composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

strike slip tectonics and granite petrogenesis transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts individual transcurrent shear zones may not only control the ascent paths siting and emplacement mechanisms of plutons but they may also cause the genesis of the granitoids themselves two contrasting situations are examined in the hercynian shear zones of iberia thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile gondwanan sediments to produce syntectonic granites in the northern part of the british caledonides the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the moho

high temperature arc parallel normal faulting and transtension at the roots of an obliquely convergent orogen structural and kinematic data from northern coastal british columbia 54 5 n document intra arc deformation patterns at mid crustal levels during and after emplacement of the coast mountains batholith major arc parallel displacements occurred along high temperature 700 c ductile normal faults and steep sinistral transtensional shear zones within an obliquely convergent margin strain patterns and pluton emplacement were controlled by 1 structural anisotropies in host rocks that predate batholith emplacement and 2 kinematic compatibility requirements created by simultaneous motion on curved pluton bounding shear zones these controls superseded a partitioning of the arc parallel and arc normal components of oblique plate convergence onto strike slip and thrust faults respectively

kinematics and tectonic significance of transpressive structures within the coast plutonic complex british columbia abstract structural data from the coast plutonic complex near prince rupert british columbia are consistent with a deformational history dominated by dextral transpression from campanian to paleocene time penetrative east side up southwest directed ductile shearing produced moderately northeast plunging overturned kilometer scale isoclinal folds these folds are dextrally sheared and refolded into kilometer scale upright northwest plunging folds and steeply dipping transposed foliations with moderate to shallow northwest plunging lineations along their western side an east side up component to the transcurrent shearing is kinematically compatible with east side up shearing found within the great tonalite sill kinematic and geometric gradients and the spatial distribution of the finite stretching direction are interpreted to result from partitioning of transpression the location of these structures and overprinting relationships suggest the great tonalite sill intruded late kinematically into a crustal scale dextral transpressive shear zone these results indicate this shear zone could form part of the baja b c fault system that would have accommodated large northward displacements of the terranes making up western british columbia and southeast alaska this conclusion is based on 1 it is favorably located to accommodate the proposed displacements 2 deformation occurred during the time period of proposed large displacement 83 59 ma 3 the 15 km thickness of the shear zone indicates it records large displacements

the southern andes between 36 and 40 s latitude seismicity and average seismic velocities abstract the project issa 2000 integrated seismological experiment in the southern andes consists of a temporary seismological network and a seismic refraction profile a network of 62 seismological stations was deployed across the southern andes at 38 s three hundred thirty three local seismic events were observed in a 3 month period p and s arrival times of a subset of high quality data were inverted simultaneously for 1 d velocity structure hypocentral coordinates and station delays seismic refraction data along a transect at 39 s provide further constraints on the crustal structure low crustal velocities beneath the forearc may be either due to subducted trench sediments or serpentinized mantle material of the continental lithosphere the continental moho is not clearly observed in this region average velocities of the crust beneath the arc are higher than those beneath the forearc crustal thickness is about 40 km crustal seismicity concentrates in the forearc region along the bio bio and gastre fault zones the area between these two prominent fault zones seems to be nearly devoid of crustal seismicity but shows highest uplift and topography in the forearc region benioff seismicity is observed down to 150 km depth resulting in the first accurate image of the benioff zone in the southern andes a maximum of seismicity at 60 km depth may be caused by dehydration embrittlement

