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

propagation of high frequency seismic waves inside the subducted lithosphere from intermediate depth earthquakes recorded in the vanuatu arc three seismic phases associated with the slab of subducted lithosphere are identified on seismograms of intermediate depth earthquakes depths 70 km recorded in the distance range up to 350 km by the vanuatu formerly new hebrides seismograph network they are interpreted as 1 a p wave refracting into the inner portion of the slab 2 a p wave traveling mainly through the outer upper portion of the slab and 3 a p to s converted wave in which the conversion occurs across the upper surface of the slab the existence of the two p arrivals from intermediate depth earthquakes suggests that these earthquakes are not located in the strong and cold inner core of the slab but within an outer zone where the velocity is lower than that within the inner portion of the slab the low frequency character of the shear wave converted from the p wave across the outer boundary of the slab indicates that either the boundary is a transition layer with a finite thickness or the material above the slab has high attenuation for high frequency shear waves large spatial variations in the transmission of high frequency shear waves also indicate that there is a localized zone of anomalous shear wave attenuation in the mantle wedge lying just above the subducted slab and beneath the active volcanos

constraints on the duration of tectonic processes protracted extension and deep crustal rotation in the grenville orogen geochronological data on metamorphic minerals within and outside the bancroft shearzone alarge scaleproterozoicshearzone provideevidenceforextensionaldisplacement over a period of 150 m y this is the longest time interval yet demonstrated for any faultrespondingtooneorogenicphase extensionaldisplacementratesalongthebancroft shear zone averaged over 150 m y from 1040 to 893 ma are 0 06 to 0 13 km m y during extension unroofingofthefootwallwasaccompaniedbyrotationas5to15kmofoverlying crustwasunloaded similarlower platerotationhasbeeninferredformanyofthetertiary metamorphic core complexes exposed in the western united states the protracted displacement and rotation history of the proterozoic bancroft shear zone contrasts with the muchfasterextensionalunroofingoftheyoungermetamorphiccorecomplexes suggesting inherent differences in the tectonic processes operating in these two geologic settings similarities however indicate that both settings may share common aspects of tectonic deformation these findings underscore the geologic importance of large deep crustal faults and indicate that present day zones of active extension such as in the himalayas could remain active many millions of years into the future

fine structure of the dipping seismic zone and subduction mechanics in the shumagin islands alaska microearthquake data from a permanent telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the shumagin islands alaska the shallow microseismicity is characterized by active interplate thrusting in the 25 45 km depth range arcward of this interplate activity microearthquakes in the overlying plate have a strike slip mechanism the near horizontal p axis of this mechanism is oriented in the direction of plate convergence suggesting that the plate interface at shallow depths is currently efficiently coupled below 45 km depth the dipping seismic zone appears to be double planed the upper plane dips at 32 from 45 to 100 km depth where it exhibits a kneelike bend below the volcanic front the lower plane begins at about 65 km depth where it is separated from the upper plane by some 25 km and appears to converge with the upper plane at about 120 km depth the inferred geometry of the plate interface suggests that the double planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate a composite fault plane solution for events in the upper plane indicates however that downdip tension is currently present rather than the downdip compression that would be expected from unbending it is proposed that because of locking of the plate interface at shallower depths slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked this model is consistent with previous interpretations that there is a high probability of a great earthquake in the shumagin islands seismic gap within the next decade or two

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

single layer buckle folding in non linear materials i experimental study of fold development from an isolated initial perturbation abstract scale model experiments involving pure shear compression of a single layer embedded in a weaker matrix were carried out to study the influence of a pre existing isolated perturbation in the shape of an otherwise planar layer on the location and shape of folds different grades of paraffin wax were used as analogues for rocks with power law rheology the wax used for the matrix flows in steady state with a stress exponent of about 3 8 the layer has a stress exponent of about 3 and strain softens viscosity ratios considered at yield stress for the layer and matrix of 30 1 and 8 1 were used bell shaped perturbations were introduced into the layer with initial average wavelengths of 8 16 and 32 times layer thickness and corresponding initial limb dips of 12 5 and 3 respectively the fold packet develops at the site of the introduced perturbation and spreads slowly along the layer with increasing shortening the shape of the initial perturbation strongly influences the fold geometry broader initial perturbations develop broader folds undergo more layer parallel shortening and develop a wider zone of contact strain the influence of the introduced perturbation shape is most marked when its average wavelength component is larger than the dominant wavelength i e the wavelength of maximum growth rate the degree of bonding between layer and matrix also influences fold development folds with welded layer matrix boundaries amplify much more slowly with a greater proportion of layer parallel shortening than folds where slip between layer and matrix is promoted

