structural evolution of the sarandi del yi shear zone uruguay kinematics deformation conditions and tectonic significance the sarandi del yi shear zone is a crustal scale shear zone that separates the piedra alta terrane from the nico perez terrane and the dom feliciano belt in southern uruguay it represents the eastern margin of the rio de la plata craton and consequently one of the main structural features of the precambrian basement of western gondwana this shear zone first underwent dextral shearing under upper to middle amphibolite facies conditions giving rise to the reactivation of pre existing crustal fabrics in the easternmost piedra alta terrane afterwards pure shear dominated sinistral shearing with contemporaneous magmatism took place under lower amphibolite to upper greenschist facies conditions the mylonites resulting from this event were then locally reactivated by a cataclastic deformation this evolution points to strain localization under progressively retrograde conditions with time indicating that the sarandi del yi shear zone represents an example of a thinning shear zone related to the collisional to post collisional evolution of the dom feliciano belt that occurred between the meso to neoproterozoic 600 ma and late ediacaran lower cambrian times

40ar 39ar thermochronology of the pan african damara orogen namibia with implications for tectonothermal and geodynamic evolution abstract forty three 40ar 39ar step heating experiments on white mica biotite hornblende and whole rocks from the damara orogen namibia revise and refine the regional perspective on cooling exhumation and tectonic reactivation across the orogen these data also document post orogenic motion on major shear zones and the thermal effects of late syn to post tectonic granite intrusions these data show regional cooling through temperatures of 300 350 c from 545 to 520 ma in the central kaoko zone 530 510 ma in the eastern kaoko zone and 495 480 ma in the southern zone of the damara belt discordant 40ar 39ar age spectra relate to 1 partial resetting of k ar isotopic system in micas by late stage activity in the major kaoko belt shear zones such as the purros and three palms mylonite zones giving apparent ages of 467 6 481 3 and 492 ma and 2 thermal effects from granitic plutions particularly in the ugab domain of the southern kaoko belt 500 475 ma and in the granite dominated central zone of the damara belt 490 460 ma phyllites of the lower grade flank regions of the orogen record apparent 40ar 39ar mica crystallization ages of 517 ma eastern kaoko zone 526 ma northern zone damara belt and 568 553 ma southern foreland zone damara belt the southern foreland samples however are maximum ages due to variable influence of detrital mica rates of cooling vary between zones within the kaoko belt in the central kaoko belt hornblende biotite and white mica apparent 40ar 39ar ages are similar suggestive of rapid cooling through the 300 550 c temperature range whereas in the orogen core or western kaoko belt hornblende and biotite apparent ages are older than the white mica ages because the white micas record the reactivation of the main structures these data support models of diachronous deformation and metamorphism of the component parts of the orogen and suggest that pan african orogenesis in the damara is broadly bracketed between 580 and 500 ma with final movements through 480 ma and spans through the terminal amalgamation of gondwana in the kuunga orogeny through the initiation of subduction under the gondwana continental margin

orogen parallel tectonic transport in the variscan belt of northeastern sardinia italy implications for the exhumation of medium pressure metamorphic rocks a transpressive crustal scale dextral shear zone is documented in the variscan basement of northeastern sardinia it indicates the presence of a shear deformation parallel to the belt overprinting previous d1 structures related to nappe stacking and top to the s and sw thrusting the l2 stretch ing lineation points to an orogen parallel stretching and to a general change in the tectonic transport from d1 to d2 phase d1 developed during initial frontal collision whereas the d2 deformation was characterized by dextral shearing during the increasing curvature of the ibero armorican arc transpressional deformation developed in a regime of decreasing pressure it caused telescoping of the barrovian isograds and the exhumation of the low to medium grade metamorphic rocks in this sector of the variscan belt exhumation is due to continuing compression with an increasing compo nent of horizontal displacement the overall change of the shortening direction in a large sector of an orogenic belt with the occurrence of increasing orogen parallel displacement may be regarded as a general mechanism affecting the exhumation of rocks and preventing the thickened collisional crust from undergoing a generalized gravitational collapse

thermally softened continental extensional zones arcs and rifts as precursors to thickened orogenic belts abstract intra continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence this enables significant thickening with formation of an orogenic root we have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra continental extension with associated heating geologically this situation is appropriate to intra continental rifts and back arc basins if thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak soft lower crust and sub arc rift mantle result this weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot these regions are associated with formation of granulites and metamorphic assemblages typical of high temperature low pressure ht lp if convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc rift domain such sites are ideal for the formation and for the exhumation of metamorphic core complexes subsequent thickening during convergence leads to ht eclogites when the previous arc rift was hot and to medium t eclogites for a thickened standard geotherm these p t paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial geotherm if the compression starts long after cessation of the extensional event and associated thermal anomaly then the geotherm of the extended area relaxes and the whole region hardens in this case no homogeneous thickening occurs and deep continental roots cannot form

