pseudotachylytes from corsica fossil earthquakes from a subduction complex subduction zones are the most seismically active tectonic environment on earth here we report fault rock evidence of palaeoearthquakes from a subduction complex pseudotachylyte veins formed by shear heating on faults at seismic strain rates contain dendrites spherulites and acicular minerals characteristic of blueschist facies conditions the veins have been found in both mantle peridotite and gabbro our discovery from corsica opens a new avenue of research towards understanding mechanisms related to subduction zone earthquakes and the conditions at which these are released

tectonothermal events in central indian tectonic zone citz and its implications in rodinian crustal assembly abstract the precambrian crust of central india comprising bundelkhand craton bkc in the north and bastar craton bc in the south were accreted along a ene wsw trending proterozoic central indian tectonic zone citz the citz is an ensemble of several low to medium grade supracrustal belts gneisses granitoids and a few linear tracts of granulite belts a number of crustal scale shear zones define the boundaries of the supracrustal belts and discrete terrains available data suggest polyphase tectonothermal events in citz spanning from palaeoproterozoic to early neoproterozoic each event is characterized by contractional tectonic regime and attendant metamorphism and magmatism a new plate tectonic model in contrast to the earlier one of yedekar et al geol surv india spec publ 28 1990 is proposed according to the present model a northward dipping subduction system leading to continent continent collision explains the growth and assembly of the citz the continent continent collision is tentatively placed at ca 1 5 ga which is comparable to similar tectonothermal event in chotanagpur gneissic complex further east while the suturing event predates the global grenvillian orogeny ca 1 3 1 0 ga the sausar tectonothermal event ca 1 1 0 9 ga of citz is correlatable with the latter

large intermediate depth earthquakes and the subduction process this study provides an overview of intermediate depth earthquake phenomena placing emphasis on the larger tectonically significant events and exploring the relation of intermediate depth earthquakes to shallower seismicity especially we examine whether intermediate depth events reflect the state of interplate coupling at subduction zones and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes historic record of large intraplate earthquakes m b 7 0 in this century shows that the new hebrides and tonga subduction zones have the largest number of large intraplate events regions associated with bends in the subducted lithosphere also have many large events e g altiplano and new ireland we compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with m 6 and depth between 40 and 200 km the final catalog includes 335 events with 47 new focal mechanisms and is probably complete for earthquakes with m b 6 5 for events with m 6 5 nearly 48 of the events had no aftershocks and only 15 of the events had more than five aftershocks within one week of the mainshock events with more than ten aftershocks are located in regions associated with bends in the subducted slab focal mechanism solutions for intermediate depth earthquakes with m 6 8 can be grouped into four categories 1 normal fault events 44 and 2 reverse fault events 33 both with a strike nearly parallel to the trench axis 3 normal or reverse fault events with a strike significantly oblique to the trench axis 10 and 4 tear faulting events 13 the focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down dip extensional stress prevails in a gently dipping plate in contrast along decoupled subduction zones great normal fault earthquakes occur at shallow depths e g the 1977 sumbawa earthquake in indonesia type 2 events with strike subparallel to the subduction zone and most of them with a near vertical tension axis occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km the increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and consequently the change in focal mechanism from type 1 to type 2 events events of type 3 occur where the trench axis bends sharply causing horizontal parallel to the trench strike extensional or compressional intraplate stress type 4 are hinge faulting events for strongly coupled zones we observed temporal changes of intermediate depth earthquake activity associated with the occurrence of a large underthrusting event after the occurrence of a large underthrusting event the stress axis orientation of intermediate depth earthquakes changes from down dip tensional to down dip compressional e g 1960 chile 1974 peru 1982 tonga and 1952 kamchatka earthquakes or the number of large intermediate events decreases for a few years e g 1964 alaska and 1985 valparaiso earthquakes we conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate depth seismicity spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate depth events

the 2000 tottori earthquake a shallow earthquake with no surface rupture and slip properties controlled by depth the m w 6 8 tottori earthquake japan does not exhibit any surface trace but was particularly well instrumented strong motion displacement records and gps coseismic data are used to constrain the evolution of the slip on the fault plane in time and space we adopt in this study a two plane fault geometry based on aftershock distributions and analysis of close station records in a first step our inversion allowed surface slip the model obtained has a significant surface slip which contradicts the absence of clear surface slip reported by geologists in a second step models with no slip at the surface buried faults compatible with geological observations have been tested the tests with different fault depths show that when slip is allowed to occur close to the surface the fit to seismological and geodetic data is increased these tests confirm that slip actually occurred at shallow depth despite the nonuniqueness of the solution all the inverted source models show 1 a large slip amplitude patch at a depth of about 4 5 km and 2 relatively small slip in the hypocentral area the rupture velocity is about 2750 m s in the asperity region the total rupture duration is about 8 s the slip distribution seems to be controlled by the variation of fault properties with depth another feature that could control the rupture of this earthquake is a fault plane almost perpendicular to the main fault nw of the epicenter which apparently inhibits further rupture propagation

interseismic coupling and asperity distribution along the kamchatka subduction zone 1 gps measurements of interseismic horizontal surface velocities reveal the degree of kinematic coupling of the plate boundary thrust along the kamchatka subduction zone from about 51 degrees to 57 degrees n latitude inversions for the distribution of aseismic slip rate along the similar to 15 degrees nw dipping underthrust suggest a nonslipping plate interface in southern kamchatka above similar to 50 km depth along the segment that ruptured in the m w 9 1952 earthquake north of similar to 53 degrees n the subduction interface experiences significant aseismic slip consistent with the lower seismic moment release in m 8 5 earthquakes along this portion of the subduction zone the gps velocities are consistent with a boundary element forward model in which historic earthquake rupture zones are represented as locked asperities surrounded by a zero shear stress subduction interface loaded by plate convergence models in which the complete rupture zones of historic earthquakes are considered locked greatly overpredict the degree of kinematic coupling reducing the area of the locked model asperities to the central 25 area of historic rupture zones fits the data well suggesting that large earthquakes involve small fully locked core asperities surrounded by conditionally stable portions of the plate interface areas of low aseismic slip rate appear to be roughly correlated with areas of low isostatic gravity anomalies over offshore forearc basins while less coupled portions of the kamchatka subduction zone coincide with high gravity anomalies offshore of two peninsulas possibly related to the subduction of the emperor meji seamount chain and the kruzenstern fracture zone

