velocity structure of the kapuskasing uplift northern ontario from seismic refraction studies a crustal scale seismic refraction experiment was conducted over the kapuskasing structural zone northern ontario in 1984 the zone cuts obliquely across the east west structural grain of the superior province in the canadian shield and has been proposed as a cross section of archean crust exposed by thrust faulting along the ivanhoe lake cataclastic zone during early proterozoic time five seismic refraction lines of 360 450 km were shot over the area there were 18 profile shots and two fan shots with a recorder spacing of 2 5 km we have modeled the travel times and amplitudes of the data from the profile lines and analyzed reflections from the crust mantle boundary on the fan shots we have imaged a low velocity zone under the abitibi greenstone belt ranging from 4 5 to 9 12 km depth that is underlain by a highly reflective zone there is a considerable deepening of the moho from 40 43 km to 50 53 km under and to the west of the southern end of the kapuskasing structural zone a high velocity anomaly of 6 6 6 7 km s has been imaged in the upper crust down to 20 km depth beneath the kapuskasing structure with a suggested dip of 15 2 to the west this corresponds well to the proposed location of a granulite zone thrust up from the middle or lower crust in the early proterozoic

from orogenic hinterlands to mediterranean style back arc basins a comparative analysis hinterland plateaux and mediterranean style back arc basins both form behind active subduction zones or collisional megathrusts and share many characteristics 1 early crustal thickening to about twice normal continental thickness 2 thin lithospheric mantle 3 mixed magmatism including asthenospheric lithospheric and crustal melts 4 late stage horizontal extension accompanied by vertical shortening horizontal extension and vertical shortening are driven by high gravitational potential energy gpe contrasts between the hinterland and surrounding lithosphere which may reach 7 10 12 n m 1 equivalent to about 2 5 times the ridge push force if extension is rapid relative to the rate of lithospheric cooling gpe may remain positive even as extension continues declining to the mid ocean ridge value as crustal thickness approaches zero this suggests that hinterland plateaux could ultimately evolve into oceanic back arc basins the rate direction and amount of extension and the rate of vertical shortening depend on the plate boundary conditions and the gpe of the surrounding lithosphere vertical shortening in tibet is limited by work required to deform the surrounding asian lithosphere whereas mediterranean back arc basins can extend at the expense of regions of thin continental or oceanic crust

experimental study of subsolidus phase relations and mixing properties of pyroxene in the system cao al2o3 sio2 abstract subsolidus phase relations in the system cao al2o3 sio2 cas were experimentally determined with tight reversals of several univariant curves and with 14 equilibration experiments containing the assemblage pyroxene anorthite where pyroxene is a binary solid solution of ca tschermak cats caal2sio6 and ca eskola caes ca0 5alsi2o6 endmembers reversals were obtained on the following reactions bar c 3an gr 2ky q p 22t 700 3an cor gr 3ky p 21 8t 950 3cats gr 2cor p 55t 53900 and 6cats 1 x caesx 2 1 2x gr 4 1 2x cor 9xan observed slopes indicate 9 8 j mol k of al si disorder in ca tschermak pyroxene and 5 3 j mol k of al si disorder in anorthite at 1300 c it is suggested that al si disorder in anorthite increases by 1 9 j mol k from 700 c to 1300 c compositions of cats caes pyroxene in equilibrium with anorthite and pbo rich liquid were experimentally determined at 1400 1430 c and 22 7 30 8 kbar microprobe measurements gave compositions which are consistent with an ideal pyroxene solution and the following parameters for the reaction 3an 2cats 2caes j bar k 2rtln xcats xcaes 60200 86 4t 5 06 13 10 7p p 0 resulting in h0j 39 8 kj mol and s0 461 8 j mol k for the ca eskola endmember at 1300 c the obtained properties of the ca eskola component are necessary for thermobarometry based on pyroxene bearing assemblages containing plagioclase quartz or kyanite

role of plutonism in low pressure metamorphic belt formation wickham and oxburgh1 recently proposed that low pressure high temperature low p high t metamorphism in the eastern pyrenees and possibly all low p high t metamorphic belts resulted from anomalously high mantle heat flow brought about by rifting their model is largely constrained by the presence of nearby synmetamorphic rift related sedimentary rocks and the interpretation that the migmatites and granites are the product of in situ melting in the presence of an anomalously steep geotherm here we present an alternative model in which low p high t metamorphism pro grade reactions at pressures near or below the al2sio5 triple point results from contact effects near sill like igneous intrusions at intermediate crustal levels low p high t conditions can be achieved through this process in regions of continent continent collision with normal mantle heat flux as well as in zones of extension our model is based on studies of the low p high t metamorphic terrane in the new england appalachians

ferrobasalts from the spiess ridge segment of the southwest indian ridge highly vesicular microporphyritic basaltic rocks have been dredged from the slow spreading spiess ridge segment of the southwest indian ridge all the samples recovered are hyalocrystalline with plagioclase clinopyroxene and olivine as phenocryst and microphenocryst phases titanomagnetite occurs as euhedral microphenocrysts in some of the more evolved samples in terms of bulk rock and quench glass chemistry the lavas are characterised by highly evolved compositions e g feo 10 3 14 2 tio2 2 0 3 4 k2o 0 50 1 1 mgo 6 0 3 5 zr 160 274ppm nb 14 32ppm and can be classified as ferrobasalts isotopic and incompatible element ratios of the lavas e g 87sr 86sr 0 70325 0 70333 zr nb 8 4 11 3 y nb 2 3 1 4 indicate their strongly enriched nature see also dickey et al 6 quantitative major and trace element modelling indicates that most of the compositional variations observed can be attributed to low pressure fractional crystallisation of plagioclase clinopyroxene and minor olivine and titanomagnetite the range in composition can be accounted for by up to 65 fractional crystallisation we suggest that the extreme differentiation of the spiess ridge lavas is related not to spreading rate but to rate of magma supply the basaltic melts appear to have evolved in a newly established zone of magmatic activity associated with the most recent northward jump of the bouvet triple junction where they were effectively isolated from significant admixture of primitive magmas

