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

nature and origin of seismic reflection fabric ruby east humboldt metamorphic core complex nevada seismic reflection profiling across exposed upper and middle crustal rocks of the ruby east humboldt metamorphic core complex delineates important characteristics of the crustal structure developed during tertiary extensional deformation the goals of this study were to trace a tertiary extensional shear zone from mylonitic surface outcrops into a seismic section and to characterize the deeper crustal fabric associated with the polyphase deformational history of the complex reflections in the shallow surface correlate with a plastic to brittle shear zone that formed during the tectonic unroofing of the middle crustal rocks constructive interference from strong planar layering in the mylonitic shear zone is considered chiefly responsible for generating the reflections east and southwest dipping reflectors in the seismic section appear to correlate with exposed opposing dipping mylonitic foliation domains the opposing dips of the mylonitic layering may reflect warping of the normal sense shear zone during tectonic exhumation coupled with an overall anastomosing character deeper in the section a heterogeneous reflection character correlates with increases in velocity shown from wide angle measurements in this region and is interpreted as penetrative fabric originating from extensional flow facilitated by broad scale pure shear and localized simple shear lower crustal rocks apparently achieved granulite facies metamorphism during extension a maximum of 6 8 km of mafic material could have been added to the crust during cenozoic extension the moho reflection has such a high amplitude that it may be caused by partially molten rocks interleaved with peridotite reflections from the moho show no significant upwarping of the base of the crust beneath the core complex this suggests that the lower crustal configuration is at least as young as the period of extensional activity that lasted from approximately 40 to 20 ma which was responsible for the exhumation of the metamorphic core complex

incremental 40ar 39ar thermochronology of mylonitic rocks from the northern snake range nevada a 40ar 39ar thermochronologic study of muscovite biotite and potassium feldspar from footwall rocks of the northern snake range decollement nevada was undertaken in order to constrain the timing and temperatures of mylonitic deformation during extension and the subsequent cooling uplift history of these mylonitic rocks mylonitic deformation has been bracketed between 37 ma and about 24 ma and minimum temperatures during ductile thinning were about 300 c mineral cooling ages along the east flank of the range suggest a minimum tmax gradient of about 55 c ma this steep thermal gradient is the result of a localized heat source and or shearing of colder rocks with older cooling histories over hotter rocks with younger cooling histories although mylonitic deformation may have spanned a 13 m y period 37 ma to 24 ma deformation was not synchronous throughout the range on the west flank of the range age spectra from muscovite and microcline samples indicate that rocks were at temperatures well below 325 c and deformation had ceased by 30 ma while on the east flank of the range lower plate rocks were at temperatures above 325 c and ductile deformation was still ongoing this is also reflected in chrontours of published k ar muscovite ages along with 40ar 39ar ages which monotonically decrease in age from west to east these data provide evidence for a temperature difference across the range within the same structural horizon from about 30 to 24 ma these relations suggest a lateral thermal gradient the east flank of the range cooling more slowly than the west flank alternatively the data suggest either an eastward dip of lower plate units prior to the onset of tertiary mylonitic deformation or that layering dipped eastward due to deformation in an east dipping top to the east shear zone both cases result in differential cooling of lower plate units as they pass through a series of subhorizontal isotherms the 40ar 39ar data along with published finite strain structural and quartz petrofabric data provide constraints on the formation and geometric evolution of metamorphic core complex detachment faults

detachment folding fold amplification and diapirism in thrust wedge experiments 1 the relations between detachment folding fold amplification and salt diapirism in contractional settings have been investigated by means of scaled analogue models the viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak type 1 models and strong type 2 models decollements deformation patterns in the roof sequence exhibited two contrasting styles 1 outward propagation of detachment folding along the decollement ofp mode and 2 passive roof duplex prd mode in type 2 models detachment folding propagated away from the most external thrust in the floor sequence while in type 1 models long lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization a silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and ofp models best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both ofp prd and amplified detachment folds box folds transitions a quantitative comparison of model results with nature has been attempted by plotting salt based fold and thrust belts data points on the scaled transition curves obtained from the modeling such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core by reducing the roof sequence strength pore fluid pressure b is inferred to shift the equilibrium of fold and thrust belts toward the field of ofp and diapirism

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

early miocene change in direction of least principal stress southwestern united states conflicting inferences from dikes and metamorphic core detachment fault terranes previous investigations have suggested that the direction of least principal stress in the western united states has shifted from northerly to easterly between 50 to 30 ma an effort is made to more tightly constrain the time of this shift using igneous dike host rock systems in which the isotopic age of both host and dike is known and differs by less than 10 ma in such systems the possibility that magma intruded along old fractures unrelated to the state of stress at the time of intrusion is minimized and the axis of least principal stress is considered to be normal to the strike of the subvertical dike during the time interval between the ages of the dike and host rock these systems indicate that a northerly least principal stress orientation persisted more recently than previously believed until probably after 30 ma possibly about 26 ma and conceivably as recently as 18 ma in some areas the data are insufficient and too loosely bracketed in time to decide how fast the change in state of stress occurred in any one place and if by systematic rotation or abrupt shift however the change could be diachronous earlier in the southeast and later in the northwest major easterly trending magmatic and tectonic elements of tertiary age in the great basin of utah and nevada are compatible with the dike data however numerous metamorphic core complexes and detachment fault terranes seem to indicate more or less easterly extension throughout the time period represented by the dikes this inconsistency is not clearly resolvable but may reflect differences between the regional plate stress field and gravitational stresses in a crust overthickened by mesozoic compression

the mesozoic cenozoic tectonothermal evolution of the ruby mountains east humboldt range nevada a cordilleran metamorphic core complex a combined k ar and 40ar 39ar geochronological study of the ruby mountains east humboldt range nevada a cordilleran metamorphic core complex has revealed a complex tectonothermal history the three structural subdivisions of the core complex migmatitic core mylonitic zone and cover share magmatic and deformational histories in part but also display important contrasts in structural style metamorphic paragenesis and thermal history during the late jurassic ca 160 ma miogeoclinal rocks were polyphase deformed metamorphosed under amphibolite facies conditions and pervasively intruded by peraluminous granitic magmas thereby forming an igneous and metamorphic complex late jurassic muscovite granite porphyry plugs also intruded unmetamorphosed cover rocks late cretaceous cooling of the igneous and metamorphic complex is locally suggested by poorly resolved cretaceous incremental release 40ar 39ar hornblende spectra and by u pb monazite ages from pelitic schist exposed in the northern ruby mountains between ca 45 ma and 20 ma the igneous and metamorphic rocks experienced an episode of complex extensional tectonism which involved the development of low angle normal sense simple shear zones as a result rocks metamorphosed at depth in the mesozoic were translated to higher crustal levels which resulted in cooling through temperatures required for the intracrystalline retention of argon within minerals near concordance of hornblende and biotite plateau ages from the migmatitic core of the complex suggest relatively rapid exhumation however it is not certain if this cooling followed a prolonged maintenance at elevated temperatures after the ca 160 ma metamorphism or if distinct locally superposed late mesozoic and or middle tertiary thermal overprints had affected the cooling history a major extensional simple shear zone is marked by an approximately 1 kilometer thick mylonite zone that experienced late kinematic superposed brittle deformation a geometric consequence of the development of low angle extensional fault zones is the eventual translation of colder rocks over hotter rocks such a history is supported by the 40ar 39ar data for example mylonitic mafic orthogneisses from a major allochthon record 40ar 39ar hornblende plateau ages between 44 and 48 ma and biotite plateau ages between 32 and 33 ma and are structurally positioned above mylonitic rocks of the northern ruby mountains which record biotite cooling ages of ca 22 24 ma such relations suggest either multiple episodes of tertiary mylonitization or a protracted multiphase history of mylonitization however after ca 20 ma all of the presently exposed mylonitic rocks were maintained below 100 c and have only experienced upper crustal brittle deformation this article contains supplementary material

differential stress strain rate and temperatures of mylonitization in the ruby mountains nevada implications for the rate and duration of uplift kno ledge of the m agnitude of the differential stress during the formation of mylonitic rocks provides constramts on mechanical and thermal models for the exhumation of the metamorphosed footwalls of major low angle detachment faults we have analyzed the differential flow stress during the mylonitization of quartzose rocks in the ruby mountains nevada using grain size piezometers and kinetic laws for grain g owth quartzites from mylonitic shear zones in lamoille canyon and secret creek gorge have grain sizes of 91 151 m and 42 64 m respectively the peak temperature during mylonitization was 630 i 50 c and analysis of grain growth kinetics indicates that mylonitization continued during cooling to temperatures s450 c quartz grain size piezometers suggest that the mylonitization occurred under differential stresses crl cr3 of 38 64 mpa or maximum shear stresses of 19 32 mpa extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10 10 and 10 13 1 arguments presented m the paper suggest that the likely range of strain rates is 10 11 to 10 12 1 these strain rates are compatible with displacement rates of the order of 23 mm yr l along a 1 5 km thick simple shear zone such a shear zone dipping 15 would produce an uplift rate of 5 8 km m y 1 and a horizontal extensioo rate of 22 km m y l this uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1 5 km thick simple shear zone dipping 15 in 2 6 m y

second order segmentation the relationship between volcanism and tectonism at the mar 38 n 35 40 n deep tow sidescan sonar data acquired along 240 km of the mid atlantic ridge mar between 35 40 n and 38 n have been combined with new bathymetric compilations and used to establish the recent tectonic and volcanic history of six second order segments and their bounding non transform offsets ntos the segments show a range of volcanic and tectonic types but in general the northernmost segments i e those with greater influence from the azorean hotspot are shallower and more volcanically robust than those to the south with hydrothermal activity in segment centres for example menez gwen and lucky strike nonetheless this generalisation requires some modification due to temporal variations in the balance between magmatic supply and tectonic dismemberment the ntos are broad right stepping discontinuities locally up to 25 km wide and accommodate offsets between 10 and 50 km the discontinuities are mostly sediment floored and link the spreading segment tips with a range of structures these include locally dense arrays of en echelon extensional normal faults short lengths of linear strike slip fault strands occasionally cutting basement blocks apparently stranded within the offsets basement blocks within the offsets are cross cut by complexes of intersecting faults suggesting that deformation is distributed across the zone the pervasive faulting taking place at the ntos favours fluid circulation and associated hydrothermal activity as at the rainbow site at 36 17 n

