an early history of pure shear in the upper plate of the raft river metamorphic core complex black pine mountains southern idaho abstract although commonly obscured by simple shear pure shear fabrics occur locally within many metamorphic core complexes the cover rocks to the raft river metamorphic core complex exposed within the black pine mountains display an early coaxial strain history which developed prior to the formation of low angle fault bounded allochthons at higher structural levels this is documented by pressure shadows with straight sutures and oppositely rotated antitaxial calcite veins these rocks were sub horizontally extended parallel to bedding by up to 160 in an e w direction the marbles of the lowest exposed levels exhibit well developed crystallographic preferred orientation fabrics calcite c and a axis pole figures are symmetric to slightly asymmetric with obliquities ranging from 0 to 8 which indicate that the percent pure shear ranges between 100 and 76 initial constraints on the timing of this layer parallel extension suggest that extension was occurring locally within the sevier belt hinterland during the late cretaceous these data coupled with previous studies suggest that pure shear might be a much more common component of strain in rocks deformed in extensional environments than is generally acknowledged

foreland shortening and crustal balancing in the andes at 30 s latitude excellent surface exposures known benioff zone geometry a dynamic morphology and the availability of industry seismic reflection data all make the andes at 30 s an excellent transect for investigating crustal scale balanced sections 150 170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland the thin skinned east verging precordillera of western argentina accounts for 60 75 of the total shortening and formed mostly since major volcanism ceased at 10 ma industry seismic reflection data show that the decollement of the precordillera belt is located anomalously deep at 15 km the belt is dominated by fault propagation folds and contains several prominent out of sequence thrust faults seismic stratigraphie analysis shows that miocene strata in the iglesia valley located between the precordillera and the crest of the andes accumulated in a piggy back basin onlap relations on the western side indicate that the high cordillera was uplifted as a major fault bend fold over a buried ramp thrusting in the two western belts both in the high cordillera of chile formed during the waning stages of arc volcanism 11 16 ma and account for 25 40 of the shortening the observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted nazca plate our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the andes this non unique model provides a simple explanation of the first order morphology of the andes at this latitude

correlation between melting deformation and fluid interaction in the continental crust of the high himalayas langtang valley nepal combined metamorphic stable isotope and structural studies from the high himalayan crystalline sequence in the langtang valley of north central nepal reveal a strong positive correlation between distance above the base of the section the main central thrust the amount of melt material and evidence of prolonged fluid and deformation histories thus suggesting that these processes are strongly interdependent kyanite grade rocks at the base of the section are unmelted and have undergone little syn or post metamorphic internal deformation and little prograde or retrograde fluid rock interaction by contrast sillimanite grade rocks higher in the section contain progressively larger volumes of melt have suffered increasingly complex syn and post metamorphic deformation and show increasing evidence for the presence of fluids although the factors that initiated these processes remain problematic it is suggested that fluid distributions within the langtang section have been passively controlled by the movement of melts these melts may have provided a primary control on deformation during both the magmatic stage and subsequently through the exsolution of exsolved aqueous fluids

the tectonostratigraphy and structure of the eastern nepal himalaya six years of geological research in eastern nepal has resulted in a new geological map of the eastern nepal himalaya which includes the region stretching from the sikkhim border in the east to the kathmandu valley in the west and from the summits of the higher himalaya in the north to the ganges plain in the south this research has permitted the determination of the tectonostratigraphy and structure of one section of the central himalayan arc south of the tibetan plateau the eastern nepal himalaya can be divided into three distinct thrust bound tectonic packages 1 the higher himalayan thrust sheet composed of the higher himalayan crystallines 2 the lesser himalayan thrust sheet composed of the lesser himalayan series and 3 the sub himalayan imbricate zone composed of sedimentary rocks belonging to the siwalik group the higher himalayan thrust sheet of eastern nepal has been thrust over the lesser himalayan metasediments a minimum of 140 km and possibly as much as 175 210 km along the main central thrust mct the lesser himalayan thrust sheet is overlain by the mct and is underlain by the main detachment fault mdf and the main boundary thrust mbt out of sequence thrust faults in the hanging wall of the mbt have breached and offset the presently inactive mct the sub himalayan imbricate zone is an emergent imbricate fan bounded by the mbt to the north and the main frontal thrust mft to the south and is underlain by the mdf which lies at a depth of between 5 km and 7 km a balanced cross section constructed across the higher lesser and sub himalaya of eastern nepal shows that the eastern nepal himalayan orogenic wedge has undergone a minimum of between 210 and 280 km of horizontal north south tectonic shortening since the initiation of the mct the lesser and sub himalaya have absorbed 70 km of north south shortening by thrusting along the basal mdf of which the sub himalayan imbricate zone has accommodated 25 km the sun kosi thrust has accommodated about 10 km and the mbt has accommodated the remaining 35 km of shortening since the initiation of the mct between 15 ma and 25 ma shortening across the eastern nepal himalaya has occurred at an average rate of 8 4 18 6 mm per year the structural geometry of the eastern nepal himalaya suggests an overall piggyback sequence of thrusting with motion transferred from the mct to the underlying mdf and its emergent splay thrust the mbt and with the mbt rotated to its present steep orientation by imbricate thrusting within the sub himalaya

comment on mid cretaceous extensional fragmentation of a jurassic early cretaceous compressional orogen alaska by e l miller and t l hudson