sources of tsunami and tsunamigenic earthquakes in subduction zones we classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface typical interplate events earthquakes at the outer rise within the subducting slab or overlying crust intraplate events and tsunami earthquakes that generate considerably larger tsunamis than expected from seismic waves the depth range of a typical interplate earthquake source is 10 40km controlled by temperature and other geological parameters the slip distribution varies both with depth and along strike recent examples show very different temporal change of slip distribution in the aleutians and the japan trench the tsunamigenic coseismic slip of the 1957 aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long this asperity ruptured again in the 1986 andreanof islands and 1996 delarof islands earthquakes by contrast the source of the 1994 sanriku oki earthquake corresponds to the low slip region of the previous interplate event the 1968 tokachi oki earthquake tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes tsunami hazard assessments must include such events similarity in macroseismic data from two southern kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights most moment release of tsunami earthquakes occurs in a narrow region near the trench and the concentrated slip is responsible for the large tsunami numerical modeling of the 1996 peru earthquake confirms this model which has been proposed for other tsunami earthquakes including 1896 sanriku 1946 aleutian and 1992 nicaragua

a fossil ocean continent transition of the mesozoic tethys preserved in the schistes lustres nappe of northern corsica abstract the analysis of field relationships between allochthonous crystalline slivers ultramafic units and associated sedimentary cover in the high pressure schistes lustres units of northern corsica allows reconstruction of the pre orogenic architecture of a fossil mesozoic ocean continent transition oct of the tethys basin the studied area encompassing the serra di pigno and col de teghime areas cap corse is composed of an assemblage of prealpine basement sheets made up of serpentinized mantle rocks permian gabbros and gneisses locally associated with paleozoic metabasites and micaschists the sedimentary cover includes numerous intervals of calc schists marbles quartzites micaschists and dolomitic metabreccias yielding a great variety of lithostratigraphical successions the metasedimentary sequences do not show affinities with the typical cover series of the prepiemontais margin domains this implies an origin from a more internal domain oceanward for the studied units in addition the sedimentary cover of the ultramafic basement units do not display the characters of the classical alpine ophiolites but bear lithological features showing affinities with both continental and oceanic environments we assume that the high variability of the sedimentary successions as well as the presence of a basement composed of continental and upper mantle rocks is consistent with an origin at an oct the acidic basement slivers are regarded as former extensional allochthons of continental crust abandoned during breakup on a newly exhumed seafloor of peridotite composition such extensional allochthons will then logically constitute the most internal continental slices buried at great depth during alpine subduction this work shows that a lot has to be expected from a revision of the schistes lustres lithostratigraphy in the light of the new concepts of the evolution of the oct domains

late eocene lawsonite eclogite facies metasomatism of a granulite sliver associated to ophiolites in alpine corsica abstract the metamorphic history of two samples of continental slivers from alpine corsica is reconstructed on the basis of detailed petrology trace and major element geochemistry of the bulk rock and of metamorphic minerals and u pb dating of zircon a lawsonite fels with a ca rich and si na k poor bulk composition and a garnet bearing schist were investigated they both exhibit strongly zoned garnet crystals characterised by cores with high mg 0 25 0 33 low grossular component 0 03 0 09 and ree patterns with a negative eu anomaly u pb dating of zircons from the lawsonite fels yielded scattered inherited core ages from 912 to 575 ma and a first metamorphic rim that is dated at 292 5 3 3 ma ti in zircon thermometry the trace element pattern of the zircon and the composition of the garnet cores indicate that both samples experienced permian upper amphibolite to granulite facies metamorphism these high grade metamorphic rocks are associated to serpentinites metabasalts and metasediments that are typical of the alpine tethys indicating that the continental slivers were incorporated into an ocean continent transition during rifting alpine eclogite facies metamorphism is characterised by low temperature conditions as demonstrated by lawsonite crystallisation in both samples the lawsonite fels preserved the most complex history with two stages of garnet growth corresponding to a first prograde path possibly in the lawsonite blueschist facies and a second stage at lawsonite eclogite facies conditions this second stage was marked by strong ca metasomatism that is responsible for the peculiar composition of the sample and the massive crystallisation of lawsonite the strong metasomatism is also responsible for the partial recrystallisation of zircon and the formation of alpine rims the age of these zircon rims 34 4 0 8 ma represents the first u pb age for alpine high pressure metamorphism in corsica

strain localization along the atlantis bank oceanic detachment fault system southwest indian ridge microstructural observations and mineral thermometry from in situ samples collected from the atlantis bank oceanic core complex sw indian ridge indicate that detachment faulting was initiated under hypersolidus conditions in the ductile regime and continued through subgreenschist temperatures through the ductile semibrittle and brittle regimes as strain localized along the exposed now subhorizontal fault surface ductile semibrittle and brittle fabrics are developed within dominantly gabbroic rocks footwall rocks exhibit crystal plastic fabrics distributed over a structural thickness up to 400 m below the denuded fault surface exposed at the seafloor whereas semibrittle and brittle fabrics are concentrated in the 80 and 30 m immediately below the principal slip surface of the detachment fault respectively sample fabrics suggest that strain localization was achieved by dynamic recrystallization of plagioclase at temperatures between 910 c and 650 c by amphibole accommodated dissolution precipitation creep at temperatures 750 c 450 c by chlorite accommodated reaction softening at temperatures 450 c 300 c and by brittle fracturing and cataclasis at temperatures 300 c

frictional melting of gabbro under extreme experimental conditions of normal stress acceleration and sliding velocity 1 with the advent of high velocity shear apparatus several experimental studies have been performed in recent years improving our understanding of the evolution of fault strength during seismic slip however these experiments were conducted under relatively low normal stress 20 mpa and using small cylindrical samples where a large gradient in slip velocity exists across the sliding surface given the above limitations the extrapolation of these experimental results to natural conditions is not trivial here we present results from an experimental study on gabbroic rocks using a newly developed rotary shear apparatus capable of reaching higher normal stress up to 50 mpa on ring shaped samples 30 50 mm internal external diameter and allowing precise control of the imposed slip velocity function the results confirm that steady state shear stress during the melt lubricated phase of the experiment depends on normal stress in the form of a power law equation as predicted by theoretical models however the exponent appears closer to 0 5 contrary to the theoretical prediction of 0 25 we observe no systematic dependence of shear stress on acceleration but increasing deceleration drastically decreases the recovery of friction during final slip we find that the slip weakening distance decreases inversely with increasing normal stress in agreement with theoretical considerations and decreases with increasing slip rate extrapolation of the slip weakening distance to natural conditions predicts a slip velocity for ancient seismic events of 0 3 1 m s when compared with field estimates these values compare well with seismological estimates