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites central anatolian crystalline complex turkey plagiogranites associated with the sarikaraman ophiolite of the central anatolian crystalline complex turkey closely resemble other plagiogranites from supra subduction zone type ophiolites of neotethys the ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite ca 10 by volume than is usually associated with such bodies the sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex the plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber both the deep seated trondhjemites and the volcanic rhyolites constitute the sarikaraman plagiogranite suite geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites which are characterised by morb normalized low hfs element contents with small negative nb ta anomalies and variably enhanced lil element abundances unlike other plagiogranites however the sarikaraman suite is not characterized by consistently low k2o contents a feature that reflects the variable mobilization of the lil elements under lower greenschist facies conditions the ree are uniformly enriched relative to the basic components of the complex but have similar normalized patterns exhibiting mild light ree depletion in terms of their origin the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization 70 85 of clinopyroxene feldspar amphibole or partial melting 5 15 batch melting of a gabbroic source material although only the first process can produce most of the range of the plagiogranite compositions as a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole

geologic thermobarometry of retrograded metamorphic rocks an indication of the uplift trajectory of a portion of the northern scandinavian caledonides kyanite grade pelitic schists from the efjord area 68 15 n 16 45 e northern norway show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the scandinavian caledonides graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure temperature conditions three element partitioning geothermometers and geobarometers yield consistent pt estimates in individual samples which together define a trend in pt space the sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates whereas samples exhibiting abundant reequilibration textures give lower pt estimates we suggest that the trend indicated by these samples reflects a portion of the pt trajectory assumed by the efjord area during uplift thus the judicious use of element partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains

reprocessing and reinterpretation of cocorp southern appalachian profiles abstract reprocessing of cocorp southern appalachian data was focused on basic seismic evidence for continuation of sediments beneath a master decollement the most important evidence is a nearly continuous series of subhorizontal reflections extending from the valley and ridge province into the piedmont province continuity of subhorizontal reflections becomes tenuous in the inner piedmont careful reprocessing has yielded evidence for termination of strong reflections beneath the allochthon and the beginning of a relatively weak and complex series of events termination of sedimentary rocks beneath the piedmont is interpreted from true amplitude seismic data a zone of detachment continues southeast of the sediment termination as far as the master decollement root zone research on stacking velocities has indicated that complex velocity structures could create apparent low stacking velocities this phenomenon may occur in the charlotte belt of georgia bouguer gravity can be modeled as a former craton of normal density with an accreted margin of very slightly higher density variation in crustal thickness also contributes to the bouguer gravity gradient no continuous large scale overthrust is needed southeast of the interpreted master decollement root zone located beneath the kings mountain belt and charlotte belt

origin of titanian pargasite in gabbroic rocks from the northern apennine ophiolites italy insights into the late magmatic evolution of a mor type intrusive sequence abstract gabbroic rocks from northern apennine ophiolites contain accessory titanian pargasite in interstices between plagioclase and clinopyroxene and as rims around interstitial fe ti oxide phases the origin of titanian pargasite has been evaluated by combining major volatile and trace element microanalyses titanian pargasites show variable amounts of f and low cl 0 03 0 23 and 0 03 wt respectively and mg value ranging from 0 78 to 0 70 and from 0 61 to 0 53 in mg and fe rich rocks respectively geothermometric calculations based on amphibole plagioclase equilibrium yield temperatures of 900 50 c and 840 50 c for mg and fe rich rocks respectively titanian pargasites are characterized by lree depletion nearly flat hree and variable negative eu anomalies total ree contents in titanian pargasites are higher and lower than in associated clinopyroxenes and apatites respectively in the chondrite normalized patterns of titanian pargasites ba k and sr are markedly depleted relative to lree whereas nb zr and ti are slightly enriched to slightly depleted relative to neighboring ree a separate of titanian pargasite from a fe rich gabbroic rock was analyzed for sr isotopic composition its initial 87sr 86sr falls within the range of modern n morb and is consistent with those of fresh mg rich gabbroic rocks titanian pargasite most likely formed by an igneous liquid with relatively high h2o content 4 4 6 1 wt and a slight lree enrichment the origin of this liquid has been ascribed to the percolation in the gabbroic crystal mush of a h2o rich agent of igneous origin which could be a trondhjemite liquid or an exsolved fluid probably such interaction triggered a post cumulus crystallization process that finally yielded the precipitation of titanian pargasite

crustal extension and magmatic processes cocorp profiles from death valley and the rio grande rift new crustal scale information on the interaction between normal faulting and magmatic activity is provided by recent cocorp deep seismic reflection profiles in death valley california and by reprocessing of the cocorp data from the socorro area of the rio grande rift new mexico the most striking feature on the seismic sections from these areas is a prominent subhorizontal series of reflectors at mid crustal depth previous studies have suggested that thin tabular magma bodies lie within the mid crustal reflective zones in addition because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the cocorp data these mid crustal horizons are here inferred to be detachments or zones of tectonic decoupling upper crustal cenozoic faults do not appear to penetrate deeper than 15 km in death valley and 13 km in the rio grande rift in death valley these faults are relatively planar with moderate dips 20 to 35 and appear to bound large basement rocks one such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central death valley to the surface location of a 690 000 yr old basaltic cinder cone listric and low to moderate angle normal faults are evident on the reprocessed new mexico data and constitute the structural component of upper crustal extension in particular a listric master fault traceable to depths as great as 13 km is inferred to underlie the albuquerque basin unlike the death valley data no faults are observed to merge with the socorro magma body per se rather subhorizontal to moderately west dipping packages of reflections are imaged between the base of the faulted upper crust 13 km depth and the mid crustal magma body about 20 km depth the middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion events seen in the middle and lower crust are generally subhorizontal and prominent layering is observed a band of reflections attributed to the crust mantle boundary is evident on most seismic sections the upper mantle appears seismically transparent on some of the profiles the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections at least one of which is a layer of magma this association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes