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

accreted terranes of northwestern british columbia canada lithospheric velocity structure and tectonics 1 lithospheric structure beneath the accreted terranes of the northwestern canadian cordillera is interpreted from analyses of p wave refraction and wide angle reflection data the 535 km long profile extends from the deformed and displaced ancestral north american margin across the omineca belt suture zone and intermontane superterrane to the coast belt this region is interpreted to have grown westward through a progression from thin skinned to thick skinned accretionary tectonics large lateral variations of velocities 2 0 6 4 km s within the upper 15 km correlate with the mapped geological and terrane elements fault offsets and the imbricated base of an overlap basin are clearly identified inboard of the stikine accreted terrane a zone of slow upper crustal velocities thins to the east consistent with an interpretation of thin skinned accretion leaving slivers of terranes overlying a wedge of metamorphosed proterozoic continental margin sediments however in the lower crust the proposed underlying cratonic ramp is not distinguished from outboard or overlying accreted crust by lateral p velocity contrasts or wide angle reflections the moho remains nearly horizontal beneath the intermontane and omineca belts gradually thinning to the east from 36 5 to 35 km to the southwest below the coast belt and to the northeast beneath the tintina fault the crust thins to 32 km upper mantle velocities across the coast intermontane and omineca belts are 7 8 7 9 km s the slow upper mantle and lower crustal velocities are indicative of high temperatures that may permit the flat moho or weak lower crust to partially decouple the crust from the mantle reflections in the upper mantle between 50 70 km depth may be related to the base of the lithosphere or to heterogeneities introduced through melting or accretionary subduction events

low pressure regional metamorphism in the omeo metamorphic complex victoria australia the omeo metamorphic complex forms the southern end of the wagga metamorphic belt which is the main locus of palaeozoic low pressure metamorphism in the lachlan fold belt south eastern australia it comprises metamorphosed ordovician quartz rich turbidites originally derived from precambrian cratonic rocks prograde regional metamorphism occurred in the early silurian very soon after sedimentation had ceased the sequence of metamorphic zones with increasing grade is chlorite biotite cordierite andalusite k feldspar and sillimanite k feldspar migmatites occur in the sillimanite k feldspar zone but large bodies of s type granite were derived from rocks underlying the exposed ordovician sequence p and t estimates for the highest grade rocks are t 700 c and p 3 5 kbar indicating a very high p t gradient of 65 c km the high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre metamorphic back arc basin environment and intrusion of hot mantle derived magmas into the lower and middle crust regional retrograde metamorphism coincided with a general reheating of the crust in the siluro devonian accompanied by intrusion of many i type plutons and resetting of the k ar dates of some earlier plutons the omeo metamorphic complex was exposed to erosion at this time

growth stabilization and reactivation of proterozoic lithosphere in the southwestern united states growth of proterozoic continental lithosphere in the southwestern united states involved assembly of tectonostratigraphic terranes during several pulses of convergent tectonism ca 1 74 1 70 and 1 65 1 60 ga prograde metamorphism accompanied orogenic assembly and peak metamorphic conditions outlasted deformation regions now characterized by the highest metamorphic grades underwent slow isobaric cooling and were not uplifted until more than 200 m y after assembly regions of low metamorphic grade were not uplifted substantially after assembly we suggest that 1 relatively thin lithospheric fragments were assembled into isostatically stable normal thickness continental lithosphere 2 assembly did not erase lithospheric scale heterogeneities 3 the present juxtaposition of different crustal levels reflects differential uplift related to 1 4 1 1 ga tectonomagmatic activity and 4 the boundaries between different lithospheric blocks were repeatedly reactivated from precambrian through tertiary time

