wollastonite and scapolite in precambrian calc silicate granulites from australia and antarctica abstract scapolite wollastonite calcite diopside grossular andradite garnet and sphene occur in calc silicate rocks in the granulite terrain of the arunta block central australia this assemblage buffers the co2 activity at a low value so that any coexisting fluid phase must be h2o rich and co2 poor xco2 0 2 0 3 in contrast the h2o activity in the surrounding felsic and mafic granulites was low thus fluid activities during granulite facies metamorphism were locally buffered in various rock units and fluid flow appears to have been restricted or fluid may have been absent late retrograde rims of garnet and garnet quartz separate phases formed in the high grade stage formation of these rims would have required either an influx of water rich fluid or a decrease in pressure evidence from the surrounding granulites shows that in one locality the calc silicate rocks had undergone late isobaric hydration in another locality minor uplift had occurred soon after peak p t conditions in both scapolite had partly broken down to plagioclase calite a calc silicate rock from the granulite terrain of enderby land antarctica contains scapolite wollastonite calcite diopside quartz and sphene this assemblage also indicates low co2 activities in this rock wollastonite has broken down to calcite quartz to indicate isobaric cooling without influx of hydrous fluid

nd and sr isotopic study of crustal and mantle inclusions from the sierra nevada and implications for batholith petrogenesis sm nd and rb sr systematics have been examined for eight upper mantle and lower crustal xenoliths the xenolith bearing trachyandesite and the granodiorite intruded by the trachyandesite in the sierra nevada batholith isotopic heterogeneity is shown by the xenoliths with 143 nd 144 nd ranging from 0 51169 to 0 51306 and 87 sr 86 sr ranging from 0 7031 to 0 7333 these ranges are similar to those of the plutonic rocks of the batholith this similarity suggests that the xenoliths believed to be representative of mantle and crustal reservoirs may approximate the diverse source regions from which the varied granitic rocks of the batholith were derived and that the sr nd isotopic correlation seen in granitic rocks may not be due to simple mixing of two end member components the subgranitic crust in this part of the sierra nevada apparently consists of metamorphosed sedimentary and igneous rocks which are highly variable isotopically the mantle beneath the sierra nevada is also isotopically heterogeneous both vertically and laterally

226ra 230th excess generated in the lower crust implications for magma transport and storage time scales excesses of 226 ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs followed by fast ascent rates of magma from slab to surface up to 1000 m yr however we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle derived 226 ra excesses and so reduce inferred vertical transport rates we developed an incongruent continuous melting model and both the incongruent melting reaction and ingrowth effects contribute to the 226 ra excess in particular we found that dehydration melting of amphibolite can produce modeled 226 ra excesses greater than 300 mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 u excess subducted slab signature this amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements similar to those observed in several arc settings which may distinguish these magmas from those with 226 ra excesses produced by slab dewatering alone

lithologic control of the depth of earthquakes in southern california the depth distribution of southern california earthquakes indicates that areas underlain by schist basement rocks have a shallower 4 to 10 kilometers maximum depth of earthquakes than do areas with other types of basement rocks the predominant minerals in the schists become plastic at lower temperatures and thus at shallower depths than the minerals in the other basement rocks the lateral variations in lithology will control the depth extent and thus the magnitudes of potential future earthquakes these depths can be determined from the depth of the current background seismicity

oroclinal bending of the southern sierra nevada batholith structural magmatic and isotopic features of the southern sierra nevada batholith are deflected clockwise with respect to its central and northern parts directions of magnetization at three localities in the southern sierra nevada are progressively deflected this is consistent with the hypothesis that the region was tectonically rotated in an orocline no paleomagnetic deflection was observed northwest of the white wolf kern canyon fault system oroclinal bending ofa block bounded by the san andreas garlock and white wolf kern canyon faults may have occurred before about 16 x l06 years ago the deformation may have been a response to shear at the western boundary of the north american plate caused by oblique subduction

small scale heterogeneity of phanerozoic lower crust evidence from isotopic and geochemical systematics of mid cretaceous granulite gneisses san gabriel mountains southern california an elongate belt of mid cretaceous compositionally banded gneisses and granulites is exposed in cucamonga terrane in the southeastern foothills of the san gabriel mountains of southern california banded gneisses include mafic granulites of two geochemical types type 1 rocks are similar to high al arc basalts and andesites but have higher hfse high field strength element abundances and extremely variable lile largeion lithophile element abundances while type 2 rocks are relatively low in al and similar to alkali rich mor midocean ridge or intraplate basalts intercalated with mafic granulites are paragneisses which include felsic granulites aluminous gneisses marble and calc silicate gneisses type 1 mafic granulites and calcic trondhjemitic pegmatites also oceur as cross cutting synmetamorphic dikes or small plutons small scale heterogeneity of deep continental crust is indicated by the lithologic and isotopic diversity of intercalated ortho and paragneisses exposed in cucamonga terrane geochemical and isotopic data indicate that k rb and u depletion and sm nd fractionation were associated with biotite muscovite dehydration reactions in type 1 mafic granulites and aluminous gneisses during high grade metamorphism field relations and model initial isotopic ratios imply a wide range of protolith ages ranging from early proterozoic to phanerozoic

steep tilting of metavolcanic rocks by multiple mechanisms central sierra nevada california for 200 km along the eastern sierra nevada continental magmatic arc mesozoic metavolcanogenic rocks dip steeply to the southwest 80 a feature that must reflect fundamental processes in magmatic arc construction although tight folds can account for such steep bedding tilts folds in the metavolcanogenic sections are sparse and small scale we propose that the high bedding tilts were produced by a combination of thrusting downward displacement and ductile deformation of the beds the last two processes accompanied emplacement of the sierra nevada batholith the ritter range pendant lies within this 200 km belt and provides a relatively large and well exposed mesozoic volcanic section ranging in age from late triassic to mid cretaceous detailed mapping and ages from u pb zircon dates and fossils within the volcanic section reveal five structural blocks i v that are separated by bedding parallel thrusts some of which are cryptic to explain the present difference in bedding orientations between blocks iii and iv we suggest that the thrusting may have had a duplex geometry which produced a maximum bedding dip of 45 in some blocks downward displacement of wall rock and ductile strain account for the remaining 35 of the observed average bedding dip 80 sw the exact time of thrusting and duplex formation of late triassic to early jurassic rocks in blocks i iv is uncertain but these structures developed either 1 between 105 and 164 ma well before the other rotational processes were active or 2 mostly around 105 ma and closer to the time when other rotational processes were active much of the subsequent ca 91 76 ma bedding tilting is related to downward displacement of beds associated with the emplacement of voluminous late cretaceous plutons and to regional ductile deformation of the wall rocks during that period the majority of the tilting probably took place between ca 91 and 86 ma bedding tilts of early to mid cretaceous rocks in blocks iv and v is bracketed between ca 98 and ca 90 ma comparisons with metavolcanic sections to the northwest near tioga pass and to the southeast in the mount morrison mount goddard and oak creek pendants suggest that bedding rotation by thrusting downward displacement and ductile strain of wall rock may explain the steep dips along this entire 200 km segment of the continental arc similar mechanisms may operate at midcrustal levels in other continental arcs

transpressional shearing and strike slip partitioning in the late cretaceous sierra nevada magmatic arc california the rosy finch shear zone rfsz represents an example of synmagmatic strike slip tectonics in the east central sierra nevada and provides information on transpressional tectonics within magmatic arcs the rfsz 1 4 km wide and 80 km long is contained in late cretaceous granitoids 92 83 ma of the mono pass intrusive suite dextral strike slip deformation is indicated by a continuous band of orthogneiss mylonitic and cataclastic deformation characterized by subvertical foliations with subhorizontal lineations the style of dextral shearing in the rosy finch shear varies along strike from wide zones of ductile deformation in the youngest plutons to narrow zones of cataclastic deformation in the older plutons deformation of the youngest granitoids is synmagmatic as constrained by both field observations and isotopic studies dextral shearing occurs concurrently in contemporaneous intrusive suites to the north and south tuolumne and mount whitney suggesting strike slip movement along the axis of active plutonism field studies also suggest that contraction across the arc acted concurrently with strike slip movement consistent with a transpressional setting for the late cretaceous sierra nevada magmatic arc comparison of the field data with strain models of oblique plate convergence suggests that the rfsz is a preserved ductile signature of transcurrent motion of strike slip partitioning within a transpressional orogenic system recent plate reconstructions indicate a switch from sinistral to dextral oblique convergence at 95 ma which is consistent with the timing constraints on the dextral movement on the rfsz

precise determinations of the equilibria kyanite sillimanite and kyanite andalusite and a revised triple point for al 2 sio 5 polymorphs