crustal structure of eastern nevada from cocorp deep seismic reflection data the western nevada segments of the cocorp 40 n deep seismic reflection survey of the north american cordillera reveal the geometry of structures of cenozoic and possibly earlier ages to travel times of 10 s corresponding to depths of 30 km the most striking feature of the data is a band of prominent reflections typically at traveltimes of 9 5 to 10 5 s that are present discontinuously across the entire data set few reflections are observed from beneath the base of this reflective zone which is interpreted as the crust mantle transition this reflection mono is inferred to be continuous across the survey area varying gradually in depth but without resolvable offsets it appears to have taken its present form or position during basin range crustal extension the middle to lower crust in much of the survey area is characterized by discontinuous reflections that are typically subhorizontal and locally dip gently west these reflections may represent intrusions or shear zones related to basin range or pre basin range extension but some are likely to be inherited from earlier compressional deformation reflections from the upper crust are interpreted as images of basin fill strata basin range normal faults and mesozoic and paleozoic thrusts related to back arc thrusting and accretion of oceanic and arc related rocks

pressure temperature time paths from two dimensional thermal models prograde retrograde and inverted metamorphism two dimensional thermal models of time transitive crustal thickening and subsequent unroofing in large scale overthrust terrains generate pressure temperature time ptt paths that generally resemble those produced by earlier one dimensional instantaneous models but that differ in detail these differences in ptt path morphology are most pronounced proximal to major fault zones where postthrusting geotherms are characterized by large temperature inversions in one dimensional models but generally lack inversions in two dimensional models tests using the two dimensional fault model developed here indicate that 1 burial rate proportional to dp dt not thrust fault geometry dip angle controls the topology of synthrusting pt paths but plays only a minor role in determining the maximum temperature that rocks attain during the later unroofing stage in their thermal histories 2 normal ranges of thrusting and erosion rates and fault parameters result in pt paths with the usual sense clockwise on conventional pt diagrams counterclockwise in our diagrams 3 the amount of heating and duration of heating following the end of thrusting are a function of the rate of unroofing dp dt during this period 4 fast unroofing rates lead to the attainment of lower maximum temperatures after greater amounts of unroofing 5 the initial thermal state of the lithosphere prior to thrusting has a profound effect on pt path morphologies and on the peak metamorphic conditions attained by samples 6 for excess heat distributed across the entire lithosphere e g due to increased background thermal gradients a plot of peak temperatures experienced by metamorphic rocks versus structural depth tmax plot closely represents the initial geotherm 7 excess heat in the crust only e g increased radioactive heating in a layer yields a different result with the tmax plot corresponding to the initial geotherm only near the top of the hanging wall tmax plots for both 6 and 7 show no temperature inversion which exceeds the nominal uncertainties 50 k for geothermometric data 8 shear heating can lead to significant temperature inversions at the fault zone if the frictional coefficient is 0 6 or greater and 9 simultaneous thrusting and erosion produce pt loops significantly narrower than those resulting from sequential thrusting and erosion suggesting that any formulation which fails to account for some degree of simultaneity between thrusting and erosion represents a far endmember model forward models like those presented here provide important guidelines for understanding the sensitivity of metamorphic ptt paths to various thermal mechanical and geometric factors related to tectonism but they are generally inappropriate for reconstructing metamorphic thermal histories from actual petrologic and geochronologic data analytical inversion techniques that use a postthrusting thermal regime consistent with two dimensional forward models and that integrate values of dp dt dt dt and radioactive heating rates extracted from suites of metamorphic rocks provide the best hope for furthering our understanding of the thermal evolution of metamorphic terrains

mid crustal cretaceous roots of cordilleran metamorphic core complexes thermobarometry for cretaceous to mid tertiary plutonism and deformation in the lower plate of whipple and santa catalina metamorphic core complexes shows that both crystalline terranes originated in the middle crust moreover they are characterized by a striking acceleration of tectonic decompression coincident with middle tertiary low angle detachment faulting leading to erosional and tectonic unroofing by mid miocene time depth estimates for emplacement of five intrusive suites within the whipple complex include 1 33 4 km for the peraluminous 89 ma whipple wash plutonic suite 2 29 1 km for the 73 ma axtel quartz diorite 3 16 5 km for mylonitization and synkinematic plutonism at 26 ma 4 6 2 1 9 km for the postkinematic 19 ma war eagle gabbro quartz diorite complex and 5 5 2 2 3 km for a 17 ma postkinematic granodiorite decompression initially occurred at a low rate of 0 3 mm yr from 89 to 26 ma and increased to approximately 2 mm yr during the late oligocene to middle miocene estimated depths for four pluton emplacement or deformational events in the santa catalina mountains include 1 21 1 km for the magmatic epidote bearing 68 ma leatherwood quartz diorite 2 15 3 km for the garnet two mica 47 ma wilderness granite 3 9 3 1 9 km for post wilderness mylonitzation and 4 6 3 2 6 km for the 27 ma catalina monzogranite post laramide decompression estimated at 0 3 mm yr accelerated to 1 3 mm yr prior to the cessation of detachment faulting whereas most batholithic terranes of the north american cordillera are representative of an upper crustal setting core complexes provide as a consequence of their tectonic evolution a petrological and structural view into middle crustal processes

automated editing and true amplitude stacking of seismic data lateral changes in recording conditions often require that trace amplitudes be balanced with consequent loss of information on the lateral amplitude variation of reflected energy we present a quality control and automated editing algorithm that recognizes source and geophone coupling problems and different noise levels along the survey line problem traces are discarded and true amplitude stacking of the remaining ones is possible with constant scaling factors application of the algorithm to one of the vancouver island lithoprobe profiles gives a nominal signal to noise improvement of 15 db and a better understanding of recording problems in the field our analysis shows that the varying strength of a reflection from near the top of the subducting juan de fuca plate cannot be explained by changing recording conditions alone results suggest that the extra effort involved in the automated optimization of common midpoint stacks of low signal to noise deep seismic data is warranted only if lateral amplit

raising the colorado plateau shallow marine rocks exposed on the 2 km high 45 km thick colorado plateau in the western united states indicate that it was near sea level during much of the phanerozoic isostatic calculations however illuminate the difficulty in maintaining a 45 km thick crust at or near sea level we propose that an isostatically balanced 30 km thick proto colorado plateau crust was thickened during the late cretaceous to early tertiary by intracrustal flow out of an overthickened sevier orogenic hinterland this plateau would have been supported by a thick 70 km crustal root which is proposed to have been the source region for hot and weak mid crustal material that flowed eastward from the plateau toward the low elevation proto colorado plateau