east west extension in the nw indian himalaya explaining the presence of normal faults in overall compressive settings is a challenging problem in understanding the tectonics of active mountain belts the himalayan tibetan orogenic system is an excellent setting to approach this problem because it preserves one of the most dramatic records of long term contemporaneous shortening and extension over the past decades several studies have described extensional features not only in the tibetan plateau but also in the himalaya for a long time the favored model explained the function of the southern tibetan detachment system a major fault zone in the himalaya as a decoupling horizon between the regime of crustal shortening forming the himalayan wedge to the south and the extensional regime of the tibetan plateau to the north however in recent years increasing evidence has shown that n s trending normal faults in the central himalaya crosscut not only the southern tibetan detachment system but also the main central thrust here we present new structural data and geologic evidence collected within the nw indian himalaya and combine them with previously published seismicity data sets in order to document pervasive e w extension accommodated along n s trending faults extending as far south as the footwall of the main central thrust we conducted a kinematic analysis of fault striations on brittle faults documented and mapped fault scarps in quaternary sedimentary deposits using satellite imagery and made field observations in the greater sutlej region spiti lahul kinnaur and the garhwal himalaya studies of extensional features within the regionally nw se trending nw indian himalaya provide the advantage that arc parallel and e w extension can be separated in contrast to the central himalaya therefore our observations of e w extension in the indian nw himalaya are well suited to test the applicability of current tectonic models for the whole himalaya we favor the interpretation of e w extension in the nw indian himalaya as a propagation of extension driven by collapse of the tibetan plateau

nappe tectonics extension and metamorphic evolution in the indian tethys himalaya higher himalaya se zanskar and nw lahul in the southern part of the indian tethys himalaya between se zanskar and nw lahul nappe tectonics with transport toward the wsw during the main deformation phase has been ascertained if the stratigraphy of the different units considered as nappes is combined it results in a composite section ranging from lower cambrian to middle cretaceous a metamorphism of amphibolite facies in the lowest unit is associated with the nappe stacking the lower miocene cooling ages of metamorphic biotite and muscovite suggest that the nappe emplacement starts at the oligocene or at the oligocene eocene boundary after the compressional phases an extension provokes the reactivation of older main thrusts as normal fault and the formation of a steeply dipping normal fault this interpretation is based on detailed mapping

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

tectonic evolution of the high himalaya in upper lahul nw himalaya india the upper lahul region in the nw himalaya is located in the transition zone between the high himalayan crystalline hhc to the sw and the tethyan zone sedimentary series to the ne the tectonic evolution of these domains during the himalayan orogeny is the consequence of a succession of five deformation events an early dl phase corresponds to synmetamorphic ne verging folding this deformation created the tandi syncline which consists of permian to jurassic tethyan metasediments cropping out in the core of a large scale synformal fold within the hhc paragneiss this tectonic event is interpreted as related to a ne directed nappe stacking shikar beh nappe probably during the late eocene to the early oligocene a subsequent d2a phase caused sw verging folding in the hhc this deformation is interpreted as contemporaneous with late oligocene to early miocene sw directed thrusting along the main central thrust in the tethyan zone a d2b phase is marked by a decollement thrust a system of reverse faults and gentle folds associated with sw directed tectonic movements this deformation is related to an imbricate structure characteristic of a shallow structural level and developed in the frontal part of a nappe affecting the tethyan zone units of se zanskar nyimaling tsarap nappe a later d3 phase generated the chandra dextral shear zone cdsz a large scale ductile dextral strike slip shear zone located in the transition zone between the hhc and the tethyan himalaya the cdsz most likely represents a part of a system of early miocene extensional and or dextral strike slip shear zones observed at the hhc tethyan zone contact along the entire himalaya a final d4 phase induced large scale doming and ne verging back folding

extensional collapse in the neoproterozoic aracuai orogen eastern brazil a setting for reactivation of asymmetric crenulation cleavage the aracuao orogen of eastern brazil is one of many brasiliano pan african orogens formed during the neoproterozoic assembly of gondwana its western edge bordering the sao francisco craton is the serra do espinhaco fold thrust belt in which top up to the west reverse sense faults west verging folds f1 and east dipping spaced to phyllitic cleavage s1 developed we have found that the kinematics of deformation changes markedly at the hinterland margin of this fold thrust belt here beneath a plateau known as the chapada acaua metadiamictite and fine grained pelitic schist comprise an east dipping belt that contains an assemblage of structures indicative of top down to the east normal sense shear this assemblage includes a cascade of f2 folds that refold f1 folds and verge down the dip of the belt s enveloping surfaces vertical tension gashes and top down to the east rotated clasts based on the presence of these structures we propose that the plateau exposes a regional scale normal sense shear zone here called the chapada acauashear zone casz because f2 folds refold f1 folds normal sense shear in the casz occurred subsequent to initial west verging thrusting considering this timing of motion in the casz we suggest that the zone accommodated displacement of the internal zone of the aracuao orogen down relative to its foreland fold thrust belt and thus played a role in extensional collapse of the orogen the casz trends parallel to preserved thrusts to the west and thus may represent an inverted thrust fault notably throughout the casz s1 schistosity has been overprinted by a pervasive west dipping asymmetric crenulation cleavage s2 the sigmoid shape of s1 surfaces in s2 microlithons require that slip on each s2 surface was top down to the west s2 cleavage is axial planar to the down dip verging f2 folds based on its geometry we suggest that s2 cleavage initiated either as an antithetic extensional crenulation cleavage during reverse sense shear or as a near vertical asymmetric crenulation cleavage formed during east west shortening of a preexisting east dipping schistosity subsequent normal sense shear in the casz reactivated this cleavage causing clockwise rotation of s2 domains as viewed looking along strike to the north in a manner similar to that of rotational bookshelf faults such movement could have accommodated concomitant vertical flattening of the casz during extensional collapse