thermal evolution and exhumation in obliquely convergent transpressive orogens most p t t path models to date have considered a linear erosion rate for exhumation from burial depth related to isostatic readjustment of crustal thickness a few have discussed extension enhanced exhumation erosional exhumation can only restore lower crustal rocks from the thickened mountain root to their previous original depth in the pre collisional crust one major assumption of all models to date is that the compressive forces responsible for crustal thickening cease before elevation and erosion begins however compression is often still active even if the crustal thickening has stopped further compression of the thickened and strongly deformed orogenic root is responsible for forceful exhumation extrusion of softened rocks upwards hence the rate of exhumation is related to the rate of convergence of colliding plates extrusional exhumation can elevate buried rocks to any depth depending on the action of fault shear systems in the extrusional exhumation models examined here the ascending rocks cool faster because they approach the zero temperature surface conditions more rapidly than by isostatic erosion the exhumation rate is also governed by the angle between the plate boundary and the displacement vector implying that the convergent plate boundaries are regarded as complex transpressive systems in which the degree of obliquity can be expressed by the ratio of pure to simple shear components a low ratio of pure simple shear typical for wrench dominated plate boundaries implies long distance horizontal transport from the buried position in the original orogen and longer periods during which metamorphic heating occurs a high ratio of pure simple shear characteristic for frontal like convergence implies a rapid exhumation without significant heating thus granulites barrovian type and blueschist facies metamorphism are characterised by increasing angle of obliquity 10 30 90 respectively the high temperature limit at low ratio of pure simple shear is the geotherm t and lies at temperatures that would commonly be taken to indicate that addition of mantle heat was required to generate hot geotherms and p t t paths wrench faulting over 100 s of kilometres at reasonable slow convergence rates could lead to extensive dehydration melting and granulite formation in large scale collisional orogens

tectonics of western tibet between the tarim and the indus a tarim to indus traverse provides insight into the tectonics of western tibet the kunlun was the site of a mid paleozoic collision at least three phases of post paleozoic accretion have thickened the blanket of sediments that covers western tibet sizeable parts of western qiangtang have remained stable however since the mid mesozoic since the neogene deformation and volcanism have been localized near the edges of the plateau strike slip motion along the karakorum and altyn tagh faults has been coeval with overthrusting in the himalayas and kunlun such slip partitioning and the volcanism appear to result simply from northward subduction of india and southward subduction of the tarim as tibet is extruded eastwards by india s penetration into asia

oblique collision and evolution of large scale transcurrent shear zones in the kaoko belt nw namibia abstract early structures in the central part of the kaoko orogenic belt of nw namibia suggest that the initial stage of collision was governed by underthrusting of the medium grade central kaoko zone below the high grade western kaoko zone resulting in the development of an inverted metamorphic gradient in the western zone early structures were overprinted by a second phase of deformation which is associated with localization of the transcurrent puros shear zone along the contact between the western and central zones during this second phase extensive partial melting and intrusion of 550 ma granitic bodies occurred in the high grade western zone in the central zone the second phase of deformation led to complete overprinting of the early foliation in the zone adjacent to the puros shear zone and to the development of kilometre scale folds in the more distal parts strain partitioning into transcurrent deformation along the puros shear zone and ne sw oriented shortening in the central zone is consistent with a sinistral transpressional regime during the second phase of deformation transcurrent deformation continued during cooling of the entire belt giving rise to the localized low temperature village mylonite zone that separates a segment of elevated mesoproterozoic basement from the rest of the western zone in which only pan african ages have so far been observed the data suggest that the boundary between the western and central kaoko zones represents a modified thrust zone controlling the tectonic evolution of the pan african kaoko belt