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

crustal structure of north central nevada results from cocorp deep seismic profiling deep seismic reflection data which were collected by cocorp in central nevada at 40 n latitude reveal a horizontally laminated reflective lower crust and an upper crust containing prominent moderately dipping reflections from cenozoic lava flows basin fill normal faults older thrust faults and paleozoic sediments prominent moho reflections are seen at depths of 31 35 km in agreement with refraction based moho depths for this area the strongly layered character of the lower crust is best developed near the mid miocene northern nevada rift suggesting that cenozoic magmatism and ductile extension have played an important role in the development of this fabric east dipping reflections beneath southern grass valley in lander county may locally disrupt an otherwise continuous reflection moho

rise and tilt of metamorphic rocks in the lower plate of a detachment fault in the funeral mountains death valley california the funeral mountains in eastern california preserve a record of early cretaceous metamorphism followed by ductile deformation uplift and low angle normal detachment faulting 40ar 39ar age spectra indicate that cooling and uplift of the lower plate began in cretaceous time uplift was accommodated by normal sense movement along a wide northwest dipping shear zone mylonitic fabrics some of which have been dated as late cretaceous deformed older high temperature metamorphic textures analyses of shear bands mica fish and porphyroclasts grain shape fabrics and folds indicate that the upper surfaces moved toward 299 12 top to the northwest relative to lower surfaces uplift continued until the near present the youngest phase being accommodated by top to the northwest movement along the detachment fault which formed subparallel to lower plate mylonitic fabrics fission track apatite data indicate that exposure of the lower plate to the surface occurred sometime after 6 ma reconstruction along the movement vector places the grapevine mountains over the funeral mountains having been displaced at least 40 km isograds and thermobarometry in pelitic schist from the lower plate indicate increasing pressures and temperatures of equilibration toward the northwest the maximum temperature and pressure was determined on a sample from monarch canyon using thermobarometry 700 c at a depth of 32 km at chloride cliff 5 km southeast of monarch canyon 4 samples yielded 575 600 c at depths of 19 27 km at indian pass 17 km southeast of monarch canyon a temperature of 490 c was determined in the southern funeral mountains about 50 km southeast of monarch canyon conodont color alteration indexes indicate temperatures of 325 425 c these data indicate that the lower plate is presently tilted strongly to the southeast from the orientation it maintained at the peak of metamorphism thermochronologic data k ar on muscovite biotite and hornblende 40ar 39ar on hornblende and fission track on apatite titanite and zircon indicate that both tilting and the transition from ductile to brittle styles of quartz deformation are confined to the interval 21 6 ma during the latter part of this interval 11 6 ma rapid uplift and movement along the detachment fault are documented the findings support current theories of detachment fault evolution in which a dipping fault surface undergoes rotation to a subhorizontal orientation while the lower plate undergoes a comparable tilt

three dimensional laramide deformation of the colorado plateau competing stresses from the sevier thrust belt and the flat farallon slab 1 kinematic analysis of laramide basement cored uplifts on the northern colorado plateau indicate that four of the uplifts were constructed by northeast directed shortening while at least two were constructed by southeast directed shortening available timing data suggest that all of the uplifts rose at approximately the same time 72 50 ma this paper proposes that the different shortening directions are a predictable result of deformation in a three dimensional stress field where the magnitude of 2 approaches that of 1 the concept is supported by both angelier stereographic analysis and elastic modeling tectonically 1 is may have been the result of coupling between the horizontally subducting farallon slab and the north american plate the elevated values of 2 are attributed to far field stresses generated by the topographically high sevier thrust belt which wraps around the western edge of the colorado plateau and was active at the time of deformation in the plateau

zanskar shear zone northeast southwest extension within the higher himalayas ladakh india a complete geologic section through the northwestern himalayas is well exposed in ladakh india the collision between the indian shield and the tibetan platform caused large scale intracrustal thrusting and piling up of the himalayan nappes this compressional event was accompanied by prograde regional metamorphism the metamorphism ranges from low grade anchimetamorphism in the indus suture zone in the northeast to upper amphibolite grade in the higher himalaya tectonic unit in the southwest in the zanskar area a well exposed and morphologically prominent shear zone 2 25 to 6 75 km wide can be followed from the northwest to the southeast for at least 80 km in contrast to the main compressional tectonics this shear zone indicates an extensional event within the higher himalaya tectonic unit the zanskar shear zone involves granitic rocks leucogranites of miocene age and various metasediments and in general separates the late precambrian early cambrian sedimentary sequence phe formation from the underlying crystalline basement zanskar crystalline unit a normal sense of shear has been determined by using asymmetric macroscopic and microscopic structures such as pegmatite boudins feldspar augen and crystallographic fabrics the metamorphic isograds are very close within this shear zone as a result of shear deformation a gradual transition from upper amphibolite to lower greenschist facies occurs within 200 m movement in this zone occurred late in the metamorphic history under greenschist or lower metamorphic conditions the minimum horizontal extension and vertical displacement are on the order of 16 and 19 km respectively