paleoseismicity and neotectonics of the cordillera blanca fault zone northern peruvian andes the cordillera bianca fault zone is a major west dipping normal fault that bounds the west side of a 120 to 170 km wide zone of active extension along the crest of the northern peruvian andes the fault is approximately 210 km long and exhibits continuous geomorphic evidence of repeated late pleistocene and holocene displacements but has not been the source of historical or teleseismically recorded earthquakes trenching and mapping of fault scarps provide new information on earthquake recurrence slip rate timing of the most recent events and andean neotectonics at quebrada queroccocha 55 km from the south end of the zone an 11 000 to 14 000 year old moraine is displaced 12 15 m and younger valley fill lacustrine and fluvial deposits are displaced 7 5 8 m scarp profiles tectonic terraces and trench exposures indicate five to seven scarp forming earthquakes of 2 3 m per event during the past 11 000 14 000 years at this location considering uncertainties in the number of events and in the age of the displaced moraine the average recurrence interval is estimated to be 2440 1060 years detrital charcoal from colluviums that predate and postdate the most recent event yielded ages of 2480 65 and 750 80 14c years b p respectively and stratigraphic relationships suggest that the event may be closer to the older date the late quaternary slip rate at quebrada queroccocha is 0 86 1 36 mm yr and appears to be representative of much of the fault zone at pachma bajo located 30 km from the north end of the zone on a separate segment displacement during each of the past two events was 2 to 3 m the morphology of scarps in alluvial fan and debris flow deposits indicates recurrence intervals of about 1000 3000 years relationships between pre inca walls and faulted deposits suggest that 1500 2000 years have elapsed since the most recent event fault behavior parameters particularly displacement per event and fault segment length show that the cordillera bianca fault zone is similar to seismogenic normal faults that nucleate m 7 7 earthquakes at depths of approximately 15 km the quebrada queroccocha slip rate and analysis of other normal faults in the region suggest cumulative late quaternary extensional strain rates of about 1 4 mm yr across the complete zone of andean crestal extension

contrasting metamorphic p t time paths of the kohistan granulites and tectonics of the western himalayas in the swat indus kohistan area lower crustal rocks of the kohistan arc including amphibolites granulites and ultramafic rocks are widely exposed detailed petrological structural studies reveal that the chilas jijal metamorphic complex comprises essentially intermediate to high pressure granulites with a granulite amphibolite transition controlled by extensive retrograde metamorphism and ductile deformation contrasting patterns of compositional zoning in garnet and orthopyroxene suggest complicated p t paths in the granulites geothermobarometry based on garnet bearing granulites identify two distinct p t time paths a high pressure anticlockwise path of the jijal complex and a low pressure retrograde path of the chilas complex these p t time paths represent the evolution of middle to lower crustal rocks of the kohistan arc the anticlockwise p t time path combined with published isotopic ages reveals that i the high pressure granulitefacies metamorphism is related to the kohistan asia collision and ii the crust of the kohistan arc had been thickened at least 55 km before the india asia collision

differential unroofing within the central metasedimentary belt of the grenville orogen constraints from 40ar 39ar thermochronology an 40ar 39ar thermochronological investigation of upper greenschist to granulite facies gneiss amphibolite and marble was conducted in the central metasedimentary belt cmb ontario to constrain its cooling history incremental 40ar 39ar release spectra indicate that substantial differential unroofing occurred in the cmb between 1000 and 600 ma a consistent pattern of significantly older hornblende and phlogopite 40ar 3ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation variations in hornblende 40ar 39ar age plateaus exceeding 200 ma occur over distances less than 50 km with major age discontinuities occurring across the robertson lake shear zone and the sharbot lake mylonite zone which separate the sharbot lake terrane from the elzevir and frontenac terranes extensional displacements of up to 14 km are inferred between the frontenac and elzevir terranes of the cmb no evidence for significant post argon closure vertical displacement is indicated in the vicinity of the perth road mylonite within the frontenac terrane variations of nearly 100 ma in phlogopite 40ar 39ar plateau ages occur in undeformed marble on either side of the bancroft shear zone phlogopites from sheared and mylonitized marble within the shear zone yield 40ar 39ar diffusional loss profiles but have older geologically meaningless ages thought to reflect incorporation of excess argon by 900 ma southeast directed extension was occurring throughout the cmb possibly initiated along previous zones of compressional shearing an easterly migration of active zones of extension is inferred possibly related to an earlier overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone the duration of extensional shearing is not well constrained but must have ceased before 600 ma as required by the deposition of overlying undeformed cambrian and or ordovician sedimentary rocks

kinematic evidence for extensional unroofing of the franciscan complex along the coast range fault northern diablo range california franciscan metagraywacke immediately below the del puerto ophiolite an outlier of the coast range ophiolite in the northern diablo range was sheared during top to the east displacement on the coast range fault this represents normal faulting and extensional offset it was accompanied by attenuation of the coast range ophiolite and great valley sequence in the hanging wall along layer parallel normal faults that sole into the coast range fault extension occurred as the franciscan complex moved relatively west out from under north american lithosphere and across the subducting ocean plate below this effected a lengthening and thinning in the wedge of material above the down going plate presumably in response to instability brought about by subduction shallowing krueger and jones 1989 and accretion of the franciscan central belt in the latest cretaceous to early paleocene as a result blueschist facies terranes of the uppermost part of the franciscan complex are now juxtaposed directly against hanging wall units that bear only low grade metamorphism

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

exhumation of the sanbagawa blueschist belt sw japan by lateral flow and extrusion evidence from structural kinematics and retrograde p t t paths

taiwan orogeny thin skinned or lithospheric collision abstract the taiwan orogeny is young and presently very active it provides an excellent environment for studying ongoing orogenic processes especially since the region is monitored intensively with dense seismological and geodetic networks and new studies aiming at deciphering shallow and deep structures in and around taiwan have been recently conducted or are being planned the available data can be used continually to test critically hypotheses of the taiwan orogeny hypotheses dealing with the mechanics of mountain building are basic to the understanding of taiwan orogeny and are particularly amenable to testing the widely cited thin skinned tectonics hypothesis was formulated to explain mainly the geologic and relatively shallow