structure and kinematics of upper cenozoic extensional detachment on naxos and paros cyclades islands greece this paper presents a study of the ductile and brittle deformation on naxos and paros islands cyclades greece previous maps and studies of the two islands have shown that a major low angle fault zone separates surface rocks above the contact from an initially deep seated unit below showing a metamorphic evolution from high to low pressures structural analysis as well as available stratigraphical metamorphic and geochronological data taken together demonstrate that this fault zone is a major normal sense detachment zone dipping to the north rapid denudation of footwall rocks subsequent to high temperature metamorphism at an estimated rate of 1 8 9 5 mm yr attests for tectonic unroofing during regional scale top to the north ductile shearing the change from ductile to brittle behavior of the footwall rocks together with a progressive localization of high strain intensity deformations just below the hangingwall is explained by the progressive cooling of the uprisen footwall of the detachment mio pliocene clastic sediments in the hangingwall represent the infilling of half grabens opened in between major normal faults that are synthetic to the underlying ductile shear zone these sediments are as old as aquitanian 25 ma or younger than the earliest recognized evidence of ductile extension in the footwall this provides a minimum age for the onset of extension in the cyclades which appears significantly older than maximum ages reported up to now 13 5 ma structural data strongly suggest that the detachment fault was initially rather low dipping 35 an evolutionary model is proposed in which migmatite domes in the footwall correspond to the uprise of the lower ductile crust between two separating upper crustal blocks during a process of asymmetric boudinage of the crust this detachment model applies to a previously thickened continental lithosphere which then suffers thermal relaxation and weakening allowing extensional deformation to reach a climax during and subsequent to high temperature metamorphism in the cyclades crustal scale extension started after early cenozoic thrusting while the crust was still thick or less likely before late underthrusting below the present surface

spatial and temporal variations in crustal production at the mid atlantic ridge 25 n 27 30 n and 0 27 ma we use high resolution multibeam bathymetry shipboard gravity side scan sonar images and magnetic anomaly data collected on conjugate flanks of the mid atlantic ridge at 25 n 27 30 n and out to 27 ma crust to investigate the crustal evolution of the ridge substantial variations in crustal structure and thickness are observed both along and across isochrons along isochrons within spreading segments there are distinct differences in seafloor morphology and gravity derived crustal thickness between inside and outside corners inside corners are associated with shallow depths thin crust and enhanced normal faulting while outside corners have greater depths thicker crust and more limited faulting across isochrons systematic variations in crustal thickness are observed at two different timescales one at 2 3 myr and another at 10 myr and these are attributed to temporal changes in melt supply at the ridge axis the shorter term variations mostly are in phase between conjugate ridge flanks although the actual crustal thickness can be significantly different on the two flanks at any given time we observe no correlation between crustal thickness and spreading rate thus during periods of low melt supply tectonic extension must increase to accommodate the full plate separation rate this extension commonly is concentrated in long lived faults on only one side of the axial valley resulting in strong across axis asymmetries in crustal thickness and seafloor morphology the thin crust flank has few volcanic features and exhibits elevated blocky topography with large offset often irregular faults while the conjugate thicker crust flank shows shorter offset regular faulting and common volcanic features the variations in melt supply at the ridge axis most likely are caused either by episodic convection in the subaxial mantle or by variable melting of chemically heterogeneous mantle

detailed tectonics near the tip of the galapagos 95 5 w propagator how the lithosphere tears and a spreading axis develops a model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using seamarc ii sea beam deep tow and alvin near the tip of the propagator growing westward at 52 km m y along the east west trending galapagos spreading axis near 95 5 w initial lithospheric rifting appears to be accommodated along reactivated roughly east west abyssal hill faults at the tectonic tip extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures the initial volcanism initial volcanic tip occurs about 6 5 km behind the tectonic tip as basalt erupts through these fissures covering most of the keystone block the neovolcanic axial tip occurs another 4 5 km back at the western termination of a well defined axial pillow ridge along which volcanism is localized spreading along the propagator axis accelerates to the full rate 29 mm yr half rate at the full rate tip interpreted to be 10 km behind the neovolcanic axial tip or 21 km behind the tectonic tip a complex pattern of strain in the tip area results from the interaction of plate spreading transform shear stress crack propagation dynamic depression of the western end of the axial valley and the transform zone and variations in the strength thickness heterogeneity and anisotropy of the lithosphere southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young thin weak relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction relative to the spreading direction due to the influence of shear stress from the transform zone similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older thick strong heterogeneous and anisotropic lithosphere by oblique slip along a set of left stepping reactivated preexisting east west abyssal hill structures although on a time scale of 106 years the overall process of propagation is mostly continuous rifting at the tectonic tip appears discontinuous at a scale of 1500 m or 30 000 years based on the east west spacing of grabens formed as rifting begins the scale of discontinuity of propagation of the neovolcanic axis is 3 5 km or 50 000 100 000 years based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes the extension involved in rifting is 10 20 based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting these values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting

tertiary structure and thermal history of the harquahala and buckskin mountains west central arizona implications for denudation by a major detachment fault system the harquahala and buckskin mountains lie in the footwall of the whipple buckskin bullard detachment system in the harquahala mountains mesozoic fabric and structure are progressively more intensely overprinted by penetrative tertiary deformation toward the northeastern pan of the range tertiary mylonitic deformation is recognized by the presence of deformed miocene mafic dikes and characteristic textural features lineations in the mylonite trend 040 060 and megascopic kinematic indicators mostly indicate top to the northeast sense of shear the crest of an antiform defined by tertiary mylonitic fabric does not coincide with the topographic crest of the range reorientation of mesozoic foliation in the vicinity of this antiform suggests that it originated as two shear zones as opposed to a single zone that was then bent the northeastern part of the brown s canyon granite acts as a large low strain lozenge in the southeast limb of the foliation arch mylonitic foliation along the southeast side of the range above this lozenge is strongly oblique to the trend of the detachment fault but the relative orientation is consistent with left oblique normal shear evidence of tertiary plastic deformation is absent southwest of sunset pass minor low angle normal faults in the southwestern harquahala mountains and little harquahala mountains dip northeast the low angle normal faults are cut by nw trending high angle right oblique faults in the central and southwestern harquahala mountains 40ar 39ar age spectra from k feldspar muscovite and hornblende and total gas ages from biotite indicate late cretaceous to early tertiary cooling to argon closure temperatures biotite and k feldspar from the area northeast of sunset pass record rapid early to middle miocene cooling reconstruction of the original geometry of the detachment system based on thermal differences indicated by contrasting cooling histories and the orientation of an early miocene dike swarm indicate that the initial dip of the detachment fault was most probably between 30 and 40 thus the harquahala mountains are a lilted block exposing of the order of a 10 km section through the pre tertiary crust heterogeneous proterozoic gneiss sparse paleozoic and mesozoic metasedimentary rocks and an oligocene plutonic complex are extensively overprinted by tertiary mylonitic fabrics in the buckskin mountains hornblende 40ar 39ar cooling ages suggest that most of these rocks were below hornblende closure temperature by early tertiary time except in the vicinity of the oligocene plutonic complex feldspar and biotite 40ar 39ar cooling ages suggest that the footwall of the whipple detachment system experienced a more uniform cooling history in the buckskin mountains than in the harquahala mountains cooling ages between about 13 and 20 ma are recorded throughout the range with no consistent spatial pattern of ages the eagle eye detachment fault southeasternmost extension of the whipple buckskin bullard fault system becomes a transfer fault along the southeastern side of the harquahala mountains this fault separates two distinct extensional terranes on the northwest approximately 50 km of crustal extension has been accommodated on the whipple fault system while on the southeast a similar total magnitude of crustal extension is accommodated by a series of smaller normal faults in the adjacent big horn lake pleasant extended terrane as the deformation becomes more distributed maximum slip on any one normal fault decreases to the point where no single fault has enough slip to denude crustal levels at which tertiary plastic deformation has occurred

deformation patterns and structural vergence in brittle ductile thrust wedges an additional analogue modelling perspective abstract scaled physical models of brittle ductile thrust wedges investigated the causes leading to the development of the various structural styles observed in fold and thrust belts and accretionary prisms this study focused on some potential controlling factors such as 1 relative strength between the brittle overburden and the viscous decollement layer brittle ductile coupling 2 the effect of very low basal friction and 3 pre shortening rheological stratigraphy including decollement strength type 1 models had a single basal decollement whereas in type 2 models the decollement was embedded in brittle material simulating an upper roof sequence and a lower floor sequence other models investigated the effects produced by an horizon of mechanical weakness at the base of the viscous decollement type 1a and the role of decollement offset type 3 the experiments suggest the occurrence of a genetic relationship between the absolute values of shear stresses and the development of dominating hinterland or foreland verging thrust faults or dually verging thrusts by controlling the orientation of the 1 axis basal friction and decollement offset is found to effectively influence the deformation pattern and structural vergence and could explain the development of dominating backthrusts in cascadian type margins the decoupling of a frontal monocline via the upper decollement type 2 models produced passive roof duplex structures termed composite roof duplex when the frontal monocline coexists with outward fold propagation

p wave anisotropy of mylonitic and infrastructural rocks from a cordilleran core complex the ruby east humboldt range nevada abstract in order to evaluate the possibility that compressional wave anisotropy might characterize the upper to middle crust of the north american cordillera we examined the nature and degree of seismic anisotropy of rock samples collected from the mylonite zone and underlying infrastructure of the ruby east humboldt metamorphic core complex in northeastern nevada seismic anisotropy in quartzofeldspathic rocks from all structural levels is low and generally related to the volume of mica relative to feldspar upper amphibolite facies infrastructure quartzite has a stronger and different quartz crystallographic preferred orientation cpo and thus higher seismic anisotropy than the lower grade mylonitic quartzites a feature related to the higher temperature deformation conditions that characterized the infrastructure schistose mylonite zone rocks as a group have the highest anisotropy we infer that this is in large part due to mica cpo although the anisotropy patterns suggest that quartz cpo may be a factor anisotropy is higher in infrastructural quartzites calc silicates and marbles than in mylonite zone equivalents seismic anisotropy data for other cordilleran core complex samples suggest that these patterns are common characteristics of cordilleran core complex rocks