the paleomagnetic effects of reheating the ecstall pluton british columbia abstract paleomagnetic data of plutons from the western canadian cordillera are currently interpreted to indicate thousands of kilometers of latitudinal transport in situ tilting or post intrusion folding for the ecstall pluton of british columbia the inclination of magnetic remanence directions steepens to the east from 16 to 81 butler et al j geophys res 107 2002 between 24 and 12 km west of a thermal boundary with the coast mountains batholith cmb the cmb was at 700 800 c between 60 and 52 ma the stable remanent magnetization is in ilmenohematite grains with exsolved lamellae of ferrian ilmenite it has the characteristic properties of lamellar magnetism robinson et al nature 418 2002 517 520 which is acquired with exsolution in ilmenohematite below 390 c ghiorso phys chem minerals 25 1997 28 38 thus it is thermal chemical remanent magnetization tcrm our heat flow calculations show that the thermal effects of eocene cmb on the adjacent 91 ma ecstall pluton were enough to reset the remanent magnetization directions reheating to the temperatures necessary for tcrm is supported by k ar and ar ar cooling dates on hornblende and biotite which young from west to east towards the cmb in concert with the change in inclinations thus the progressive steepening of magnetization inclinations is attributed to reheating during the eocene of the 91 ma ecstall pluton by the cmb this reheating followed post solidification northward latitudinal displacement of the pluton the amount of translational displacement of the coastal terranes of british columbia prior to 60 ma hinges on the interpretation of discordant magnetizations from plutons that may have cooled slowly after emplacement in the mid to lower crust as was the case for the ecstall pluton many of these plutons contain ilmenohematite as an accessory magnetic phase our interpretation for the low temperature acquisition of tcrm in the ecstall pluton may have far reaching implications for understanding enigmatic discordant paleomagnetic directions reported from plutons of the western canadian cordillera

formation of new continental crust in western british columbia during transpression and transtension crustal growth in the coast mountains along the leading edge of the canadian cordillera was the result of processes associated with horizontal flow of material during transpression and subsequent transtension and the vertical accretion of mantle derived melts from 85 to 58 ma as exotic terranes were translated northward during transpression the crust was thickened to about 55 km and melt that originated from a mix of mantle derived basalt with partial melt of the thickened crust intruded into crustal scale transpressive shear zones when the orogen extended between 58 and 50 ma there was large scale decompression melting in the mantle and dehydration melting in the lower crust voluminous emplacement of sub horizontal sills facilitated by ductile flow of the gneissic country rocks partially filled space created as the crust was pulled apart and as 15 to 20 km of tectonic exhumation occurred across low angle normal ductile shear zones by 50 ma the final crustal thickness of the new continental crust was about 34 km comparison of seismic data with other crustal sections suggests that the crust forming processes identified in western british columbia have general applicability to models for the formation of continental crust

strain partitioning in an obliquely convergent orogen plutonism and synorogenic collapse coast mountains batholith british columbia canada 1 we describe the crustal structure of the coast mountains batholith between 54 and 55 n within the canadian cordillera with emphasis on emplacement of the 7 km thick kasiks sill complex ksc kinematic patterns that developed during emplacement of the ksc are the result of interactions between magma transport magma accumulation and regional deformation the sills were emplaced during nw directed normal shearing and flattening of country rocks that host the ksc a 2 km thick shallowly ne dipping mylonite zone cuts the eastern side of the ksc kinematic indicators within the mylonite zone record top to the east normal displacements structural analysis shows that mylonite formation occurred during subvertical shortening and east northeast subhorizontal extension u pb zircon age dates show that ene directed normal shearing along the eastern side of the ksc and wnw directed normal shearing within the ksc occurred contemporaneously between 54 and 51 ma indicating strong strain partitioning between the mylonite and the ksc this pattern of strain partitioning is interpreted to have been driven by return flow of melt laden crust in response to tectonic denudation of the upper crust seismic profiling shows that many of these structures extend to mid and lower crustal depths comparison of our results with other regions within the canadian cordillera indicates that orogen scale right lateral strike slip faults deformed synchronously with wide spread magmatism and formation of extensional gneiss domes thus the crustal structure of the coast mountains batholith was the result of early tertiary batholith construction during dextral oblique convergence and synorogenic collapse