cause and effect among thrust and normal faulting anatectic melting and exhumation in the himalaya the transition from compression to extension in a regime of overall convergence in a narrow mountain belt requires a change with time in at least one of the following horizontal normal stress applied to the belt by its surroundings the distribution of shear traction on the base of the belt the constitutive relation between stress and strain rate of material in the belt itself the surface height of the belt the following processes could have caused one or other of these changes in the himalaya in miocene time and thus could have triggered the onset of extension by normal faulting on planes parallel to thrust faults that were active at the same time the shear stress on the thrust fault at the base of the himalaya could have decreased for instance because of melting on the fault plane alternatively the creation of a new deeper thrust fault and hence a new locus of convergence could have initiated extensional faulting if the distribution of shear stress on the new thrust fault were unable to support the himalaya above it the himalaya could have been weakened by an increase in pore fluid pressure for instance owing to metamorphic dehydration reactions within the wedge the himalaya could have been elevated for instance following the detachment of a tongue of mantle lithosphere from beneath it or by the underthrusting of a lens of material beneath its base the apparent simultaneity of thrust and normal faulting on nearly parallel planes of anatexis and of exhumation in the himalaya in miocene time can be explained if normal faulting was responsible for most of the exhumation and through decompression melting for anatexis existing data suggest that the most probable cause for miocene normal faulting was either uplift of the himalaya or the cessation of slip on the main central thrust at the base of the orogenic wedge and the creation of a new deeper thrust fault and a new base to the deforming himalaya 1993 the geological society

subduction of continental margins and the uplift of high pressure metamorphic rocks abstract the mechanism by which high pressure metamorphosed continental material is emplaced at high structural levels is a major unsolved problem of collisional orogenesis we suggest that the emplacement results from partial subduction of the continental margin which because of its high flexural rigidity produces a rapid change in the trajectory of the descending slab we assume a two fold increase in effective elastic thickness of the lithosphere as the continental margin approaches the subduction zone and calculate the flexural profile of a thin plate for progressive downward migration of the zone of increased rigidity we assess the effect of changes in the flexural profile on the overlying accretionary prism and mantle wedge as the continent approaches by estimating the extra stresses that are imposed on the wedge due to the bending moment exerted by the continental part of the plate the wedges overlying the subduction zones and the subducting slab itself experience substantial extra compressional stress at depths of around 100 km and extensional stress at shallower depths as the continental margin passes through the zone of maximum curvature the magnitudes of such extra stresses are probably adequate to effect significant deformation of the wedge and or the descending plate and are experienced in a time interval of less than 5 m y for typical subduction rates the spatial variation of yield stresses in the region of the wedge and descending slab indicates that much of this deformation may be taken up in the crustal part of the descending slab which is the weakest region in the deeper parts of the subduction zone this may result in rapid upward migration of the crust of the partially subducted continental margin against the flow of subduction high pressure metamorphosed terranes emplaced by the mechanism envisaged in this paper would be bounded by thrust faults below and normal faults above movement on the faults would have been coeval and would have resulted in rapid unroofing of the high pressure terranes synchronous with arrival of the continental margin at the subduction zone and therefore relatively early in the history of a collisional orogen

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

compressional metamorphic core complexes low angle normal faults and extensional fabrics in compressional tectonic settingscompressional core complexesm searle t lamont metamorphic core complexes mccs are interpreted as domal structures exposing ductile deformed high grade metamorphic rocks in the core underlying a ductile to brittle high strain detachment that experienced tens of kilometres of normal sense displacement in response to lithospheric extension extension is supposedly the driving force that has governed exhumation however numerous core complexes notably himalayan karakoram and pamir domes occur in wholly compressional environments and are not related to lithospheric extension we suggest that many mccs previously thought to form during extension are instead related to compressional tectonics pressures of kyanite and sillimanite grade rocks in the cores of many of these domes are c 10 14 kbar approximating to exhumation from depths of c 35 45 km too great to be accounted for solely by isostatic uplift the evolution of high grade metamorphic rocks is driven by crustal thickening shortening regional barrovian metamorphism isoclinal folding and ductile shear in a compressional tectonic setting prior to regional extension extensional fabrics commonly associated with all these core complexes result from reverse flow along an orogenic channel channel flow following peak metamorphism beneath a passive roof stretching fault in naxos low angle normal faults associated with regional aegean extension cut earlier formed compressional folds and metamorphic fabrics related to crustal shortening and thickening the fact that low angle normal faults exist in both extensional and compressional tectonic settings and can actively slip at low angles c 60 is needed

tectonic mode switches and the nature of orogenesis abstract the birth and death of many mountain belts occurs in lithosphere that over rides major subduction zones here the tectonic mode shortening versus extension can abruptly switch even during continuous and otherwise smooth convergence if the hinge line of the foundering slab rapidly retreats i e rolls back the foundering slab creates a gravitational potential well into which the orogen collapses this motion coupled with stress guides can pull the orogen apart a slowing of roll back or of hinge retreat means that the subduction flexure may subsequently begin to be pushed back or be pushed over by the advancing orogen the consequence of such changes in relative motion is that orogenic belts are affected by abrupt tectonic mode switches the change from push to pull leads to a sudden change from horizontal extension to horizontal shortening potentially throughout the entire mass of the orogenic lithosphere that over rides the subducting slab the sequencing of these tectonic mode switches affects the thermal evolution of the orogen and thus fundamentally determines the nature of orogenesis this insight led to us to our quite different views as to how orogens work it is evident that orogens affected by abrupt push pull mode switches are characterized by high pressure metamorphism whereas orogens affected by abrupt pull push mode switches are characterized by high temperature metamorphism magmatism and anatexis

pressure temperature deformation time constraints on the south tibetan detachment system in the garhwal himalaya nw india a peculiar feature of the himalaya is the occurrence of a system of low angle normal faults and shear zones the south tibetan detachment system stds at the mountain crests the stds was active during syn convergent tectonics we describe the stds related sheared rocks along the dhauli ganga valley in the garhwal himalaya nw india where the malari granite reported as an undeformed igneous body cross cutting the stds occurs a detailed multidisciplinary study integrating field based microstructural petrographic and geochronological analyses was carried out on rocks along this valley we demonstrate how the non coaxial ductile portion of the stds affected the upper part of the greater himalaya sequence migmatite which experienced peak pressure p temperature t conditions of 0 9 1 1 gpa and 750 c at 24 ma this migmatite has been reworked structurally upwards leading to the formation of high t sillimanite bearing mylonites further upward medium t shearing deformed the malari granite and leucogranite dykes forming medium t mylonites ductile shearing was temporally constrained based on new in situ monazite datings and previously published ar ar geochronology between 20 15 ma we demonstrate that a preserved ductile to brittle spatial and temporal transition of the stds deformation exists with the brittle features overprinting ductile ones our data shed new light on the geological evolution of the stds in the nw himalaya with implications for the relationship and relative timing of partial melting granite emplacement and deformation along low angle normal faults