deep crustal ductile extension following thrusting in the southwestern grenville province ontario two lithotectonic assemblages in southern britt domain have different histories of plutonism metamorphism structural development and mafic dyke emplacement these differences are accounted for by postulating that a cryptic grenvillian thrust separates the assemblages amphibolite facies extensional shear along the central britt shear zone cbsz overprinted the thrust obscuring kinematic evidence for its existence the structurally lower bayfield nadeau island assemblage contains orthogneiss suites of disparate age lesser amounts of supracrustal rocks deposited before intrusion of the youngest orthogneiss pre grenvillian and grenvillian metamorphic assemblages and at least three mafic dyke suites the overlying ojibway sand bay assemblage contains only younger orthogneiss with grenvillian metamorphic assemblages volumetrically important supracrustal rocks that are younger than the youngest orthogneiss and lacks cross cutting mafic dykes comparable tectono stratigraphic changes are present acros

seismic images of the grenville orogen in ontario in 1990 lithoprobe acquired 240 km of seismic reflection data across parts of the central gneiss belt cgb and the central metasedimentary belt cmb within the western grenville province of southern ontario interpretation of these data in conjunction with geological constraints provided by bedrock mapping supports a model of northwest directed thrusting and crustal shortening for the grenville orogen within the cgb the parry sound shear zone is imaged as a 3 km wide zone of reflections dipping southeastward at 20 25 and soling at depths 7 km in the north and 3 km in the south beneath parry sound domain parry sound domain and the immediately adjacent domains are underlain by a gently southeast dipping reflective zone at 4 5 12 0 km depth interpreted as a detachment surface likely associated with the central britt shear zone this zone may have accommodated northwesterly transport of parry sound southern britt and northwestern rosseau domains over britt domain during grenvillian thrusting wit

seismic transmission survey mapping in two layered media i propose a new concept of a seismic survey which takes advantage of mode converted transmission waves this method furnishes us with valuable information independent of any other survey which have been used to explore the earth s structure in the seismic reflection survey both not mode converted and mode converted reflection waves are used in order to obtain the section of the reflectivity in the seismic transmission survey refraction waves with mode changes from p to sv or from sv to pin propagating through the interfaces are used in order to obtain the section of the transmissivity the model consists of only a horizontally layered elastic medium to correct the normal move out of the transmission waves i apply the method which have been developed to process the mode converted reflection waves the approximation error is estimated by comparing with the exact solution in the case of a horizontally two layered medium numerical simulations prove that this method is available for the two layered medium with the discontinuity which has the inclination of 30 degrees

rapid map and inversion of p sv waves multicomponent seismic recordings are currently being analyzed in an attempt to improve conventional p wave sections and to find and use rock properties associated with shear waves e g dohr 1985 danbom and dominico 1986 mode converted p sv waves hold a special interest for several reasons they are generated by conventional p wave sources and have only a one way travel path as a shear wave through the typically low velocity and attenuative near surface for a given frequency they will have a shorter wavelength than the original p wave and thus offer higher spatial resolution this has been observed in several vertical seismic profiling vsp cases e g geis et al 1990 however for surface seismic data converted waves are often found to be of lower frequency than p p waves e g eaton et al 1991

evidence for seafloor spreading in the laxmi basin northeastern arabian sea abstract marine magnetic anomaly data from the laxmi basin for the first time reveal the presence of fairly correlatable nnw trending magnetic lineations these magnetic lineations are symmetric about a central negative magnetic anomaly and the axis of symmetry coincides with a characteristic short wavelength free air gravity low the anomalies are interpreted as representing a two limbed seafloor spreading sequence which can be equated to the a28 a33 interval of the geomagnetic polarity reversal timescale these results suggest that the laxmi basin is underlain by an oceanic crust evidence of seafloor spreading in this basin possibly implies a pre a27 spreading episode in the evolutionary history of the arabian sea