physical properties of deep crustal reflectors in southern california from multioffset amplitude analysis analysis of reflection waveforms before stack can constrain the physical properties of reflectors in the deep crust to simplify this analysis recorded amplitudes are assumed to be reflections from weak elastic heterogeneities with these assumptions trends in reflection amplitudes with offset may indicate whether the signs of a reflector s density and rigidity contrast agree with or oppose the sign of its lame s parameter contrast the slope of the trend indicates the degree of poisson s ratio contrast no attempt is made to invert for the individual modulus or density contrasts by examining only gross amplitude versus offset avo trends deep reflections constrain some crustal properties two seismic reflection surveys in the mojave desert recorded deep reflections that show amplitude changes with offset both the 1985 calcrust ward valley survey in the eastern mojave and the 87 km cocorp mojave line 3 in the western mojave incorporate long offsets of 10 km or more prestack traces are equalized usi

seismic reflection coefficients from mantle fault zones summary several bright reflections from structures within the mantle can be seen on birps deep seismic reflection profiles we have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0 1 it is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle these large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite the brightest reflections seem to be best explained as major faults or shear zones within the mantle

experimental study of liquid evolution in an fe rich layered mafic intrusion constraints of fe ti oxide precipitation on the t f o 2 and t ϱ paths of tholeiitic magmas the newark island layered intrusion a composite intrusion displaying a similar fractionation sequence to the skaergaard has both dikes which preserved liquids fed into the intrusion and chilled pillows of liquids resident in the chamber this study reports experimentally determined one atmosphere liquid lines of descent of these compositions as a function of oxygen fugacity which varies from qfm quartz fayalite magnetite to 0 5 log10 units above iw iron wustite these experiments reveal a strong oxygen fugacity dependence on the order of appearance and relative abundances of the fe ti oxide minerals titanomagnetite saturates prior to ilmenite at qfm but the order is reversed at lower oxygen fugacities in the layered series of the newark island intrusion ilmenite arrives shortly before titanomagnetite and the titanomagnetite ilmenite ratio decreases monotonically after the cumulus appearance of titanomagnetite comparison of the crystallization sequence in the intrusion with that of the experiments requires that the oxygen fugacity in the intrusion increased relative to qfm before titanomagnetite saturation and decreased afterward but always remained between the qfm and iw buffers similar trends in the modes of the fe ti oxides ilmenite and titanomagnetite in the skaergaard kiglapait and somerset dam intrusions along with fe2o3 feo ratios in morbs suggest that such a temperature oxygen fugacity path may be typical of tholeiitic magma differentiation calculations of the temperature density paths of the experimental liquids indicate that at all possible oxygen fugacities the density must have decreased abruptly after fe ti oxide saturation accordingly liquids replenishing the intrusion after fe ti oxide saturation should pond at the bottom of the chamber quenching against older cumulates field observation at the newark island intrusion confirm this prediction the similarities in the fractionation paths of several other layered intrusions to that of the newark island intrusion suggest that the density of the liquids in these intrusions also decreased after fe ti oxide saturation experiments on a suggested initial skaergaard liquid are consistent with this model

limitations of quantitative analysis of deep crustal seismic reflection data examples from glimpce amplitude preservation in seismic reflection data can be obtained by a relative true amplitude rta processing technique in which the relative strength of reflection amplitudes is preserved vertically as well as horizontally after compensating for amplitude distortion by near surface effects and propagation effects quantitative analysis of relative true amplitudes of the great lakes international multidisciplinary program on crustal evolution seismic data is hampered by large uncertainties in estimates of the water bottom reflection coefficient and the vertical amplitude correction and by inadequate noise suppression processing techniques such as deconvolution f k filtering and migration significantly change the overall shape of amplitude curves and hence calculation of reflection coefficients and average reflectance thus lithological interpretation of deep crustal seismic data based on the absolute value of estimated reflection strength alone is meaningless the relative strength of individual events however is preserved on curves generated at different stages in the processing we suggest that qualitative comparisons of relative strength if used carefully provide a meaningful measure of variations in reflectivity simple theoretical models indicate that peg leg multiples rather than water bottom multiples are the most severe source of noise contamination these multiples are extremely difficult to remove when the water bottom reflection coefficient is large 0 6 a condition that exists beneath parts of lake superior and most of lake huron

extensional tectonics in the death valley area california transport of the panamint range structural block 80 km northwestward in the hypothesis presented here the panamint range is a structural block that was detached during late cenozoic time from underlying rocks that now form the black mountains and was transported tectonically an estimated 80 km to the northwest transport occurred along a single low angle westward dipping detachment fault or perhaps along a system of such faults the estimate of 80 km of transport distance is based on the apparent right lateral offset of late precambrian and paleozoic facies and thickness trends along the furnace creek fault zone that bounds the detached block on the northeast the death valley turtleback surfaces are considered to be gigantic mullions related to the detachment surface

a comparison of common midpoint single shot and plane wave depth migration a comparison of common midpoint cmp single shot and plane wave migration was made for simple two dimensional structures such as a syncline and a horizontal reflector with a laterally variable reflection coefficient by using synthetic seismograms the seismograms were calculated employing the finite difference technique cmp sections were simulated by 18 fold stacking and plane wave sections by slant stacking by applying a finite difference scheme the synthetic wave field was continued downward the usual imaging condition of cmp migration was extended in order to carry out migration of single shot and plane wave sections the reflection coefficient was reconstructed by comparing the migrated wave field with the incident wave field at the reflector the results are 1 all three migration techniques succeeded in reconstructing the reflector position 2 as a consequence of the finite aperture of the geophone spread only segments of the reflector could be reconstructed by single shot and plane wave