abrupt strike slip fault to subduction transition the alpine fault puysegur trench connection new zealand swath bathymetry and other geophysical data collected over the fiordland margin southwest of new zealand are used to investigate the mechanism of transform subduction transition between the alpine fault and the puysegur trench two segments of the pacific australian plate boundary in this region the cenozoic southeast tasman basin which obliquely underthrusts fiordland at the puysegur trench is separated from the cretaceous tasman basin by the resolution ridge system a major lithospheric discontinuity of the downgoing plate interpretation of seafloor morphology shows that the alpine fault extends offshore along the fiordland coast and splits into west and east branches the west branch cuts obliquely across the margin and connects sharply to the puysegur subduction front at the northeastern tip of the resolution ridge system earthquake and seismic reflection data indicate that the west branch is genetically controlled by downgoing plate structures associated with the resolution ridge system hence the west branch is interpreted as the surface trace of the plate boundary segment extending between the alpine fault and the puysegur trench we conclude that the development of the strike slip segment of the plate boundary and its sharp transition to the puysegur subduction are controlled by inherited structures of the australian plate furthermore according to geophysical data presented here a tearing of the downgoing plate can be interpreted beneath the west branch a review of geophysical data along the region of the alpine fault hikurangi trough northeast new zealand shows a progressive transform subduction transition that is accommodated by motion partitioning between the subduction interface and strike slip faults this transition is accounted for by an interplate coupling that progressively increases toward the alpine fault in relation with a gradual thickening of the downgoing crust the comparison between the fiordland and the hikurangi strike slip subduction transitions show that presence of inherited downgoing plate crustal faults properly oriented with respect to the plate motion facilitates a sharp strike slip subduction transition

explosion seismic tomography of a magmatic body beneath etna volatile discharge and tectonic control of volcanism to constrain the upper depth range of lateral heterogeneities beneath mt etna a seismic tomography method with artificial sources is designed to undershoot the target and restrict the depth of seismic propagation by reflection at the top of the basement a strong high velocity anomaly thus imaged southeast of the summit interpreted as a magmatic body is reliably constrained to reach at least as shallow as 2 km depth where exsolution of so2 from magma may occur the high gas output rate of etna allard 1997 hence appears as the normal complement of the huge magma body seismically imaged this magma body is located beneath the volcanic edifice rather than within which attests to the control of volcanism by extension these volatile output and internal structure distinguish mt etna volcanism among current types and may relate to the peculiar context of a recent regional change from subduction related compression to extension

transition from strike slip faulting to oblique subduction active tectonics at the puysegur margin south new zealand abstract south of new zealand the pacific australia pac aus plate boundary runs along the intracontinental alpine fault the puysegur subduction front and the intraoceanic puysegur fault the puysegur fault is located along puysegur ridge which terminates at ca 47 s against the continental puysegur bank in a complex zone of deformation called the snares zone at puysegur trench the australian plate subducts beneath puysegur bank and the fiordland massif east of fiordland and puysegur bank the moonlight fault system mfs represents the eocene strike slip plate boundary interpretation of seafloor morphology and seismic reflection profiles acquired over puysegur bank and the snares zone allows study of the transition from intraoceanic strike slip faulting along the puysegur ridge to oblique subduction at the puysegur trench and to better understand the genetic link between the puysegur fault and the mfs seafloor morphology is interpreted from a bathymetric dataset compiled from swath bathymetry data acquired during the 1993 geodynz survey and single beam echo soundings acquired by the nz royal navy the snares zone is the key transition zone from strike slip faulting to subduction it divides into three sectors namely east nw and sw sectors a conspicuous 3600 m deep trough the snares trough separates the nw and east sectors the east sector is characterised by the ne termination of puysegur ridge into right stepping en echelon ridges that accommodate a change of strike from the puysegur fault to the mfs between 48 s and 47 s in the nw sector and the snares trough a series of transpressional faults splay northwards from the puysegur fault between 49 50 s and 48 s thrusts develop progressively at puysegur trench into a decollement north of 48 s the snares trough develops between two splays of the puysegur fault indicating superficial extension associated with the subsidence of puysegur ridge seismic reflection profiles and bathymetric maps show a series of transpressional faults that splay northwards across the snares fault and terminate at the top of the puysegur trench slope between ca 48 s and 46 30 s the relative plate motion appears to be distributed over the puysegur subduction zone and the strike slip faults located on the edge of the upper plate conversely north of ca 46 s a lack of active strike slip faulting along the mfs and across most of puysegur bank indicates that the subduction in the northern part of puysegur trench accounts for most of the oblique convergence hence active transpression in the snares fault zone indicates that the relative pac aus plate motion is transferred from strike slip faulting along the puysegur fault to subduction at puysegur trench the progressive transition from thrusts at puysegur trench and strike slip faulting at the puysegur fault to oblique subduction at puysegur trench suggests that the subduction interface progressively developed from a western shallow splay of the puysegur fault it implies that the transfer fault links the subduction interface at depth a tectonic sliver is identified between puysegur trench and the puysegur fault its northwards motion relative to the pacific plate implies that is might collide with puysegur bank