near isothermal conditions in the middle and lower crust induced by melt migration simple one dimensional thermal models that include the effects of melt migration have been developed the models show that long lived plutonism results in a quasi steady state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust the thermal structure of the crust strongly influences deformation metamorphism and plutonism1 2 3 models for the geothermal gradient in stable crust predict a steady increase of temperature with depth this thermal structure however is incompatible with observations from high temperature metamorphic terranes exhumed in orogens1 4 5 6 global compilations7 of peak conditions in high temperature metamorphic terranes define relatively narrow ranges of peak temperatures over a wide range in pressure for both isothermal decompression and isobaric cooling paths here we develop simple one dimensional thermal models that include the effects of melt migration these models show that long lived plutonism results in a quasi steady state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust the models also predict that the upward advection of heat by melt generates granulite facies metamorphism and widespread andalusite sillimanite metamorphism in the upper crust once the quasi steady state thermal profile is reached the middle and lower crust are greatly weakened due to high temperatures and anatectic conditions thus setting the stage for gravitational collapse8 exhumation and isothermal decompression after the onset of plutonism near isothermal conditions in the middle and lower crust result from the thermal buffering effect of dehydration melting reactions that in part control the shape of the geotherm

application of lu hf garnet dating to unravel the relationships between deformation metamorphism and plutonism an example from the prince rupert area british columbia abstract the tectonic history of the prince rupert area is marked by profound crustal thickening from thrusting that produced inverted metamorphic field gradients and transpression in crustal scale shear zones syn tectonic garnet in the prince rupert area has lu hf ages of 102 6 3 7 ma and 108 3 4 1 ma 2 porphyroblast matrix relationships in these samples and samples from the same outcrops indicate syn tectonic garnet growth these relationships imply that the garnet ages directly date the development of the metamorphic foliations a third sample of migmatitic garnet amphibolite from the contact aureole of the 94 90 ma ecstall pluton had complex isotope systematics interpreted to indicate a garnet growth episode 105 ma similar to the ages obtained from the other samples and a growth or equilibration event at 94 ma during pluton emplacement the data show that the older lu hf garnet ages date prograde metamorphism during foliation development and modification during pluton emplacement the ecstall pluton was emplaced 10 to 15 m y after regional metamorphism and thrust stacking in the prince rupert area in order to place our samples in a regional tectonic context we compare our results to patterns of regional deformation metamorphism and plutonism throughout the north american cordillera in the time period between 110 and 85 ma contractional and transpressional deformation occurred throughout much of the north american cordillera at this time from southeastern alaska to the baja peninsula in mexico left lateral transpression dominated the canadian cordillera whereas right lateral transpression affected areas south of the idaho salmon river suture zone including the sierra nevada batholith this reversal in kinematics in the northern and southern cordillera within coeval magmatic belts appears to be a first order feature of the geology of the north american cordillera during the cretaceous

structural and geochronologic relations along the western flank of the coast mountains batholith stikine river to cape fanshaw central southeastern alaska abstract geologic and u pb geochronologic studies in central southeastern alaska provide constraints on the mid cretaceous to mid tertiary deformation west of the coast mountains batholith the ne dipping sumdum and fanshaw faults record the w directed emplacement of the ruth assemblage yukon tanana terrane and taku terrane over the subjacent gravina belt and alexander terrane ductile fabrics of the sumdum fanshaw fault system truncate early formed foliation and thrust faults in albian and older gravina belt strata u pb age data from syn and post tectonic plutons suggest that deformation was ongoing at 92 9 3 0 ma but had mostly ceased by 90 ma the sumdum fanshaw fault system marks the tectonic boundary between the alexander wrangellia terrane and inboard yukon tanana and stikine terranes and lies within a thrust belt that extends from southern alaska to northern washington mid cretaceous structures are truncated to the east by ne dipping ductile fabrics of the coast shear zone sheet like paleocene tonalites were emplaced into and deformed within the shear zone undeformed mid eocene dikes cross cut the tonalites kinematic relations in the coast shear zone suggest a complex displacement history that includes both east side up reverse and west side up normal shear the shear zone probably accommodated the collapse of overthickened crust developed during mid cretaceous shortening