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

fast cooling following a late triassic metamorphic and magmatic pulse implications for the tectonic evolution of the korean collision belt we discuss the evolution of korea in the context of a relatively short lived tectonically induced magmatic and metamorphic pulse that affected large portions of the crust of the peninsula s southern part during the late triassic recent 40ar 39ar single grain laser step heating dates imply a prolonged metamorphic recrystallization between 243 and 220 ma which occurred in distinct phases that were not coeval throughout the peninsula we obtained identical plateau ages between 231 4 0 8 and 228 9 0 8 ma 1 85 95 39ar release on single grains of detrital muscovite from jurassic sandstones gimpo group a literature review shows that the ages of detrital muscovites are identical to 1 concordant 40ar 39ar ages of biotite 228 ma and amphibole 230 ma in amphibolites of the deokjeongri gneiss formation and the weolhyeonri complex pointing to very rapid cooling of 100 150 c ma and 2 231 229 ma muscovite from the low grade metamorphic mid paleozoic turbidites of the taean formation the efficiency of cooling is further underlined by the near coincidence of these 40ar 39ar ages with 243 229 ma average 234 6 ma zircon u pb ages in the gyeonggi massif and the hongseong belt in the literature it is argued that the late triassic magmatic and metamorphic pulse is superimposed on an earlier tectono metamorphic event possibly related to collision indicated by 1 243 237 ma muscovite ages or age components in age spectra and 2 two generations of folds and associated tectonic foliations truncated by 229 5 ma old syenites and earlier mafic dykes the late triassic thermal pulse could have been the result of post collisional delamination of the lower crust and uppermost mantle and or oceanic slab break off which is also suggested by almost coeval widespread mantle sourced mg rich potassic magmatism continuing ductile deformation is shown by mylonitization of late triassic magmatic rocks an 220 ma muscovite age may be related to this

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

propagating rift model for the v shaped ridges south of iceland 1 we present new marine geophysical data which constrain the seafloor spreading history of the reykjanes ridge near iceland and the origin of its flanking v shaped topographic and gravity ridges contrary to the geometry assumed in pulsing plume models the v shaped ridges are not symmetric about the reykjanes ridge axis and seafloor spreading has not been symmetric about a stable axis thus existing models must at least be modified to include an additional asymmetry producing mechanism the best understood and documented such mechanism is rift propagation one possibility is that plume pulses drive the propagators however rift propagation also produces v shaped wakes with crustal thickness variations suggesting the possibility that a pulsing iceland plume might not be necessary to explain the reykjanes v shaped ridges scarps and troughs

probable anticlockwise p t evolution in extending crust hlinsko region bohemian massif in the hlinsko region variscan bohemian massif czech republic a major extensional shear zone separates low grade metasedimentary series hlinsko schists and high grade rocks of the moldanubian terrane svratka crystalline unit during late variscan extension a tonalite intruded syntectonically into the normal ductile shear zone and caused contact metamorphism of the overlying schists concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists in order to decipher the detailed petrological evolution of the hlinsko unit situated in the hangingwall of this tectonic contact a phase diagram approach and pctrogenetic grids calculated with the thermocalc computer program were used the crystallization deformation relationships and the paragenetic analysis of the hlinsko schists define a p t path with an initial minor increase in pressure followed by cooling calculated pseudosections constrain this anticlockwise p t evolution to the upper part of the andalusite field between 0 36 and 0 40 gpa for temperatures ranging from 570 to 530 c a low uhz0 is required to explain the presence of andalusite biotite bearing assemblages and could be related to the presence of abundant graphite in contrast the footwall rocks of the svratka crystalline unit record decompression from around 0 8 gpa at a relatively constant temperature followed by cooling thus the footwall and the hangingwall units display opposite but convergent p thistories decompression in the footwall rocks is related to a rapid exhumation we propose that the inverse anticlockwise p t path recorded in the hangingwall pelites is related to the rapid extension controlled sedimentation of the overlying flysch series

basin geometry and salt diapirs in the flinders ranges south australia insights gained from geologically constrained modelling of potential field data abstract the adelaide basin in australia is a complex of late neoproterozoic to early cambrian rift and sag basins which was inverted during the cambro ordovician delamerian orogeny the deposition of evaporitic sediments during the earliest stage of basin development in the late neoproterozoic willouran age played a major role in the subsequent tectonic evolution of the basin previous studies have shown that early mobilization vertical transport and withdrawal of the evaporites influenced the sedimentation during the late neoproterozoic and early cambrian the evaporites also influenced deformation during the inversion of the basin and the development of the delamerian fold and thrust belt however the control exerted by basement structures in the deposition of the evaporitic beds and the role of these tectonic structures in the later inversion of the basin have been poorly constrained in this work we use a combination of published and original geological observations along with the interpretation of potential field datasets total magnetic intensity and bouguer anomaly data to better constrain the geology of the basin at depth we construct two and a half dimensional forward models of the potential field data along selected profiles across the adelaide basin these models are constrained by the geology at the surface drill hole data and measured petrophysical properties specific gravity and magnetic susceptibility we achieved the best fit between observed and modelled potential fields with a model favouring thick skinned deformation where the diapirs are not randomly distributed but located directly above basement penetrating normal faults furthermore these normal faults were probably active during the sedimentation of the late neoproterozoic and early cambrian sediments and underwent partial or total inversion during the cambro ordovician delamerian orogeny mobilisation and withdrawal of the evaporites was therefore initiated and facilitated by the extension coeval with the opening of the basin and continued during the subsequent delamerian orogeny

crustal thickness anomalies in the north atlantic ocean basin from gravity analysis gravity derived crustal thickness models were calculated for the north atlantic ocean between 76 n and the chain fracture zone and calibrated using seismically determined crustal thickness about 7 of the ocean crust is 7 km thick and is interpreted to have been affected by excess magmatism thin crust probably reflects reduced melt production from relatively cold or refractory mantle at scales of up to hundreds of kilometers along the spreading axis by far the most prominent thick crust anomaly is associated with iceland and adjacent areas which accounts for 57 of total crustal volume in excess of 7 km much smaller anomalies include the azores 8 cape verde islands 6 canary islands 5 madeira 4 and new england great meteor seamount chain 2 all of which appear to be associated with hot spots hot spot related crustal thickening is largely intermittent suggesting that melt production is episodic on time scales of tens of millions of years thickened crust shows both symmetrical and asymmetrical patterns about the mid atlantic ridge mar axis reflecting whether melt anomalies were or were not centered on the mar axis respectively thickened crust at the bermuda and cape verde rises appears to have been formed by isolated melt anomalies over periods of only 20 25 myr crustal thickness anomalies on the african plate generally are larger than those on the north american plate this most likely results from slower absolute plate speed of the african plate over relatively fixed hot spots

relation between extensional geometry of the northern grant range and oil occurrences in railroad valley east central nevada in the northern grant range heterogeneous neogene extension was dominated by synchronous arching and attenuation attenuation was accomplished along a stacked set of attenuation faults that formed at low angles to bedding as the paleozoic carbonate and paleogene rocks arched about a north northwest axis the style and amount of attenuation was controlled by lithologic character and structural depth of rock units and by geometry of the arch on the steeper west side of the grant range arch the curviplanar low angle attenuation faults converge into a single shallowly west dipping fault zone along which the stratigraphic juxtaposition of mississippian units over middle cambrian units and late cretaceous granite marks the zone of maximum attenuation the distinct geometry o the arched westward converging low angle fault array is seen in windows into the deeper structure low on the west side of the range we conclude that arching and heterogeneous extension resulted from uplift of the grant range relative to the structural basin of railroad valley to the west this structural differentiation is expressed as a complex zone of subparallel to bedding shallow dipping attenuation faults rather than as a simple high angle range front fault seismic and drill hole data indicate that low angle attenuation faults in the range extend into railroad valley and control the structure buried in the valley mississippian and paleocene to eocene petroleum source rocks and devonian to oligocene reservoir rocks in railroad valley oil fields are in extensively fractured rocks of the upper plate to the major extensional fault system thus relatively cold upper plate rocks immature with respect to hydrocarbon generation were brought relatively down into contact with hotter lower plate rocks by neogene attenuation faulting oil in railroad valley which is sourced from rocks as young as eocene was probably generated by this juxtaposition during neogene crustal attenuation and subsequently migrated into upper plate fractured reservoi s

late cretaceous extension in the hinterland of the sevier thrust belt northwestern utah and southern idaho cover rocks of the raft river metamorphic core complex located in the sevier belt hinterland preserve a structural and metamorphic history that predates the middle tertiary extension of the region in the eastern raft river mountains cambrian permian rocks form two allochthons that occur in the hanging wall of the mid miocene raft river detachment fault dramatically attenuated metamorphosed cambrian pennsylvanian strata of the lower allochthon yield late cretaceous muscovite sup 40 ar sup 39 ar plateau ages ca 82 90 ma that are interpreted to date cooling during final stages of layer parallel extension devonian permian rocks exposed in the black pine mountains underwent east west extension approximately 160 associated with syntectonic growth of white mica along cleavage sup 40 ar sup 39 ar whole rock spectra of associated slates also indicate a late cretaceous metamorphism together field and sup 40 ar sup 39 ar results suggest that late cretaceous extension occurred in the sevier belt hinterland at the same time as shortening in the eastern foreland and at depth in the hinterland sufficient topography must have been present to drive upper crustal extension in the eastern hinterland