initiating intermediate depth earthquakes insights from a hp lt ophiolite from corsica abstract the hypocentres of intermediate depth earthquakes have been shown to overlap with the regions in subducting slabs that contain high abundances of hydrous minerals this relationship was initially revealed using geophysical and numerical modelling and until recently has lacked corroboration from direct field based research we evaluated the relationship of the coincidence of intermediate depth earthquakes with hydrous minerals in the slab by undertaking detailed geochemical analyses of blueschist to lawsonite to eclogite facies pseudotachylytes and their hostrocks located within an exhumed ophiolite the eocene schistes lustres complex in corsica these units comprise incompletely metamorphosed metagabbro and peridotite the wallrocks of the pseudotachylytes contain variable amounts of hydrous minerals tremolite mg hornblende glaucophane in the metagabbro and serpentine tremolite and chlorite in the peridotite back scatter electron images show that the hydrous minerals entrained in the melt undergo fusion rather than dehydration vesicular and h 2 o rich melt veins are observed cross cutting partially molten pseudotachylyte fault veins and show evidence of h 2 o exsolution during melt solidification the crystallisation products of these melts indicate formation under high temperature high pressure conditions 1400 1700 c 1 5 gpa the peridotite hosted pseudotachylytes crystallised olivine orthopyroxene and diopside which are surrounded by interstitial al and h 2 o rich glass the metagabbro pseudotachylyte is dominated by al rich omphacite ilmenite and high fe anorthite xrf bulk analyses of the wallrock of the pseudotachylyte and electron microprobe analyses of the pseudotachylyte matrix entrained survivor clasts and the crystallisation products show that near total disequilibrium melting took place the peridotite hosted pseudotachylyte composition is skewed strongly towards chlorite however the preservation of delicate dendritic diopside and olivine hopper crystals suggests that the pseudotachylyte is unaltered indicating that preferential fusion of chlorite took place the metagabbro hosted pseudotachylyte matrix composition is very similar to the bulk wallrock composition but slightly skewed by the preferential melting of mg hornblende and tremolite not all the pseudotachylytes are hydrous as the h 2 o content of the melts is highly variable the metagabbro pseudotachylyte ranges from 0 to 4 wt and the peridotite pseudotachylyte ranges from 0 to 14 wt the range in h 2 o content of the pseudotachylytes has lead us to conclude that the localised dehydration of hydrous minerals may be a second order factor in initiating intermediate depth seismicity however we have observed that the pseudotachylytes with the most chaotic vein networks thickest fault veins and most comminuted material have the highest abundances of hydrous wallrock minerals possibly owing to repeated fluid ingress in between pseudotachylyte generating events this implies that free fluids enhance pseudotachylyte generation and possibly seismicity but are not a first order requirement microtextural and geochemical results from this study suggest that the presence of abundant h 2 o rich minerals in the slab exerts a strong rheological control during high strain rate deformation facilitating thermally triggered localising shear instabilities these field based observations allow us to explore the assumption of the causal link between slab hydration and earthquake nucleation and offer fresh insight into the debate of how intermediate depth earthquakes take place

characterization and origin of aluminous a type granites from the lachlan fold belt southeastern australia the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are a distinctive group of igneous rocks on the basis of chemical and mineralogical criteria those granites that contain 65 72 sio2 can be distinguished from other types on the basis of higher abundances of fetotal fetotal mg high field strength elements trivalent rare earth elements ga and zn mineralogically they contain fe rich hydrous mafic minerals and primary ilmenite and hence are reduced relative to the ni nio buffer however the extremely felsic a type granites sio2 72 have the same chemical and mineralogical characteristics as felsic fractionated i type granites recent analyses indicate that the lachlan fold belt a type granites have sc f alkali element trace transition element and h2o contents similar to those of other unfractionated i type granites rb sr and nd sm isotopic compositions are highly variable probably reflecting source region heterogeneity the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are distinct from peralkaline rocks in terms of chemical composition petrography and field associations although these rocks have been grouped together as a single type in current classification schemes we propose that the metaluminous to weakly peraluminous a type granites such as those of the lachlan fold belt should be defined as aluminous a type granites and should not be grouped with peralkaline granites the lachlan fold belt aluminous a type granites have relatively high calculated zircon saturation temperatures we suggest that these granites were produced by high temperature partial melting of a felsic infracrustal source

the geochemistry of phosphorus in granite rocks and the effect of aluminium phosphorus has been shown to have a profound influence on the evolution of silicate melt systems it also controls the geochemical behavior of rare earths uranium thorium strontium and other elements through the relationships between melt and accessory phosphates based on experimental data harrison and watson 1984 proposed a model of saturation behavior of apatite in crustal melts as a function of temperature and silica content subaluminous granites fit in well with such a model but departures are evident in the case of peraluminous granites a systematic study of the distribution of p2o5 in granitoids from all over the world shows that there are two modes to describe phosphorus enrichment in peraluminous granitic rocks 1 the hoyos type trend which is parallel to the one in harrison and watson s hw model but with higher p2o5 concentrations at the same silica contents and 2 the pedrobernardo type trend implying an increase in phosphorus in the most sio2 rich differentiates of strongly peraluminous low ca granites pedrobernardo type leucogranites are characterized by the presence of rare phosphates or p rich feld spars since they have excess p2o5 over cao to form normative apatite we here propose the term perphosphorous granites high phosphorus contents appear to be due to elevated apatite solubility in peraluminous granitic melts rather than to high amounts of restitic apatite both the hoyos and pedrobernardo type departures from the hw model can be corrected by using the expression p2o5corrected p2o5hw efactor where p2o5hw is the prediction according to the hw model and factor is asi 1 6429t asi being the al2o3 cao na2o k2o molar ratio we suggest that such departures could be caused by a decrease in the ca activity in the melt as a result of increased peraluminousity the pedrobernardo type trend is a limiting case in which ca activity in the melt is nearly zero and calcium instead of phosphorus becomes the essential structural constituent esc of the apatite when this happens phosphorus starts behaving as an incompatible element that is concentrated in late fluids high p2o5 contents are more characteristic of s than other granite types no significant departures from the hw model have been found for i a or m type granites in central iberia granites associated with u sn and w mineralizations usually have high phosphorus contents values of p2o50 5 wt at sio270 wt are good indicators of economic mineralization