paleomagnetism of the mount stuart batholith revisited again what has been learned since 1972 we have collected 20 new paleomagnetic sites from the mount stuart batholith and the adjacent beckler peak stock using thermal and low temperature demagnetization and rock magnetic tests we have found that the remanence in most of the batholith is carried by single domain magnetite the mean of the new mount stuart batholith sites is d 354 2 i 46 2 k 87 2 a95 4 6 n 11 and is similar to those of the beck and noson 1972 and beck and others 1981 studies examination of ar geochronology of hornblende and biotite from the mount stuart batholith finds that the southern part of the batholith cooled through the blocking temperature of magnetite at 91 ma the northern part cooled through the blocking temperature of magnetite at 86 ma and of pyrrhotite at 83 ma from these combined results we conclude that the paleomagnetic directions from the southern part of the batholith were acquired within 0 5 to 1 0 ma of the time at which the ah barometers cooled below their closure temperature use of the ah barometry to establish paleohorizontal for these rocks is thus well justified correcting the mount stuart direction for the tilt indicated by the aluminum in hornblende ah barometry the mean becomes d 355 9 i 50 6 this result places the mount stuart batholith at a latitude of 31 3 3 8 3 4 n at 91 ma by identifying the carrier of remanence in the mount stuart batholith as being single domain sd magnetite we have removed the major remaining uncertainty in interpreting the paleomagnetism of these rocks the characteristic magnetization repeatedly found in the mount stuart rocks clearly supports the microplate tectonic model of beck and noson 1972 presently manifest as the baja british columbia baja bc hypothesis

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

response of continental magmatic arcs to regional tectonic changes recorded by synorogenic plutons in the middle crust an example from the coast mountains of british columbia abstract the coast mountains of british columbia record an increase in magmatic activity acceleration in exhumation rate and a change from transpression to extension between 60 and 52 ma structural analysis of fabrics in three mid crustal plutons and country rocks leads to conclusions about pluton emplacement mechanisms and strain partitioning during changing tectonic conditions the quottoon and kitlope plutons 60 ma have steep foliations and lineations consistent with partitioned transpressional deformation the chief matthew s pluton 57 55 ma intruded during the formation of a sub horizontal transposition foliation and has radially distributed lineations consistent with sub vertical flattening during extension the change in orientation of the foliation represents an almost orthogonal rotation of the shortening direction from sub horizontal to sub vertical the chief mathew s pluton is interpreted to intrude initially into gently dipping fractures perpendicular to the steeply dipping foliation these melt filled fractures acted as conduits for melt triggering horizontal flow and eventually formed kilometer scale sills the steeply dipping fabrics of the coast shear zone provided pathways for melt to ascended from lower to middle crustal depths partitioning deformation into three dimensional domains of flattening simple shear and constrictional strain created the space for plutons this pattern of deformation reflects the interaction of regional deformation with magma emplacement

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

field geochemical and isotopic evidence for magma mixing and assimilation and fractional crystallization processes in the quottoon igneous complex northwestern british columbia and southeastern alaska the quartz dioritic quottoon igneous complex qic is a major paleogene 65 56 ma magmatic body in northwestern british columbia and southeastern alaska that was emplaced along the coast shear zon

seismic velocity structure of the queen charlotte basin beneath hecate strait seismic refraction data across hecate strait in the northern queen charlotte basin were collected in a coincident reflection and refraction survey crustal velocity models provide a framework to help understand the formation of the sedimentary basin and the processes occurring near the queen charlotte fault a major ocean continent transform fault beneath the sediments which have a maximum thickness of 6 km a velocity gradient extends from about 5 to 8 km depth within which velocities increase typically from 6 3 to 6 4 km s 1 a thick constant velocity region was found down to a depth varying from 14 to 22 km with the smallest depths located beneath the central basin the base of the constant velocity layer was marked by a distinct mid crustal interface across which velocities increased from 6 4 6 5 km s 1 to approximately 6 8 6 9 km s 1 moho was interpreted to be at a near uniform depth of 26 28 km beneath hecate strait and the eastern queen charlotte islands the associated variation in crustal t