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

the distribution of radiogenic heat production as a function of depth in the sierra nevada batholith california geochemical analyses and geobarometric determinations have been combined to create a depth vs radiogenic heat production database for the sierra nevada batholith california this database shows that mean heat production values first increase then decrease with increasing depth heat production is 2 aw m 3 within the 3 km thick volcanic pile at the top of the batholith below which it increases to an average value of 3 5 aw m 3 at 5 5 km depth then decreases to 0 5 1 aw m 3 at 15 km depth and remains at these values through the entire crust below 15 km below the crust from depths of 40 125 km the batholith s root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of 0 14 aw m 3 this is higher than the rates from most published xenolith studies but reasonable given the presence of crustal components in the arc root assemblages the pattern of radiogenic heat production interpreted from the depth vs heat production database is not consistent with the downward decreasing exponential distribution predicted from modeling of surface heat flow data the interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the sierra nevada could be generated within the 35 km thick crust this requires a very low heat flux from the mantle which is consistent with a model of cessation of sierran magmatism during laramide flat slab subduction followed by conductive cooling of the upper mantle for 70 m y the heat production variation with depth is principally due to large variations in uranium and thorium concentration potassium is less variable in concentration within the sierran crust and produces relatively little of the heat in high heat production rocks because silica content is relatively constant through the upper 30 km of the sierran batholith while u th and k concentrations are highly variable radiogenic heat production does not vary directly with silica content d 2005 elsevier b v all rights reserved

significance of composite lineations in the mid to deep crust a case study from the north cascades washington abstract the origin of broadly orogen parallel nw se mineral lineations in cretaceous and paleogene arc plutons and amphibolite facies metamorphic rocks paleodepths of

effects of diffusional modification of garnet growth zoning on p t path calculations the effect of intragranular diffusion on chemical zoning in garnet and on p t paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth syn and post growth diffusion of mg fe mn ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635 c compositions from zoned garnet as modified by diffusion hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure temperature p t paths from isobaric and polybaric model histories p t path calculations based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635 c show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path the magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition the effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating only models that reached peak temperatures approximately 15 30 c higher compositions of garnets with radii less than 1 mm that reached actual peak temperatures of 605 c along temperature time histories characteristic of regional metamorphism experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories path distortions were significant at heating cooling rates less than 10 c ma but rapidly diminished for rates faster than this in polybaric simulations diffusion effects were least noticeable when the actual pressure temperature conditions changed in a clockwise sense i e convex to higher p and higher t but apprecciable modification was seen in path models that underwent counterclockwise changes in p and t reequilibration of garnet rim compositions occurred during cooling on all paths and temperature maxima obtained from garnet biotite geothermometry underestimated actual peak conditions by 40 to 70 c calculations suggest that p t path trajectories calculated from garnets of at least 1 mm size and that experienced actual thermal maxima below 585 c are not likely to be distorted by diffusional effects during regional metamorphism however p t path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history the partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling seriously complicating the task of quantitatively estimating diffusion effects on path calculations

structures in the jackass lakes pluton host rock system central sierra nevada california and inferred mid cretaceous farallon north america plate kinematics

slab flattening trigger for isotopic disturbance and magmatic flare up in the southernmost sierra nevada batholith california the san emigdio schist of the southwestern sierra nevada batholith snb permits examination of partial melting and devolatilization processes along a late cretaceous shallow subduction zone detrital and metamorphic zircon of the structurally highest and earliest subducted portions of the san emigdio schist constrain the depositional age to between ca 102 and 98 ma zircon oxygen isotope data from both lower plate schist and upper plate batholithic assemblages reveal a 18 o shift of 1 5 between igneous 5 5 and metamorphic 7 domains these results taken with previous zircon and whole rock 18 o measurements provide evidence for massive devolatilization of the san emigdio schist and fluid traversal of upper plate batholithic assemblages thereby altering the isotopic composition of overlying material furthermore the timing of fluid rock interaction in the southwestern snb is coincident with eastward arc migration and an associated pulse of voluminous magmatism we posit that during flattening of the farallon slab the schist was rapidly emplaced beneath the magmatic source where ensuing devolatilization triggered a magmatic flare up in the southeastern snb this short lived 15 myr high flux event was followed by the termination of arc magmatism as the shallow subduction zone approached thermal equilibrium

the transition from magmatic to high temperature solid state deformation implications from the mount stuart batholith washington criteria for syntectonic emplacement of plutons are commonly ambiguous the strongest single criterion is probably the preservation of a continuous transition from submagmatic to high temperature solidstate deformation this transition is not commonly documented an exception is the mount stuart batholith which displays a variety of syntectonic structures within its sheared northeastern margin including the following 1 s c fabrics that range from submagmatic to high temperature solid state 2 solid state foliation and lineation defined by amphibolite facies assemblages that overprint and are parallel to magmatic foliation and lineation 3 folds of magmatic and solid state foliation that have hinge lines parallel to magmatic and solid state lineation and to equivalent structures in wall rocks 4 dikes that were folded and boudinaged over a wide range of rheological states with boudin necks locally filled by tonalite 5 pegmatite dikes and mineralized joints that are sub perpendicular to magmatic and solid state lineation and 6 submagmatic and high temperature solidstate ductile shear zones we suggest that many plutons emplaced during regional deformation do not preserve evidence for syntectonic deformation because of the transitory nature of the submagmatic state and the obscuring effects of postemplacement deformation syntectonic features are most likely preserved in plutons cooled at slow to moderate rates or in plutons deformed at high strain rates during emplacement the optimum conditions for preservation may occur in plutons emplaced along fault zones in the mid crust such as the mount stuart batholith and in intrusions at deeper levels that were rapidly exhumed and or intruded during the waning stages of regional deformation

protolith age of the swakane gneiss north cascades washington evidence of rapid underthrusting of sediments beneath an arc 1 the metamorphic core of the north cascades largely comprises island arc and oceanic terranes juxtaposed prior to circa 96 ma magmatism however the tectonic affinity of the structurally deepest terrane the 9 12 kbar swakane gneiss is distinctly different from other terranes in the core it is not intruded by arc related plutons and contains abundant precambrian zircons new u pb analyses of detrital zircons from the swakane gneiss yield dates from 73 ma to 1610 ma with a dominant late cretaceous population these data indicate that the swakane protolith was deposited as late as 72 5 0 6 ma the 206pb 238u date of the youngest detrital grain following deposition the gneiss was intruded by peraluminous leucogranite sheets that may represent partial melt derived from the gneiss at near to postpeak p t conditions one sheet yielded a u pb crystallization age of 68 36 0 07 ma which indicates that the gneiss was deeply buried within 5 myr of deposition two possible mechanisms considered for this rapid burial 7 mm yr are overthrusting of a forearc or back arc basin by older crystalline rock or underthrusting of trench sediments during low angle subduction the model involving overthrusting of a forearc or back arc basin is most consistent with thermobarometric and isotopic data and the regional geologic setting of the cascades core rapid burial of the swakane protolith is coincident with burial of sediments that formed the pelona orocopia and rand schists of southern california however differences in peak metamorphic temperatures indicate that conditions of burial must have varied along the plate margin

tectonomagmatism in continental arcs evidence from the sark arc complex abstract the island of sark channel islands uk exposes syntectonic plutons and country rock gneisses within a precambrian cadomian continental arc this sark arc complex records sequential pulses of magmatism over a period of 7 ma ca 616 609 ma the earliest intrusion ca 616 ma was a composite sill that shows an ultramafic base overlain by a magma mingled net vein complex subsequently deformed at near solidus temperatures into the amphibolitic and tonalitic tintageu banded gneisses the deformation was synchronous with d 2 deformation of the paragneissic envelope with both intrusion and country rock showing flat top to the south ls fabrics later plutonism injected three homogeneous quartz diorite granodiorite sheets the creux moulin pluton 150 250 m ca 614 ma the little sark pluton 700 m 611 ma and the northern pluton 500 m 609 ma similar but thinner sheets in the south derrible hogsback dixcart and west port es saies brecqhou are interpreted as offshoots from the creux moulin pluton and little sark pluton respectively all these plutons show the same ls fabric seen in the older gneisses with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling the cooling rate increased rapidly with decreasing crystallisation age 9 ma for the oldest intrusion to cool to lower amphibolite conditions 7 8 ma for the creux moulin pluton 5 6 ma for the little sark pluton and 2 the initiation of the d 2 event is suggested to have been a response to the intrusion of the tintageu magma which promoted a rapid increase in strain rate 10 14 s 1 that focussed extensional deformation into the sark area the increased rates of extension allowed ingress of the subsequent quartz diorite granodiorite sheets although strain rate slowly declined as the whole complex cooled during exhumation the regional architecture of syntectonic cadomian arc complexes includes flat lying sark type and steep guernsey type domains produced synchronously in shear zone networks induced by oblique subduction a pattern seen in other continental arcs such as that running from alaska to california

nd sr and pb isotopic characteristics of cretaceous intrusive rocks from deep levels of the sierra nevada batholith tehachapi mountains california the cretaceous gabbroic to granitic intrusive rocks of the tehachapi mountains were emplaced at depths of 25 30 km and thus afford a view of deep processes in the sierra nevada batholith they consist of the 115 ma tehachapi suite and the 100 ma bear valley suite new zircon u pb age data reveal the presence of the latter as far west as grapevine canyon the nd sr pb and o isotopic whole rock data and zircon pb inheritance patterns for the bulk of the suites suggest an origin by mixing between depleted mantle derived magmas and metasedimentary material with a substantial component of old continental material however this mixing is not evident in variations between isotopic ratios and chemical and lithologic parameters this implies that isotopic hybridization of magmas took place deeper than 30 km and that fractionation processes are likely responsible for the bulk of the chemical variation in this part of the sierra nevada batholith consideration of the isotopic data in the context of the sierra nevada batholith as a whole suggests that the well known east to west isotopic gradients in the batholith may reflect a change in the average isotopic character of the preintrusive framework rather than a change in amount of crustal component on the other hand the lack of areal gradients in sr and nd isotopic ratios in the main study area may indicate a lack of pronounced gradation at deep levels at least within the western batholith

mineralogy and pressure temperature time path of cretaceous granulite gneisses south eastern san gabriel mountains southern california mid cretaceous granulite gneisses crop out in a narrow belt in the cucamonga region of the south eastern foothills of the san gabriel mountains southern california interlayered mafic granulites and pelitic carbonate calc silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700 800 c regional deformation and formation of banded gneisses ceased by c 108 ma although mafic intermediate magmatism and high grade metamorphism continued locally as late as c 88 ma garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near isobaric cooling but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo chronological data it is more likely that the entire terrane remained at granulite facies p t conditions until 88 ma followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes uplift occurred between c 88 and 78 ma at rates of approximately 1 2 km ma 1 the geotectonic evolution of the cucamonga granulites is similar to mid cretaceous high p granulites in the sierra nevada and salinian block of central california late cretaceous uplift common to these granulites may provide an important tectonic link between dismembered mesozoic batholithic terranes in the california cordillera