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

the upper boundary of the philippine sea plate beneath the western kanto region estimated from s p converted wave abstract in the kanto tokai district of central japan the configuration of the subducting philippine sea plate has been estimated using three dimensional velocity inversions of the seismic wave velocity structure and the distribution of microearthquakes however it is difficult to show the configuration of the philippine sea plate from the distribution of microearthquakes beneath the western kanto region i e the yamanashi and nagano prefectures because seismic activity is quite low there a clear x phase between p and s phases on seismograms from an earthquake is observed at temporary seismological stations located in this area this x phase is identified as the s to p converted wave at the upper boundary of the subducting philippine sea plate thus we have been able to deduce the shape of the philippine sea plate and to show that it dips northwest

tectonic and metamorphic events in the westernmost grenville province central ontario new results from high precision u pb zircon geochronology the u pb isotopic ages of metamorphic zircons from deformed amphibolitized metadiabase dykes and associated unstrained anatectic pegmatites have been determined to constrain the times of postdyke ductile flow related recrystallization and deformation samples from the central gneiss belt of the grenville province in the southern georgian bay region provide a means of linking areas formed during common episodes of ductile flow three zircon fractions from one attenuated dyke in the moon river domain gave identical concordant ages of 1062 1065 and 1064 2 ma while those from another gave identical concordant ages of 1052 1051 and 1050 2 ma for the time of metamorphism zircons from a folded dyke from the go home domain gave identical concordant ages of 1049 1047 and 1046 2 ma while a single overgrowth fragment from an unstrained anatectic pegmatite in the fold hinge gave a concordant age of 1047 2 ma as if folding metamorphic zircon growth and postfolding pegmatite are part of the same proc

seismic transmission survey application to real data from microearthquakes two discontinuities have been discovered which are closely related to the philippine sea phs plate that subducts beneath the kanto district in central honshu japan a seismic transmission survey method is used to image discontinuities in the earth from mode converted transmission mct waves and this method is applied to aftershocks of an event which occurred beneath the southern tokyo bay at a depth of 100 km as a result two predominant interfaces at depths of 24 km and 35 km appear in the section of p sv mct waves this example demonstrates that the seismic transmission survey method proposed by katsumata 1992 should be very useful in studying fine structure in the earth moreover it indicates that even microearthquakes can be used as sources for seismic surveys

kinematic and mechanical discontinuity at a coherent interface abstract across a coherent interface displacements are continuous but some components of large strain rotation stretching spin stress and rheology need not be so this accounts for refraction of cleavage and other geological structures the general theory of deformation and motion is used to analyze how discontinuities develop a measure of discontinuity in total deformation gradients strain and rotation is the ratio k of amounts of shear above and below the interface and in directions parallel to it it is shown that k is equal to the ratio of simple shearings above and below the interface provided i the latter ratio is constant in time and ii no volume changes occur these kinematic conditions are shown to hold in newtonian fluids and incompressible neo hookean solids where k is exactly equal to the inverse viscosity ratio or to the inverse rigidity ratio the conditions do not hold in general in reiner rivlin fluids in power law fluids the ratio of simple shearings is constant for two special classes of motion one with little simple shearing along the interface the other with simple shearing alone therefore the rheological contrast can be determined the theoretical results can be used to determine rheological contrasts in nature or experiment provided there is some knowledge of the nature of the flow laws that operate under these conditions an interface is an inbuilt rheometer

a simple method for the determination of the dilation direction of intrusive sheets abstract it is shown that the occurrence of offsets of various orientations along the contacts of an intrusive sheet permits the rapid establishment of the net dilation direction for the sheet the application of the method to some dykes on arran shows that oblique opening of a fissure was sometimes accompanied by a vertical component of opening the technique provides a tool for the analysis of patterns of crustal displacement associated with dyke intrusion

synorogenic collapse a perspective from the middle crust the proterozoic grenville orogen structural petrological and geochronological studies of the middle to late proterozoic grenville orogen in ontario canada indicate that a major extensional fault developed synchronously with late thrusting this fault zone was initiated during peak metamorphism and extended into the crust to depths of at least 25 kilometers the temporal and spatial relations among faulting metamorphism and regional compression indicate that synorogenic collapse initiated because the crust exceeded the maximum physiographic height and thickness that could be supported by its rheology comparison of grenville with recent himalayan orogenic activity suggests that during proterozoic times physiographic height crustal thickness and crustal strength were similar to modern conditions in orogenic belts