fifteen new discriminant function based multi dimensional robust diagrams for acid rocks and their application to precambrian rocks abstract for the discrimination of four tectonic settings of island arc continental arc within plate continental rift and ocean island together and collision we present three sets of new diagrams obtained from linear discriminant analysis of natural logarithm transformed ratios of major elements immobile major and trace elements and immobile trace elements in acid magmas the use of discordant outlier free samples prior to linear discriminant analysis had improved the success rates by about 3 on the average success rates of these new diagrams were acceptably high about 69 to 97 for the first set about 69 to 99 for the second set and about 60 to 96 for the third set testing of these diagrams for acid rock samples not used for constructing them from known tectonic settings confirmed their overall good performance application of these new diagrams to precambrian case studies provided the following generally consistent results a continental arc setting for the caribou greenstone belt canada at about 3000 ma sao francisco craton brazil at about 3085 2983 ma penakacherla greenstone terrane dharwar craton india at about 2700 ma and adola ethiopia at about 885 765 ma a transitional continental arc to collision setting for the rio maria terrane brazil at about 2870 ma and eastern felsic volcanic terrain india at about 2700 ma a collision setting for the kolar suture zone india at about 2610 ma and korpo area finland at about 1852 ma and a within plate likely a continental rift setting for malani igneous suite india at about 745 700 ma these applications suggest utility of the new discrimination diagrams for all four tectonic settings in fact all three sets of diagrams were shown to be robust against post emplacement compositional changes caused by analytical errors element mobility related to low or high temperature alteration or fe oxidation caused by weathering

comparison of source areas of m4 8 0 1 repeating earthquakes off kamaishi ne japan are asperities persistent features abstract m4 8 0 1 earthquakes have occurred regularly since 1957 with a recurrence interval of 5 52 0 68 yr at the same location of the plate boundary off kamaishi iwate prefecture the last event m4 7 occurred on 11 13 2001 16 44 jst and the previous one m4 8 on 3 11 1995 10 30 jst waveforms of these events are similar to each other to examine the hypothesis that this characteristic earthquake sequence is caused by repeating ruptures of an asperity frictionally locked patch surrounded by creeping areas we compared the rupture area of the 2001 event with that of the 1995 event by a waveform inversion method estimated spatial extents of the rupture areas of the 2001 and 1995 events are almost the same and are estimated to be about 1 5 1 5 km 2 the rupture area of the 2001 event is almost completely overlapped with that of the 1995 event although the ruptures of the two events were initiated from different points the present observations clearly show that the 1995 and 2001 events are caused by the repeating ruptures of the same asperity patch and support the hypothesis of persistent asperities

earthquakes in the ductile regime pseudotachylytes from a crustal scale shear zone in central australia have developed in a cyclical manner once developed an individual pseudotachylyte is deformed in a ductile manner only to be overprinted at a later stage by a new generation of pseudotachylytes such cyclic generation and deformation of pseudotachylyte has been interpreted in the past as representing conditions at the brittleductile transition a different interpretation however is presented here it is proposed that psuedotachylytes and associated ultramylonites can develop entirely within the ductile regime as ductile instabilities such instabilities are different in nature to those previously discussed at length in the geophysical literature but are identical in principle with the instabilities that develop for velocity weakening frictional behavior in spring slider systems at a given strain rate a critical temperature tc is defined at which the transient work hardening equals the product of stress relaxation due to a thermal fluctuation and the heat generated by shearing a necessary condition for ductile instability at a given strain rate is that the temperature is belowtc then the rate of change of stress with respect to strain is negative an additional requirement is that this rate of change exceeds in magnitude the effective elastic stiffness of the loading system ductile instabilities are marginally possible at geological strain rates in quartzites but are possible at mid crustal temperatures in other rock types on the basis of these observations a new interpretation is presented for the base of the seismogenic zone in crustal regions

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

polyphase ductile brittle deformation along a major tectonic boundary in an ophiolitic nappe alpine corsica insights on subduction zone intermediate depth asperities in an ophiolitic nappe of alpine corsica a major fault zone superimposes metagabbro over serpentinite and peridotite ductile and brittle deformation structures are observed in the fault damage zones in the metagabbro damage zone early deformation culminates in blueschist or eclogite facies conditions and consists of west verging mylonitization alternating with pseudotachylyte forming faulting with undetermined vergence this early deformation is likely coeval with west verging seismic pseudotachylyte forming reverse faulting in the footwall peridotite or with aseismic distributed cataclastic deformation of footwall serpentinite these early events aseismic mylonitization or distributed cataclasis and seismic faulting are interpreted as reverse faulting shear in an east dipping subducting oceanic lithosphere in cretaceous to eocene times late deformation events consist of ductile shear and seismic faulting having occurred under retrograde greenschist conditions kinematics of the ductile shear is top to the east these events are interpreted as the result of syn to post collision extension of alpine corsica in eocene to miocene times the heterogeneous distribution of pseudotachylyte veins along the fault zone abundant at peridotite metagabbro interfaces rare or absent at serpentinite metagabbro interfaces is interpreted as the consequence of contrasted frictional properties of the rocks in contact high friction peridotite metagabbro contacts could correspond to asperities whereas low friction serpentinite metagabbro contacts could correspond to creeping zones

the frictional behavior of serpentinite implications for aseismic creep on shallow crustal faults serpentine is common in many active faults and may be responsible for aseismic creep along segments of these faults to test this we have conducted a series of velocity stepping experiments to determine the frictional velocity dependence of serpentinite we slid initially bare rough surfaces of antigorite serpentinite at room temperature with velocities from 0 0032 to 10 0 m s 1 0 102 to 3 2 105 mm yr and normal stresses from 25 to 125 mpa we find that the velocity dependence of serpentinite undergoes a transition from velocity weakening at fast loading velocities to velocity strengthening at slow velocities and that this change is accompanied by other changes in the constitutive behavior these results suggest that serpentinite should not be the site of instability initiation during sliding at plate velocities but may permit propagation of unstable slip initiated elsewhere