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

accelerating exhumation in the eocene north american cordilleran hinterland implications from detrital zircon u th he pb double dating

zircon and apatite u th he evidence for paleogene and neogene extension in the southern snake range nevada usa despite decades of study the timing rates and magnitude of extension in the basin and range are poorly quantified in some areas this study integrates new zircon and apatite u th he analyses zrnhe and aphe with published thermochronologic data to quantify these extensional parameters in the southern snake range ssr of east central nevada the new zrnhe dates range from 40 7 4 9 ma in the western ssr to 21 0 3 3 ma near the present day trace of the southern snake range decollement ssrd and the aphe dates range from 15 1 2 4 ma in the central ssr to 13 6 0 7 ma closest to the ssrd trace these new and previously published low temperature thermochronologic cooling ages were inverted for the extensional history of the ssr using a bayesian monte carlo method incorporating pecube the posterior extensional histories indicate three significant pulses of extension occurred during the paleogene and neogene 1 50 45 to 38 ma eocene 2 33 30 to 23 ma oligocene and 3 23 20 to 10 8 ma miocene modeled rates of extension were low at 0 5 mm a 1 however more rapid rates possibly occurred during the eocene and the miocene based on posterior histories net cumulative extension from posterior histories is 19 8 to 34 9 km with a mean of 29 7 km about 10 18 km of extension occurred during the eocene and oligocene model results indicate no relationship between extension and magmatism in the ssr our new model results and interpretations also indicate extensional collapse of the nevadaplano initiated prior to 17 ma

experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement abstract this paper discusses the differences in mechanical properties and kinematics between fold and thrust belts detaching on evaporitic decollements and those detaching on stronger detachments physical experiments are described that model shortening of a thick brittle cover overlying a weak viscous decollement to gain a better understanding of these differences we tested the influence of 1 the decollement layer thickness and 2 the presence of a deformable backstop on the hinterland side and of a decollement pinch out on the foreland side because of the very low shear strength of the viscous decollement folds and thrusts did not propagate according to a piggy back sequence but by centripetal back and forth propagation additional shortening was accommodated by growth of all existing structures fold symmetry and thrust vergence varied between experiments models confined between two rigid vertical endwalls always deformed by symmetric folding and thrusting in models having a deformable backstop and a foreland decollement pinch out forethrusting initially dominated folds were asymmetric and fault blocks rotated in models having a thick decollement layer folds kept growing asymmetrically diapirism also depended on the initial decollement geometry diapirs formed only in models having a deformable backstop and were restricted to the hanging wall of a fault related fold finally the implications of such kinematics on the thermal history are discussed as well as the likelihood of hydrocarbon maturation and preservation

morphogenesis of the sw balearic continental slope and adjacent abyssal plain western mediterranean sea we present the seafloor morphology and shallow seismic structure of the continental slope south east of the balearic promontory and of the adjacent algero balearic abyssal plain from multibeam and chirp sonar data the main purpose of this research was to identify the sediment pathways from the balearic promontory to the algero balearic deep basin from the early pliocene to the present the morphology of the southern balearic margin is controlled by a sw ne structural trend whose main expressions are the emile baudot escarpment transform fault and a newly discovered wsw ene trend that affects the sw end of the escarpment and the abyssal plain we relate the two structural trends to right lateral simple shear as a consequence of the miocene westward migration of the gibraltar arc newly discovered steep and narrow volcanic ridges were probably enabled to grow by local transtension along the transform margin abyssal plain knolls and seahills relate to the subsurface deformation of early stage halokinetic structures such as salt rollers salt anticlines and salt pillows the limited thickness of the overburden and the limited amount of deformation in the deep basin prevent the formation of more mature halokinetic structures such as diapirs salt walls bulbs and salt extrusions the uppermost sediment cover is affected by a dense pattern of sub vertical small throw normal faults resulting from extensional stress induced in the overburden by subsurface salt deformation structures shallow gas seismic character and the possible presence of an active polygonal fault system suggest upward fluid migration and fluid and sediment expulsion at the seafloor through a probable mud volcano and other piercement structures one large debris flow deposit named formentera debris flow has been identified on the lower slope and rise of the south formentera margin based on current observations we hypothesize that the landslide originating the formentera debris flow occurred in the holocene perhaps in historical times

segmentation and crustal structure of the western mid atlantic ridge flank 25 25 27 10 n and 0 29 m y we conducted a detailed geological geophysical survey of the west flank of the mid atlantic ridge between 25 25 n and 27 10 n and from the ridge axis out to 29 ma crust acquiring hydrosweep multibeam bathymetry hawaii mr1 sidescan sonar imagery gravity magnetics and single channel seismic reflection profiles the survey covered all or part of nine spreading segments bounded by mostly nontransform right stepping discontinuities which are subparallel to flow lines but which migrated independently of one another some discontinuities alternated between small right and left stepping offsets or exhibited zero offset for up to 3 4 m y despite these changes the spreading segments have been long lived and extend 20 m y or more across isochrons a large shift 9 in relative plate motion about 24 22 ma caused significant changes in segmentation pattern the nature of this plate boundary response together with the persistence of segments through periods of zero offset at their bounding discontinuities suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt within segments there are distinct differences in seafloor depth morphology residual mantle bouguer gravity anomaly and apparent crustal thickness between inside corner and outside corner crust this demands fundamentally asymmetric crustal accretion and extension across the ridge axis which we attribute to low angle detachment faulting near segment ends cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal thickness changes of at least 1 2 km every 2 3 m y these are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension controlled by retention and quasiperiodic release of melt from the upwelling mantle detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes creating crustal domes analogous to turtleback metamorphic core complexes that are formed by low angle detachment faulting in subaerial extensional environments

positive fault inversion triggering intrusive diapirism an analogue modelling perspective understanding the formation and the development of salt structures is very important especially because they are of significant economical interest for hydrocarbon trapping and for long term storage of radioactive waste and energy reserves generally the activity of normal faults developed in extensional regimes is considered the most efficient mechanism for salt diapirs the results of analogue models reported in this paper suggest a new triggering mechanism for the rise of salt structures during basin inversion this mechanism relates the localization of ductile diapirs to early normal faults only after their inversion during later shortening in this case diapiric growth is related to the strong dip slip reactivation component along the fault extruding the silicone simulating salt upward some natural cases in which the timing and the mechanism of diapiric growth is not clear can be re interpreted in the light of these analogue model results

petrogenesis of garnet two mica granites in the ruby mountains nevada garnet two mica granites of jurassic 160 3 m y and cretaceous 83 1 3 m y ages intrude amphibolite facies metasedimentary rocks of precambrian age and lower paleozoic sedimentary rocks in the northern part of the ruby mountains nevada high initial 87sr 86sr and high values of 18o for minerals suggest a dominantly sedimentary source for the magmas total rare earth contents are low and less fractionated cen ybn 10 than granitic rocks of similar composition in the sierra nevada and show a negative eu anomaly eu eu 0 57 these data are consistent with melting in the stability field of plagioclase with little contribution to the melt from refractory minerals such as zircon sphene and apatite minor euhedral garnets are manganese rich and magnesium poor al60sp34py3 6gr2 8 compared to garnet al79sp6py13gr2 in the intruded metamorphic rocks and show an increase in manganese and a decrease in calcium in the outer few microns fractionation of magnesium iron between garnet and biotite and of sodium potassium between k feldspar and plagioclase in the granites suggest submagmatic equilibration temperatures in the range of 365 to 505 c whereas oxygen isotope equilibration temperatures for two granites are higher at 480 and 570 c the compositions of the granites plot near minimum melting compositions in the water saturated quartz albite orthoclase system comparison of muscovite plus quartz stability the water saturated granite solidus and experimental garnet melt equilibria suggests pressures of crystallization no lower than about 3 5 kbar this pressure is compatible with that estimated from garnet plagioclase sillimanite quartz equilibria for the intruded metasedimentary rocks

structural evolution of the whipple and south mountains shear zones southwestern united states the whipple and south mountains of the southwestern united states have undergone a strikingly similar sequence of deformations in both ranges gently dipping mylonitic fabrics have been overprinted by successively more brittle structures associated with a low angle detachment fault kinematic indicators reveal that the mylonitic rocks brittle structures and detachment faults are kinematically coordinated and were all formed by top to the northeast shear the structural evolution of both areas can be explained in terms of major shallow dipping shear zones that accommodated tertiary crustal extension we suggest that detachment faults and associated zones of brecciation cataclasis and seismic slip were originally continuous downdip along the low angle shear zones into mylonitic gneisses formed below or near the ductile brittle transition as the mylonites were drawn out from beneath the brittlely extending upper plate they were progressively uplifted above the ductile brittle transition and were overprinted by successively more brittle structures

paleomagnetic evidence for the age and extent of middle tertiary counterclockwise rotation dixie valley region west central nevada paleomagnetic data obtained from oligocene to lower miocene igneous rocks and middle miocene basaltic rocks at fifteen localities from a region surrounding dixie valley in west central nevada indicate that parts of the area experienced counterclockwise vertical axis rotation and these data provide constraints on the extent and timing of rotation counterclockwise vertical axis rotation probably exceeding 30 is indicated for oligocene to lower miocene rocks in the central part of the study area calculated rotations increase with increasing age of the oligocene to lower miocene rocks indicating that vertical axis rotation was concurrent with ash flow deposition about 23 33 ma paleomagnetic data indicate that middle miocene basaltic rocks about 10 17 ma postdate significant counterclockwise vertical axis rotation although this interpretation is complicated because the rocks were erupted episodically in pulses that apparently were short lived with respect to geomagnetic secular variation counterclockwise rotation was related to a deformation event that predated development of the present basin and range physiography of the area the total amount of crustal rotation in tertiary time in the area is poorly known rotation estimates calculated from time averaged mean directions that incorporate data from a broad range of the oligocene to lower miocene units probably underestimate total tertiary rotation because the mean directions include data from units that postdate much of the rotation for example a rotation estimate for a composite sequence from the central part of the study area is 23 15 whereas estimates from older and younger halves of the sequence are 37 21 and 11 16 respectively paleomagnetic data indicate that oligocene to lower miocene rocks at some localities in the northern and southern parts of the study area e g the golconda canyon locality probably did not experience significant tertiary counterclockwise rotation although the paleomagnetic data alone are insufficient to fully define the geographic area subjected to tertiary counterclockwise rotation the data suggest that a discrete subregion was affected rather than the entire basin and range province

dynamics and structural development of metamorphic core complexes the development of metamorphic core complexes mccs in a thickened continental lithosphere is studied using fully coupled thermomechanical numerical code accounting for elastic brittle ductile properties of constituent rocks for mccs to develop the middle lower crust and the sub moho mantle are required to be weak enough to flow laterally so as to simultaneously feed the exhuming dome and enable the moho to keep a flat geometry the conditions are satisfied with initial moho temperatures of 800 c or higher with crustal thicknesses of 45 km or greater and with initial effective viscosities lower than 1020 pa s and 1022 pa s in the lower crust and the underlying mantle respectively a compositional mainly density anomaly with the properties of granite is placed centrally in the crust to localize strain at the onset of deformation during a first stage of upper crust necking the deformation pattern is relatively symmetrical and dominated by graben formation in the upper crust when the first ductile layers reach the surface a second stage of dome amplification and widening occurs dome amplification is accommodated by horizontal flow in the ductile crust giving an early symmetrical pattern of conjugate shear zones with no obvious detachment zone the system then rapidly becomes asymmetric with the localization of a detachment zone along one dome limb further accommodating dome widening thus the exhumation process of a metamorphic dome results in the progressive development of a detachment zone depending on initial moho temperature the detachment zone can migrate in space or die out and be replaced by a new one with an opposite dip