metamorphism of the deepest exposed arc rocks in the cretaceous to paleogene cascades belt washington evidence for large scale vertical motion in a continental arc the swakane gneiss and the overlying napeequa complex in the north cascade range washington were metamorphosed and deformed during development of a cretaceous paleogene continental arc and are among the structurally deepest exposed rocks within the cordilleran arcs of north america peak metamorphic conditions in both the swakane gneiss and napeequa complex were c 640 750 c 9 12 kbar clockwise paths and widespread evidence for high p metamorphism in meta supracrustal rocks burial to 40 km document major vertical tectonic motion during arc construction and unroofing these and other moderately high pressure rocks in the north cascades coast mountains experienced a dramatically different tectonometamorphic history than metamorphic rocks within other cordilleran arcs the exhumed arc complexes of the sierra nevada and peninsular ranges are dominated by relatively low p metamorphic and plutonic rocks typically 6 kbar there is no evidence that the northern cordillera was thickened to a greater degree than these other belts suggesting that the greater magnitude of vertical motion in the cascades may have been related to exhumation mechanisms eocene extension in the northern cordillera vs erosional unroofing in the central and southern cordillera

regional variations in bulk chemistry mineralogy and the compositions of mafic and accessory minerals in the batholiths of california we define regional variations in mafic and accessory mineral assemblages and compositions and expand the current understanding of spatial variations in whole rock geochemistry in the batholiths of california in so doing we gain new insights into the nature of magmatic source rocks and mechanisms of magma generation in volcano plutonic arcs of active continental margins little studied metaluminous to strongly peraluminous granites containing fe rich biotite with log x mg x fe f oh and mn in biotite and amphibole increase on a regional scale from western i wc types to eastern i mc and i sc types parallel to eastward increases in incompatible elements and decreases in compatible elements in the plutons in contrast the belts of western i scr granites and eastern i wc quartz diorites and granodiorites disrupt the regional west to east systematics in both mineral and whole rock geochemistry spatial variations in the al content of amphibole are regional in scale and reflect pressures of pluton crystallization we conclude that significant previously unrecognized complexity exists in regional geochemical systematics in the california batholiths

is the southeast coast plutonic complex the consequence of accretion of the insular superterrane evidence from u pb zircon geochronometry in the northern washington cascades zircon u pb geochronometry of orthogneisses and plutons in the southwestern crystalline core of the north cascades coupled with fabric and textural studies of the orthogneisses plutons and their metamorphic host rocks indicates extensive synmetamorphic plutonism at 89 96 ma metamorphic mineral assemblages define a culmination composed of an axial kyanite sillimanite zone rimmed by lower grade zones high grade index minerals are typically syntectonic to posttectonic metamorphic fabrics are characterized by an orogen parallel northwest striking steep foliation that contains a subhorizontal stretching and mineral lineation interpreted to be the product of ductile strike slip deformation this fabric is crosscut by 96 92 ma plutons yet is imprinted on 92 89 ma orthogneisses suggesting spatially diachronous fabric development during orogeny documentation of the spatial and temporal coincidence of magmatism with the peak of orogeny together with the kinematic significance of the metamorphic fabric precludes generation of the metamorphic fabric and plutons in response to thrust loading the authors suggest that this part of the coast plutonic complex evolved as a transpressional magmatic arc

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

lateral displacement during neogene convergence in the western and central alps although major deformation in the western alps is clearly the result of north northwest directed thrust tectonics there is evidence suggesting that orogen parallel deformation may have been important in the late tectonic history of the western alps the authors propose that this northeast trending deformation includes 1 southwest directed normal fault movement along the simplon line 2 a diffuse zone of northeast striking dextral strike slip deformation along the rhone valley in switzerland between the mont blanc and aiguilles rouges massifs and through the belledonne massif and 3 southwest directed thrusting in the embrunais ubaye and digne nappe systems of southeastern france the correlation of these three broad regions of deformation is based on similar amounts of minimum displacement consistent kinematics and timing of deformation

tremolite and h 2 o activity attending metamorphism of hornblende plagioclase garnet assemblages estimation of the activity of tremolite component atr in calcic amphiboles is an important problem in igneous and metamorphic petrology because equilibria involving tremolite are used in the estimation of the activity of h2o attending crystallization of igneous and metamorphic rocks estimated values of atr from hornblende analyses using both ionic and coupled substitution crystalline solution models can be compared to values of atr calculated from vapor absent mineral equilibria in addition these values of atr can be used in calculations of ah2o for rocks for which there is an independent estimate of ah2o the values of atr calculated from vapor absent equilibria are generally consistent with those estimated from the different crystalline solution models but uncertainties in the calculations preclude choosing a preferred solution model from computed mineral equilibria it is clear that mineral assemblages with low values of atr can be in equilibrium with high values of ah2o consequently the low values of ah2o estimated from hornblende bearing high grade rocks with low values of atr may be real rocks from mica creek british columbia probably achieved extremely low values of ah2o by vapor absent metamorphism

fold controlled quartz textures in the pennine mischabel backfold near zermatt switzerland abstract quartz fabric analyses along the basal thrust of the dent blanche nappe and in the footfall pennine units indicate south directed thrusting for the last stage of austro alpine pennine nappe emplacement quartz fabrics in particular those from the pennine mischabel backfold imply complex spatial and temporal texture developments with crystallographic preferred orientations cpo that are commonly in disagreement with mesoscopic grain shape lineations in consequence kinematic directions inferred by quartz textures do not necessarily coincide with those predicted by mesoscopic structural elements such as grain shape lineations in our particular case orientation of grain shape lineations is rather complex in the study area while principal axes inferred from the quartz textures remain geographically constant and exhibit a simple geometric relationship to the mischabel backfold it is suggested that the grain shape lineations correspond to the total accommodated finite strain whereas the quartz fabrics reflect late increments of deformation quartz cpos in the hinge of the mischabel fold are similar to those along the thrust revealing asymmetric single girdles to c axes point maxima subgrain rotation is the predominant recrystallization mechanism suggesting steady state conditions in fabric development during progressive rotational deformation in contrast asymmetric cross girdles dominate the upper limb and uneven small circle c axes distributions characterize the lower limb although recrystallization in both limbs is characterized by grain boundary migration this clear separation of quartz textures and recrystallization deformation mechanism conflicts with the common view on heterogeneous deformation patterns in folds where the major component of rotational shear is expected in the fold limbs our study suggests that fabric development in the limbs of the recumbent mischabel fold was reset during progressive deformation shortly prior to the transition from an active buckling to passive fold amplification while the hinge suffered continuous bulk deformation provided by the back thrusting of the dent blanche nappe

chapter 3 mid crustal emplacement ofmesozoic plutons san gabriel mountains california and implicationsfor the geologic history of the san gabriel terrane

driving mechanisms for 40 km of exhumation during contraction and extension in a continental arc cascades core washington 1 in the nw north american cordillera the cascades core region of the coast plutonic complex underwent late cretaceous 96 ma to locally 73 ma sw ne contraction and crustal thickening followed by dextral transpression 73 to 55 ma then transtension 3 mm yr by local thrusting in regions undergoing crustal thickening in the central part of the core chelan block 40 km of exhumation occurred between 91 and 45 ma about half of which occurred during early contraction driven by thrusting and half during top to north arc oblique shear during reactivation of a midcrustal cretaceous thrust the dinkelman decollement the footwall of this thrust consists of the swakane biotite gneiss a cretaceous metaclastic assemblage with recorded pressures of 10 12 kbar no arc related magmatism and structures dominated by pervasive top to north shearing the hanging wall consists of the napeequa complex an oceanic assemblage with recorded pressures of 6 12 kbar voluminous arc related magmatism and complex structures indicating early top to wsw shearing younger top to north shearing and widespread folding in the napeequa top to north shearing started by 73 ma during melt present conditions at pressures 6 kbar top to north shearing in both hanging wall and footwall continued during exhumation 1 6 mm yr and cooling to greenschist facies conditions during which slip became increasingly localized eventually resulting in formation of pseudotachylite on discrete slip surfaces we suggest that arc oblique extension was driven by along arc heterogeneity in displacements erosion initially during transpression and underplating of continental sediments and later during transtension

fragments of the mantle and crust from beneath the sierra nevada batholith xenoliths in a volcanic pipe near big creek california a miocene volcanic pipe that cuts cretaceous granodiorite is exposed in the central sierra nevada near the town of big creek california the volcanic magma began its ascent from a mantle source as an alkalic basalt but was extensively contaminated by incorporation and assimilation of silicic plutonic rocks during ascent resulting in a pseudo andesitic composition the conduit cut through the entire thickness of the sierra nevada batholith as well as the underlying lower crust and upper mantle and the magma entrained a diverse polygenetic assemblage of igneous and metamorphic xenoliths including peridotites eclogites and granulites schists and hornfels and gabbroids and granitoids some xenoliths of the high grade metamorphic xenolith group are of particular interest in elucidating the nature of the uppermost mantle and crust beneath the sierra nevada garnet granulites the most abundant rock type of the mafic high grade metamorphic assemblage and eclogites are restricted to the central area of the pipe these xenoliths are essentially bimineralic consisting of colorless to pale green clinopyroxene and garnet eclogites contain brilliant emerald green clinopyroxene the clinopyroxenes which have a broad range of compostions mg mg fe 0 45 0 90 have a 20 jadeite component in eclogites garnets also have a range of compositions mg mg fe 0 30 0 70 with those from eclogites being the most fe rich one of the xenoliths contains ca rich garnet grs adr 90 feldspathic granulites contain plagioclase of an 45 70 and some lack garnet hornblende occurs in some granulites and a few amphibolites are present likely protoliths are ocean floor basalts for the eclogites and precambrian limestone for the grossular clinopyroxene rock the big creek xenoliths attest to the former presence of subducted oceanic lithosphere beneath the western sierra nevada