shear zones and magma ascent a model based on a review of the tertiary magmatism in the alps 1 the alpine oligocene plutons are spatially and temporally associated with the activity of the periadriatic fault system pfs an orogen parallel crustal scale transpressive mylonitic belt excellent three dimensional exposure combined with a wealth of structural seismic petrological geochronological geochemical and paleomagnetic data collected over the last decades help to constrain the relationships between deformation ascent and emplacement of the plutons magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the periadriatic fault system which was used as ascent pathway to cover vertical lengths of 20 to 40 km therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth ascent of the melts is controlled by the mylonitic foliation of the pfs which forms the only steep anisotropy continuously traversing the entire alpine crust in contrast the flow direction is not influenced by the specific kinematics of the faults final emplacement of the plutons occurred by extrusion from the periadriatic fault system into the adjacent country rocks the transition from ascent to final emplacement is favored by partitioning of transpressive deformation

seismic and laboratory constraints on crustal formation in a former continental arc accrete southeastern alaska and western british columbia 1 the accrete project studies continent formation by assemblage of exotic terranes and arc magmatism a marine land seismic profile crosses the terranes with dense sampling and remarkably strong s waves as well as p waves unique high quality s wave seismic data provide further constraints on interpretation and are used together with new laboratory p and s wave velocity measurements corrected for high temperatures within and at the base of the crust to make a lithologic and petrologic interpretation vp vs ratios indicate three distinct terranes that were assembled to form continental crust these terranes are the outboard accreted alexander wrangellia terrane the coast mountains batholith cmb magmatic arc and the inboard stikinia terrane below the cmb vp and vp vs to a depth of 10 15 km are appropriate for tonalite and diorite the vp and vp vs increase with depth indicating that the rocks become more mafic but vp vs is not high enough for gabbro the vp and vp vs of the lower crust are consistent with a mixture of mafic garnet granulite and restite whose quartz content lowers the vp vs ratio below that of gabbro when corrected for high temperature and uplift due to exhumation the crust under the cmb exhibits seismic properties of an average continental crust these results suggest that gabbro could have intruded a metasedimentary pile in the deep crust to cause melting of the metasediments to form intrusions of the cmb leaving behind a mixture of mafic garnet granulite and sillmanite garnet quartz restite and generating the deeper part of what becomes an average continental crustal section

geometry evolution and tectonic framework of the skeena fold belt north central british columbia the intermontane belt of the canadian cordillera has long been viewed as a passive relatively rigid block between two metamorphic plutonic belts the coast plutonic complex and the omineca belt however the skeena fold belt which spans most of the width of the northern intermontane belt exhibits shortening comparable with that in the rocky mountain fold and thrust belt the skeena fold belt has many features of thin skinned fold and thrust belts such as thrust faults which sole into a detachment a wide variety of structural styles which depend on rock type a foreland basin which was cannibalized by continued deformation a frontal triangle zone and a hinterland of metamorphic and plutonic rocks coast plutonic complex the skeena fold belt thus is comparable with the rocky mountain fold and thrust belt but rather than deforming a continental terrace wedge it developed in a terrane stikinia which had accreted to north america in the early mesozoic and in jurassic and cretaceous clastic successions bowser lake and sustut groups which overlie stikinia structural and stratigraphic relationships show that the earliest deformation occurred between oxfordian and albian time and that the last folds developed in latest cretaceous or early tertiary time as much as 160 km of northeastward shortening in the skeena fold belt was broadly contemporaneous with crustal thickening in the coast plutonic complex and omineca belt with dextral strike slip faulting east of the skeena fold belt and with shortening in the rocky mountain fold and thrust belt therefore between latest jurassic and early tertiary times horizontal shortening occured across most of the width of the northern canadian cordillera concurrent shortening across the cordillera suggests that a common detachment or detachments fed all of these zones as far east as the rocky mountain fold and thrust belt