the kaoko belt namibia part of a late neoproterozoic continental scale strike slip system the neoproterozoic kaoko belt of northwestern namibia to the west of the congo craton consists of basement gneisses and a sequence of highly deformed metasedimentary and metavolcanic rocks the metasedimentary rocks are interpreted as the infill of a narrow north trending basin floored by attenuated continental crust similar basins are represented by the western continuation of the kaoko belt the dom feliciano and ribeira belts of eastern brazil on the basis of consideration of data from these belts the kaoko belt rocks indicate the absence of a large neoproterozoic oceanic basin between the congo and southwestern american cratons deformation structures in the kaoko belt show transpressional kinematics with strain partitioning at around 550 ma north northwest south southeast sinistral transcurrent shear in the west passes eastward into east southeast directed thrusting the transcurrent shear component dominates temporally and in strain intensity dextral transcurrent motion at 550 ma is recorded in the east northeast trending schlesien mwembeshi shear zone bordering the congo craton to the south accordingly the congo craton extruded northeastward along the kaoko belt and the schlesien mwembeshi shear zone relative to the kalahari craton in the southeast and the south american cratons to the west this extrusion probably resulted from a final phase of convergence between east and west proto gondwana

upper plate contraction north of the migrating mendocino triple junction northern california implications for partitioning of strain geologic measurement of permanent contraction across the cascadia subduction margin constrains one component of the tectonic deformation along the convergent plate boundary the component critical for the seismic hazard assessment of crustal faults a comprehensive survey of active faults in onshore subduction margin rocks at the southern end of the cascadia subduction zone indicates that these thrust faults accommodate 10 mm yr of convergence oriented 020 045 seismotectonic models of subduction zones typically assign this upper plate strain to the estimate of aseismic slip on the megathrust geodetic models include this permanent crustal strain within estimates of elastic strain accumulation on the megathrust both types of models underestimate the seismic hazard associated with crustal faults subtracting the observed contraction from the plate convergence rate 40 50 mm yr directed 040 055 leaves 30 40 mm yr of convergence to be partitioned between slip on the megathrust contraction within the southern juan de fuca plate and crustal contraction outside the subduction complex rocks this simple estimate of slip partitioning neglects the discrepancy between the plate convergence and contraction directions in the vicinity of the mendocino triple junction the san andreas and cascadia limbs of the mendocino triple junction are not collinear the eastern edge of the broad san andreas boundary is 85 km east of the cascadia subduction boundary and across this zone the pacific plate converges directly with the north america plate the skewed orientation of crustal structures just north of the leading edge of the pacific plate suggests that they are deforming in a hybrid stress field resulting from both juan de fuca north america motion and pacific north america motion the composite convergence direction 50 mm yr directed 023 is consistent with the compressive stress axis 020 inferred from focal mechanisms of crustal earthquakes in the humboldt region deformation in such a hybrid stress field implies that the crustal faults are being loaded from two major tectonic sources the slip on crustal faults north of the mendocino triple junction may consume 4 5 mm yr of pacific humboldt convergence the remaining 17 18 mm yr of convergence may be consumed as distributed shortening expressed in the high rates of uplift in the cape mendocino region or as northward translation of the continental margin north of the triple junction

geochemical character and petrogenesis of pan african amspoort suite of the boundary igneous complex in the kaoko belt nw namibia abstract in the kaoko belt of northwestern namibia syn tectonic granitoids of the boundary igneous complex intruded a nnw sse trending suture between the neoproterozoic coastal terrane and archaean neoproterozoic congo craton derived foreland typical representatives of this syn collision igneous association are ca 550 ma old k feldspar phyric bt cam granites granodiorites of the amspoort suite with minor cpx gabbro and rare two pyroxene dolerite bodies the scarce opx cpx bt dolerites could come from a chur like mantle source 87sr 86sr550 0 7045 e550nd 0 the rest of the amspoort suite is interpreted as a product of ht anatexis of a heterogeneous lower crust built mainly by immature metapsammites rich in arc derived detritus with minor metabasite and intermediate metaigneous bodies the most likely source appears to be the anatectic coastal terrane gneisses yet partial melting of so far little constrained congo craton cover if formed by immature and youthful detritus unrelated to the basement cannot be discounted in any case the rather primitive sr nd isotopic compositions of the amspoort suite apart from dolerites e550nd 3 4 to 5 3 87sr 86sr550 0 7063 0 711 rule out any major role for the congo craton basement derived material which is in sharp contrast to other more southerly pan african granitoid complexes of the kaoko belt