crustal structure in the elko carlin region nevada during eocene gold mineralization ruby east humboldt metamorphic core complex as a guide to the deep crust the deep crustal rocks exposed in the ruby east humboldt metamorphic core complex northeastern nevada provide a guide for reconstructing eocene crustal structure 50 km to the west near the carlin trend of gold deposits the deep crustal rocks in the footwall of a west dipping normal sense shear system may have underlain the pinon and adobe ranges about 50 km to the west before tertiary extension close to or under part of the carlin trend eocene lakes formed on the hanging wall of the fault system during an early phase of extension and may have been linked to a fluid reservoir for hydrothermal circulation the magnitude and timing of paleogene extension remain indistinct but dikes and tilt axes in the upper crust indicate that spreading was east west to northwest southeast perpendicular to a paleozoic and mesozoic orogen that the spreading overprinted high geothermal gradients associated with eocene or older crustal thinning may have contributed to hydrothermal circulation in the upper crust late eocene eruptions upper crustal dike intrusion and gold mineralization approximately coincided temporally with deep intrusion of eocene sills of granite and quartz diorite and shallower intrusion of the harrison pass pluton into the core complex rocks stacked mesozoic nappes of metamorphosed paleozoic and precambrian rocks in the core complex lay at least 13 to 20 km deep in eocene time on the basis of geobarometry studies in the northern part of the complex the presently exposed rocks had been even deeper in the late mesozoic to 30 km depths before losing part of their cover by eocene time nappes in the core plunge northward beneath the originally thicker mesozoic tectonic cover in the north part of the core complex mesozoic nappes and tectonic wedging likely occupied the thickened midlevel crustal section between the deep crustal core complex intrusions and nappes and the overlying upper crust these structures as well as the subsequent large displacement cenozoic extensional faulting and flow in the deep crust would be expected to blur the expression of any regional structural roots that could correlate with mineral belts structural mismatch of the mineralized upper crust and the tectonically complex middle crust suggests that the carlin trend relates not to subjacent deeply penetrating rooted structures but to favorable upper crustal host rocks aligned within a relatively coherent regional block of upper crust

misalignment of quartz c axis fabrics and lineations due to oblique final strain increments in the ruby mountains core complex nevada abstract to interpret c axis stereonet patterns of deformed quartz the patterns must be viewed in the xz strain plane which is usually assumed to contain the elongation lineation this study describes several c axis patterns from a region of complex amphibolite facies deformation in the northern ruby mountains where the late xz plane is oblique to most of the lineations without the lineation as a guide the proper orientation for interpretation of each pattern is determined by rotating the c axis data around the normal to the foliation until the c axes within the foliation plane lie at the center of the stereonet this new view is inferred to be the xz plane for the final increments of strain the rotation produces an internal symmetry in the patterns in addition the rotated views across the area show a more consistent geographic orientation than the lineations this geographic orientation is aligned with strain in a mylonitic shear zone which represents the latest macroscopic evidence for plastic strain along the edge of the study area the complete reorientation of the quartz textures during the final strain increments implies that they record very little of the finite strain history quartz is the only mineral recrystallized during the late strain thereby providing insight into the geometry of strain increments that cannot be determined from other minerals

synextensional granitoids and detachment systems within cycladic metamorphic core complexes aegean sea greece toward a regional tectonomagmatic model within deforming continental regions where metamorphic core complexes mccs and synextensional granitoids are closely associated deciphering the link between detachment faulting and magmatism often remains complex as 1 the rheological weakness of magma may stimulate mechanisms of strain localization and conversely 2 tectonic processes may open close drains where magmas can intrude here we tackle this issue by focusing on the cyclades with the comparison of five granitoid cored mccs tinos mykonos ikaria naxos and serifos and their flanking detachment systems in this region granitoids were emplaced into the middle upper crust over a relatively short time period 15 9 ma while metamorphic domes were largely exhumed after more than 10 myr of extension none of those intrusions thereby proves to be a real candidate for the genesis of mccs but would rather be a consequence of a warmer regime during lithospheric thinning however all collected structural and kinematic data converge toward a regional scheme in which magmatic activity played a more pivotal role than previously postulated indeed late evolution stages of mccs were dynamically impacted by intrusions along which local and transient heterogeneities of the mechanical strength occurred interfering with the sequential development of detachments during their tectonically controlled emplacement magmatic products intruded already formed detachments at depth locally inhibiting their activity associated with a contemporaneous upward migration of extensional deformation that tended to localize through time within intrusion roofs along rheological discontinuities the newly formed detachments are expressed within granitoids through a continuum of deformation from comagmatic to ductile conditions followed by cataclasis along detachments

fission track analysis of the footwall of the catalina detachment fault arizona tectonic denudation magmatism and erosion new apatite and zircon fission track ages obtained from the footwall of the catalina metamorphic core complex record a complicated cooling history associated with mid tertiary extension zircon fission track ages record the progressive unroofing of the catalina metamorphic core complex along the catalina detachment fault zircon fission track ages range from 31 9 to 19 4 ma generally decrease in the hanging wall slip direction and yield slip rates along the catalina detachment fault ranging from 1 2 to 12 km myr 1 in contrast apatite fission track ages increase in the hanging wall slip direction samples from the main range of the santa catalina mountains yield apatite fission track ages of 20 5 to 14 6 ma samples from the santa catalina mountain forerange located closer to the detachment fault yield apatite fission track ages of 21 4 to 18 8 ma rapid cooling 40 to 60 c myr 1 related to detachment faulting is best recorded by zircon fission track ages and higher temperature thermochronometers in the main range and by nearly concordant zircon and apatite fission track ages in the forerange slower cooling 3 7 c myr 1 of the footwall is recorded by shortened mean confined fission track lengths 14 m and is related to erosional unroofing approximately 2 km of late tertiary erosion played a significant role in the unroofing of the footwall of the catalina metamorphic core complex in contrast to metamorphic core complexes in western arizona where detachment faulting is the dominant unroofing mechanism

monazite zircon and garnet growth in migmatitic pelites as a record of metamorphism and partial melting in the east humboldt range nevada monazite and zircon thermometry and geochronology were applied to anatectic pelites of the northern east humboldt range nevada the study area is an exhumed portion of the sevier orogenic root and is characterized by two crustal blocks with different prograde metamorphic histories the winchell lake nappe and the underlying lizzies basin block u th pb secondary ion mass spectrometry shrimp results from zircon and monazite indicate that in the lizzies basin block prograde metamorphism began by 96 5 8 0 ma cooling and melt crystallization was initiated by 80 1 1 4 ma possible reheating and a monazite growth event occurred again at 76 2 2 8 to 68 2 2 0 ma in the upper limb of the overlying winchell lake nappe prograde metamorphism began by 82 8 1 3 ma cooling and melt crystallization recorded by zircon and monazite growth recrystallization at 77 4 12 4 to 58 9 3 6 ma melt may have been present in the winchell lake nappe upper limb for a protracted period or may have formed during several unresolved melting events thin eocene oligocene 40 32 ma high y monazite rims are found in both crustal blocks these probably represent a phase of heating during eocene oligocene magmatism and extensional deformation u th pb geochronology results are consistent with differential burial heating and exhumation of different crustal blocks within the east humboldt range significantly fast exhumation of upper limb rocks of the winchell lake nappe and somewhat slower exhumation of lizzies basin block rock occurred prior to the core complex phase of the exhumation of the ruby mountains east humboldt range metamorphic core complex differences in the timing and tectonic significance of exhumation episodes within the east humboldt range may reflect how the localized presence of partial melt can affect the evolution of an exhumed orogenic terrane

oxygen and carbon isotope profiles in metasediments from lizzies basin east humboldt range nevada constraints on mid crustal metamorphic and magmatic volatile fluxes 18o 16o ratios vary systematically in the 1 000 m section of interlayered metasediments and granitoids at lizzies basin the deepest structural level exposed in the east humboldt range metamorphic core complex in the lower 300 m of the section the lower zone 18o is homogeneous and low 6 6 to 8 8 in silicate rocks 8 7 to 12 1 in marbles a detailed oxygen isotope profile across an individual marble layer within the lower zone has a similar range in 18o 9 5 to 11 9 with the highest values preserved in the middle of the layer in the upper 700 m of the section the upper zone metasediments have been less strongly 18o depleted the 18o values are higher and more heterogeneous and in a profile across a marble layer of similar thickness rise steeply from marginal values of 12 to core values of 23 quartz from silicate metasediments throughout the upper zone ranges from 11 to 13 and is thus fairly homogeneous particularly where detailed profiles have been measured adjacent to the 18o rich marble layers covariation of 18o 16o and 13c 12c ratios in marbles suggests that the isotopic composition of these elements has been altered by exchange with infiltrating water rich co2 h2o fluids with mantle like isotopic composition in most cases marble cores preserve protolith 13c values and these vary systematically throughout the section according to structural level including some exceptionally 13c rich values up to 12 this range may reflect stratigraphic variation in 13c of the proterozoic sedimentary protoliths of these rocks the upper lower zone boundary and the contrast in isotope systematics to either side has been previously explained either as an impermeable barrier to fluid flow or as an infiltration front wickham and peters 1990 the second alternative has been tested using a numerical model in which low 18o aqueous fluid flows vertically upward through an alternating sequence of monomineralic quartz 18o 12 and calcite 18o 22 layers in a regime in which the oxygen isotopic composition is controlled by advection diffusion in the fluid and fluid solid exchange solutions to the relevant transport equations indicate that the abrupt change in the average 18o value of the layers at the upper lower zone boundary can be reproduced by the model with a uniform porosity throughout the system but the observed contrast in the shapes of the marble layer profiles in the upper and lower zones cannot be reproduced under these conditions however if the calcite layers have two orders of magnitude lower porosity in comparison with the quartz layers and exchange within them is diffusion dominated as opposed to advection dominated in the quartz the contrasting shapes of the marble profiles in the two zones are reproduced as well as the upper lower zone discontinuity the range of conditions that generates both an infiltration front at the appropriate scale and contrasting marble profiles at the appropriate scale is quite narrow but requires a volatile flux that could be generated by plausible volumes of mantle derived magma crystallizing at depth beneath the area providing support for this mechanism as a viable agent of 18o depletion in the deep level rocks of this terrane