evolution of the southern san joaquin basin and mid tertiary transitional tectonics central california a cenozoic tectonic and sedimentary history is proposed for the southern san joaquin basin ssjb and tehachapi mountains that evolved adjacent to the plate margin off central california seismic reflection borehole field biostratigraphic and paleomagnetic data are integrated into geologic and fault structure maps cross sections and geohistory plots and are analyzed with previous work in the region to develop a model relating the sequence timing and distribution of complex tectonically linked events the largely buried structures and strata in the ssjb preserve an unusually complete record of the mid tertiary transition from convergent to transform plate boundary as well as the regional transition to contraction during the pliocene significant structural relief existing across both extensional and contractile features is preserved in the subsurface and an active fold thrust belt propagates basinward along the margin of the u shaped tejon embayment the cenozoic evolution of the ssjb reflects the regional deformation of central california as different tectonic events followed each other along the adjacent north american plate margin five oligocene miocene basin phases are identified in the ssjb 1 late oligocene early miocene extensional subsidence with high and low angle normal faulting accompanied by volcanism and deposition of coarse syntectonic conglomerates 2 middle miocene uplift 3 later mid miocene transtensional subsidence to lower bathyal depths 4 alternating subsidence and uplift until the late miocene and 5 flexural subsidence due to pliocene to recent contraction reconstructions of mid tertiary california place the southern san joaquin tehachapi extensional terrane as a paleotectonic block located between the western mojave terrane then to the east and the western california terrane then to the south and west regional extension occurred during a long transition period between convergent and transform boundaries along the north american plate margin significant slip along the san andreas transform postdates this extensional event its origin apparently coincided with a regional middle miocene uplift event indicating that the san andreas fault is younger than previously supposed the tertiary ssjb has subsided due to extension transtension crustal tilting and thrust related loading the maricopa subbasin floor is likely composed of ensimatic and mafic rocks like those along the west side of the sierra nevada and locally has subsided beyond 12 km in contrast the relatively stable tejon embayment underlain by sierran crystalline crust achieved its maximum subsidence in the miocene since pliocene time the ssjb has filled and continued to deepen after the basin was tectonically shut off from the pacific ocean

contribution of ductile flow to exhumation of low temperature high pressure metamorphic rocks san juan cascade nappes nw washington state the san juan cascade sjc nappes were subducted to a depth of 18 km metamorphosed under low temperature high pressure conditions and then exhumed all within 16 m y during exhumation penetrative deformation by solution mass transfer smt resulted in a widespread spaced cleavage strain directions were determined for 27 sandstone samples and absolute strain measurements for a subset of 19 samples z directions generally plunge moderately to the ne and x and y directions are scattered in the plane perpendicular to z smt deformation is constrictional at the local scale but the tensor average indicates plane strain uniaxial shortening at the regional scale with sx sy and sz equaling 1 01 0 91 and 0 58 respectively the average flattening plane xy dips 30 to the ne and the average x direction plunges 20 to the north the large shortening in z was compensated by a mass loss volume strain of 47 we present a simple one dimensional model that illustrates the relationship between finite strain and ductile exhumation for a steady state convergent wedge assuming depth dependent ductile flow and no reversal of principal strain rates with depth this model indicates that ductile thinning of the sjc nappes accomplished only 13 of the total exhumation despite a vertical shortening strain of 36 there is no evidence that normal faulting contributed significantly to exhumation we conclude that erosion operating at an average rate of 1 1 km m y 1 was the dominant exhumation process

early history and reactivation of the rand thrust southern california abstract the rand thrust of the rand mountains in the northwestern mojave desert separates an upper plate of quartz monzonite and quartzofeldspathic to amphibolitic gneiss from a lower plate of metagraywacke and mafic schist rand schist the rand thrust is considered part of the regionally extensive vincent chocolate mountain thrust system which is commonly believed to represent a late cretaceous subduction zone the initial direction of dip and sense of movement along the vincent chocolate mountain thrust are controversial microfabrics of mylonites and quartzites from the rand mountains were analyzed in an attempt to determine transport direction for this region but the results are ambiguous in addition the southwestern portion of the rand thrust was found to have been reactivated as a low angle normal fault after subduction reactivation might have occurred shortly after subduction in which case it could account for the preservation of high pressure mineral assemblages in the rand schist or it could be related to mid tertiary extension in the western united states in either event the reactivation might be responsible for the complicated nature of the microfabrics the rand schist exhibits an inverted metamorphic zonation isograds in the schist are not significantly truncated by the reactivated segment of the rand thrust this indicates that other segments of the vincent chocolate mountain thrust should be re evaluated for the possibility of late movement even if they show an apparently undisturbed inverted metamorphic zonation

magnetotelluric evidence of lithospheric mantle thinning beneath the southern sierra nevada a wideband 0 01 1000 s magnetotelluric survey across the southern sierra nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10 20 km the eastern zone underlies the highest topography of the range and extends eastward this eastern conductive zone extends to depths in excess of 100 km based on model sensitivities and lies well below the moho depth of 32 38 km from seismological studies therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt such an interpretation is consistent with gravitational seismological and geologic evidence estimates of the partial melt fraction range from 2 to 5 at depths of 40 70 km which are consistent with fractions of melt inclusions observed in xenoliths from the eastern sierra nevada this partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations the other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes the western conductive zone also straddles the moho and extends into the mantle beneath western foothills of the sierra nevada and the great valley sediments to the west however xenoliths from this region indicate high velocity crustal rocks to depths in excess of 60 km and we therefore attribute the enhanced conductivity to graphitic metasediments and or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the sierran plutons

evaluation of biotite garnet geothermometers application to the english river subprovince ontario in recent years there have been numerous calibrations of the biotite garnet fe mg exchange geothermometer the eastern lac seul region of the english river subprovince ontario provides an excellent field area in which to compare these calibrations

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

geochemical constraints on the petrogenesis of the salinian arc central california implications for the origin of intermediate magmas article i nfo magmatic arcs are the primary locations where continental crust is distilled to an intermediate calc alkaline composition the root zones of continental arcs are thought to be the primary sites of magmatic differentiation yet few deeply exhumed arc sections are available for direct study the coast ridge belt of central coastal california provides an exceptional opportunity to directly observe the cumulative effects of melting mixing assimilation and homogenization related to construction of the latest cretaceous california arc we present new major and trace elementchemistry aswellasradiogenicisotopicratiosdeterminedoncoastridgebeltassemblagesrepresentative of 20 to 30 km crustal levels late cretaceous ca 93 to 81 ma gabbro diorite tonalite and granodiorite of the coast ridge belt are calc alkaline some exhibit cumulate characteristics and all show enriched isotopic composi tions sri 0 7061 to 0 7092 and end 1 4 to 5 9 rare earth element patterns in igneous and metaigneous rocks of the coast ridge belt suggest that they are sourced deeper than the 25 km paleodepth of the exposed sec tion but probably not significantly below 40 km underplating of basaltic melts derived from evolved lithospheric mantle provides the most satisfactory mechanism explaining geochemical and field evidence for partial melting and assimilation of metasedimentary framework rocks to yield gabbroic to dioritic magmas followed soon there afterby remelting to produce more silicic magmas we suggest that basaltic underplating provided a source of heat to the base of the salinian crust leading to thermal weakening and downward flow of melt fertile intra arc supracrustal assemblages thereby igniting the magmatic pulse that formed the salinian arc

geochemical characteristics of crustal anatexis during the formation of migmatite at the southern sierra nevada california we provide data on the geochemical and isotopic consequences of nonmodal partial melting of a thick jurassic pelite unit at mid crustal levels that produced a migmatite complex in conjunction with the intrusion of part of the southern sierra nevada batholith at ca 100 ma field relations suggest that this pelitic migmatite formed and then abruptly solidified prior to substantial mobilization and escape of its melt products hence this area yields insights into potential mid crustal level contributions of crustal components into cordilleran type batholiths major and trace element analyses in addition to field and petrographic data demonstrate that leucosomes are products of partial melting of the pelitic protolith host compared with the metapelites leucosomes have higher sr and lower sm concentrations and lower rb sr ratios the initial 87sr 86sr ratios of leucosomes range from 0 7124 to 0 7247 similar to those of the metapelite protoliths 0 7125 0 7221 however the leucosomes have a much wider range of initial end values which range from 6 0 to 11 0 as compared to 8 7 to 11 3 for the metapelites sr and nd isotopic compositions of the leucosomes migmatites and metapelites suggest disequilibrium partial melting of the metapelite protolith based on their sr nd and other trace element characteristics two groups of leucosomes have been identified group a leucosomes have relatively high rb pb ba and k2o contents rb sr ratios 0 15 rb sr 1 0 and initial end values group b leucosomes have relatively low rb pb ba and k2o contents rb sr ratios 0 15 and initial end values the low rb concentrations and rb sr ratios of the group b leucosomes together suggest that partial melting was dominated by water saturated or h2o fluxed melting of quartz feldspar assemblage with minor involvement of muscovite breakdown of quartz and plagioclase with minor contributions from muscovite resulted in low rb sr ratios characterizing both group a and group b leucosomes in contrast group a leucosomes have greater contributions from k feldspar which is suggested by 1 their relatively high k concentrations 2 positive or slightly negative eu anomalies and 3 correlation of their pb and ba concentrations with k2o contents it is also shown that accessory minerals have played a critical role in regulating the partitioning of key trace elements such as sm nd nb and v between melt products and residues during migmatization the various degrees of parent daughter fractionations in the rb sr and sm nd isotopic systems as a consequence of nonmodal crustal anatexis would render melt products with distinct isotopic signatures which could profoundly influence the products of subsequent mixing events this is not only important for geochemical patterns of intracrustal differentiation but also a potentially important process in generating crustal scale as well as individual pluton scale isotopic heterogeneities