pan african rifting and orogenesis in southern to equatorial africa and eastern brazil abstract the upper proterozoic pan african belts of africa and brasiliano belts of south america are assumed to have evolved from an elaborate system of continental rifts which formed on the west gondwana continent 1100 1000 ma ago reviews of the damara belt kaoko belt gariep belt saldania belt west congolian belt lufilian arc and zambezi belt of southern and equatorial africa and the ribeira mantiqueira belt of brazil show the applicability of the continental rifting model to all these belts strict application of the rifting model describing the formation and further evolution of rift structures in the continental crust leads to the conclusion that the west congolian belt as traditionally described is incomplete and has to be supplemented by the mayumbian belt and a rift structure situated farther to the west the lufilian arc is assumed to have evolved from two rift structures separated from each other by the later domes region the golfe du katanga which branches off northeastwards from the lufilian arc is interpreted as an aulacogen shaba aulacogen the pan african brasiliano rift system is comparable to the mesozoic arctic north atlantic rift system with regard to size and distribution of rifts ocean floor spreading and opening of a proto south atlantic ocean most probably occurred along a main line of rifting situated between the west coast of africa and eastern brazil the wedge like ocean may have terminated at the sao francisco congo cratonic bridge between northeastern brazil and gabon cameroon there is no evidence of ocean floor spreading in the zambezi belt the lufilian arc and the traditional west congolian belt which are arranged along another major line of rifting closure of the pan african brasiliano rift system took place during two successive orogenic episodes the zambezi belt lufilian arc and west congolian belt underwent their main orogenic deformation with prevailing ene and wsw oriented tectonic transportation directions during the katangan episode at 900 750 ma the transcontinental sinistral mwembeshi shear zone adjusted opposite movement directions in the zambezi belt and lufilian arc and at its western prolongation concurrently supported opening of the khomas trough in the southern damara belt the proto south atlantic ocean still opening during the katangan episode was gradually closed during the damaran episode at 750 500 ma closure proceeded from north to south and was accompanied by northwest directed subduction underneath the brazilian plate and southeast directed tectonic transportation in the kaoko damara and gariep belts in the previously formed west congolian belt lufilian arc and zambezi belt a second deformation phase characterized by strike slip faulting and local emplacement of nappes occurred during this episode

time constraints on magmatism along the major gercino shear zone southern brazil implications for west gondwana reconstruction the dom feliciano belt situated in southernmost brazil and uruguay contains a large mass of granite gneissic rocks also known as florianopolis pelotas batholith formed during the pre syn and post orogenic phases of the brasiliano pan african cycle in the ne extreme of this granitic mass pre syn and post tectonic granites associated with the major gercino shear zone mgsz are exposed the granitic manifestation along the mgsz can be divided into pre kinematic tonalitic gneisses peraluminous high k calc alkaline early kinematic shoshonitic and metaluminous post kinematic granites u pb zircon data suggest an age of 649 10 ma for the pre tectonic gneisses and a time span from 623 6 ma to 588 3 ma for the early to post tectonic magmatism negative ehf t values ranging from 4 6 to 14 6 and hf model ages ranging from 1 64 to 2 39 ga for magmatic zircons coupled with whole rock nd model ages ranging from 1 24 to 2 05 ga and end t values ranging from 3 84 to 7 50 point to a crustal derivation for the granitic magmatism the geochemical and isotope data support a continental magmatic arc generated from melting of dominant paleoproterozoic crust and a similar evolution for the granitic batholiths of the eastern dom feliciano belt and western kaoko belt