oblique crustal detachment in the variscan schwarzwald southwestern germany the schwarzwald is part of the central polymetamorphic crystalline belt of the variscan orogen moldanubian belt from north to south it consists of four terranes the metasedimentary zone of baden baden the polymetamorphic central schwarzwald gneiss complex the sedimentary metamorphic zone of badenweiler lenzkirch and the hotzenwald complex the largest of these terranes is the central schwarzwald gneiss complex csgc whose rocks record a history of protracted regional metamorphism and anatectic melt generation during variscan convergence between 350 and 325 ma the csgc became detached from a high temperature lower crustal substratum and was emplaced southeastward over paleozoic clastics volcanic rocks and crystalline slivers of the zone of badenweiler lenzkirch and the hotzenwald complex kinematic indicators suggest that these early convergent movements on retrograde shear zones and the concomitant crustal thickening were superseded by movements on divergent shear zones the ascent of voluminous granitic plutons from a mid crustal zone of melt generation into the upper crust was probably triggered by a change in the crustal kinematics from overall convergence to overall divergence at about 325 ma in detail this process was probably diachronous detachment of upper crust and large scale melt generation in the middle crust of the schwarzwald was probably facilitated by the tectonic stacking of water rich pelitic clastics and gneiss slivers with relatively even proportions of crystalline and pelitic materials

soft sediment deformation structures and their implications for late quaternary seismicity on the south tibetan detachment system central himalaya uttarakhand india abstract the south tibetan detachment system stds defines the lithological and tectonic boundary between the higher himalayan crystallines and the tethyan sedimentaries earlier studies have suggested that the stds has been dormant since its inception during the miocene along with the main central thrust mct however recent studies indicate that the stds was active during the pleistocene holocene period we provide additional support for this more recent activity based on the occurrence of seismically induced soft sediment deformation structures ssds preserved in relict lake sediments in the dhauli ganga gori ganga and kali ganga river basins of the central himalaya the relict lakes are located on the hanging wall of the stds an optical chronology of the lake sediments brackets the seismically induced ssds between 20 ka and 11 ka with a major seismic event of magnitude 6 5 occurring between 17 ka and 13 5 ka since mct and stds are considered to be the coupled structures our observation supports the hypothesis that the stds is providing accommodation space to the strain gradient arising due to the north south compression in the himalaya during the late quaternary

flow to a well in an unconfined fractured and leaky wedge shaped aquifer system abstract wedge shaped aquifers are usually formed where two linear boundaries intersect these types of aquifers are commonly seen in river basins and alluvial fans but have received much less attention in the hydrological sciences community fundamental understanding of the effects of lateral boundaries on the hydrodynamic process in such aquifers is quite poor particularly for unconfined fractured and leaky wedge shaped aquifer systems which will be the concern of this study solutions of flow to a well in an unconfined fractured wedge shaped two aquifer system are obtained using laplace hankel and fourier transforms an infinitesimal radius pumping well partially penetrates the overlying unconfined aquifer both aquifer layers are homogeneous and anisotropic bounded by two intersecting linear lateral boundaries of constant head or barrier types and the aquifer system is semi infinite in radial direction the time domain drawdowns are obtained via the stehfest algorithm and gaussian quadrature the inter porosity flow in the underlying fracture aquifer the instantaneous water table drainage the horizontal and vertical hydraulic conductivity anisotropy and the wedge angle are taken into account eliminating the inter porosity flow in the underlying fractured aquifer the presented solution reduces to a leaky wedge shaped aquifer case the influences of the underlying fractured aquifer hydraulic parameters on the overlying unconfined aquifer for different wedge angles and linear lateral boundaries types are explored using dimensionless drawdown and scaled sensitivity curves the results of this study can be utilized to obtain hydraulic parameters of an alluvial and river basin unconfined fractured wedge shaped aquifer systems the presented solution can also be used for water budget assessment of river basin aquifers

a review on the lithospheric structures in the tibetan plateau and constraints for dynamics in the last decade several international joint projects were conducted in the tibetan plateau by chinese american and french geophysicists and geologists in the present review the results from vertical reflections wide angle reflections and broadband digital seismic recordings are reviewed and compared constraints for the dynamics of continent continent collision from the lithospheric structures seismicity focal mechanism and anisotropy are discussed

incipient extension along the active convergent margin of nubia in sicily italy cefalu etna seismic zone 1 recent geodetic data are compatible with nne ssw tectonic extension at a rate of 5 mm yr in sicily southern italy within a broader region of net active compression along the nubian plate margin northern africa the structures that accommodate such extensional regime and its cause are still unknown from field structural surveys and seismological analyses the geometry kinematics structural architecture and seismic potential of an extensional seismic zone linking cefalu and mount etna in central eastern sicily are defined the zone includes high angle wnw striking normal and right lateral strike slip faults and subordinate north and nne striking strike slip faults either right or left lateral the occurrence of small discontinuous faults and the absence of related depressions and sedimentary basins suggest that the extensional regime is still in an incipient stage the ongoing seismic activity possibly reactivates preexisting faults instrumentally and historically recorded earthquakes are lower than about 6 in magnitude and destructive events are historically unknown since at least 1300 a d this apparent upper bound of earthquake magnitudes is consistent with the maximum magnitude values estimated from the length of the longest mapped faults and sources of seismic swarms which all together suggest a value between 6 and 6 5 as the maximum expected magnitude that can be proposed at the present stage of investigation for earthquakes in the study area lateral extension on preexisting faults and upwelling of melt mantle material beneath mount etna are considered viable processes to explain at least in part the active extensional tectonics along the cefalu etna seismic zone strike slip seismic faulting beneath mount etna may be part of a previously proposed diffuse transfer zone affecting northeastern sicily and including the tindari fault