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

migration why doesn t it work for deep continental data summary conventional migration of deep seismic reflection data often produces disappointingly poor results even when the original unmigrated data are of high quality and are relatively noise free surprisingly deep data often subjectively appear to be best migrated at velocities which are up to 50 less than appropriate interval velocities derived from crustal refraction experiments or directly from stacking velocities the explanation for this behaviour is that near surface features distort and attenuate the seismic wave field and produce apparent discontinuities in deep reflections since these discontinuities are spurious they are not associated with the appropriate large diffractions which real discontinuities at depth would produce during the process of migration reflections are invented in order to cancel out the missing diffractions thereby producing a smiley section which appears overmigrated since the lateral extent of individual smiles increases with increasing migration velocity increasing two way time and increasing seismic wavelength the effect is almost unnoticeable on conventional shallow seismic data but is overwhelming for deep crustal data

crustal structure of the midcontinent rift system results from glimpce deep seismic reflection profiles interpretation of great lakes international multidisciplinary program on crustal evolution glimpce seismic reflection profiles indicates that the midcontinent keweenawan 1100 ma rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km about 10 5 s reflection time along profiles crossing western central and eastern lake superior and the northern end of lake michigan this area may overlie the greatest thickness of intracratonic rift deposits on earth times to moho reflections vary along strike from 11 5 to 14 s about 37 46 km depth in the west to 17 s about 55 km in the center and 13 to 15 s about 42 49 km in the eastern end of lake superior the prerift crust however was thinned 25 30 km beneath the central rift compared with its flanks providing evidence for crustal extension by factors of about 3 4 the midcontinent rift system differs from phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding presumably unextended regions

tectonomagmatic evolution of cenozoic extension in the north american cordillera the spatial and temporal distributions of cenozoic extension and magmatism in the cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt 100 km pre extension and followed the onset of intermediate and silicic magmatism by no more than a few million years extension began in early eocene time in southern british columbia northern washington idaho and montana farther s extension began at about the eocene oligocene boundary in the great basin and slightly later in the mojave sonora desert region the intervening area at the latitude of las vegas remained quiescent until mid miocene time compositional and isotopic characteristics of most pre miocene magmas are consistent with their containing major components of melted continental crust in mid miocene time two major changes occurred widening of the area of extension and the widespread appearance of basaltic magmas the area affected by extension from southwestern montana to the lake mead region widened to several hundred kilometres by this time extension in southern british columbia northern washington and northern montana had ceased probably before the end of the eocene and extension s of lake mead except in the gulf of california had waned regions affected by the broader belt of extension during late miocene pliocene and quaternary time experienced basaltic magmatism which began along a central rift zone in the northern part of the region and which had within a few million years spread to include most of the region later basaltic activity has tended to concentrate in restricted zones especially near the margins of the extended area we recognize a correlation between the amount of earlier crustal thickening and cenozoic extension and between the length of time after shortening but before extension and the degree to which a given region was intruded by late cretaceous plutons the localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal mechanical model developed in a companion paper sonder et al this raises the possibility that stresses inherent in the north american plate dominated over plate interaction forces as controls of the cenozoic tectonomagmatic evolution of the north american cordillera especially in its earlier stages

a new hypothesis for the amount and distribution of dextral displacement along the fish lake valley northern death valley furnace creek fault zone california nevada 1 the fish lake valley northern death valley furnace creek fault zone a 250 km long predominantly right lateral structure in california and nevada is a key element in tectonic reconstructions of the death valley area eastern california shear zone and walker lane and central basin and range province total displacement on the fault zone is contested however with estimates ranging from 30 to 63 km or more here we present a new synthesis of available constraints preextensional thrust faults folds and igneous rocks indicate that offset reaches a maximum of 50 km neogene rocks constrain its partitioning over time most offset is interpreted as 13 10 ma accruing at 3 5 mm yr in the middle of the fault zone and more slowly toward the tips the offset markers imply 68 14 km of translation between the cottonwood mountains and resting spring nopah range 60 14 km since 15 ma through a combination of strike slip and crustal extension this suggests that a previous interpretation of 104 7 km based on the middle miocene eagle mountain formation is an overestimate by 50 our results also help to mitigate a discrepancy in the 12 0 ma strain budget for the eastern california shear zone displacement has previously been estimated at 100 10 km and 67 6 km for the basin and range and mojave portions of the shear zone respectively our new estimate of 74 17 km for the basin and range is within the uncertainty of the mojave estimate

the pinto shear zone a laramide synconvergent extensional shear zone in the mojave desert region of the southwestern united states the pinto shear zone is one of several late cretaceous shear zones within the eastern fringe of the mesozoic magmatic arc of the southwest cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening we demonstrate an extensional origin for the shear zone by describing the shear zone geometry and kinematics hanging wall deformation style progressive changes in deformation temperature and differences in hanging wall and footwall thermal histories deformation is constrained between w74 and 68 ma by 40 ar 39 ar thermochronology of the exhumed footwall including multi diffusion domain modeling of k feldspar we discount the interpretations applied in other areas of the mojave desert region that widespread late cretaceous cooling results from refrigeration due to subduction of a shallowly dipping laramide slab or to erosional denudation and suggest alternatively that post intrusion cooling and exhumation by extensional structures are recorded widespread crustal melting and magmatism followed by extension and cooling in the late cretaceous are most consistent with production of a low viscosity lower crust during anatexis and or delamination of mantle lithosphere at the onset of laramide shallow subduction q 2005 published by elsevier ltd

the origin of oceanic plagiogranites from the karmoy ophiolite western norway both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the karmoy ophiolite of western norway the plutonic zone of this ophiolite can be subdivided into three complexes the east karmoy igneous complex the visnes high level complex and the veavagen igneous complex and plagiogranitic rocks are well developed in the first two of these within the east karmoy igneous complex plagiogranites are associated with high temperature pre basic dyke shear zones rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite hydrated diabase assuming a starting material consisting of 40 hornblende and 60 plagioclase and that batch melting occurred within the stability field of hornblende in comparison plagiogranite occurs in a number of bodies in the upper part of the visnes high level complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes microgabbros and magnetite gabbros and a floor assemblage of layered and non layered gabbros the r e e modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari textured gabbros although the distribution of highly hygromagmatophile elements such as k rb ba etc cannot be explained satisfactorily by this model alone depletion in these elements appears to be an autometasomatic effect