metamorphic response and crustal architecture in a classic collisional orogen the damara belt namibia abstract the damara belt is a well exposed mid crustal section through a collisional orogen of cambrian age that closed the khomas ocean basin between passive margins on the congo and kalahari cratons collision resulted in a bi vergent orogen with a distinct paired metamorphic pattern of foreland vergent high p low t orogenic margins and a broad high grade low p high t orogen core spatial and temporal patterns of the metamorphic response to collision have been characterized for all parts of the belt using a large dataset n 240 of internally consistent quantitative pt determinations evolution of mineral parageneses and semi quantitative p t paths metamorphic mapping and quantitative metamorphic field gradients integration with deformation history structural profiles metamorphic chronology magmatic history and stratigraphy constrains a dynamic model of crustal architecture during peak metamorphic events the pattern of zonal metamorphic response is demarcated by three major metamorphic discontinuities md with steep pressure gradients inferring crustal scale structures that accommodated lateral exhumation of crustal wedges discontinuities are confirmed by deformation features in the field and metamorphic mineral growth indicates that vertical flattening at the peak of metamorphism progressing through ductile to brittle extensional structures crustal wedges along the orogenic margins experienced steep clockwise p t paths with peak pt conditions terminated by isothermal decompression during rapid exhumation in isostatic response to deep burial metamorphic chronology and over printing metamorphic fabrics constrain a sequence of foreland propagating out wedging of crustal thrust wedges that resulted in telescoping of the orogenic margins peak metamorphism at a geothermal gradient of 20 25 c km and 8 9 kb in the southern zone wedge i was attained between 517 530 ma followed by south directed out wedging at the uis pass suture accommodated by relative extension md1 at high structural levels near the boundary with the high grade orogen core out wedging of the southern zone further buried the southern margin zone wedge ii in the footwall below the uis pass suture peak metamorphism at 17 c km and 9 5 11 5 kb in the southern margin zone was attained at 517 4 ma and followed by out wedging on basal thrusts accommodated by vertical flattening and extensional reactivation of the uis pass suture md2 peak metamorphism at 17 c km and 10 5 kb in a high p low t crustal wedge in the northern margin wedge iii occurred at 510 4 ma peak metamorphism in this wedge was terminated by isothermal decompression during north directed thrusting accommodated by extension at higher structural levels near the high grade orogen core md3 granulite facies metamorphism in the high grade orogen core which formed the upper plate to the deeper thrust wedges occurred at significantly lower pressures 4 3 6 0 kb higher t depth ratios 38 47 c km and low p t clockwise p t paths indicating only moderate burial and protracted high grade conditions metamorphic chronology confirms high grade conditions 540 505 ma persisted beyond isostatic adjustment of the high p low t orogenic margins high heat flow conditions and long thermal lag are best explained by lithospheric breakoff during collision in contrast to the high p low t margins that experienced lateral exhumation in a convergent system the high grade orogen core was exhumed vertically as a broad core complex during extensional collapse of the orogen peak metamorphic conditions in the central part of the orogen core record higher pressures than marginal lower grade zones to both the north and south indicating vertical exhumation as a broad massif this post peak vertical exhumation of the central core occurred after 505 ma was accommodated by brittle extensional detachments at high stratigraphic levels with km scale omission of crustal section

oblique plate motion and continental tectonics three dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins kinematic models of strike slip partitioning for transpression and transtension allow the exact relation among three critical parameters plate motion instantaneous strain axes and degree of strike slip partitioning to be calculated application to two end member tectonic environments characterized by a low south island new zealand and high central california degree of strike slip partitioning demonstrates a remarkable consistency among the three parameters suggesting that strike slip partitioned transpression is a valid model for deformation in these regions the extreme degree of strike slip partitioning in wrench dominated systems such as central california is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes

u pb zircon ages for granitoid gneisses in northern namibia and their significance for proterozoic crustal evolution of southwestern africa abstract the pre damara 900 ma basement gneisses and granitoids of northern namibia and southern angola constitute the southern margin of the congo craton and are poorly known geochronologically we report u pb zircon concordia intercept ages for an augengneiss of the epupa complex 1800 ma a banded quartzofeldspathic gneiss of the huab complex 1810 ma a post huab granitic gneiss 1750 ma and for inherited zircons 2120 ma from a damaran syenite that has a true emplacement age of 800 840 ma these ages suggest that the epupa and huab complexes are broadly coeval and their formation together with the emplacement of the widespread fransfontein granitoid suite constitutes a significant and regionally extensive crust forming event in northern namibia and southern angola the huabian episode that corresponds to the eburnian event in western central and eastern africa older crust in excess of 2120 ma is likely to be present in the region but has not been documented geochronologically similarities in age rock association and broad geologic evolution for pre damara basement complexes in northern central and southern namibia suggest that the entire territory may have been part of the congo craton in mid proterozoic times whose southwestern portion was largely reworked during the late precambrian namaqua and pan african events

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

two phase evolution of accretionary margins examples from the north american cordillera abstract the coast mountains orogen southeastern alaska and the salmon river suture zone western idaho record the accretion of oceanic crustal fragments to the north american cordilleran margin structural elements associated with these boundaries include a western accretion related thrust belt and steeply dipping post accretionary shear zone the shear zones the coast shear zone southeastern alaska and western idaho shear zone idaho are steeply dipping crustal scale structures characterized by steep isotopic and geophysical gradients the presence of syndeformational tabular calc alkaline tonalitic plutons concordant with shear zone fabrics locally preserved dextral strike slip indicators and pervasive down dip mineral and elongation lineations are consistent with an intra arc transpressional origin for both structures in addition to strike slip translation the structures accommodated exhumation of high pressure moderate temperature metamorphic rocks within the thrust belt to the west and moderate pressure high temperature batholithic rocks to the east similarities between the structures in alaska and idaho suggest that the observed two stage history of underthrusting of accreting crustal fragments followed by modification by intra arc transpressional shear zones is a common tectonic process our analysis implies that recognition of processes active during the accretion of crustal fragments to continental or cratonal margins is commonly inhibited by post accretion deformation and provides insight into lower crustal processes presently operative under modern strike slip accretionary boundaries in addition the intra arc transpressional shear zones e g the coast and western idaho shear zones are structures that could have accommodated paleomagnetically determined large magnitude transcurrent displacements