three dimensional seismic model of the sierra nevada arc california and its implications for crustal and upper mantle composition a three dimensional p wave velocity model of south central california from the coast ranges to the sierra nevada shows that the crust under most of the southern sierra nevada batholith has seismic velocities 5 9 6 3 km s below the continental average the crust is not much thicker on average about 35 km than in the adjacent great valley and basin and range province apart from a small northward thickening crustal root under the western sierra nevada that reaches a depth of 42 km crustal velocities above the continental average are observed beneath much of the great valley due to a high velocity body underlying the sedimentary basin and the foothills metamorphic belt 6 4 7 0 km s upper mantle velocities are generally low 7 8 km s but span a wide range 7 4 8 2 km s we display the velocity model in several cross sections and maps of moho depth and average crustal velocity the measured velocities in the upper and mid crust of the sierra nevada batholith are in good agreement with laboratory measurements on sierra nevada tonalites after corrections for density and temperature peridotite xenoliths from the eastern sierra nevada suggest strong upper mantle anisotropy which could explain some of the velocity heterogeneity in the sierra nevada mantle by the time cretaceous subduction related magmatism ceased the sierra nevada arc must have had a thick mafic lower crust yet a principal result of our work is that today the batholith has a crust of mainly felsic composition throughout a subcrustal layer with velocities below normal pn velocities 7 6 km s may indicate the presence of lower crustal material in eclogite facies

hydrothermal rare earth element xenotime mineralization at maw zone athabasca basin canada and its relationship to unconformity related uranium deposits a discussion

role of extrusion of the rand and sierra de salinas schists in late cretaceous extension and rotation of the southern sierra nevada and vicinity the rand and sierra de salinas schists of southern california were underplated beneath the southern sierra nevada batholith and adjacent mojave salinia region along a shallow segment of the subducting farallon plate in late cretaceous time various mechanisms including return flow isostatically driven uplift upper plate normal faulting erosion or some combination thereof have been proposed for the exhumation of the schist we supplement existing kinematic data with new vorticity and strain analysis to characterize deformation in the rand and sierra de salinas schists these data indicate that the schist was transported to the ssw from deep to shallow crustal levels along a mylonitic contact the rand fault and salinas shear zone with upper plate assemblages crystallographic preferred orientation patterns in deformed quartzites reveal a decreasing simple shear component with increasing structural depth suggesting a pure shear dominated westward flow within the subduction channel and localized simple shear along the upper channel boundary the resulting flow type within the channel is that of general shear extrusion integration of these observations with published geochronologic thermochronometric thermobarometric and paleomagnetic studies reveals a temporal relationship between schist unroofing and upper crustal extension and rotation we present a model whereby trench directed channelized extrusion of the underplated schist triggered gravitational collapse and clockwise rotation of the upper plate

magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of california effects of assimilation crustal thickness and depth of crystallization in order to better understand geologic fac tors controlling pronounced regional variations in whole rock chemistry mineralogy and mineral chemistry in the batholiths of california we calculate the magmatic intensive variables f hf f h 2 o f hf f hcl and f o 2 regional scale west to east increases in f oh in mafic silicates corresponding to the systematic i wc to i mc and i sc progression reflect orders of magnitude increase in f hf f h 2 o attending pluton crystallization low f hf f h 2 o of formation of western i wc types is consistent with their derivation from low fluorine source rocks in subducted oceanic slabs or the upper mantle in contrast higher f hf f h 2 o of crystallization of i mc and i sc types to the east implies the involvement of 1 progressively greater amounts of continental crustal source material such as biotite bearing metamorphic rocks their unweathered sedimentary derivatives retaining f rich mafic minerals or their fusion products and or 2 source materials which become more f rich toward the continental interior the regional distribution of i mc and i sc types suggests that the precambrian craton of western north america or derivative sediments may extend farther north in california and be morphologically more complex than previously thought from seemingly out of place occurrences of i wc plutons on the eastern slopes of the sierra nevada batholith we infer the existence of regions where precambrian basement was thin or absent in mesozoic time which prevented extensive cratonal contamination of subducted slab or upper mantle derived magmas new methods for estimating t f o 2 relations in the magmas demonstrate that i scr granites crystallize at oxygen fugacities as much as five orders of magnitude lower than those of i wc i mc and i sc types under conditions at or below the maximum stability limit of graphite in equilibrium with a c o h s gas phase the local scale formation of i scr granites in plutonic belts within specific wall rock terranes containing highly reducing sediments or metasediments may occur by contamination of i types with graphitic pelite or in some cases by the direct fusion of this reducing pelitic wall rock the spatial distribution of i scr granite provinces therefore is controlled simply by wall rock lithology amphibole geobarometry demonstrates a general west to east decrease in crystallization pressure across the sierra nevada batholith in contrast the peninsular ranges batholith displays a west to east crystallization pressure increase the bulk of the california batholiths crystallized at pressures less than 4 5 kb and depths less than about 15 19 km in the southern sierra nevada batholith the san gabriels and the eastern peninsular ranges however plutons crystallized at pressures exceeding 6 kb at deep crustal levels 23 km reconstruction of the pre erosion top of the batholith shows that in an east west cross section the central sierra nevada batholith was a horizontal tabular body with an aspect ratio of at least five to one

late cretaceous intra arc thrusting in southern california thick zones of mylonitic rock are exposed in or between a number of the suspect crystalline terranes in the transverse ranges and eastern peninsular ranges of southern california after palinspastic reconstruction of strike slip displacements along neogene faults in the region the mylonite zones appear in a semicontinuous belt several hundred kilometers long mylonitic deformation in these areas is of a similar style and age lineation trends in the mylonitic rocks are closely aligned when palinspastic reconstruction takes into account the block rotations in the region and the sense of shear is consistently west directed the mylonite zones appear to have formed concurrently in a major midcrustal thrust system that juxtaposed the basement terranes during the late cretaceous a kinematic model is proposed in which a west directed ductile thrust zone along the eastern side of the peninsular ranges is offset by a sinistral tear fault or lateral thrust ramp along the southern margin of the san gabriel mountains from a ductile thrust underlying crystalline basement terranes to the north this model requires the suspect basement rocks of the peninsular and transverse ranges to be linked tectonically by the latest cretaceous this late cretaceous thrust system also appears to continue directly into autochthonous basement rocks in the san bernardino mountains the late cretaceous westward displacement of basement rocks in the transverse ranges relative to the northern end of the peninsular ranges batholith is mirrored by similar tectonic movement around the southern end of the sierra nevada batholith it is argued here that a formerly continuous batholithic belt was breached by ductile west directed thrusting during the late cretaceous between the two still recognizable batholiths the salinia terrane represents the westernmost portion of this westward escaping arc segment and would have been established as a crustal salient by the latest cretaceous this model implies that the basement terranes involved in this thrusting episode are parautochthonous fragments of a cretaceous continental arc and refutes suggestions that they have been accreted in the cenozoic

thermobarometric constraints on the depth of exposure and conditions of plutonism and metamorphism at deep levels of the sierra nevada batholith tehachapi mountains california we present thermodynamic estimates of pressures temperatures and volatile activities in variably deformed gabbroic to granitic cretaceous 115 100 ma batholithic and framework rocks of the tehachapi mountains southernmost sierra nevada california al contents of hornblende in granitoids imply igneous emplacement at 8 kbar in the southernmost tehachapi mountains with lower pressures 3 7 kbar to the north metamorphic pressures and temperatures for garnet bearing paragneisses and metaigneous rocks were estimated on the basis of garnet hornblende plagioclase quartz and garnet biotite plagioclase quartz thermobarometers disparate results for the metaigneous rocks from the latter system point to the difficulty of applying pelite based thermobarometers to rocks of contrasting composition and mineralogy preferred pressures cluster at 7 1 9 4 and 3 6 4 3 kbar incomplete knowledge of reaction histories however limits our interpretation of the lower pressures because they are minimum estimates the 4 kbar samples are all from a small area and if our interpretation is correct they imply a local more shallow event superimposed on crust once residing at deeper structural levels garnet hornblende and garnet biotite temperatures are less coherent likely owing to retrograde fe mg exchange and range from 570 to 790 c the majority of the rocks are igneous and affected by recrystallization and metamorphism during subsolidus cooling they are not granulites country rock paragneisses are typically migmatized at peak metamorphic conditions near that of the wet granite solidus 690 c veinlike paragenesis of garnet in the metaigneous rocks suggests formation related to the presence of a fluid phase thermodynamic estimates of volatile activities in these garnet bearing assemblages suggest variable mostly co 2 rich fluid compositions in the absence of any pervasive fluid flux the igneous rocks of the tehachapi mountains were thus intruded at depths of 30 km making them the deepest known exposed components of the cretaceous sierra nevada batholith metamorphism occurred at these great depths and perhaps locally after 15 km of uplift before 87 ma implying an uplift rate of 1 2 mm yr a minimum uplift rate is 0 6 mm yr this original uplift and possible subsequent uplift events may have been related to underthrusting of a block of rand schist from what is now the southeast with concomitant widespread ductile deformation the deduced pressure temperature and uplift history is similar to those of high pressure high temperature cretaceous batholithic rocks in salinia and the san gabriel mountains but direct correlation is not wan anted when compared with higher level intrusive rocks from analogous portions of the sierra nevada batholith to the north the tehachapi rocks reveal a deep batholith that is more heterogeneous and somewhat more mafic on average but displaying a similar level of isotopic hybridization involving mantle and crustal sources the batholith is quartz rich at these levels suggestive of a weak ductile middle crust susceptible to prolonged deformation and possible delamination