style and age of late oligocene early miocene deformation in the southern stillwater range west central nevada paleomagnetism geochronology and field relations paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late oligocene to early miocene episode of large magnitude extension in the southern stillwater range and adjacent regions of west central nevada the southern stillwater range was the site of an oligocene to early miocene volcanic center comprising 1 28 3 to 24 3 ma intracaldera ash flow luffs lava flows and subjacent plutons associated with three calderas 2 24 8 to 20 7 ma postcaldera silicic dikes and domes and 3 unconformably overlying 15 3 to 13 ma dacite to basalt lava flows plugs and dikes the caldera related luffs lava flows and plutons were tilted 60 70 either west or east during the initial period of cenozoic deformation that accommodated over 100 extension directions of remanent magnetization obtained from these extrusive and intrusive caldera related rocks are strongly deflected from an expected miocene direction in senses appropriate for their tilt a mean direction for these rocks after tilt correction however suggests that they were also affected by a moderate 33 4 11 8 component of counterclockwise vertical axis rotation paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24 8 to 20 7 ma postcaldera dikes and domes in detail an apparent decrease in rotation with decreasing age of individual isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24 4 and 24 2 ma the onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked deformation was not driven by magma buoyancy however because tilting equally affected the caldera systems of different ages including their plutonic roots it is more likely that regional extension was focused in the southern stillwater range due to magmatic warming and reduction of tensile strength of the brittle crust faults that accommodated deformation in the southern stillwater range initially dipped steeply and cut deeply to expose more than 9 km of crustal section the exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early miocene brittle ductile transition

some observations on the interpretation of 40ar 39ar age spectra several recent applications of the 40ar 39ar age spectrum technique have utilized the ability of this technique to resolve the internal ar isotope distribution in crystals for the purpose of placing constraints on the timing of thermal events additionally a few of these studies have attempted to use certain properties of the age spectrum to estimate the temperature and duration of heating experienced by the sample during the disturbance although these successful applications are encouraging the interpretation of age spectra of disturbed samples is in many ways still an art form with the beholder apparently relatively free of constraints that might limit an assessment in fact there are a number of restrictions on interpretation imposed by 1 multiple episodic loss 2 slow cooling 3 mixed phases 4 resolution of the age spectrum 5 excess 40ar 6 recoil artifacts 7 grain distribution and size and 8 phase changes while in the vacuum extraction system these effects are discussed with a view to understanding possible ambiguities in ar release patterns in most cases only with independent geochronological knowledge an understanding of the release systematics and complete sample characterization can these constraints be successfully dealt with in a quantitative framework

homogenization and lowering of 18o 16o in mid crustal rocks during extension related magmatism in eastern nevada the oxygen isotope systematics of tertiary volcanic rocks of east central nevada and of plutonic and metamorphic rocks of the ruby mountains east humboldt range core complex provide complementary evidence for major18o depletion and 18o 16o homogenization of mid crustal rocks during metamorphism and magmatism the 18o value of crustal source material for silicic volcanic rocks decreased from between 9 and 11 to between 7 and 8 over 5 ma mid crustal metasedimentary and granitic rocks in the east humboldt range have 18o values very similar to the volcanic rocks and values are lower and more homogeneous at deeper structural levels exchange with deep seated mantle derived igneous rocks or fluids derived therefrom is the most plausible18o depletion mechanism intrusion of these mafic magmas promoted crustal melting and fluid migration homogenization of 18o 16o resulted from migration of high temperature fluids and melts at mid crustal levels and was less effective at higher structural levels where the crust was dominated by less permeable carbonate rocks

heterogeneous and asymmetric crustal accretion new constraints from multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 36 15 n at slow spreading mid ocean ridges crustal accretion style can vary significantly along and across ridge segments in magma poor regions seafloor spreading can be accommodated largely by tectonic processes however the internal structure and formation mechanism of such highly tectonized crust are not fully understood we analyze multibeam bathymetry and potential field data from the rainbow area of the mid atlantic ridge 35 40 n 36 40 n a section of the ridge that shows diverse accretion styles we identify volcanic tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain t and the fraction of magmatic accretion m estimated t values range from 0 2 0 4 on ridge segments to 0 6 0 8 at the rainbow nontransform discontinuity ntd at segment ends t is asymmetric reflecting asymmetries in accretion rate topography and faulting between inside and outside offset corners detachment faults have formed preferentially at inside corners where tectonic strain is higher we identify at least two oceanic core complexes on the fossil trace of the ntd in addition to the rainbow massif which occupies the offset today a gravity high and low magnetization suggest that the rainbow massif which hosts a high temperature hydrothermal system was uplifted by a west dipping detachment fault asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis which may have caused emplacement of magma into the footwall hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep penetrating second generation faults

interpretation and tectonic implications of cooling histories an example from the black mountains death valley extended terrane california abstract in the death valley extended terrane of california the black mountains have long been considered unique because they largely lack the miogeoclinal cover rocks characteristic of the surrounding ranges fission track ages presented here are combined with published40ar 39ar ages and used to construct cooling path envelopes for samples of precambrian crystalline basement and miocene plutonic rocks collected across the entire range the cooling history reconstructions are used to differentiate between contrasting miocene unroofing histories proposed for this range apatite and zircon fission track ages from the southeastern portion of the range suggest unroofing occurred there at 13 8 5 ma from temperatures well below 300 c cooling age data from the central black mountains indicate major unroofing at 8 5 6 0 ma from temperatures greater than 300 c old cooling ages from directly beneath the highly extended amargosa chaos rocks are consistent with the chaos rocks being part of an allochthonous slice that was tectonically transported from high crustal levels onto deeper crustal levels scenarios for the miocene unroofing history of this range rely heavily on interpretations of the depth of emplacement of miocene plutons in the core of the range thermochronologic and geobarometric data and thermal modeling of intrusion cooling suggest emplacement of an 11 6 ma pluton into the crystalline core at a depth of 10 15 km both the cooling age data and considerations of the local geology seem to preclude an unroofing history dominated by erosion of the overlying miogeoclinal section the morphology of the cooling path envelopes constructed here are similar to those constructed for detachment fault terranes the data are most consistent with unroofing involving tectonic denudation 10 15 km along a single westerly dipping detachment zone diachronous rapid cooling from southeast to northwest within the range is interpreted as a result of the lower plate undergoing flexural deformation as it pulls out from underneath a relatively rigid scoop shaped hanging wall block the rolling hinge model migration of unroofing and tilting within the range mimics the overall east to west sequential tilting and unroofing of range blocks in the death valley extended terrane similar thermochronologic evidence for sequential range scale tilting has been obtained from the lake mead extended region suggesting that this style of extension might provide an explanation for strongly extended domains in the basin and range province

shallow seismic reflection profiling over a mylonitic shear zone ruby mountains east humboldt range metamorphic core complex ne nevada seismic reflection profiling carried out with a sledgehammer source has imaged tertiary extensional structures over a depth range of 45 500 m within lower plate rocks of the ruby mountains east humboldt range metamorphic core complex the 400 m cmp profile straddles an exposed contact between tectonic slices of dolomitic marble and metaquartzite emplaced by low angle ductile brittle normal faulting subhorizontal reflections from layering within the tectonic slices give way at 160 ms 160 220 m depth to reflections that dip 15 45 to the east in contrast with dips indicated in a poorly imaged segment of a coincident regional seismic line but in agreement with dips of foliation mapped for nearby up plunge exposures of a late proterozoic early cambrian sequence of metaquartzites marbles schists and granitic rocks that forms the bulk of the underlying shear zone differences with the regional profile are attributed to the higher frequencies 30 100 hz generated by the smaller hammer source and the enhanced lateral resolution provided by the straighter profile and much smaller shot receiver offsets 46 157 m contributing to the stack for each cmp the results suggest that the near surface east dipping component of the anastomozing shear zone extends at least 2 km farther east than previously interpreted rough estimates of interval velocities 1500 4500 m s inferred from stacking velocities are consistent with velocities of mylonitic rocks measured perpendicular to foliation at low confining pressures when the effects of macroscopic fractures and joints are taken into account peaks in amplitude spectra of stacked traces suggest long wavelength components of layering resolved at scales from 5 8 m depth 50 m to 15 25 m depth 500 m

thermochronologic evidence of major tectonic denudation associated with detachment faulting northern ruby mountains east humboldt range nevada fission track ft thermochronology studies on igneous and metamorphic rocks from the ruby mountains east humboldt range metamorphic core complex provide important constraints on the timing and nature of major mid tertiary extension in northeast nevada rocks from within the ruby east humboldt detachment brittle ductile normal sense shear zone were analyzed the dominant lithology studied was variably mylonitic mafic orthogneiss nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the oligocene circa 36 ma harrison pass pluton were also studied ft ages on apatite zircon and sphene except one 18 4 ma apatite from the basal portion of the detachment zone are concordant and range in age from 26 5 ma to 23 6 ma all ages overlap at 1 between 25 4 ma and 23 4 ma these data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above 285 c sphene closure temperature to below 70 c lower temperature limit of track stability in apatite near the beginning of the miocene minimum cooling rate of 40 c ma the length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly fish canyon ash flow tuff the lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval but apparently only cooled to a temperature within the zone of partial annealing of apatite 70 130 c residence of these rocks in this zone led to a partial retention of fission tracks thus resulting in a reduced age confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history remained hotter longer than structurally higher levels of the zone ft data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23 4 ma rapid cooling of the region is considered to reflect large scale tectonic denudation intracrustal thinning the vertical complement to crustal extension rocks originating in the middle crust 10 15 km were quickly brought near the surface along the ruby east humboldt detachment fault brittle ductile simple shear zone and juxtaposed against brittlely extended rocks deformed under upper crustal conditions

dating the detachment fault system of the ruby mountains nevada significance for the kinematics of low angle normal faults 1 the mechanics of low angle normal faulting and metamorphic core complexes continue to be a subject of debate we investigate the conditions timing and kinematics of slip in the late upper crustal stages of core complex evolution of the ruby mountains detachment fault at the well exposed secret pass locality with an x ray diffraction xrd and ar ar study of clay gouge samples from three separate faults two from the low angle detachment system and one from a high angle normal fault that soles into the main detachment fault system xrd analysis and modeling of xrd analysis show that authigenic illite rich illite smectite i s in gouge at secret pass is distinguishable from clay phases in hanging wall rocks because the i s in the gouges contains only one water layer as opposed to the more common two water i s phases found in both the hanging wall and footwall ar ar ages for the monomineralic one water i s found in the hanging wall high angle fault the main detachment and a low angle normal fault structurally above the main detachment are 11 6 0 1 ma 12 3 0 1 ma and 13 8 0 2 ma respectively the not quite flat ar ar spectra indicate the gouge illites grew over some interval of time and not in discrete events the nearly overlapping ages indicate that gouge formation and thus the last major period of activity on the detachment were at 11 13 ma and were active coevally as part of a kinematically linked fault system with the main detachment active at dips 45 and possibly as low as 22