the metamorphic architecture of a transpressional orogen the kaoko belt namibia s 64 61 goscombe b hand m 2001b metamorphic model for the inland branch of the damara orogen a paired metamorphic mountain belt pmmb geological society of australia specialist group in tectonics and structural geology conference ulverstone

kinematics of oblique collision and ramping inferred from microstructures and strain in middle crustal rocks central southern alps new zealand the hanging wall of the alpine fault near franz josef glacier has been exhumed during the past similar to2 3 m y providing a sample of the ductilely deformed middle crust of a modem obliquely convergent orogen presently exposed rocks of the pacific plate are inferred to have undergone several phases of ductile deformation as they moved westward above a mid crustal detachment initially they were transpressed across the outboard part of the orogen resulting in oblate fabrics with a down dip stretch later they encountered the alpine fault experiencing an oblique slip backshearing on vertical planes this escalator like deformation tilted and thinned the incoming crust onto that crustal scale oblique ramp this style of hanging wall deformation may affect only the most rapidly uplifting central part of the southern alps because of the low flexural rigidity of the crust in that region and its displacement over a relatively sharp ramp angle at depth a 3d transpressive flow affected mylonites locally near the fault but their shear direction remained parallel to plate motion ruling out ductile extrusion as an important process in this orogen outside the mylonite zone late cenozoic shortening is inferred to be modest 30 40 as measured from deformation of younger biotite grains oblique collision is dominated by translation on the alpine fault and rocks migrate rapidly through the deforming zone preventing the accumulation of large finite strains transpression may play a minor role in oblique collision c 2001 elsevier science ltd all rights reserved

climatology of mesospheric gravity wave activity over urbana illinois 40 n 88 w a 5 year 1991 1995 climatology of mesospheric gravity wave activity observed with the urbana medium frequency mf radar is presented we focus on the monthly mean horizontal wind variances associated with gravity waves with observed periods between 10 min and 8 hours these results are compared with the seasonal characteristics from other gravity wave observations the total wave variance displays a dominant semiannual variation maxima summer winter minima spring fall below 87 km with little seasonal variation above 87 km the zonal wave variance exceeds the meridional wave variance except around regions of zonal wind reversal in all seasons the observed variance scale height is greater than that of a nondissipating wave field a simple zero mean wind model based on linear saturation theory is found to be inadequate in reproducing the seasonal characteristics of the observed wave variances an alternative model based on critical layer filtering by a height dependent wind field can reproduce the observed semiannual oscillation in the wave variance given a specific source input

timing and sources of pre collisional neoproterozoic sedimentation along the sw margin of the congo craton kaoko belt nw namibia abstract isotopic dating of detrital zircon populations from metasediments and of magmatic zircon from intercalated metavolcanic layers in the medium to high grade part of the kaoko belt in namibia provides first robust constraints on the age of neoproterozoic sedimentation along the southwestern margin of the congo craton dating of detrital zircons from metasediments directly overlying the cratonic basement shows maximum sedimentary ages of c 1 00 and c 1 45 ga and age populations comparable with known protolith ages from the gneisses of the congo kalahari cratons dating of zircon from associated metavolcanics constrains the age of the earliest preserved sediments at c 740 710 ma detrital zircon populations from the samples collected from the upper parts of the metasedimentary succession contain only small proportion of grains with ages similar to those from the congo craton these samples show dominance of c 1 00 ga c 750 ma and c 650 ma old zircon grains that are probably derived from the punta del este coastal terrane dom feliciano and kaoko belts that acted as an arc back arc domain at c 650 630 ma neodymium model ages for the samples of metasedimentary cover of the cratonic basement provide another evidence that the youngest preserved sediments could not have been derived from the congo craton recognition of the punta del este coastal terrane crust as a source region for the youngest pre collisional sediments of the kaoko belt suggests that this terrane must have been in close proximity to the congo craton passive margin already some time prior to their mutual collision at c 570 550 ma this is in accord with an interpretation that the c 650 630 ma arc back arc punta del este coastal terrane has developed directly on top of or very close to the attenuated congo craton passive margin

