive heat flow on the south flank of the costa rica rift conventional slug and constant rate injection tests were made below a hydraulic packer set at various depths in the hole the packer was first set in a massive flow unit 37 m below the sediment basement interface the bulk permeability of the 172 5 m of pillow basalts and basaltic flows below the packer was found to be about 40 millidarcys 4 10 10 cm2 measurements over 3 and 15 m intervals at the bottom of the hole in an altered pillow zone indicated a bulk permeability of 2 4 millidarcys these values are thought to be accurate to 30 formation pore pressures were found to be approximately 8 bars 2 below hydrostatic interpretation of the data with respect to simple numerical convection models suggests that the transition from convective to conductive heat flow is controlled by the cessation of convective heat transport through the sedimentary layer rather than the cessation of convection in the sediment furthermore the agreement between observed and modeled underpressures implies that hole 504b penetrated an active ocean crustal convection system the thick sedimentary layer layers of basal chert and massive flow basalts above the layer 2a pillow flows apparently form an impermeable lid effectively isolating the convection system from the ocean

analysis and application of in situ pore pressure measurements in marine sediments recent measurements of in situ subseafloor pore pressure in sediment ponds on the flank of the mid atlantic ridge using a pop up pore pressure instrument have provided important information regarding fluid exchange processes at the seafloor as well as in situ estimates of physical properties of marine sediments the in situ pore pressure recordings of 4 5 days duration contain three distinct components the early part of the record is dominated by the decay of a pressure pulse associated with probe penetration the rate of decay of this pulse is used to estimate in situ permeabilities which range from 3 1 10 16 m2 to 6 5 10 15 m2 the recording also exhibits a low amplitude oscillation that is associated with tidal pressure variation on the seafloor a study based on biot s theory shows that amplitudes and phases of tidally induced pore pressure oscillations are determined by both the frame compressibility and the permeability of the sediments and thus the oscillation components are used to estimate these parameters the frame compressibilities and the permeabilities estimated from tidal oscillations range from 1 2 10 8 pa 1 to 4 1 10 8 pa 1 and from 1 1 10 15 m2 to 4 8 10 15 m2 respectively laboratory measured permeabilities range from 7 6 10 16 m1 to 1 2 10 15 m2 permeabilities determined by these three methods are comparable the third component is the ambient equilibrium pore pressure negative pore pressure gradients were identified on all deployments when combined with the permeabilities of the sediments the estimated average rates of drawdown of water through the sedimentary layers in the ponds are 2 7 mm yr the pore pressure gradient near the seafloor can be used to construct a pore pressure profile through the sediment extrapolated pore pressure at the top of the basement is about 80 kpa 0 8 bar below the hydrostatic pressure

geometry and kinematics of the darjeeling sikkim himalaya india implications for the evolution of the himalayan fold thrust belt abstract the darjeeling sikkim himalaya lies in a zone where the convergence vector between the indian and eurasian plates is almost perpendicular to the himalayan deformation front current outcrop pattern preserves high grade hanging wall rocks of the main central thrust mct and the pelling thrust pt within 5 km of the mountain front u pb shrimp analysis on zircon grains from mct sheet record two 206pb 238u age groups of ca 2575 and ca 943 ma and from the pt 1835 13 ma the lesser himalayan duplex in the footwall of the mct is composed of two duplex systems a structurally higher hinterland dipping duplex and a lower duplex system that varies from hinterland dipping in the north to an antiformal stack in the middle and foreland dipping duplex in the south the foreland dipping horses and reactivation of the roof thrust resulted in large translation of the overlying mct and pelling thrust sheets the duplex also resulted in a plunge culmination forming a broad n s trending anticline a transport parallel restorable balanced cross section reveals a minimum shortening of 450 km 81 south of the south tibetan detachment system and an average long term shortening rate 20 mm yr although the study area lies in a zone of high convergence angle with the deformation front the surface structures do not record the greatest shortening of the orogen the shillong plateau and a tranverse zone lying southeast of the study area have accommodated a part of the convergence related shortening thereby under representing the shortening in the sikkim himalayan fold thrust belt lateral variations in the original width of the lesser himalayan basin may have played a role in partitioning the shortening in the fold thrust belt

shortening structures in eastern and north western himalayan rocks w schwan p s saklani series editor today and tomorrow s printers and publishers new delhi 1980 62 pp 12 00