parallel stretching lineations and fold axes oblique to a shear displacement direction a model and observations abstract spatial variations in shear strain rate are expected in ductile shear zones where the variation is a change in shear strain perpendicular to the displacement direction the effect is to rotate the shear slip planes this is a mechanism for giving a rotation of fold axes into parallelism with the slip and extension direction in a rock if such a variation in shear strain affects rocks with a strong planar anisotropy it is possible to produce a fabric with an apparent stretching lineation parallel to fold axes but both significantly oblique to the slip direction a possible example of this is seen in strongly deformed quartz mica schists from syros greece where a stretching lineation is seen parallel to fold hinges over a range of fold axes orientations of at least 40

qingyuan high grade granite greenstone terrain in the eastern north china craton root of a neoarchaean arc abstract the qingyuan high grade granite greenstone terrain is one of the first recognized granite greenstone terrains in the eastern north china craton similar to other archaean terrains its tectonic environment is debated was it an intra continental rift or a modern analogy of arc occurrence and petrogenesis of major igneous series the ultramafic mafic and felsic volcanic rocks 2510 ma and the plutons of the quartz diorite 2570 2510 ma ttg 2570 2510 ma and quartz monzodiorite 2510 2490 ma series in xinbin area are investigated the mafic intrusives and volcanic rocks have high mgo content 5 4 7 5 wt and mg number 48 61 they show slightly depleted to flat trace element patterns the ultramafic rocks serpentinite could be genetically related to the mafic rocks the meta dacite rhyolite is adakitic with enriched light and middle rees and liles but significantly depleted hfses the quartz diorite has high mg number 60 64 moderately enriched light and middle rees and liles the ttg shows distinct light but moderate middle ree enrichment prominent nb ta depletion but zr hf enrichment zr sm 100 the quartz monzodiorite has moderate light and middle ree enrichment significant nb ta depletion and negative eu sr anomalies the ttg has more depleted sr nd isotopes endt 2 6 87sr 86srt 0 700 than all the others endt 0 2 87sr 86srt 0 701 703 their petrogenesis can be explained by an archaean style subduction defined as a mantle wedge absent flat hot subduction with significant vertical tectonism in the overriding slab the ultramafic mafic rocks were originated from primitive mantle the meta dacite rhyolite was originated from the eclogite facies overriding crust the quartz diorite was a mixture of melts from mantle and the overriding crust the ttg was from the subducting slab under amphibolite to amphibole bearing eclogite facies and the quartz monzodiorite was from the subducting slab after the derivation of the ttg the qingyuan terrain represents remnants of the root of a neoarchaean arc its anticlockwise p t path recorded the initiation of the arc to its cession and exhumation cratonization at 2480 ma

mafic gneissic complex batholithic root in the southernmost sierra nevada california a mafic complex of hornblende rich gneissic to granoblastic amphibolite to granulite grade metamorphic rocks and associated mafic to felsic magmatic rocks is exposed over an area of some 300 km2 in the southernmost sierra nevada the mafic complex probably reflects a deeper exposed crustal level than is represented by the dominantly granitic terrane of the sierra nevada batholith to the north and it may be the upper part of the root zone and metamorphic substrate of the sierra nevada batholith xenoliths of gneiss amphibolite and granulite from subbatholithic levels transported upward and preserved in volcanic rocks in the central sierra nevada are possible analogues of some of the rocks of the southernmost sierra nevada mafic complex

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

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

metamorphic fluids in the deep crust most of the inferences about fluid action in the deep crust are drawn from field petrographic and geochemical studies in the granulite facies terrains deductions on the nature of fluids are made from major and minor element distributions and isotopic patterns in metamorphic rocks calculations of fluid mineral equilibria based on observed assemblages fluid inclusions and field evidence of fluid pathways discussion of these features has focused attention on several major problems concerning the timing flow mechanisms and origin of the fluids a key issue concerns episodicity versus secular activity of fluids there are strong arguments that most fluid action in the deep crust is linked to thermal deformational events including magmatism and regional meta morphism discrete radiometric ages of high grade terrains often define relatively short periods of crystallization or recrystallization the late archean granulite facies metamorphism of southern india is an example where u pb systems of zircons buhl et al 1983 whole rock u pb data peucat et al 1 987 and sm nd systems of whole rocks and minerals bernard griffiths et al 1 987 all show a major high temperature recrys tallization event inferred by several workers to have involved pervasive fluid action centered closely around 2 5 ga another geochemical indi cation of episodic fluids comes from stable isotope studies of progressive metamorphic sequences as in the damara orogen namibia where whole rock b 180 increases slightly at isograds representing arrested dehydration events but is monotonous between isograds hoernes hoffer 1 985

significance of seismic reflections beneath a tilted exposure of deep continental crust tehachapi mountains california the focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface with the tehachapi mountains in california being the case in point as a prime example of the lower crust these mountains expose cretaceous gneisses that formed 25 30 km down in the sierra nevada batholith and appear to be underlain by the ensimatic rand schist integrated geophysical and geological studies by the calcrust program have produced a cross section through this post mid cretaceous structure and suggest a general model for its development seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the san joaquin basin s tejon embayment two south dipping reverse faults on the rim of the tejon embayment were discovered in the reflection data and verified in the field the faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the rand schist the upper and lower crusts are separated by a zone of laterally discontinuous reflectors reflections from the lower crust form a wedge the base of which is a nearly flat moho at 33 km regional geological relations and gravity models both suggest that the reflective zone corresponds to the rand schist and the newly recognized faults account for its neogene exposure alternatively the reflective zone maybe part of the gneiss complex suggesting that the schist either lies deeper or is not present under the gneisses if the rand schist underlies the tehachapi mountains and mojave region to their south a model for their evolution can be constructed from regional geological relations it seems that during late cretaceous laramide subduction the protolith of the schist was thrust eastward beneath the mojave along this portion of the cordilleran batholithic belt the subduction was evidently at very low angles the bottom of the batholith was removed and replaced by a thick section of schist fluids from which weakened the overlying batholith this thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat lying slabs when emplacement of the schist ended in latest cretaceous earliest paleocene the underlying mantle rose compensating for the extension and providing material for magmatic underplating in the neogene transpression and rotation of the upper crust along the san andreas and garlock faults resulted in the exposure of the schist

in defense of magmatic diapirs abstract diapirism as a crustal magma ascent mechanism has been recently criticized we contend that this reflects an overly simplistic view that diapirs must resemble modeled hot stokes diapirs and the perception that magma ascent in dikes is a more problem free mechanism for the construction of plutons we describe four cordilleran plutons that have characteristics much more compatible with diapirs than dike fed chambers these plutons were emplaced at depths ranging from 10 to 30 km and record different parts of diapiric ascent paths most ascended during complex visco elastic flow of host rock during regional deformation have narrow structural aureoles indicating power law behavior of host rock and were constructed of multiple batches of magma attributes enabling them to ascend greater distances than single hot stokes diapirs some features of these plutons are not typically attributed to diapirs and thus we introduce the term visco elastic diapir for bodies consisting of one or more batches of magma rising together with length to width ratios

p t t evidence for mechanisms of vertical tectonic motion in a contractional orogen north western us and canadian cordillera the 1800 km long coast mountains north cascades orogen of the canadian cordillera and north western us developed as a continental magmatic arc metamorphic rocks in the orogen contain widespread evidence for burial of supracrustal rocks to depths of c 40 km followed by nearly isothermal decompression to depths of 10 km near many shallowly emplaced mid cretaceous plutons low pressure contact metamorphic effects were overprinted by high pressure regional metamorphic minerals and textures as evidenced by kyanite staurolite pseudomorphs after andalusite in metapelitic rocks therefore near pluton rocks record the loading history of the orogen metapelitic rocks not associated with plutons only preserve evidence for high pressure conditions and or high temperature decompression as indicated for example by sillimanite and cordierite after kyanite and garnet respectively petrological evidence for burial and decompression is therefore recorded in different rocks various regions of the orogen differ in timing of metamorphism the overall shape of p t paths and the relative timing and regional extent of the high pressure event but most of these data and observations are consistent with thrusting and or pure shear thickening as primary loading mechanisms throughout the orogen as opposed to magma dominated loading this interpretation is further supported by comparison with thermal models which demonstrate that the p t paths are consistent with simultaneous thrusting and folding at a high initial geothermal gradient 35 40 c km 1 in much of the orogen a high geothermal gradient supports tectonic models invoking intra arc contraction and suggests that magmatism played an important role in regional temperature time paths this tectonic thermal history may be typical of other contractional orogens and illustrates the importance of large vertical displacement of crust in magmatic arcs