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

the sources of energy for crustal melting and the geochemistry of heat producing elements crustal melting to produce granite magmas requires a tremendous amount of energy in principle there are three main mechanisms of heating that can extensively melt a fertile crust radiogenic heat caused by the decay of 40k 230th 235u and 238u increased subcrustal heat flux caused by the upwelling of deeper therefore hotter mantle materials advection of heat caused by the emplacement and crystallization of hot mantle magmas two dimensional finite elements modeling reveals that a fertile crust thickened to 65 70 km would produce copious granite magmatism after 30 40 m y if its average heat production is a 1 2 w m 3 but it would scarcely melt if a 0 65 w m 3 increasing the subcrustal heat flow from the normal value of qm 0 025 w m 2 to qm 0 04 w m 2 may also lead to extensive crustal melting especially if the crust does not thin to less than 30 35 km very high qm 0 06 w m 2 affecting the continental crust is unlikely but the combination of moderately high qm 0 04 w m 2 and a thick fertile crust with a 1 2 w m 3 such as often happens in the volcanic and back arc areas of subduction zones is ideal to produce copious granite magmatism lastly the emplacement of hot mantle magmas in a fertile crust can produce crustal melts in just a few thousand years but the volume of these is equal to or less than the volume of the intruding magma a clue for understanding the relative importance of each of these three mechanisms comes from the radiogenic heat production of granite rocks calculated from the concentration of 40k 230th 235u and 238u at the time of their formation this parameter estimated on more than 3400 granites samples of different ages and provenance reveals a strongly asymmetric distribution peaking around 2 4 w m 3 a value much higher than the average continental crust about 1 1 2 w m 3 and certainly much higher than the average lower continental crust about 0 4 0 8 w m 3 only those granite rock types that are clearly connected with mantle heat sources such as the archean ttg post archean subduction related trondhjemites and recent adakites have a heat production equal to or smaller than the lower continental crust since the bulk melt solid partition coefficient of the heat producing elements hpe k th and u is k 1 the elevated hpe contents of granites indicates that most of them have been derived from hpe rich sources we conclude that radiogenic heating is often essential and always advantageous for generating large volumes of granite magmas and that granite magmatism is the main cause of the accumulation of hpe in the upper crust

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

use of grain size and magnetic fabric analyses to distinguish among depositional environments the combined use of grain size and magnetic fabric analyses provides the ability to discriminate among depositional environments in deep sea terrigenous sediments we analyzed samples from three different depositional settings turbidites pelagic or hemipelagic interlayers and sediment drifts results indicate that sediment samples from these different environments can be distinguished from each other on the basis of their median grain size sorting as well as the intensity and shape of magnetic fabric as determined from an examination of the anisotropy of magnetic susceptibility we use these discriminators to interpret downcore samples from the bermuda rise sediment drift we find that the finer grains of the bermuda rise relative to the blake outer ridge do not result from lower depositional energy current speed and so may reflect a difference in the nature of sediment being delivered to the site i e distance from source between the two locations

pseudotachylyte generated in the semi brittle and brittle regimes bench canyon shear zone central sierra nevada pseudotachylyte cuts cretaceous plutonic and volcanic rocks in the bench canyon shear zone central sierra nevada california as documented by optical and scanning electron microscopy the pseudotachylyte displays evidence for a friction melt origin including vesicles amygdules crystallites flow fabrics and embayed crystal fragments based on fault related rock association vein morphology and inferred crustal level of generation two distinct types of pseudotachylyte are documented in the shear zone pseudotachylyte associated with cataclasite was generated at shallow crustal levels 1 10 km via unstable velocity weakening abrasive wear friction pseudotachylyte associated and interlayered with mylonite was generated at deeper crustal levels 10 15 km most likely via stable velocity strengthening adhesive wear friction this pseudotachylyte underwent ductile deformation in the solid state interlayered pseudotachylyte and mylonite records cyclic aseismic seismic slip at a transitional crustal level between the semi brittle field of quartzofeldspathic rocks and the base of the seismogenic zone the cyclicity is attributed to strain rate fluctuation effected by strain resistant phacoids pore fluid pressure variation ductile instabilities or downward fault propagation the mode of deformation within the shear zone progressed from i production of mylonite during amphibolite to greenschist facies conditions to ii generation of pseudotachylyte by intermittent seismic faulting within a background of aseismic shear in the upper realm of the semi brittle field 300 400 c to iii generation of pseudotachylyte through seismic events in the brittle 300 c during uplift exhumation and cooling some of the latter may have been generated at shallow enough crustal levels 4 km to allow vesicle formation the exhumation of a crustal level exposing such an assemblage provides a rare opportunity to study the enigmatic nature of the transitional change from semi brittle flow to seismogenic rupturing

the diversification of granitoids and plate tectonic implications at the archaean proterozoic boundary in the bundelkhand craton central india

fossil oceanic core complexes recognized in the blueschist metaophiolites of western alps and corsica tethyan ophiolites show an apparent poorly organized association of ultramafic andmafic rocks by contrast tothe complete mantle crustal sections of semail type ophiolite sheets tethyan ophiolites are characterized by asmaller amount of mafic rocks gabbros and basalts by the absence of any sheeted dyke complex and by thefrequent occurrence of oceanic sediments stratigraphically overlying mantle derived peridotites and associatedgabbroic intrusions therefore they are considered as typical remnants of oceanic lithosphere formed in slowspreadingenvironment or in ocean continent transition at distal passive margins in the very first models offormation of the tethyan ophiolites in the years 1980 the geodynamical processes leading to mantle unroofingwere poorly understood due to the paucity of data and concepts available at that time from the present dayoceans in particular at that time little work had focused on the distribution origin and significance ofmafic rocks with respect to the dominant surrounding ultramafics here we reconsider the geology of sometypical metaophiolites from the western alps and corsica and we show how results from the past decadeobtained in the current oceans ask for reassessing the significance of the tethyan ophiolites in general revisitedexamples include a set of representative metaophiolites from the blueschists units of the western alps queyrasregion and from alpine corsica golo valley field relationships between the ophiolitic basement andthe metasedimentary metavolcanic oceanic cover are described outlining a typical character of the tethyanophiolite lithological associations jurassic marbles and polymictic ophiolite metabreccias are unconformablyoverlying the mantle gabbo basement in a way strictly similar to what is observed in the non metamorphicappennine ophiolites or chenaillet massif this confirms that very early tectonic juxtaposition of ultramafic andmafic rocks occurred in the oceanic domain before subduction this juxtaposition resulted from tectonic activitythat is now assigned to the development of detachment faults and to the formation of oceanic core complexes occs at the axis of slow spreading ridges this fundamental plate tectonics process is responsible for the exhumationand for the axial denudation of mantle rocks and gabbros at diverging plate boundaries in addition fieldrelationships between the discontinuous basaltic formations and the ultramafic mafic basement indicate that thistectonic stage is followed or not by a volcanic stage we discuss this issue in the light of available field constraints

clustering and periodic recurrence of microearthquakes on the san andreas fault at parkfield california the san andreas fault at parkfield california apparently late in an interval between repeating magnitude 6 earthquakes is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters 20 meter radius of microearthquakes within these clusters which account for 63 of the earthquakes in a 1987 92 study interval virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second the rate of earthquake occurrence particularly at depth increased significantly during the study period but the fraction of earthquakes that were cluster members decreased