40ar 39ar geochronology and p t t paths from the cordillera darwin metamorphic complex tierra del fuego chile ar 39ar data collected from hornblende muscovite biotite and k feldspar constrain the p t t history of the cordillera darwin metamorphic complex tierra del fuego chile these data show two periods of rapid cooling the first between c 500 and c 325 c at rates 225 c ma and the second between c 250 and c 200 c for high t cooling 40ar 39ar ages are spatially disparate and depend on metamorphic grade rocks that record deeper and hotter peak metamorphic conditions have younger 40ar 39ar ages sillimanite and kyanite grade rocks in the south central part of the complex cooled latest 40ar 39ar hbl 73 77 ma ms 67 70 ma bt 68 ma and oldest kfs 65 ma thermobarometry and p t path studies of these rocks indicate that maximum burial of 26 30 km at 575 625 c may have been followed by as much as 10 km of exhumation with heating of 25 50 c staurolite grade rocks have intermediate 40ar 39ar ages hbl 84 86 ma ms 71 ma bt 72 75 ma and oldest kfs 80 ma thermobarometry on these rocks indicates maximum burial of 19 26 km at temperatures of 550 580 c garnet grade rocks have the oldest ages ms 72 ma and oldest kfs 91 ma peak p t conditions were 525 550 c and 5 7 kbar regional metamorphic temperatures for greenschist facies rocks south of the beagle channel did not exceed c 300 325 c from 110ma to the present although the rocks are only 2 km from kyanite bearing rocks to the north one dimensional thermal models allow limits to be placed on exhumation rates assuming a stable geothermal gradient of 20 25 c km i the maximum exhumation rate for the st grade rocks is c 2 5 mm yr i whereas the minimum exhumation rate for the ky sil grade rocks is c 1 0 mm yr uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades and so early differential exhumation is inferred to have occurred petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c 20 km of overburden this behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid to deep crustal levels the p t r data from cordillera darwin 1 are best explained by a wedge extrusion model in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the magallanes sedimentary basin 2 suggest that differential exhumation occurred initially with st grade rocks exhuming faster than ky sil grade rocks and 3 show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions

models of ocean ridge lithospheric deformation dependence on crustal thickness spreading rate and segmentation we use three dimensional 3 d temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid ocean ridge lithospheric structure we find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along axis gradients in crustal temperature and a pita pocket shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle in contrast fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis the overthickened crust produced by ridge hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling creating an extremely weak lithosphere in a steady state model we apply a simple two dimensional 2 d cyclic faulting model to across axis sections of temperature and rheological structure slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing in contrast fast spreading and hotspot affected segments are predicted to have much smaller variations in faulting styles except in the immediate vicinity of a major transform fault the presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness spreading rate and the ability of rift bounding normal faults to propagate along axis for fast spreading and hotspot affected segments the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma chamber supported isostatic features this is in contrast to most slow spreading segments where axial rift valleys are predicted to be a dominant morphological feature for a slow spreading segment of great length however the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center

an oxygen isotope discontinuity in high grade rocks of the east humboldt range nevada recent studies of metamorphic terranes document sharply contrasting 18o 16o systematics1 some regions are isotopically homogeneous but with decreased 18o 16o ratios see for example refs 2 4 whereas others preserve steep gradients in oxygen isotope composition5 7 these results indicate that some terranes experience very large pervasive fluid fluxes whereas others may at least locally remain virtually fluid absent here we report a very detailed oxygen isotope study of a high grade metamorphic terrane revealing striking systematic variations that are closely correlated with structural depth in particular metamorphic rocks that are low in 18o first detected at deep structural levels in a preliminary study of the area8 are now shown to be restricted to a clearly defined zone several hundred metres thick and extending for at least 3 km laterally the most striking feature is the existence of a very sharp oxygen isotope discontinuity marking the upper boundary of the low 18o zone and separating it from an isotopically heavier and more heterogeneous although still high grade overlying region

late cretaceous ductile deformation metamorphism and plutonism in the piute mountains eastern mojave desert late cretaceous deformation in the piute mountains involved ductile thrusting on a network of anastomosing northeast and southwest directed shear zones the shear zones often separate proterozoic lithologies with strong competency contrasts suggesting that proterozoic crustal anisotropies were important in controlling the geometry of the shear zone network shear zones now divide the crust into wedge and lozenge shaped blocks the piute mountains can be separated into two kinematic domains each characterized by a dominant sense of shear thrusting in the southwest directed kinematic domain ended at 85 7 ma the age of the late synkinematic east piute pluton thrusting in the northeast directed kinematic domain could have lasted until 74 3 ma the age of the lazy daisy pluton postthrusting northwest southeast shortening produced a second generation of upnght open folds with a northeast striking subvertical crenulation cleavage f2 strain increases toward the south and is concentrated around the margins of the lazy daisy pluton this deformation was synchronous with pluton emplacement late cretaceous peak metamorphism outlasted all ductile deformation peak metamorphic grade increases from upper greenschist facies in the north to upper amphibolite facies as the lazy daisy pluton is approached temperature and pressure of approximately 450 c and 2 5 4 0 kbar were achieved during ductile thrusting temperatures of 500 540 c existed at the onset of upright open folding and peak temperatures as high as 620 c near the lazy daisy pluton occurred after f2 upright folding all penetrative deformation ended before 72 71 ma by which time the area had cooled to below 300 c foster et al 1989

two and three dimensional thermal modeling of a low angle detachment exhumation history of the simplon fault zone central alps 1 two alternative models have been proposed to explain footwall exhumation along major low angle detachments 1 crustal scale exhumation along a detachment fault that maintained a low dip angle or 2 exhumation along a high angle fault passively rotated by isostatic rebound rolling hinge model these proposed models were tested against a well documented example of a low angle detachment fault in the european central alps the simplon fault zone sfz an extensive thermochronological data set provides the basis for 2 and 3 d thermokinematic models pecube coupled with a stochastic inversion algorithm the neighbourhood algorithm model results establish that the thermochronological pattern is better reproduced by a low angle detachment that maintained a 30 dip rather than by a rolling hinge model although a range of histories involving either steady state or variable exhumation rates is possible the preferred model of highest probability is for a variable rate with the fault zone initiated at 18 5 2 5 ma and active until the present day footwall exhumation was relatively fast until 14 5 1 5 ma 1 4 mm yr 1 this enhanced sfz footwall exhumation is similar in timing and kinematics to orogen parallel extension reported throughout the alpine orogen after 14 5 ma sfz footwall exhumation continued at a reduced rate 0 7 mm yr 1 until 4 ma the subsequent increase to 1 mm yr 1 reflects enhanced regional erosion rates across both footwall and hanging wall after circa 4 ma from 0 35 0 15 mm yr 1 to 0 70 0 15 mm yr 1 probably in response to climate changes during the pliocene

recognition of extraneous argon components through incremental release 40ar39ar analysis of biotite and hornblende across the grenvillian metamorphic gradient in southwestern labrador 40ar39ar incremental release ages have been determined for 15 hornblende and 20 biotite concentrates separated from rocks collected across the garnet and kyanite zones of grenvillian metamorphism in southwestern labrador most hornblende spectra from the kyanite zone are slightly discordant with low temperature increments yielding ages older than the ca 1000 ma date suggested for culmination of grenvillian metamorphism in the area however all the hornblende concentrates record well defined plateau ages these range from 968 to 905 ma across the kyanite zone and date times of diachronous post metamorphic cooling the discordant spectra are interpreted to result from low temperature liberation of excess 40ar components from grain margins two hornblende concentrates from the garnet zone display very discordant spectra total gas ages of 2100 and 3017 ma in which incremental dates systematically decrease during analysis this pattern of discordance suggests that excess argon components are inhomogeneously distributed throughout these hornblende grains most biotites from the garnet and kyanite zones record total gas or plateau ages in excess of 1000 ma 2066 857 ma reflecting the widespread presence of excess argon components because most of the 40ar39ar age spectra are internally concordant the ratios of excess 40ar relative to radiogenic 40ar must have been uniform in the various gas fractions liberated from each sample this is also reflected in the inability of isotope correlation diagrams to differentiate between excess radiogenic and atmospheric argon components the biotite total gas or plateau dates show marked local variation this is interpreted to indicate that the biotite grains were in contact with a post metamorphic intergranular vapor phase that was characterized by large and variable 40ar36ar ratios such ratios most likely resulted from widespread diffusion of the argon liberated from adjacent archean basement gneisses during the grenvillian metamorphic overprint

major late miocene cooling of the middle crust associated with extensional orogenesis in the funeral mountains california the funeral mountains of the death valley region consist of a metamorphic core complex that underlies the miocene boundary canyon detachment fault fissiontrack age determinations on sphene zircon and apatite from the highest grade portions of the funeral mountains collected in monarch canyon indicate rapid cooling from above 285 c at 9 10 ma the steep dt dt slope implied by the cooling path envelope as well as its synchroneity with development of the boundary canyon detachment suggests tectonic unroofing of between 9 5 and 17 km since 10 ma assuming typical geothermal gradients prior to detachment development of between 20 30 c km previous studies report muscovite 40ar 39ar plateau ages from the same area indicating that the funeral mountains core may have cooled below 350 c between 110 55 ma these data when combined with the fission track ages suggest residence of the metamorphic terrain at 285 350 c from late cretaceous to late miocene time recent studies using petrologic constraints have suggested that much of the unroofing of the funeral mountains as well as development of tectonite fabrics occurred during mesozoic extension recognition of temperatures in the 300 c range during miocene time suggests some of the ductile extensional fabrics may be late miocene in age