bulk arc strain crustal thickening magma emplacement and mass balances in the mesozoic sierra nevada arc quantifying crustal deformation is important for evaluating mass balance material transfer and the interplay between tectonism and magmatism in continental arcs we present a dataset of 650 finite strain analyses compiled from published works and our own studies with associated structural geochronologic and geobarometric information in central and southern sierra nevada california to quantify the arc crust deformation our results show that mesozoic tectonism results in 65 arc perpendicular bulk crust shortening under a more or less plane strain condition mesozoic arc magmatism replaced 80 of this actively deforming arc crust with plutons requiring significantly greater crustal thickening we suggest that by 85 ma the arc crust thickness was 80 km with a 30 km thick arc root resulting in a 5 km elevation most tectonic shortening and magma emplacement must be accommodated by downward displacements of crustal materials into growing crustal roots at the estimated downward transfer rate of 2 13 km myr the downward transfer of crustal materials must occur in active magma channels or in escape channels in between solidified plutons that decrease in size with time and depth resulting in an increase in the intensity of constrictional strain with depth we argue that both tectonism and magmatism control the thickness of the crust and surface elevation with slight modification by surface erosion the downward transported crustal materials initially fertilize the mash zone thus enhancing to the generation of additional magmas as the crustal root grows it may potentially pinch out and cool the mantle wedge and thus cause reduction of arc magmatism

arc composition at mid crustal depths insights from the coast ridge belt santa lucia mountains california 1 the coast ridge belt crb santa lucia mts central california comprises mid crustal rocks 750 800 c and 0 8 gpa of the california magmatic arc we estimated the bulk composition of the crb and converted our results to seismic velocities expected at the observed pressures and temperatures this transformation allows us to compare calculated velocities for the crb with in situ measurements in similar arcs the bulk composition of this arc section changes abruptly at 25 km depth from a granodiorite to a quartz diorite or diorite these data are in agreement with geophysical results from other cordilleran batholiths and suggest 1 5 to 2 times thicker felsic columns than usually interpreted for modern continental arcs and a relatively sharp transition between a felsic upper crustal batholith and a mafic deep crust this implied rheological boundary may have significant implications for intracrustal faulting or convective removal of the roots of batholiths

u pb zircon strontium and oxygen isotopic and geochronological study of the southernmost sierra nevada batholith california the southernmost sierra nevada offers a view into the deep levels of the mesozoic batholithic belt which constitutes much of the range to the north and represents one of the major tectonic features of western north america the main crystalline rocks of the study area are 1 the intrusive suite of bear valley a middle cretaceous tonalite batholith complex with coeval gabbroic intrusives and 2 the gneiss complex of the tehachapi mountains which consists of early cretaceous orthogneiss and subordinate paragneiss with local domains having granulite facies metamorphic assemblages the orthogneisses are dominantly tonalitic in composition with significant layers of granodioritic to granitic and lesser dioritic to gabbroic gneiss quartz rich and psammitic metasedimentary rocks with subordinate marble constitute the main framework assemblage into which the plutonic rocks were emplaced field relations demonstrate assimilation of metasedimentary material into the orthogneiss and tonalite batholith magmas and magma mixing between mafic tonalitic and granitic materials significant domains of both homogenization and inhomogenization are recognized isotopically within the mixed rocks u pb zircon studies have resolved two major igneous suites and a third suite of postdeformational intrusives all lying between 90 and 120 ma the first suite gneiss complex of the tehachapi mountains was emplaced at 115 ma and exhibits penetrative high temperature deformation developed at or near solidus conditions a number of discordance patterns along with the physical properties of the zircon suggest minor inheritance of proterozoic zircon and limited open system behavior in response to a major 100 ma plutonic event the 100 3 ma intrusive suite of bear valley crosscuts the older suite but also exhibits significant synplutonic deformation mainly concordant zircon ages indicate the igneous crystallization age but some discordances occur due to inheritance or entrainment of proterozoic zircon the high temperature deformation fabrics in these suites and within the metasedimentary framework rocks were crosscut by the granodiorite of claraville 90 ma and pegmatite dikes 95 ma the granodiorite of claraville shows strong inheritance of proterozoic zircon and high initial 87 sr 86 sr and 18 o zircon populations from paragneiss and quartzite samples are dominated by proterozoic detrital grains strontium and oxygen isotopic data on the zircon geochronology sample suite suggest simple twocomponent mixing of mantle derived gabbroic to tonalitic magmas with partial to complete melt products from the metasedimentary framework rocks sedimentary admixtures for some granitic rocks may be as high as 45 but for the tonalitic batholithic complex are no higher than about 15 modeled values of 10 20 metasediment are typical for the orthogneisses initial 87 sr 86 sr correlates directly with 18 o and generally correlates inversely with sr content some subtle complexities in the sr and o isotopic data suggest the involvement of a third cryptic component such a component could be early phanerozoic ensimatic accretionary terranes that were structurally beneath the observed metasedimentary sequence or altered oceanic crust and sediments introduced into the mantle magma source area by subduction one of the initial aims of this study was to seek out remnants of proterozoic sialic crystalline rocks within the gneiss complex of the tehachapi mountains no such remnants were found and our studies strongly suggest that sialic components within this link of the mesozoic batholithic belt were introduced into mantle derived magraatic systems by anatexis of continent derived sedimentary rocks

tectonic and magmatic development of the salinian coast ridge belt california 1 we present new field structural petrographic and geochronologic data on a rare midcrustal 25 km exposure of a cordilleran arc the coast ridge belt located in the santa lucia mountains of central california the study area is composed primarily of a deformed suite of upper amphibolite to granulite facies rocks the sur series which is dominated by metaigneous tonalites diorites and gabbros with subordinate metasedimentary quartzite and marble inherited zircons in magmatic rocks suggest that the provenance of framework rocks is drawn heavily from miogeoclinal formations and that sedimentation occurred in the late paleozoic or later minor magmatism in the coast ridge belt began in the early or middle cretaceous but magmatic activity was most intense during a short period time from 93 to 81 ma based on u pb zircon ages of a felsic gneiss and two less deformed diorites the time period 93 81 ma also brackets a period of extensive thickening and high temperature ductile deformation while a thrusting cause for ductile deformation cannot be ruled out we favor the hypothesis that the exposed rocks correspond to a zone of return flow of supracrustal rocks locally displaced by granitoid plutons in the shallower crust magmatism ended throughout salinia between81and76ma coincidentwiththeattainmentof peak pressure and temperature conditions of 0 75 gpa and 800 c exhumation followed immediately bringing the coast ridge belt to the surface within 8 my at a rate of at least 2 3 mm yr exhumation was coincident with an episode of extensional collapse that has been documented elsewhere in the southern california arc during the early laramide orogeny and that may be related to underthrusting of the forearc at that time index terms 8102 tectonophysics continental contractional orogenic belts 8035 structural geology pluton emplacement 3640 mineralogy and petrology igneous petrology 8030 structural geology microstructures 8124 tectonophysics earth s interior composition and state 1212

generation of tonalitic and dioritic magmas by coupled partial melting of gabbroic and metasedimentary rocks within the deep crust of the famatinian magmatic arc argentina the source regions of dioritic and tonalitic magmas have been identified in a deep crustal section of the famatinian arc sierras pampeanas of western argentina the source zones of intermediate igneous rocks are located at the transition between a gabbro dominated mafic unit and a tonalite dominated intermediate unit in the upper levels of the mafic unit mafic magmas intruded into metasedimentary wall rocks crystallized mainly as amphibole gabbronorite and caused the partial melting of the surrounding metasediments in turn the leucogranitic melts sourced from the metasedimentary rocks intruded into the newly crystallized but still hot mafic layers and catalysed the process of partial melting of the gabbroic plutonic rocks the gabbroic rocks became mafic migmatites comprising amphibole rich pyroxene bearing mesosomes and leucotonalitic veins significantly most of the mafic migmatites have isotopic compositions sr sr t 50 7063 and end t 1 4 0 94 to 2 24 similar to those of the gabbroic rocks and distinct from those of their complementary leucotonalitic veins sr sr t 1 4 0 7075 0 7126 and end t 5 2 65 providing evidence for the idea that melting of the mafic rocks was triggered by the intrusion of leucogranitic anatectic melts sr sr t 1 4 0 715 and end t 1 4 6 21 mass balance calculations show that the model reaction plagioclase amphibole leucogranitic melt leucotonalitic melt clinopyroxene orthopyroxene can better explain the partial melting of the gabbroic rocks based on field observations we argue that the coalescence of leucotonalitic veins in the mafic migmatites led to breakdown of the solid matrix to form melt dominated leucotonalitic pools however the leucotonalitic veins that crystallized before leaving behind the mafic migmatitic rock are chemically elemental and isotopic more evolved than the dioritic and tonalitic rocks we envisage that once detached from their source region the leucotonalitic magmas were able to react commingle and mix with entrained fragments of both mafic and metasedimentary rocks this process gave rise to melts that became tonalitic and dioritic magmas this study concludes that the generation of intermediate magmas is a multistage process with three critical steps 1 influx and emplacement of hydrous mafic magmas into a deep crust containing metasedimentary country rocks 2 physically and chemically coupled melting of mafic and metasedimentary rocks leading to the formation of a leucotonalitic vein and dyke system that coalesces to form leucotonalitic or tonalitic magma bodies 3 retrogression of the leucotonalitic magmas by partially assimilating entrained fragments of their mafic and metasedimentary precursors the dimensions of the