neoproterozoic igneous complex emplaced along major tectonic boundary in the kaoko belt nw namibia ion probe and la icp ms dating of magmatic and metamorphic zircons granitoid intrusions of the boundary igneous complex separate segments with different ages of high grade metamorphism in the kaoko belt nw namibia two granitoids of this complex were dated at 575 10 ma secondary ionization mass spectrometry sims or 571 9 ma laser ablation inductively coupled plasma mass spectrometry la icp ms and 562 11 ma sims or 572 4 ma la icp ms respectively the age of granulite facies metamorphism in the eastern part of the western kaoko zone was established at 549 5 ma sims by analysing metamorphic overgrowths of older c 1850 1000 ma zircons from melt segregations in amphibolites the coastal part of the western kaoko zone consists of horizons of migmatitic metasedimentary rocks that are intercalated with fine grained orthogneisses and amphibolites resembling metamorphosed sequences of bimodal volcanic rocks zircons from felsic members of two bimodal suites have sims ages of 805 4 ma and 810 840 ma respectively that are interpreted as dating their respective igneous protoliths melt segregations in the mafic member of the lower bimodal suite contain two populations of zircon dated at 650 5 ma sims or 645 5 ma la icp ms and 629 6 ma sims or 630 5 ma la icp ms respectively the later age is indistinguishable from the age of 630 4 ma sims or 625 10 ma la icp ms obtained from melt patches present in overlying metagreywackes the available age data suggest that the boundary igneous complex masks the suture between the coastal terrane and the rest of the kaoko belt ages of granitoid intrusions in this igneous complex are indicative of magmatic activity between 580 and 550 ma

vertical extent of the newborn san andreas fault at the mendocino triple junction three dimensional seismic refraction data reveal the structure of the plate boundaries at the mendocino triple junction first arrival traveltimes were inverted to obtain a three dimensional seismic velocity model the tomography results indicate a strong lateral velocity contrast in the lower crust this velocity contrast is beneath the surface trace of the san andreas fault and tracks along the fault to the triple junction and then along the mendocino fracture zone velocity values appropriate for mafic rocks are more than 5 km shallower on the pacific plate than east of the san andreas fault and 1 3 km shallower than under the cascadia accretionary prism the model suggests that the san andreas fault which is growing as the triple junction migrates northward cuts steeply through the entire crust pacific oceanic crust does not extend beneath north america at this location the slabless window south of the triple junction and east of the san andreas fault must be filled with upwelling asthenosphere the sharp corner of the pacific plate migrates northward displacing the cascadia accretionary prism and forcing the prism material to move north or east

riedel shear control on the development of pennant veins field example and analogue modelling abstract the wall rocks of a crustal scale sinistral ductile shear zone in namibia the purros mylonite zone contain two types of asymmetric quartz veins bedding surfaces contain sigmoidal quartz veins with limited thickness along their symmetry axes that can be classified as tension gashes a second type of veins consists of a striated central fault vein separating pennant type quartz filled terminations the tips of these pennant veins have a different orientation to those of the tension gashes analogue experiments were carried out using a sheet of silicone powder suspended on a slab of poly dimethyl siloxane pdms both deformed in simple shear these experiments produced open fractures very similar to the pennant veins that form by intersection of r and r riedel shear fractures these fractures rotate and slip during progressive deformation opening pennant shaped gaps we interpret the natural pennant veins to form by the same mechanism of r and r shear fracture initiation and subsequent rotation and opening since this mechanism differs from that of previously described vein types such as wing cracks tension gashes and swordtail or fishmouth termination veins which mainly open as tension veins we consider pennant veins as in new independent class of asymmetric mineral filled veins

high temperature deformation in the neoproterozoic transpressional ribeira belt southeast brazil abstract the neoproterozoic ribeira belt is subdivided in two domains with contrasting tectonic characteristics the northern domain is dominated by shallowly dipping foliations and orogen normal thrust tectonics the southern domain is characterized by a 1000 km long network of anastomosing transcurrent shear zones parallel to the belt this contrast is interpreted as reflecting continent continent convergence that is almost orthogonal to the margins in the northern domain and significantly oblique in the southern domain the central transitional domain of the ribeira belt displays the northern termination of the transcurrent shear zone network the alem paraiba padua shear zone system appss the 250 km long alem paraiba padua system involves granulites facies mylonites deformed through transpression a detailed study of the microstructure and lattice preferred orientation lpo of the rock forming minerals in these granulite mylonites allow a better understanding of deformation mechanisms active at high temperature in the crust plagioclase crystals are plastically deformed they display curved twins and cleavages mechanical twins and evidence of dynamic recrystallization lpo of plagioclase is consistent with activation of the 010 100 and 010 001 slip systems lpo of orthopyroxene and amphibole indicates that these minerals have been deformed through dislocation creep with the activation of the 100 001 slip system quartz in granulite mylonite displays evidence of extensive growth through grain boundary migration the lpo of quartz is therefore the result of a static transformation of an initial syn kinematic lpo and cannot be straightforwardly interpreted in terms of deformation mechanisms active during mylonitization