the thermal response of a metamorphic belt to extension constraints from laser ar data on metamorphic micas abstract metamorphic rocks may be exhumed to the surface either by isostatic rebound uplift and erosion or tectonically via thrusting or extension the latter mechanism is particularly important in the himalayas where a large extensional detachment system appears to be responsible for very fast unroofing at 20 ma here we present the results of a laser argon study of micas and quartz aimed at investigating the thermal response of an orogenic belt to such extension the data when combined with existing constraints on the prograde evolution lead to a uniquely complete thermal history for a metamorphic belt laser argon data are reported for quartz and micas from semi pelitic metamorphic rocks from the zanskar himalayas nw india the samples derive from three traverses extending from low grade rocks near the detachment to high grade rocks in the core of the belt most samples demonstrably contain no excess component and the ages of muscovite and biotite show a correlation with distance from the extensional detachment the ages themselves as well as the difference between the muscovite and biotite ages both increase as the detachment is approached muscovite and biotite ages from within the shear zone date deformation associated with extension at 20 25 ma the remaining data are most readily interpreted in terms of early slow cooling of the lower grade rocks near the detachment in response to erosional denudation before the detachment became active rocks in the core of the orogen were still heating at this time later fast cooling of the high grade rocks in the core of the belt ensued when the extensional regime became active simple heat transfer calculations based on the distance of the high grade rocks from the fault and the argon ages of their micas provide further constraints on the timing of extension and suggest that the fault first moved 23 25 ma ago and that it abruptly terminated metamorphism in the core of the metamorphic belt our analysis of the thermal history suggests that cooling and exhumation rates determined from metamorphic belts beneath extensional detachments will often be minima

continental underplating model for the rise of the tibetan plateau abstract subduction of continental lithosphere entrained in an oceanic plate for distances of several hundred kilometers can lead to continental underplating whereby the continental crust with or without its lithospheric mantle is detached at depth from the main part of the sinking slab and rises buoyantly beneath the overriding plate the rising continental material causes first extension in the overriding plate as the asthenosphere flows laterally and later rapid isostatic uplift when it reaches the base of the overriding lithosphere this continental underplating model is applied to the geology of the tibetan region and the following stages are identified 100 ma to 50 ma oceanic subduction leading to continental collision 50 ma to 20 ma pause in convergence with possible delamination of indian crust 20 ma to 5 ma continental subduction initially at an angle of 30 of indian lithosphere along the main central thrust 5 ma to 2 ma rise of the continental lithosphere displacing the asthenosphere beneath tibet and causing widespread graben formation 2 ma to present contact between the underplating indian continental lithosphere and the overlying asian crust leading to rapid rise of the surface altitude by 3 4 km present day convergence between india and tibet albeit at a rate much less than the total india eurasia convergence continues as continental underthrusting

kinematic history of the main central thrust zone in the langtang area nepal abstract this paper describes for the first time evidence for a northeastward brittle extensional movement of the main central thrust mct zone overprinting the southward ductile thrust movement the mct zone is the major tectonic boundary between the lesser himalayan metasedimentary sequence lhs and higher himalayan crystalline sequence hhs and contributes to the kinematic and tectonic evidence of continent continent collision in the neogene the langtang area 50 km north of kathmandu is a matter of interest in considering the movement of the kathmandu nappe because the general trend of foliation of the hhs and the underlying mct zone is turned from wnw ese to n s or nne ssw the major tectono lithostratigraphic units of the mct zone in the study area are divided into three the lower and middle units included in the lhs and the upper unit even included in the hhs characteristic mesoscopic quartz lenses are developed in the schists of the lower unit forming asymmetric boudins this asymmetry indicates an extensional top to the ne shear sense however a thrust sense of top to the sw is also preserved in microscopic ductile shear bands and mica fish the extensional shear movement in the lower unit took place after the major thrust movement in the mct zone as a negative inversion the middle unit contains ulleri type augen gneiss syabru bensi augen gneiss which is widely distributed in the mct zone in the other parts of the nepalese himalaya asymmetric clast tails or clast pressure shadows and asymmetric boudins observed in the syabru bensi augen gneiss give a dextral thrust oblique sense of shear this dextral component is regarded as movement of a n s trending lateral ramp during the overthrusting of the kathmandu nappe the upper unit above the mct is commonly made up of kyanite bearing medium pressure type amphibolite facies pelitic gneiss asymmetric deformational features such as mica fish and shear bands in the pelitic gneisses commonly show a thrust top to the w sense of movement after the emplacement of the kathmandu nappe 9 6 ma brittle extensional shear occurred only in the crystalline schists of the lower unit presumably associated with layer parallel slip this extensional movement in the lower unit is probably caused by the change in strike of foliations e g from a general trend of e w in the other areas to n s as seen in the study area this is because n s contraction during continued continent continent collision produced e w extension normal to the contraction axis n s thus the relative east directed movement on normal faults took place only in the n s trending domain of the mct zone another possibility for the extensional movement is squeezing of the upper part of the lhs between the mct above and an out of sequence thrust or the main detachment fault below