pre and poststack migration of glimpce reflection data abstract glimpce deep seismic reflection profiles across the midcontinent rift system beneath lake superior reveal a central asymmetric rift with an enormous thickness of volcanic and sedimentary rocks true amplitude cmp processing poststack and prestack migration and forward modelling are used to improve images of steeply dipping faults unconformities and other discontinuities in the deep seismic data with prestack migration important steeply dipping structural features of the lake superior data set are revealed for the first time improved images of high angle normal faults later reactivated as reverse faults provide key structural information for understanding the evolution of the rift system results illustrate that structural interpretations of complex deep reflection records such as those recorded by glimpce should always be based on migrated data furthermore depth migrated sections provide useful starting models for forward velocity modelling of seismic data

relative amplitude preserving processing for crustal seismic reflection data an example from western canada abstract most crustal scale seismic reflection profiles are processed using methods that preserve reflection geometries but not true relative amplitudes and therefore do not provide direct constraints on the magnitude of impedance contrasts at major crustal boundaries the effects of attenuation in the crust or the maximum time of signal penetration here we describe a new relative amplitude preserving rap processing scheme tailored for high fold land based acquisition systems that can be used to provide some of these constraints our processing philosophy employs a statistical treatment of amplitudes and is based on the following assumptions 1 temporal decay of recorded amplitudes is proportional to the product of zero offset geometrical spreading in a layered earth and attenuation based on a constant q model and 2 spatial variations in recorded amplitudes are surface consistent we apply this methodology to a profile from western canada where well logs from oil and gas exploration are available to calibrate observed seismic reflections using a spectral ratio method tailored for noisy data we obtain an estimate of 450 for apparent q in the sub phanerozoic crust signal penetration is frequency dependent and is estimated to decrease from 20 to 13 s over the frequency range of 10 to 40 hz application of our processing methodology leads to a stack section in which lower crustal reflectivity is much more prominent than reflectivity in the middle crust compared with conventional processing of the same data which yields a stack section characterized by conspicuous mid crustal reflections the p wave reflection coefficient for the moho is estimated to be about 0 1

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

jurassic barrovian metamorphism in a western u s cordilleran metamorphic core complex funeral mountains california large magnitude retroarc shortening of cretaceous age is well documented in the sevier orogenic belt of the western united states and has been associated with eastward franciscan subduction that began in the middle late jurassic but evidence for major late jurassic retroarc shortening has been lacking here we report new lu hf garnet geochronology pressure temperature p t paths and 40 ar 39 ar thermochronology data that document tectonic burial of late jurassic age in a barrovian metamorphic terrain the funeral mountains metamorphic core complex california located in the hinterland of the sevier orogenic belt the p t paths determined from growth zoned garnets in upper greenschist facies pelitic schist show steep p t trajectories consistent with metamorphism during thrust loading the age of thrust loading is constrained by a five point lu hf garnet isochron to be 158 2 2 6 ma 2 partial exhumation recorded in 146 153 ma 40 ar 39 ar muscovite cooling ages closely followed garnet growth late jurassic barrovian metamorphism has not been previously recognized in cordilleran metamorphic core complexes possibly due to being obscured by late cretaceous to tertiary deformation magmatism and metamorphism this study finds that the period of large magnitude crustal shortening in the retroarc extended into the late jurassic and may have closely followed the formation of the coherent orogenic system associated with east dipping franciscan subduction

variations in the reflectivity of the moho transition zone beneath the midcontinent rift system of north america results from true amplitude analysis of glimpce data true amplitude processing of the great lakes international multidisciplinary program on crustal evolution seismic reflection data from the midcontinent rift system of north america shows large differences in the reflectivity of the moho transition zone beneath the axial rift beneath the rift flanks and outside of the rift the moho reflection from the axial rift has a discontinuous diffractive character marginally stronger several decibels than an otherwise transparent lower crust and upper mantle beneath the axial rift moho is interpreted to be a synrift igneous feature beneath the rift flanks the reflectivity of the moho transition is generally well developed with two identifiable boundaries although in places it is weakly reflective to nonreflective similar to moho outside the rift the two boundaries are interpreted as the base of essentially intact although stretched prerift archean crust upper boundary and new synrift moho 1 2 s 6 7 km deeper lower boundary beneath the rift flanks the layered reflection moho transition results from the preexisting crustal composition and fabric modified by synrift igneous processes and extensional tectonic metamorphic processes the geologic evidence for extensive basaltic magmatism in the rift is the basis for interpreting the moho signature as a keweenawan structure that has been preserved for 1 1 b y extension and magmatism appear to enhance reflectivity in the lower crust and moho transition zone only where stretching factors are moderate rift flanks and not where they are extreme axial rift this leads to the prediction that the reflectivity across analogous volcanic passive continental margins should be greatest beneath the moderately stretched continental shelves and should decrease towards the ocean continent boundary

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

metamorphic evolution of exhumed middle to lower crustal rocks in the mojave extensional belt southern california usa the waterman metamorphic complex of the central mojave desert was exposed as a consequence of early miocene detachment dominated extension however it has evidence consistent with a more extensive geological history that involves collision of a crustal fragment s tectonic thickening by overthrusting and two periods of extension the metamorphic complex contains granitoid intrusives and felsic mylonitic gneisses as well as polymetamorphic rocks that include marble calc silicate quartzite mafic granulite pyribolite amphibolite migmatite and biotite schist the latter group of rocks was affected by an initial series of high grade metamorphic events m1 and m2 and a localized lower grade overprint m3 the initial metamorphism m1 can be separated into two stages along its high grade p t path m1a a granulite facies metamorphism at 800 850 c and 7 5 9 kbar and mlb an upper amphibolite facies overprint at 750 800 c and 10 12 kbar m1a developed mineral assemblages and textures consistent with granulite facies conditions at a reduced activity of h2o and is associated with intense ductile deformation d1 and minor local partial melting m1b overprinted the granulite assemblages with a series of hydrous phases under conditions of increasing pressure and h2o activity and is accompanied by little or no deformation m2 developed at lower pressures and temperatures 650 750 c 4 5 5 5 kbar and is distinguished by a second local overprint of hydrous phases that reflects an input of aqueous fluids probably associated with the intrusion of a series of granitic dykes and veins effects of m3 are confined to the mitchel detachment zone an anastomosing early miocene detachment fault and are characterized by local ductile brittle deformation d2 of the pre existing high grade rocks and granitoid intrusives and by the production of mylonites and mylonitic gneisses under greenschist facies conditions 300 350 c 3 5 kbar the initial overprint m1a represents metamorphism devolatilization and minor partial melting of supracrustal rocks under granulite facies conditions as a consequence of tectonic and possibly magmatic thickening the increasing pressure transition of m1a to m1b reflects a period of continued compressional tectonism thrusting and influx of h2o in part locally related to crystallization of partial melts the near isothermal decompression between m1b and m2 probably represents a pre 112 ma extensional episode that may have been the result of a decompressional readjustment of a thickened crust following the initial extensional event the metamorphic complex remained at depths of 10 17 km for at least 90 ma until it was uplifted following miocene extension m3 develops locally in response to this second extensional period resulting from the early miocene detachment faulting