archaean ttgs as sources of younger granitic magmas melting of sodic metatonalites at 0 6 1 2 gpa two natural low k2o na2o ttg tonalitic gneisses one hornblende bearing and the other biotite bearing were partially melted at 0 8 1 2 gpa fluid absent the chief melting reactions involve the breakdown of the biotite and hornblende the hornblende tonalite is slightly less fertile than the biotite tonalite but melt volumes reach around 30 at 1 000 c this contrasts with results of most previous work on more potassic ttgs which generally showed much lower fertility though commonly producing more potassic melts garnet is formed in biotite bearing tonalitic protoliths at p 0 8 gpa and at 1 0 gpa in hornblende bearing tonalitic protoliths all fluid absent experiments produced peraluminous granitic to granodioritic melts typically with sio2 70 wt for the biotite tonalite increasing t formed progressively more melt with progressively lower k2o na2o however the compositions of melts from the hornblende tonalite do not vary significantly with t with increasing p melts from the biotite tonalite become less potassic due to the increasing thermal stability of biotite for the hornblende tonalite again there is no consistent trend fluid absent melting of sodic ttgs produces melts with insufficient k2o to model the magmas that formed the voluminous late potassic granites that are common in archaean terranes reconnaissance fluid present experiments at 0 6 gpa imply that h2o saturated partial melting of ttgs is also not a viable process for producing magmas that formed these granites the protoliths for these must have been more potassic and less silicic nevertheless at granulite facies conditions sodic ttgs will produce significant quantities of broadly leucogranodioritic melt that will be more potassic than the protoliths upward abstraction of this melt would result in some lile depletion of the terrane younger k rich magmatism is unlikely to represent recycling of ttg crust on its own and it seems most likely that evolved crustal rocks and or highly enriched mantle must be involved

on ferroan a type granitoids their compositional variability and modes of origin we recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry these include alkalic granitoids that may be metaluminous or peralkaline alkali calcic granitoids that may be metaluminous peraluminous or peralkaline calc alkalic granitoids that may be metaluminous or peraluminous and rare calcic ferroan granitoids these granitoids may form by two distinct end member processes extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali calcic or metaluminous alkalic alkali calcic and calc alkalic with alkalinity increasing with increasing pressure of differentiation partial melting of tonalitic to granodioritic crust produces alkali calcic to calc alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures it is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids most granitoids that are referred to as ferroan however the term has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis for this reason we recommend that the term be discontinued and ferroan used in its stead

trace element and nd isotopic evidence for sediment sources in the mid proterozoic vindhyan basin central india abstract ancient sediments reflect the compositions of old eroded continental crustal material and preserve geochemical signatures of paleo tectonic and volcanic events geochemical study of such sediments provides insight into tectonic events that shaped the face of the earth nd isotopes are best suited for such studies because rare earth elements ree specifically sm and nd are not fractionated during post depositional processes such as erosion transportation and diagenesis of organic poor sediments and hence reflect the source composition of these sediments sedimentation in the proterozoic vindhyan basin of central india with a 4 km thick succession of unmetamorphosed and mostly undeformed sediments lasted over 1000 ma from 1630 ma up to the precambrian cambrian boundary these sediments provide an ideal opportunity to study the evolution of ancient continental crust as well as the tectonic evolution of peninsular india the occurrence of porcellanites in the lower part of the basin indicates active volcanism during deposition of the vindhyan sediments these tuffs show large ion lithophile element lile enrichment as well as depletions in nb and ta in primitive mantle normalized plots characteristic of an arc volcanic source in combined trace element ratio plots these volcanic rocks indicate mixing between mantle derived melts and the continental crust and overlap with the composition of global arcs their initial nd at 1630 ma ranging from 5 7 to 0 9 is similar to those observed in andean type continental margin arcs trace element concentration patterns as well as nd isotopic compositions of the clastic and non clastic sediments of the vindhyan basin from one of the type localities of the vindhyan stratigraphy in the son valley are vastly different from those of the bundelkhand granite which is the basement for these sediments indicating that these granitic rocks were not a major source of sediments for the vindhyan basin our nd isotopic study clearly indicates that sedimentation in the vindhyan basin which lasted over 1 ga was not continuous the boundaries between the semri and kaimur groups as well as the rewa and bhander groups are characterized by abrupt changes in the nd isotopic compositions indicating changes in provenance with sediments in the kaimur and bhander groups being derived from more juvenile sources compared to the semri and rewa groups respectively this inference is also supported by changes in t dm and f sm nd for the sediments across these boundaries the strikingly different trace element patterns of the rewa group sediments compared to the underlying kaimur sediments indicates that sediment sources also changed across the kaimur rewa boundary multiple changes in provenance in the vindhyan sediments indicate that this basin was tectonically active during the entire span of sedimentation while there is some uncertainty in determining the exact sources of sediments for the vindhyan basin we argue on the basis of our geochemical in particular nd isotopic ratios data and available paleocurrent data that majority of the sediments were derived from a now extinct andean type arc source in the south that resulted from a southerly dipping subduction prior to the collision of the bhandara and bundelkhand cratons according to this reconstruction the vindhyan basin was initiated as a foreland basin after the collision of these two cratons

slow and fast deformation in the dora maira massif italian alps pseudotachylytes and inferences on exhumation history abstract the dora maira massif dm western alps is a type example of an ultra high pressure uhp metamorphic terrain the occurrence of pseudotachylytes within the val gilba area of dm led to speculation regarding the role of seismic slip in the exhumation of the massif in this study new field microstructural and geochemical data on the pseudotachylyte veins and their mylonitic gneiss host are presented the formation of pseudotachylyte veins occurred late in the exhumation history at 20 1 0 5 ma and at shallow depths

late orogenic post orogenic and anorogenic granites distinction by major element and trace element chemistry and possible origins granites classified into four categories based solely on tectonics of occurrence and associated rock types also have compositional characteristics that are consistent within groups and different am