petrogenetic and tectonic significance of young leucogranites from the northwestern himalaya pakistan in the himalaya of northwestern pakistan small discordant bodies of leucogranite intrude metamorphosed basement of the indian crust as the first step in assessing the tectonic significance of these granites we have used the shrimp ion microprobe to obtain u pb dates on zircon zircons of igneous appearance are scarce in the leucogranites and with one exception are associated with xenocrystic components which occur as discrete grains and as cores rimmed by young zircon zircons of igneous appearance are high in u 5000 to 40 000 ppm low in th u 0 01 to 0 05 and frequently colored a distinct blue xenocrystic components give a range of proterozoic ages consistent with ages previously reported for indian crust in sharp contrast zircons of igneous appearance yield clusters of concordant tertiary ages two leucogranite dikes from southern localities yield rather old emplacement ages of about 35 ma swat and about 50 ma naran together with previously determined 40ar 39ar and fission track cooling ages these ages document immediate and rapid metamorphism and denudation following eocene collision three leucogranite dikes from northern localities within the nanga parbat haramosh massif are very young having emplacement ages of about 2 3 ma 5 ma and 7 ma the formation and emplacement of these leucogranites during the rapid late tertiary denudation of the nanga parbat region suggests to us that decompression melting may be a viable mechanism for leucogranite genesis

kinematics and vorticity in kangmar dome southern tibet testing midcrustal channel flow models for the himalaya 1 kinematic kinematic vorticity wm and deformation temperature analyses were completed to test the hypothesis that midcrustal rocks exposed in the core of the kangmar gneiss dome southern tibet record ductile deformation patterns of a frozen segment of a southward flowing midcrustal channel microscopic and mesoscopic kinematic indicators exhibit a downward transition from a subequal mix of top north and top south shear in garnet zone rocks to dominantly top north shear in staurolite kyanite zone and deeper rocks kinematic vorticity values indicate an increase in pure shear component with depth from 48 pure shear in chloritoid zone rocks through 62 in garnet zone to staurolite kyanite zone rocks to 68 pure shear in an orthogneiss the deepest exposed rocks deformation temperatures inferred from grain scale microstructures and quartz lattice preferred orientations increase from 300 c 400 c in chloritoid zone rocks to 600 c in the deepest exposed rocks these temperatures are equivalent to temperatures derived from garnet biotite thermobarometry indicating that wm was recorded during peak metamorphism this ductile deformation zone was cut by the brittle southern tibetan detachment system stds that juxtaposed metasedimentary rocks upon the orthogneiss on the basis of these relations midcrustal rocks in the core of kangmar dome record 1 general shear vertical thinning and n s horizontal extension with a component of top north shear during peak metamorphism within a ductile shear zone corresponding to the northern and deeper portion of the stds 2 an increase in pure shear with structural depth a consequence of an increase in lithostatic load and 3 displacement of the high temperature shear zone by the brittle stds our data are compatible with the deformation patterns predicted for the top part of a southward flowing midcrustal channel

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

dominant simple shear deformation during peak metamorphism for the lower portion of the greater himalayan sequence in west nepal new implications for hybrid channel flow type mechanisms in the dolpo region abstract i conducted new vorticity and deformation temperatures studies to test competing models of the exhumation of the mid crustal rocks exposed in the dolpo region west nepal my results indicate that the main central thrust is located 5 km structurally below the previous mapped locations deformation temperature increasing up structural section from 450 c to 650 c and overlap with peak metamorphic temperature indicating that penetrative shearing was responsible for the exhumation of the ghs occurred at close to peak metamorphic conditions i interpreted the telescoping and the inversion of the paleo isotherms at the base of the ghs as produced mainly by a sub simple shearing wm 0 88 1 pervasively distributed through the lower portion of the ghs my results are consistent with hybrid channel flow type models where the boundary between lower and upper portions of the ghs broadly corresponding to the tectonometamorphic discontinuity recently documented in west nepal represents the limit between buried material affected by dominant simple shearing and exhumed material affected by a general flow dominates by pure shearing this interpretation is consistent with the recent models suggesting the simultaneous operation of channel flow and critical wedge type processes at different structural depth

thrust tectonics crustal shortening and the structure of the far eastern nepal himalaya balanced and restored structural sections across the far eastern nepal himalaya have been constructed in order to determine the structure and evolution of the himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the main central thrust mct the far eastern nepal himalaya is comprised of three distinct thrust bound tectonic packages the higher himalayan crystalline thrust sheet the lesser himalayan metasediment thrust package and the sub himalayan imbricate fan the higher himalayan crystallines consisting of kyanite and sillimanite bearing gneisses intruded by the miocene jannu leucogranites have been thrust over the lesser himalayan metasediments along the mct for a distance of 140 km to 175 km the lesser himalayan metasediments are a 12 km thick unit consisting primarily of phyllites metaquartzites and mylonitic augen gneisses in which garnet biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels the lesser himalayan metasediment thrust package is underlain by a decollement the main detachment fault mdf which lies at a calculated depth of between 6 and 10 km underneath the mahabharat lekh and at a calculated depth of 20 to 25 km north of the tamar khola dome the tamar khola dome overlies a footwall ramp along the mdf where the mdf cuts upsection through the lesser himalayan metasediments the lesser himalayan thrust package probably has an internal structure aproximating a hinterland dipping duplex with the mct and the mdf corresponding to the roof and floor thrusts respectively both the tamar khola thrust an out of sequence breach thrust and the main boundary thrust mbt are splay thrusts off of the mdf the sub himalaya consisting of nonmetamorphosed sedimentary rocks displays an emergent imbricate fan geometry and is underlain by the southern continuation of the mdf which lies at a depth of 5 5 km to 6 km beneath the siwalik hills folding and thrusting within the lesser himalayan thrust package and the sub himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening total north south shortening across the higher lesser and sub himalaya of far eastern nepal south of the tibetan plateau has been of the order of 185 km to 245 km and has occurred at an average rate of 7 4 mm to 15 3 mm per year since the initiation of the mct between 16 and 25 ma