wide angle seismic imaging across accreted terranes southeastern alaska and western british columbia abstract this study addresses the question of crustal moho and uppermost mantle structure across an accreted terrane continental arc and fold and thrust belt in southeastern alaska and western british columbia the 186 km long portland canal line of the accrete wide angle seismic dataset across the coast mountains is analyzed using a combination of travel time analysis techniques including delay time tomography turning ray tomography reciprocal time analysis intercept time inversion and forward ray tracing additional constraints on the model are obtained from a coincident vertical incidence seismic section and from correlation with the mapping of geologic structures plunging into the seismic section the study reveals moderately high crustal velocities low velocity gradient in the middle crust and decreasing average crustal velocity and a north deepening moho termination of crustal reflectivity across a vertical zone the coast shear zone csz indicates that the csz is most likely a strike slip fault associated with a transpressive tectonic regime a mid cretaceous thrust system mapped on the surface to the west of the csz is imaged by several groups of mid to lower crustal reflectors extending close to the moho indicating it was a thick skinned thrust system ne dipping fabric imaged within the coast mountains batholith cmb is associated with a ductile deformation during early eocene crustal extension the crustal section under the cmb which has an average velocity of 6 55 km s and shallower than average crustal thickness of 31 km can be considered as corresponding to the lower two thirds of an average crustal section which has been inflated by intrusions of high velocity tonalite to gabbro sills

the great tonalite sill emplacement into a contractional shear zone and implications for late cretaceous to early eocene tectonics in southeastern alaska and british columbia the late cretaceous to early tertiary great tonalite sill of southeast alaska and british columbia is a very long 1 000 km and thin 25 km orogen parallel composite batholith which lies between the insular superter rane including the alexander and wrangellia terranes and the intermontane superterrane including the stikine and cache creek terranes the batholith is delineated by many steep sheet like plutons which are dominated by northwest southeast striking concordant fabrics with steep lineations that formed during deformation in a country rock shear zone prior to the complete crystallization of the magmas deformation in this shear zone is dominated by northeast southwest directed contraction orthogonal to the orogenic strike associated with a component of northeast over southwest high angle shear the steep multiple dike like nature of the body and its emplacement during orogenic contraction imply that ascent and emplacement have been achieved by dike wedging mechanisms along the deep reaching shear zone the remarkable narrowness of the great tonalite sill is probably the result of melting at the base of a very localized zone of thickened crust produced by the associated narrow contractional shear zone extending along the orogen length such a shear zone of late cretaceous to early tertiary age lying along 800 km of the possible boundary between the insular and intermontane superterranes implies that it may represent the actual boundary between them if this hypothesis is correct it implies that the large scale tectonic regime during emplacement of the great tonalite sill was predominantly orthogonal and not obliquely dextral as has been indicated from paleomagnetic data

crustal structure across the central alaska range anatomy of a mesozoic collisional zone a first order process in the growth of continents is the collision and accretion of terranes against continental margins collision leads to the formation of a suture zone between the accreted terrane and the former continental margin new insights on the suturing process are observed from two receiver function transects across the mesozoic alaska range suture zone three distinct crustal sections are identified from observations of crustal thickness intracrustal discontinuities and vp vs a northern section with 27 km thick crust of felsic to intermediate composition a central section that is 37 km thick that exhibits intracrustal discontinuities and has felsic to intermediate composition and a southern section that is 30 km thick and has a more mafic composition we interpret these sections to correspond with the former continental margin yukon composite terrane the suture zone proper and the allochthonous oceanic terrane wrangellia composite terrane the boundary between the yukon composite terrane and the suture zone appears to be a subhorizontal discontinuity that accommodated underthrusting of crust from the suture zone beneath the former continental margin the boundary between the suture zone and the wrangellia composite terrane in contrast appears to be a relatively discrete vertical boundary the observed variability in the crust across the alaska range suture zone is likely controlled by the differing compositions of the terranes involved which influences how each section responds to precollisional syncollisional and postcollisional deformation