40ar 39ar incremental release ages of biotite from a progressively remetamorphosed archean basement terrane in southwestern labrador abstract gneisses within an archean basement terrane adjacent to the southwestern portion of the labrador trough were variably retrograded during a regional metamorphism of grenville age ca 1000 ma biotites from non retrograded segments of the gneiss terrane record 40 ar 39 ar plateau and isochron ages which date times of cooling following an episode of the kenoran orogeny 2376 2391 ma a suite of gneiss samples displaying varying degrees of retrograde alteration was collected across the grenville metamorphic gradient biotites in these samples show no petrographic evidence of retrograde alteration however they do record internally discordant 40 ar 39 ar age spectra although the extent of internal discordance is variable the overall character of the release patterns is similar with younger apparent ages recorded in intermediate temperature gas fractions the total gas dates range from 2257 27 ma northwest to 1751 23 ma southeast suggesting that variable quantities of radiogenic argon were lost from the archean biotites during grenville metamorphism the saddle shaped nature of the discordant spectra indicates that argon loss was not accomplished through single stage volume diffusion processes biotites in portions of the gneiss terrane which were completely recrystallized during grenville metamorphism are petrographically and texturally distinct a representative of this phase records a 40 ar 39 ar plateau age of 2674 28 ma this date is markedly inconsistent with regional constraints on the timing of grenville metamorphism and indicates the presence of extraneous argon components both the extraneous and radiogenic argon components must have been liberated in constant proportions during experimental heating because the argon isotopic data yield a well defined 40 ar 36 ar vs 39 ar 36 ar isochron corresponding to an age 2658 23 ma similar to that defined by the plateau portion of the spectrum the 40 ar 39 ar biotite dates suggest that the effects of grenville metamorphism extent 15 20 km northward into the superior province the limit of this overprint is approximately coincident with the northernmost development of grenville age thrust faults in the archean terrane therefore it is proposed that the northern margin of the grenville province in southwestern labrador should be located along the northernmost grenville thrust fault because this represents both a structural and a thermal discontinuity

thermal evolution of a portion of the sevier hinterland the northern ruby mountains east humboldt range and wood hills northeastern nevada conventional thermobarometry and gibbs method modeling have been used to obtain new information pertaining to the mesozoic tertiary thermal evolution of the ruby mountains east humboldt range and wood hills metamorphic complexes northeastern nevada kyanite grade assemblages from clover hill in the extreme northeastern part of the east humboldt range were produced at temperatures of 780 810 k and depths in excess of 35 km previously published geochronologic data suggest an early cretaceous or late jurassic age for this event the clover hill area subsequently experienced a major unroofing interval prior to late cretaceous time during which it was brought roughly 20 km closer to the surface the lack of stratigraphic evidence for substantial cretaceous erosion in the region and the topology of the pressure temperature paths for the clover hill samples suggest that the unroofing mechanism may have been tectonic denudation related to the relaxation of large topographic gradients produced by crustal thickening in the hinterland of the sevier orogen metamorphic rocks at clover hill and within the wood hills core complex equilibrated at 820 900 k and 500 640 mpa 18 24 km in cretaceous time prior to 115 ma other parts of the ruby mountains east humboldt range metamorphic complex presently structurally separated from the clover hill sequence were intruded extensively by late cretaceous granitic magmas and attained substantially higher temperatures than the clover hill wood hills block sillimanite grade assemblages from these areas apparently underwent roughly 10 km of unroofing and 130 k of cooling prior to final equilibration at 820 920 k and 360 430 mpa 13 16 km during oligocene extension

chronology of polyphase extension in the windermere hills northeast nevada fission track and 40 ar 39 ar dating and chemical correlation of volcanic strata exposed in the windermere hills and northern pequop mountains northeast nevada indicate a protracted polyphase history of tertiary late eocene late miocene extension along the northern margin of a major cordilleran metamorphic core complex early extension is recorded by a west tilted half graben filled with early oligocene 34 79 0 18 39 18 0 12 ma sedimentary rocks in the eastern windermere hills above the lowangle black mountain detachment fault the early oligocene half graben conformably overlies a widespread suite of late eocene 39 18 0 12 40 38 0 06 ma calc alkaline volcanic rocks reflecting a temporal link between early extension at a high structural level and the end of the ignimbrite flare up these strata are cut by east west striking normal faults which are exposed along and parallel to the northern margin of the metamorphic complex available age data e g between 14 93 0 08 and 34 79 0 18 ma permit the interpretation that the east west striking faults formed at the same time as or after large magnitude unroofing of highgrade rocks we interpret the east west striking faults to accommodate differential uplift of greenschist grade metamorphic rocks in the upper crust above a lateral ramp in a west northwest directed mylonitic shear zone subsequent extension in the windermere hills is defined by deep rapidly filled half grabens of middle miocene 7 42 2 0 to 14 93 0 08 ma age that unconformably overlie older faults and synextensional deposits these are the youngest half grabens in the region and are inferred to be initiated by extensional stresses imparted to the base of the lithosphere by a laterally spreading mantle plume e g the yellowstone hotspot located in southeastern oregon at this time

coeval extensional shearing and lateral underflow during late cretaceous core complex development in the nigde massif central anatolia turkey 1 the nigde massif at the southern tip of the central anatolian crystalline complex consists of two structural units foliations in the lower unit define a dome cored by migmatites the contact between the two units bears all the elements of a ductile to brittle extensional detachment hence the nigde massif represents an extensional metamorphic core complex top to ne ene shearing at higher levels of the lower unit relates to displacement along the detachment deeper levels of the lower unit display high temperature top to ssw ductile shearing the two shearing deformations show a difference in the mean trend of stretching lineations of up to 58 new 40ar 39ar ages combined with previously published data enable us to infer that the two shears were contemporaneous in our favored interpretation oblique shearing in the core of the dome reflects lateral underflow i e horizontal flowing of the lower crust in a direction highly oblique to the direction of extension as a result of the interaction between lateral underflow and downdip shearing along the overlying detachment distinct structural domains are expected to exist within the migmatitic part of the core complex with observed counterparts in the nigde dome lateral underflow may reflect inward flow on the scale of the core complex regional scale channel flow is an alternative that would better account for the record of non coaxial deformation in the core of the dome more generally we suspect that the development of lateral underflow in a metamorphic core complex more likely reflects regional channel flow rather than local inward flow

crustal thickness map of the western united states by partitioned waveform inversion the partitioned waveform inversion method nolet 1990 was used to obtain a new crustal thickness map of the western united states we fitted 200 seismograms containing higher and fundamental mode rayleigh waves for events and stations in the western united states which provided average crustal thickness constraints between each source receiver pair all these constraints were combined in a linear inversion to obtain a crustal thickness map local crustal thickness estimates from published seismic reflection refraction work were used as additional constraints to overcome deficiencies of path coverage in some areas the resulting crustal thickness map shows thicker 45 km crust beneath the cascades thick 40 km crust beneath the sierra nevada thin 30 km crust beneath the basin and range and thick greater than 45 km crust beneath the colorado plateau the boundary between the colorado plateau and the southern leg of the basin and range is a prominent feature of the model thick crust is seen at the bend of the san andreas fault near los angeles our model is broadly consistent with the crustal thickness map of mooney and weaver 1989 a quantitative comparison with the crustal thickness model 3smac nataf and ricard 1996 was performed by inverting for a model that deviates least from it results show that we are able to retain most of the features of that model while maintaining a good fit to the surface wave and point constraint data

thermal constraints on the tectonic evolution of a metamorphic core complex santa catalina mountains arizona abstract we present an analysis of the thermal evolution of the metamorphic core complex of the santa catalina rincon and tortolita mountains in south arizona the structural and kinematic data are briefly reviewed granite mylonites result from three successive shearing events respectively compressional top west during paleocene and top ne during mid eocene and extensional top sw after eocene in a preliminary part of the paper available radiometric data are reviewed in detail for both deformed and undeformed granites with closing temperatures from 565 to 105 c these data are then used to constrain the temperature history for three intrusions which were emplaced respectively during paleocene eocene and miocene times the general temperature time curve shows 1 rapid cooling for the three intrusions just after emplacement and 2 a large and rapid cooling of the entire complex between 35 and 22 ma from 530 to 100 c in the final part this thermal history is used to constrain the tectonic history of the metamorphic core complex the thermal effects of erosion tectonic denudation homogeneous and heterogeneous thinning are computed using a finite difference scheme in one dimension an analysis of the parameters involved in the models shows that the crust of the santa catalina metamorphic core complex has undergone mainly a thinning of its upper part of at least 15 km and more reasonably 20 25 km in less than 10 ma this estimation is discussed considering thermal effects of magmatism

alternating contraction and extension in the hinterlands of orogenic belts an example from the raft river mountains utah combined macroscopic to microscopic structural analyses detailed geologic mapping and thermochronology were used to study the structural evolution of midcrustal rocks of the sevier orogenic belt hinterland in northwestern utah these results when combined with previous studies provide new insight into the structural history of this region and suggest alternating tectonic contraction and extension during mesozoic to early cenozoic time two allochthons form the upper plate of the miocene raft river detachment fault in the eastern raft river mountains the lower allochthon comprises neoproterozoic ordovician and pennsylvanian strata and is bounded below by the raft river detachment fault and above by the middle detachment fault strata within the lower allochthon were dramatically attenuated by two episodes of ductile deformation the first deformation d1 took place at metamorphic temperatures of 500 c and resulted in penetrative fabrics throughout these rocks that record combined flattening and top to northeast shearing strains the second deformation d2 resulted in significant stratigraphic attenuation along discrete top to the west shear zones that are generally parallel to lithologic contacts separates of synkinematic muscovite from the penetrative fabric yield 40ar 39ar cooling ages that indicate that d1 deformation occurred prior to cooling ca 90 ma both fabrics were subsequently folded about d3 kilometer scale recumbent folds the areally extensive middle allochthon composed chiefly of pennsylvanian and permian rocks metamorphosed in the greenschist facies was emplaced d4 along the low angle middle detachment fault this fault cuts across various structural levels of the recumbently folded lower allochthon in its footwall and juxtaposes greenschist facies over amphibolite facies metamorphic rocks the lower and middle allochthon were subsequently deformed d5 into open folds with north trending axes neogene extension d6 produced an 200 m thick top to east ductile shear zone in precambrian rocks and formed the younger raft river detachment fault which forms the present upper contact of the ductile shear zone northeast vergent d1 fabrics probably record shortening deformation on the basis of fabric correlations with the grouse creek and albion mountains deformation kinematics and synkinematic prograde metamorphism d2 attenuation faults have been interpreted to record crustal extension of late cretaceous age if d3 recumbent folds developed during extension then the deformation sequence records early shortening followed by protracted extensional unroofing with variable structural styles the favored alternative is that d3 recumbent folds developed during shortening in this case d1 through d4 record episodic alternations of contraction and extension such alternations are consistent with observations from analog and theoretical models of contractional mountain belts and suggest that the hinterland of the sevier orogenic belt underwent dynamic adjustments in crustal thickness and deformation kinematics in response to changes in the boundary conditions of the orogenic belt