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

correlated sediment thickness temperature gradient and excess pore pressure in a marine fault block basin measurements of temperature gradient and excess pore pressure in the surficial sediment of a fault block basin in the guatemala basin correlate with sediment thickness the temperature gradient is smaller and the excess pore pressure gradient is more negative in areas of thinner sediment this correlation is explained by postulating downward pore water advection within the sediments with flow velocities on the order of 10 9 to 10 8 m s in the thinnest sediments and much less flow in the thickest sediments sediment physical properties and pore water chemistry also support this interpretation since the conductive heat flow of the basin as a whole is less than one third that predicted by sea floor spreading models the oceanic basement may be the site of a vigorous hydrothermal circulation system the pore water advection in the sediments may be driven by this larger scale circulation

strain distribution in a fold in the west norwegian caledonides abstract strain has been measured from clasts within a deformed conglomerate layer at 17 localities around an asymmetric fold in the rundemanen formation in the bergen arc system west norwegian caledonides strain is very high and a marked gradient in strain ellipsoid shape exists to either side of the fold strain within the conglomerate bed is of the extreme flattening type in the fold especially on the lower fold closure the strain is constrictional mathematical models of perturbations of flow in glacial ice have produced folds of the same geometry as this fold with a strikingly similar pattern of finite strain the fold geometry and strain pattern as well as other field observations suggest that the fold developed passively as the result of a perturbation of flow in a shear zone where the strain was accommodated by simple shear accompanied by extension along y

extensional shearing deformation and kilometer scale a type folds in a cordilleran metamorphic core complex raft river mountains northwestern utah cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations in the raft river mountains a mylonitic zone and a late superimposed detachment fault separate precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous paleozoic rocks which suffered a brittle extensive deformation we suggest that the mylonitic ductile deformation is the result of tertiary extensional tectonics strain analysis shows that 1 strain results from large scale progressive shearing deformation in a non coaxial regime 2 e w regional stretching lineations represent the direction of shearing and transport 3 shear criteria indicate an east directed movement 4 strain magnitude is increasing eastward 5 kilometer scale folds parallel to the lineations are interpreted as a type folds formed by eastward shearing of a previously northward dipping layering measurement of striated faults in the upper plate gives an e w direction of extension parallel to the lineations thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics a model is proposed to explain the observed geometry and kinematics of the structures the mesozoic to paleocene cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes important uplift abnormal heating and granite emplacement occurred later during the collapse of the former belt the combined effects of thermal and gravitational instabilities during tertiary extensional tectonics enabled the development of a ductile shear zone localized at the precambrian basement paleozoic cover interface the brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

evidence for excess pore pressures in southwest indian ocean sediments brown clay cores from the madagascar and crozet basins show the following evidence of excess pore pressures large amounts of flow in increasing average sedimentation rate with age and nonlinear temperature gradients additionally many hilltops in these basins have no visible sediment cover the bare hilltops may result from periodic slumping caused by excess pore pressures calculated excess pore pressures which equal or exceed the overburden pressure were inferred from water fluxes predicted by nonlinear temperature gradients and laboratory permeability measurements by using darcy s law since pore pressures which exceed the overburden pressure are unreasonable we attribute this discrepancy to laboratory measures which underestimate the in situ permeability the widespread presence of overpressured sediments in areas of irregular topography provides a process for resuspension of clay sized particles this mechanism does not require high current velocities for the erosion of clay and therefore can be applied to many areas where no strong currents are evident carbonate rich sediments from the madagascar ridge the mozambique ridge and the agulhas plateau had almost no flow in and occurred in areas where all topography was thickly draped with sediment since the age and tectonic location of the ridges and plateaus preclude water circulation in the basement wemore attribute these differences between the brown clay and the carbonate rich material to an absence of significant excess pore pressures in the plateau and ridge sediments less

pre tertiary felsic magmatism of the nepal himalaya recycling of continental crust abstract at least seven different groups of felsic magmatic rocks have been observed in the lesser and higher himalayan units of nepal six of them are pre himalayan the ulleri lower proterozoic augen gneiss extends along most of the length of the lesser himalaya of nepal and represents a precambrian felsic volcanism or plutono volcanism mainly recycling continental crustal material this volcanism has contributed sediment to the lower group of formations of the lesser himalaya the ampipal alkaline gneiss is a small elongated body appearing as a window at the base of the lesser himalayan formations of central nepal it originated as a precambrian nepheline syenite pluton contaminated by lower continental crust the lesser himalayan granitic belt is well represented in nepal by nine large granitic plutons these cambro ordovician peraluminous generally porphyritic granites only occur in the crystaline nappes they were probably produced by large scale melting of the continental crust at the northern tip of the indian craton during a general episode of thinning of gondwana continent with heating and mantle injection of the crust the formation iii augen gneisses of the higher himalaya such as the augen gneiss of the higher himalayan crystalline nappes gosainkund are coeval to the lesser himalayan granites and their more metamorphic lower amphibolite grade equivalents limited outcrops of cretaceous trachytic volcanism lie along the southern limb of the lesser himalaya and are coeval with spilitic volcanism in the higher himalayan sedimentary series this volcanism foreshadows the general uplift of the indian margin before the himalayan collision the predominance of felsic over basic magmatism in the 2 5 ga long evolution of the himalayan domain constitutes an unique example of recycling of continental material with very limited addition of juvenile mantle products