two stage exhumation of midcrustal arc rocks coast mountains british columbia 1 new geologic data from the central gneiss complex along douglas channel help delineate the burial and exhumation of the late cretaceous to eocene coast mountains magmatic arc arc plutonism was on going between 90 and 60 ma replaced by formation of dikes to 52 ma supracrustal rocks were buried to midcrustal levels by 90 ma and garnet and kyanite grew from 90 through 70 ma subsequent exhumation was nearly isothermal and took place in two stages the first occurred from 70 to 59 ma as the arc contracted obliquely and sillimanite replaced kyanite exhumation was slow 0 5 mm yr probably accomplished by erosion aided by coaxial crustal thinning exhumation rates about doubled in the second stage of exhumation after 59 ma and before 52 ma penetrative deformation ended prior to intrusion of the quottoon pluton at 59 ma and 6 5 kbar production of cordierite rims on garnet at 4 5 kbar and 700 c signaled the end of near isothermal decompression prior to 52 ma rapid cooling 100 c 106 years of the central gneiss complex followed cordierite growth temperatures dropped to 250 by 48 ma the increase in exhumation rate and the subsequent rapid cooling are attributed to excision of 6 km of crust on a detachment system on the northeastern side of the central gneiss complex comparison to other parts of the coast mountains arc the sierra nevada and fiordland new zealand shows that the amount and tempo of exhumation vary greatly within and between arcs suggesting that the processes accommodating exhumation vary significantly

generation of new continental crust and terrane accretion in southeastern alaska and western british columbia constraints from p and s wave wide angle seismic data accrete abstract the accrete study addresses the question of continental assemblage in southeastern alaska and western british columbia through accretion of exotic terranes and generation of new crust by magmatic addition in a former continental arc we present results of wide angle p and s wave seismic interpretation of a 300 km long marine land seismic line across the contacts between accreted terranes and coast mountains additional constraints on the model are obtained from correlation with geologic mapping our results indicate that the coast shear zone csz is a nearly vertical fault zone probably related to a transpressive regime west of the csz the mid cretaceous 90 ma thrust belt is rooted in the deep crust and is truncated by the csz from the interpretation of the imaged sub vertical reflecting zones we infer the positions of the alexander wrangellia terrane boundary awb and of tertiary extensional grabens within dixon entrance near its intersection with the profile the observed values of vp and vp vs in the lower crust of the alexander terrane are similar to those of oceanic crust and distinctly different from the coast mountains batholith cmb to the northeast the crust under the cmb 32 km is thinner than the average continental crust and the moho is sharp 200 m and highly reflective the low velocity mantle 7 9 km s suggests high temperature consistent with the stability of garnet in mafic rocks in the lower crust the lower crustal velocity of 6 9 km s supports a lower crust composed of interlayered garnet pyroxene granulite and quartzofeldspathic restite related to batholith generation the crustal section under the cmb is seismically identical to the lower two thirds of normal crust heated and inflated by intrusions of tonalite and gabbro interlayered with restites from batholith generation and uplifted during exhumation

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

constraints and interpretation of 87sr 86sr ratios in cenozoic dolomites replacement dolomitization by seawater has been modeled in order to quantify the sr isotope signature in cenozoic dolomites as a function of precursor mineralogy and 87sr 86sr ratio reaction stoichiometry and 87sr 86sr ratio of the dolomitizing fluids high sr carbonates such as aragonite may introduce a significant precursor memory into an otherwise seawater dominated sr isotope signature if small quantities of seawater per unit volume of precursor carbonate are involved dolomitization of low sr carbonates i e low mg calcite are shown to create an isotopic signature indistinguishable from that of the seawater involved in the reaction therefore by comparison with the sr isotope evolution curve of seawater the 87sr 86sr ratios of the dolomites can be used to record the oldest possible age of dolomitization and the youngest age of deposition the implications for this approach have been applied to data obtained from a dolomitized core from little bahama bank bahamas two periods of dolomitization are recognized one in the early late miocene involving middle miocene or older rocks and a second one around 2 4 ma ago affecting early pliocene carbonates