death valley bright spot a midcrustal magma body in the southern great basin california a previously unrecognized midcrustal magma body may have been detected by cocorp deep seismic reflection profiles in the death valley region of the southern great basin high amplitude relatively broad band reflections at 6 s 15 km are attributed to partially molten material within a subhorizontal intrusion this bright spot extends laterally at least 15 km beneath central death valley a moderately dipping normal fault can be traced from the inferred magma chamber upward to a 690 000 yr old basaltic cinder cone the fault zone is inferred to have been a magma conduit during the formation of the cinder cone vertical variations in crustal reflection character suggest that the death valley magma body may have been emplaced along a zone of decoupling that separates a faulted brittle upper crust from a more ductile and or intruded lower crust the death valley bright spot is similar to reflections recorded by cocorp in 1977 in the rio grande rift where both geophysical and geodetic evidence support the inference of a tabular magma chamber at 20 km depth

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

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

thermal modeling of extensional tectonics application to pressure temperature time histories of metamorphic rocks one and two dimensional finite difference models are used to generate theoretical pressure temperature time ptt paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere pure shear and via movement of crustal blocks along rooted large scale low angle normal faults simple shear the temperature time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear and experience a greater amount of posttectonic cooling the depth temperature paths of rocks uplifted from deep crustal levels 30 km depth via pure shear extension are often characterized by a period of nearly isothermal high dp dt uplift during the early stages of extension a modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces ptt paths characterized by a more protracted period of nearly isothermal uplift in contrast simple shear uplift along low angle normal faults produces almost linear depth temperature paths with moderate dp dt and isothermal uplift is not observed for a given initial geotherm the single factor controlling the ptt paths of rocks unroofed below gently dipping normal faults that dip less than 30 is the rate at which unroofing takes place unroofing rate is defined as the rate of uplift relative to the surface and the syntectonic ptt paths are insensitive to fault dip or horizontal displacement rate current geothermometric and geobarometric techniques yield typical uncertainties in real pressure temperature and temperature time data of 1 kbar and 50 c thus it will be difficult to distinguish between these various extensional mechanisms on the basis of ptt data derived from a single sample or from a single structural level although use of data from samples collected from a broad range of structural levels may improve resolution ultimately the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature measurements to 0 5 kbar and 25 c

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

evidence for east west crustal extension in the eastern alps implications for the unroofing history of the tauern window the contact between the western border of the tauern window and the overlying austroalpine nappes austria and italy is marked by a structural petrologic and geochronologic discontinuity that developed in the early stages of unroofing of the window within the window pressure temperature time paths j selverstone 1985 indicate that significant ductile thinning occurred shortly after the cessation of thrusting paleocene eocene sense of shear indicators rotated porphyroblasts asymmetric augen and pressure shadows s c fabrics associated with a west plunging stretching lineation indicate that this thinning occurred in response to top to the west low angle normal shear during metamorphism the same sense of normal shear is present in the mesozoic members of the austroalpine sequence west of the window in these rocks however west directed shear occurred at low temperatures 300 c after the thermal peak of metamorphism structural data and the regional geometry imply that the western end of the tauern window is a low angle normal fault brenner line that juxtaposed brittlely deformed rocks of the austroalpine nappes against more ductilely deformed rocks from the window biotite k ar and rb sr data suggest a miocene age for final movement on the brenner line these data indicate a prolonged history of east west extension in the eastern alps that affected all crustal levels early ductile thinning of lower crust followed by mid to upper crustal low angle normal faulting a model is presented that relates the young east west extension to displacement transfer from dextral movement on the periadriatic lineament in this model the brenner line and by analogy the simplon line can be thought of as detachment faults that tectonically unroofed the tauern window and lepontine thermal highs respectively

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

petrologic constraints on the unroofing history of the funeral mountain metamorphic core complex california this is the published version copyright 1990 american geophysical union all rights reserved

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

a technique for analyzing the thermal and uplift histories of eroding orogenic belts a scandinavian example geothermometric and geobarometric data from pelitic schists in the upper plate of the forsa thrust in the northern norwegian caledonides are interpreted as recording part of the uplift trajectory of this area during erosion and slow cooling a method developed in this paper for reconstructing the thermal history of orogenic belts from such data is based on a general mathematical description of thermal conduction in a lithospheric slab with an arbitrary initial temperature distribution and uplifted at a constant rate with certain restrictions the uplift trajectory tz path of any rock parcel can be calculated from only a few temperature depth points along that uplift trajectory by applying this method to geothermometry and geobarometry data from near the forsa thrust we can 1 extrapolate tz paths measured over a limited depth range upward to the surface 2 recover geotherms present at various times during the uplift and erosion history of the mountain belt 3 obtain tz paths for rock parcels above and below those for which pt data are available and 4 use pt data in combination with geochronological data to obtain accurate estimates of uplift rates our analysis suggests that shortly after nappe emplacement the nappe stack was hot near 800 c in the lower part of the nappe stack while the underlying autochthon was cold near thermal equilibrium prior to nappe emplacement the maximum burial depth of rocks near the forsa thrust was probably between 35 and 60 km