supracrustal input to magmas in the deep crust of sierra nevada batholith evidence from high δ 18 o zircon oxygen isotope ratios of zircon zc from intrusives exposed in the tehachapi mountains southern california reveal large inputs of high 18 o supracrustal contaminant into gabbroic and tonalitic magmas deep 30 km in the cretaceous sierra nevada batholith high 18 o zc values 7 8 0 7 predominate in the deep parts of the batholith but lower values 6 1 0 9 occur in shallower parts this indicates a larger gradient in 18 o with depth in the batholith than occurs from west to east across it oxygen sr and nd isotope data show that the supracrustal contaminant was likely young paleozoic or mesozoic hydrothermally altered upper oceanic crust or volcanic arc sediments such rocks were subducted or underthrust beneath the sierran arc during accretion of oceanic terranes onto north america this component yielded high 18 o magmas that were added to the base of the batholith on average gabbros in the southern sierra contain at least 18 of the subducted supracrustal component some tonalite and granodiorite magmas were additionally contaminated by kings sequence metasedimentary rocks as evidenced by 18 o zc and initial 87 sr 86 sr that trend toward values measured for the kings sequence besides high 18 o values in the southern sierra xenoliths in the central sierra also have elevated 18 o which confirms the widespread abundance of supracrustal material in the sub arc lithospheric mantle in contrast to 18 o zc whole rock 18 o values of many samples have undergone post magmatic alteration that obscures the magmatic contamination history of those rocks such alteration previously prevented confident determination of the mass of young hydrothermally altered mantle rocks that contributed to sierran granitoids

late cretaceous juxtaposition of metamorphic terranes in the southeastern san gabriel mountains california new structural petrological and geochronological data from the southeastern san gabriel mountains define the amalgamation history of suspect terranes in this region four metamorphic terranes constitute the area the cucamonga san antonio san gabriel and baldy terranes and were juxtaposed in two discrete episodes during the late cretaceous the cucamonga san antonio and san gabriel terranes were juxtaposed along subparallel east west trending left lateral mylonitic fault zones during the first episode the cucamonga terrane consists of lower cretaceous granulite facies gneissic rocks these rocks were intruded in the late cretaceous by granitic rocks as the cucamonga terrane was faulted ductilely against the southern margin of the san antonio terrane mylonitized units were subjected concurrently to retrograde amphibolite facies metamorphism metasedimentary pendants in the san antonio terrane were metamorphosed under upper amphibolite facies conditions as plutonic rocks were emplaced syntectonically along its northern boundary the san antonio terrane was juxtaposed concurrently against the san gabriel terrane a terrane consisting of precambrian gneiss and precambrian and mesozoic intrusive rocks the boundary is represented by middle fork complex a ductile zone intermingling rocks of the two terranes after palinspastic restoration of local cenozoic fault displacements the left lateral mylonitic fault zones separating the terranes appear as parts of a major sinistral transform system along the southern margin of the san gabriel mountains displacement for each of the mylonitic fault zones was approximately several tens of kilometers and was part of a broad left lateral shear system this system is modeled as a lateral ramp or tear fault to a west directed synplutonic ductile thrust system underlying the san gabriel terrane crosscutting syn and post tectonic intrusives indicate that most of the displacement occurred between 88 and 78 m y ago no lithologic isotopic or structural data support previous assertions that these terranes were accreted to north america during the tertiary the late cretaceous synplutonic juxtaposition of the cucamonga san antonio and san gabriel terranes was followed by underthrusting of oceanic rocks of the baldy terrane that is pelona schist along the vincent thrust precise timing of this episode is as yet uncertain

intra arc crustal loading and its tectonic implications north cascades crystalline core washington and british columbia widespread high pressure cretaceous metamorphism in the coast plutonic complex of southeast alaska and british columbia and its southeast extension the cascades crystalline core is commonly attributed to thrust loading during superterrane collision however terrane boundaries within the cascades core are intruded by relatively shallow mid cretaceous plutons and crustal loading of 2 5 kbar postdates these plutons these observations are not consistent with proposed collisional models and we suggest that loading occurred by structural and or plutonic processes operating within a magmatic arc such loading may be an important process in arc tectonics although it has not been widely reported

quaternary reactivation of the kern canyon fault system southern sierra nevada california the kern canyon fault the longest fault in the southern sierra nevada is an active structure and has been reactivated at discrete times over the past 100 m y in response to changing lithospheric stresses after initiation as a cretaceous transpressional structure the kern canyon fault transitioned into a dextral strike slip shear zone that remained active as it was exhumed into the brittle regime during regional late cretaceous uplift of the sierra nevada batholith the kern canyon fault was reactivated during miocene regional extension as part of a transfer zone between two differentially extending domains in the southern sierra nevada subsequent normal displacement along the fault began in pliocene time new evidence for fault activity which continued into late quaternary time includes its current geomorphic expression as a series of meters high west side up scarps that crop out discontinuously along the fault s 130 km length relocated focal mechanisms of modern earthquakes confirm ongoing normal faulting and geodetic measure ments suggest that the sierra nevada is uplifting relative to the adjacent valleys this evidence for recent activity overturns a long held view that the kern canyon fault has been in active for more than 3 5 m y its reactivation indicates that deformation repeatedly localized along a pre existing crustal weakness a cretaceous shear zone we propose that a system of inter related normal faults including the kern canyon fault is responding to mantle lithosphere removal beneath the southern sierra nevada region the location of the active kern canyon fault within the sierra nevada great valley microplate indicates that deformation is occurring within the microplate

analysis and interpretation of composite foliations in areas of progressive deformation abstract in areas of progressive deformation where successive structures develop during a relatively continuous deformation within a geologically short time period traditional chronological notations of structural elements e g s 1 s 2 etc can give erroneous impressions of how large rock masses evolve in time and space we demonstrate from field examples that successive structures can develop which a are comparable in morphology and orientation but of different ages b are different in morphology but of comparable age and c show rapid morphological changes over short distances under such conditions correct identification of the relative age of structures is often difficult to impossible we consider the concepts of a composite foliation and of transposition cycles as vehicles to objectively evaluate the significance of different sets of structures in the evolution of larger rock masses we suggest that 1 structural elements be labeled using morphological notation adding numerical subscripts only when independent evidence is available 2 geologists more fully acknowledge and integrate the concept of deformation partitioning into their models and 3 when analyzing areas of multiple deformation more emphasis is placed on the relationship between domains of differing complexity integration of these three perspectives in the analysis will lead to a more realistic basis upon which to model the structural evolution of large rock masses

apparent isobaric cooling paths from granulites two counterexamples from british columbia and new hampshire textural evidence for the reaction histories of pelitic granulites from british columbia and new hampshire shows that pressure temperature paths obtained from thermobarometry on these samples are an artifact of the calculation technique samples from both localities show breakdown of garnet to form cordierite a reaction that occurs during decompression additional reaction textures and fluid inclusion data also imply decompression at high temperatures for both localities however core rim thermobarometry yields nearly isobaric cooling paths similar cooling paths have been used by other authors to argue for a magmatic underplating model for regional mafic granulites we show that mineral homogenization in pelitic granulites can lead to erroneous isobaric cooling paths and hence to incorrect conclusions concerning tectonic similarities between different types of granulites

kinetic control of metamorphic imprint during synplutonic loading of batholiths an example from mount stuart washington the superimposition of barrovian type metamorphism on low pressure contact aureoles in magmatic arcs is subject to the kinetic constraints of temperature and influx of h 2 o and may be cryptically developed or not at all the spatial distribution of detectable overprint will reflect these kinetic factors rather than the geometry of the imposed load many of the features believed indicative of magma loading including apparent steep baric gradients may be produced in this way on the basis of adding areas of cryptic metamorphism to the area of obvious barrovian overprint on the northeast side of the mount stuart batholith we conclude that loading 2 3 m y after emplacement was probably much more extensive sheetlike than believed previously

mafic injection as a trigger for felsic magmatism a numerical study 1 the origin of crustal scale silicic magmatism remains a matter of debate and notable uncertainty exists concerning the physical mechanisms that drive ascent and emplacement of felsic magmas in upper crustal regions a 2 d numerical model demonstrates that injection of mantle derived mafic magma into a partially molten hot zone in the lower crust can drive felsic magma ascent and intrusion into upper crustal levels the injection of mafic magma induces overpressure in the reservoir which increases crustal stresses and triggers development of brittle plastic shear zones and can drive significant surface uplift the emerging topography causes a nonuniform overpressure distribution in the reservoir and can trigger felsic magma ascent along crustal shear zones based on systematic numerical experiments we investigate the influence of crustal strength and injection rate the initial upper crustal strength controls the degree of crustal faulting and surface uplift and therefore whether felsic magma ascent can be initiated or not the final upper crustal strength influences the depth and final style of felsic intrusion the injection rate of mafic magma determines the time scale of overpressure growth and surface uplift stage in contrast the duration of the subsequent felsic ascent and intrusion emplacement stages remains nearly constant our results imply that mafic underplating and intrusion into the lower crust may not only be a prime control for the generation of felsic magmas in the lower crust but may also be an important physical driving mechanism for felsic magma ascent and intrusion into upper crustal levels

rotation of the southernmost sierra nevada california structural paleogeographic and magmatic trends of the sierra nevada batholith swing from a characteristic nnw trend in the northern and central parts of the batholith to a nearly e w trend in the tehachapi and san emigdio mountains paleomagnetic results from eight sites in the upper cretaceous 80 86 m y bear valley springs pluton in the tehachapi mountains yield a mean direction d 022 i 56 k 97 95 5 6 this mean direction is significantly different from the expected late cretaceous field direction for this locality and suggests that the pluton has undergone a clockwise rotation of 45 14 in accord with the change in structural trend in that area lower to middle miocene volcanic and sedimentary rocks of the kinnick formation overlie the plutonic rocks and yield a mean direction d 354 i 64 a paleomagnetic pole derived from this mean direction is not significantly different from the expected miocene pole for north america this constrains the time of rotation of the pluton to be between about 80 and 20 m y probably predating establishment of the present san andreas transform in this area we suggest that the observed rotation is part of a large oroclinal bend affecting to different degrees the entire southern sierra nevada the driving mechanism for such a bending may be right lateral shear along a proto san andreas transform for which there is now a significant body of evidence