orogeny and large scale strike slip faulting on venus tectonic evolution of maxwell montes the ishtar terra highland region of venus contains mountain belts morphologically similar to terrestrial orogenic belts akna montes and freyja montes are long linear mountain belts with about 4 km of relief above the 3 to 4 km high lakshmi planum the maxwell montes mountain belt however is more rectangular in plan view and has relief of approximately 8 km we have investigated the nature of this latter range using complementary radar images obtained by the arecibo observatory and the venera 15 16 spacecraft as well as allimetry data obtained by the venera 15 16 and pioneer venus spacecraft geological and structural maps have been compiled on the basis of these data the individual features and their stratigraphic relationships interpreted and a model for the origin and evolution of maxwell montes is proposed the dominant features of this mountain range are long parallel ridges and valleys that are intepreted to be anticlines and synclines and to have resulted from ene wsw oriented compression two major shear zones bound maxwell montes on the north and south converging toward the west nine narrow linear parallel features several hundreds of kilometers long identified as cross strike discontinuities csds cut obliquely across the strike of the ridges and valleys in a nw se direction and disrupt the structural fabric of the mountain range dividing maxwell montes into 10 crustal domains among several possible origins we find that the data are most consistent with their interpretation as right lateral strike slip faults with offsets of up to 125 km retrodeformation and reconstruction of maxwell montes using the offsets determined for individual domains produces a long linear mountain range similar to akna montes using geologic unit maps and topographic maps of the present configuration of maxwell similar reconstructions were made these reconstructions restored several linear tectonic elements topographic trends and sinuous unit boundaries to more contiguous positions on the basis of these data and observations we suggest that akna montes may represent the initial form of compressional orogenic belts on venus while maxwell montes attained its present morphology through a second stage of deformation involving large scale strike slip faulting we examine several models for this second stage and favor the interpretation that it occurred as the akna montes like proto maxwell montes was transported to the west between two converging shear zones accompanied by rotation of the entire mountain range thirty percent shortening of the mountain range is implied by the geometries of the present and reconstructed maxwell montes on the basis of the deforamation in maxwell montes and the evidence for large scale transport we conclude that large scale horizontal a movement of crustal materials is a significant aspect of the tectonics of venus

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

large magnitude continental extension an example from the central mojave metamorphic core complex the central mojave metamorphic core complex is defined by a belt of miocene brittle ductile extension and coeval magmatism the brittle ductile fault zone defines a basin and dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes one fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km the axial surfaces of these folds can be traced from the footwall mylonites through the brittle detachment and into hanging wall strata however folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime possibly after movement across the fault zone ceased mesoscopic fabrics related to the transport parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction we interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large magnitude extension the other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km mesoscopic fabrics related to these folds include northwest striking joints kink bands and en echelon tension gash arrays these fabrics formed after mylonitization and record both layer parallel extension and northeast side up subvertical shear the postmylonitic fabrics are kinematically compatible with rolling hinge style isostatic rebound of the footwall following tectonic denudation the relative timing of extension related magma intrusion and ductile deformation varies through the central mojave metamorphic core complex on the scale of the small mountain ranges that make up the central mojave metamorphic core complex no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension related igneous rocks this suggests that models that invoke a single upper crustal genetic relationship such as magmatism triggering extension or vice versa do not apply to the central mojave metamorphic core complex systematic variation in the relative timing of dike emplacement and mylonitization suggests that at the time of dike emplacement rocks in the mitchel range were below the brittle ductile transition while those in the hinkley hills were above it the hinkley hills and mitchel range are separated by 2 km in the dip direction of the fault zone which suggests that the vertical thickness of the brittle ductile transition probably was between 100 and 950 m

thermochronologic evaluation of postcollision extension in the anatolide orogen western turkey 1 to better understand the driving mechanisms behind the transition from collision to extension in a convergent orogen data from multiple low temperature thermochronometers were obtained from the simav detachment fault sdf the earliest developed major extensional structure recognized in the western anatolide orogen of western turkey twenty two zircon fission track ft 26 apatite ft 12 apatite u th he ages and 26 apatite ft track length analyses are presented the data establish that the sdf was a major extensional fault active between 25 and 19 ma the coincidence in timing of magmatism with cessation of sdf activity implies the detachment became locked owing to doming induced by magma emplacement zircon ft ages away from the influence of miocene magmatism record rapid footwall cooling between 25 and 21 ma at a slip rate of up to 15 km myr and demonstrate that active ductile deformation along the sdf migrated northward with time apatite ft ages from the footwall of the sdf are spatially invariant over 100 km and consistently 2 3 myr younger than the zircon fission track ages from the same samples these data are consistent with a regional but relatively rapid erosion linked cooling phase that removed 2 3 km of overburden following cessation of sdf activity but before deposition of sedimentary rocks on the detachment surface at 16 4 ma postcollision extension of the anatolide orogen with development of the sdf can be best explained as the result of postcollision magmatism and thermal weakening of the crust inducing the extensional reactivation of an earlier major out of sequence thrust

are the alps collapsing abstract orogenic collapse is a process that transfers gravitational potential energy from regions of high potential energy to regions of lower potential energy this transfer is classically considered to be accomplished by extension in the orogenic core and by synchronous shortening in foreland regions of the orogen not all extensional features in collisional mountain belts need however reflect orogenic collapse normal faulting thrust faulting and strike slip faulting are all active in different parts of the alps today and reflect complex local responses to ongoing europe adria convergence the western alps is the only area today where extension and shortening radial to orogen trend occur synchronously and where orogenic collapse may be an important process elsewhere in the alps normal faults are oriented at a high angle to orogen trend and were primarily active in oligocene and miocene time most present day activity in the central and eastern alps is on strike slip faults that are accommodati

partial melting of rocks observed by the sound velocity method and the possibility of a quasi dry low velocity zone in the upper mantle abstract the effect of glass contained in rocks on the sound velocity is remarkable at high temperatures much less than 1 of glass included in the rock specimen can be easily detected by the present sound velocity method if the glass geometry is a film or tube of small aspect ratio the velocity temperature relations the v t for lehrzolite as well as several igneous rock types were studied up to 900 950 c at moderate constant pressure under quasi dry conditions above 900 c it was observed that the downward deflection of the v t from a linear relation becomes remarkable for lehrzolite but not for olivine xenoliths this indicates that the partial melting of anhydrous lehrzolite commences at 900 c corresponding to the temperature of the upper rim of the low velocity zone this means that the existence of a quasi dry low velocity zone is possible