exposed plate interface in the european alps reveals fabric styles and gradients related to an ancient seismogenic coupling zone 1 we present observations from a continuous exposure of an ancient plate interface in the depth range of its former seismogenic zone in the central alps of europe related to late cretaceous early tertiary subduction and accretion of the south penninic lower plate underneath the adriatic upper plate the material forming the exposed plate interface zone has experienced flow and fracturing over an extended period of time followed by syncollisional exhumation thus reflecting a multistage evolution fabric formation and metamorphism however chiefly record the deformation conditions of the precollisional setting along the plate interface we identify an unstable slip domain from pseudotachylytes occurring in the temperature range between 200 and 300 c this zone coincides with a domain of intense veining in the subduction melange with mineral growth into open cavities indicating fast possibly seismic rupture evidence for transient near lithostatic fluid pressure as well as brittle fractures competing with mylonitic shear zones continues into the region below the occurrence of pseudotachylytes possibly reflecting a zone of conditionally stable slip the zone above the unstable slip area is devoid of veins but displays ample evidence of fluid assisted processes similar to the deeper zone solution precipitation creep and dehydration reactions in the melange matrix hydration and sealing of the base of the upper plate seismic rupture here is possibly expressed by ubiquitous localized deformation zones we hypothesize that trenchward sealing of parts of the plate interface as well as reaction enhanced destruction of upper plate permeability is an important component localizing the unstable slip zone this relation may result from the competition of the pervasive presumably interseismic pressure solution creep destroying permeability and building elevated fluid pressure until the strength threshold is reached with seismic failure

origin of post collisional a type granites in the mahakoshal supracrustal belt central indian tectonic zone india zircon u pb ages and geochemical evidences abstract there are several a type granitic plutons in the central and western parts of the mahakoshal supracrustal belt msb india we report new whole rock elemental and nd isotope data along with zircon u pb ages on these granites the id tims zircon u pb ages of these granites are 1645 25 ma 1695 9 ma and 1796 17 ma these granites are metaluminous to weakly peraluminous alkali calcic to calc alkalic and ferroan in composition they are characterized by enrichment in rees except eu and hfses zr y and depletion in liles ba sr they have high ga al 2 6 5 4 ratios and elevated magmatic temperatures 900 c elemental ratios such as y nb nb ta th nb la nb and ce pb coupled with negative nb sr eu and ti anomalies suggest their derivation from crustal sources they have very low end t values 11 4 to 5 6 and older nd model ages tdm 2481 3225 ma confirming their crustal derivation we propose that partial melting of the archean tonalitic granodioritic crust of basement bundelkhand craton in low pressure high temperature conditions generated the parental magma of these granites their a2 type affinity y nb 1 2 further suggests their emplacement in an extensional environment related to post collisional tectonic setting

dynamic weakening of serpentinite gouges and bare surfaces at seismic slip rates to investigate differences in the frictional behavior between initially bare rock surfaces of serpentinite and powdered serpentinite gouge at subseismic to seismic slip rates we conducted single velocity step and multiple velocity step friction experiments on an antigorite rich and lizardite rich serpentinite at slip rates v from 0 003 m s to 6 5 m s sliding displacements up to 1 6 m and normal stresses n up to 22 mpa for gouge and 97 mpa for bare surfaces nominal steady state friction values nss in gouge at v 1 m s are larger than in bare surfaces for all n tested and demonstrate a strong n dependence nss decreased from 0 51 at 4 0 mpa to 0 39 at 22 4 mpa conversely nss values for bare surfaces remained 0 1 with increasing n and v additionally the velocity at the onset of frictional weakening and the amount of slip prior to weakening were orders of magnitude larger in gouge than in bare surfaces extrapolation of the normal stress dependence for nss suggests that the behavior of antigorite gouge approaches that of bare surfaces at n 60 mpa x ray diffraction revealed dehydration reaction products in samples that frictionally weakened microstructural analysis revealed highly localized slip zones with melt like textures in some cases gouge experiments and in all bare surfaces experiments for v 1 m s one dimensional thermal modeling indicates that flash heating causes frictional weakening in both bare surfaces and gouge friction values for gouge decrease at higher velocities and after longer displacements than bare surfaces because strain is more distributed

possible carlin type disseminated gold mineralization in the mahakoshal fold belt central india abstract the paper records the first occurrence of dolomite hosted disseminated gold mineralization at barhi and jhal in a late archean early proterozoic metavolcano sedimentary belt mahakoshal fold belt in central india gold mineralization is hosted by dolomite that occurs as discontinuous bands interbedded with phyllite hydrothermal alteration styles of the host rock include decalcification silicification and argillization pyrite is the most common sulfide whereas stibnite and realgar are rare mineralization is characterized by persistent gold from 0 20 to 0 62 ppm and a consistent association of anomalous arsenic antimony and mercury with gold

paleomagnetism of ca 2 3 ga mafic dyke swarms in the northeastern southern granulite terrain india constraints on the position and extent of dharwar craton in the paleoproterozoic abstract the pole positions reported for the mafic dykes occurring in the northeastern southern granulite terrain and easterly trending dykes of dharwar craton are alike while their ages differ by ca 700 ma confounding tectonic reconstruction to resolve the discordance and to constrain the position of the indian continent in the proterozoic paleomagnetic and geochronological studies have been carried out on the mafic dyke swam in the northeastern southern granulite terrain two distinct pole positions are obtained one from the group of tiruvannamalai dykes as 27 7 s 231 5 e dp dm 12 14 with corresponding mean declination dm and inclination im as 125 and 73 8 95 22 7 6 respectively and second from the east coast dykes as 2 32 s 188 2 e dp dm 5 8 with corresponding mean dm and im as 88 9 and 33 8 95 48 7 1 respectively the remanent efficiency of magnetization values between 0 01 and 0 08 suggest that the dykes have been cooled through curie temperature of magnetite which is the carrier of magnetization the baked contact test confirms that the characteristic remanent magnetization is the primary magnetization the sm nd mineral whole rock isochron from one of the tiruvannamalai dykes yielded an age of 2318 60 ma mswd 1 9 this age is the time of intrusion of the tiruvannamalai dykes and is similar within errors to the age of dykes in the dharwar craton thus the contiguity of dharwar craton with the northern block of southern granulite terrain as early as 2 3 ga ago is established the pole position thus obtained corresponds to a high latitudinal position of dharwar craton during early paleoproterozoic

flash weakening of serpentinite at near seismic slip rates 1 to investigate the processes responsible for dynamic frictional weakening in antigorite serpentinite we conducted single velocity and velocity stepping friction experiments at slip rates v from 0 01 to 0 4 m s 1 sliding displacements up to 40 mm and a normal stress of 5 mpa single velocity experiments demonstrate an approximately 1 v dependence of friction on velocity above a characteristic weakening velocity vw 0 1 m s 1 consistent with theoretical predictions for flash heating and subsequent weakening of asperities velocity stepping experiments impose stepwise increases in slip rate show stepwise weakening at slip rates above vw scanning electron microscopy of experimental fault surfaces reveals nanoparticle gouge textures at raised sites 10 m in diameter in tests that exhibit dramatic weakening furthermore x ray diffraction analyses of fault gouge from the sliding surface document the formation of significant talc in these tests indicating that weakening temperatures reached 500 c 700 c in contrast no talc is observed in samples for which v remained significantly below vw the observed value for vw is consistent with published microphysical models for flash weakening when independent constraints on the thermal stability and contact strength of antigorite are considered finally while serpentinite displays velocity strengthening behavior at plate tectonic slip rates our results indicate that seismic ruptures propagating into serpentinized regions in the shallow lithosphere may trigger seismicity or slow earthquakes after limited amounts of displacement

geochemical evidence for the paleoproterozoic arc back arc basin association and its importance in understanding the evolution of the central indian tectonic zone abstract the central indian tectonic zone citz is considered to be one sector in a series of paleoproterozoic orogens that stitched together the supercontinent columbia the paleoproterozoic mahakoshal belt forms the foundation of the citz metabasalts and metaclastic rocks of the mahakoshal belt have been studied for major elements and trace elements including the rare earth elements ree metabasalts of the lower saleemanabad formation are grouped into two types however both types of metabasalt depict a back arc basalt character by showing enrichments in large ion lithophile elements liles like k rb cs and ba and light rare earth elements lrees but show depletions in high field strength elements hfses like nb ta y and ti and hree heavy rare earth elements in comparison to the n morb this is further supported by some key elemental ratios like ba nb nb ta la nb eu eu nb nb and ree patterns the geochemistry of the metaclastic rocks of the lower saleemanabad middle parsoi and upper dudhmaniya formations of the mahakoshal belt show an arc signature rather than the cratonic signature in the bundlekhand ttg and n morb normalised diagrams the arc signature calc alkaline series of metaclastic rocks is further supported by various tectonic discrimination diagrams the overall study indicates that the mahakoshal belt was formed as a back arc rift basin very close to continental arcs on the hinterland of the bundelkhand craton at the initial stage of subduction as a result of trench rollback followed by thermal relaxation and basin inversion the sediment supply started from the continental arcs lying south of the mahakoshal basin furthermore the present study strengthens the mechanism of subduction related outgrowth of columbia along its continental margins and implies that the citz is a major precambrian crustal boundary that began as a mahakoshal back arc rift by northward subduction

interrelations between intermediate depth earthquakes and fluid flow within subducting oceanic plates constraints from eclogite facies pseudotachylytes field evidence preserved in ancient subducted oceanic crust documents that eclogite facies frictional failure with melting pseudotachylyte formation during intermediate depth earthquakes was accompanied and followed not preceded by infiltration of external fluids and progressive vein formation in the eclogites eclogitization began during seismic failure and fluid passage through the shear zone subsequent fluid flow produced hydraulic fracturing and continuous vein formation during ongoing burial we suggest that this kind of shear zone may allow channellized fluid flow within and out of slabs

proterozoic ferroan feldspathic magmatism abstract the proterozoic eon is characterized by an unusual abundance of ferroan feldspathic rocks that range in composition from granite to quartz syenites to feldspathoid bearing syenites three associations of ferroan rocks may be recognized on the basis of major element compositions 1 metaluminous alkalic to alkali calcic syenites and granites associated with anorthosites and iron rich basaltic rocks 2 alkaline intrusions in continental rifts and 3 calc alkalic metaluminous to peraluminous granitoid plutons that are less strongly ferroan than the other two groups the first two groups form by differentiation of tholeiitic to alkali basaltic parental magmas the third by partial melting of quartzo feldspathic crust although all three associations may form in intraplate settings only the alkaline intrusions which typically contain high incompatible trace element abundances plot entirely within the within plate granite fields on trace element discrimination diagrams processes of crustal melting and magma mixing have had a greater effect on the trace element compositions of ferroan granitoids from the other two associations causing some rocks to plot outside this field of these three associations the granitoids associated with anorthosites are the most voluminous and are essentially restricted to the proterozoic special conditions perhaps related to the presence of a long lived supercontinent under which widespread mantle upwelling took place appear required to explain the abundance of these rocks ferroan alkaline rocks although rarely preserved in archean crust are present in proterozoic and phanerozoic rifts the earliest alkaline intrusions thus may mark the establishment of rigid continents capable of rifting the formation of ferroan calc alkalic granites does not appear to be time dependent these granites have been generated throughout earth history in response to crustal melting

pseudotachylyte and the development of ultramylonite bands in the saint barthelemy massif french pyrenees abstract in mylonitic gneisses of the saint barthelemy massif in the french pyrenees evidence was found that ultramylonite bands developed by ductile deformation of pseudotachylyte in the gneiss a mylonitic shape fabric was produced by a continuous structural event during retrogression from amphibolite to upper greenschist facies decrease in temperature caused gradual hardening in the gneiss which led to the development of pseudotachylyte bands by intermittent seismic slip these acted as weak zones in which ductile deformation was concentrated a microstructure typical of ultramylonite was produced in these deformed pseudotachylyte bands

crustal evolution in eastern labrador constraints from precise u pb ages abstract new u pb ages for rocks of eastern labrador complement previous u pb dates for the grenville and makkovik provinces and substantiate the following conclusions eastern labrador consists of two distinct crustal blocks that are different in age as well as magmatic metamorphic and structural overprint the northern block includes archaean to lower proterozoic basement rocks 1900 ma and two distinct episodes of rhyolitic volcanism at 1860 and 1807 ma high grade metamorphism deformation and anatexis affected this block between 1860 and 1760 ma ago makkovikian orogeny the southern block comprises the trans labrador batholith and the various terranes of the grenville province in contrast to the northern region the southern block shows no proof for the existence of continental crust older than 1710 ma this suggests that large amounts of juvenile material were added between 1710 and 1630 ma ago as dated by a large series of precise rock ages at least towards its end this crust forming event was associated with high grade metamorphism deformation and anatexis labradorian orogeny the 1710 1630 ma period of activity was followed by three distinct phases of anorogenic plutonism causing the emplacement of pegmatites 1500 ma a widespread suite of cotonitic metagabbros michael gabbro 1426 ma and granitic syenitic bodies 1296 ma during the grenvillian orogeny the region achieved its present structural configuration including the emplacement of granitoid plutons the structurally highest regions the mealy mountains and the hawke river terranes were thrust over the lake melville terrane and part of the groswater bay terrane and they were only marginally affected by grenvillian metamorphism the more deeply buried lake melville and groswater bay terranes however were significantly modified during tectonothermal events at 1030 ma and 980 960 ma respectively

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process