accretion and subduction tectonics in the chugach mountains and copper river basin alaska initial results of the trans alaska crustal transect geologic seismic gravity and magnetic data from the northern chugach mountains and southern copper river basin alaska indicate that the chugach terrane cgt and the composite peninsular wrangellia terrane pet wrt are thin 10 km rootless sheets bounded on the south by north dipping thrust faults that sole into a shallow horizontal low velocity zone the cgt has been thrust at least 40 km beneath the pet wrt along the border ranges fault system brfs adjacent to the brfs uplift and erosion of 30 40 km since jurassic time have exposed blueschist facies rocks in the cgt and mafic and ultramafic cumulate rocks in the pet wrt four paired north dipping layers of low and high seismic velocities extend beneath the northern cgt and southern pet wrt and may be slices of subducted oceanic crust and upper mantle the upper two pairs may now be joined to the continental plate 15 references 5 figures

crustal structure of accreted terranes in southern alaska chugach mountains and copper river basin from seismic refraction results seismic refraction data were collected along a 320 km long transect line in southern alaska crossing the prince william chugach peninsular and wrangellia terranes and along several shorter lines within individual terranes velocity structure in the upper crust less than 9 km depth differs among the four terranes in contrast layers in the middle crust 9 to 25 km depth in some cases extend across projected terrane boundaries the following observations can be made 1 an intermediate velocity layer 6 4 km s at 9 km depth extends across the deep projection of the suture between the chugach and peninsular terranes suggesting that the northern chugach and southern peninsular terranes are detached and rest on a deeper terrane of unknown origin 2 the top of a gently north dipping sequence of low and high velocity layers 5 7 7 8 km s more than 10 km thick extends from near the surface in the southern chugach terrane to more than 20 km depth beneath the southern peninsular terrane this sequence truncated by the suture between the prince william and chugach terranes is interpreted to be an underplated terrane made up of fragments of the kula plate and its sedimentary overburden that were accreted during subduction in the late mesozoic and or early tertiary during or between times of accretion of the prince william and chugach terranes 3 a thick crustal root with a laminated sequence at its top extends from a depth of 19 km to as much as 57 km beneath the northern peninsular and wrangellia terranes this root extends across the deep projection of the suture between the peninsular and wrangellia terranes although resolution of this apparent crosscutting relationship is relatively poor this root may represent tectonically or possibly magmatically emplaced rocks the lower crust beneath the prince william chugach and southern peninsular terranes includes a north dipping 3 to 8 km thick section of subducting oceanic crust

upper crustal structure of the accreted chugach terrane alaska we have developed a model for the upper crustal structure of the chugach terrane of southcentral alaska from u s geological survey seismic refraction data using two dimensional asymptotic ray tracing the refraction profile acquired as part of the trans alaska crustal transect program extends approximately 135 km in an east west direction along the local strike of the chugach mountains the refraction survey consists of four shots recorded by 120 portable seismic instruments spaced at 1 km intervals we observe a velocity depth profile which has unusually high velocities at shallow depths and at least two velocity reversals the average velocity increases from 4 0 km s at the surface to 6 9 km s at a depth of 9 km near surface velocities in cretaceous valdez group rocks increase eastward following the metamorphic gradient observed by hudson and plafker 1982 lower layers may be correlated with rocks mapped to the south of the contact fault zone a suture boundary between the chugach and prince william terranes and units not having surface expression the first velocity reversal occurs at a depth of 9 km this 1 to 2 km zone has velocities which vary laterally from 6 0 to 6 9 km s and pinches out to the east in the profile this unit may represent a zone comprising tertiary intrusives and country rock it is followed at a depth of 10 km by a layer with velocities of 7 2 7 23 km s the second velocity reversal occurs at a depth of 12 km velocities within this layer range from 6 6 to 6 7 km s and may correlate with underplated oceanic crust higher velocities of 7 35 7 4 km s are attributed to a thin unit at a 14 km depth associated with ultramafic and mafic rocks the deepest unit is modelled as a 5 km layer having velocities of 7 2 7 25 km s this layer may be underplated prince william terrane rocks oceanic crust or some portion of the yakutat block the refraction data alone do not resolve deeper structure

the crustal structure of the axis of the great valley california from seismic refraction measurements abstract in 1982 the u s geological survey collected six seismic refraction profiles in the great valley of california three axial profiles with a maximum shot to receiver offset of 160 km and three shorter profiles perpendicular to the valley axis this paper presents the results of two dimensional raytracing and synthetic seismogram modeling of the central axial profile the crust of the central great valley is laterally heterogeneous along its axis but generally consists of a sedimentary section overlying distinct upper middle and lower crustal units the sedimentary rocks are 3 5 km thick along the profile with velocities increasing with depth from 1 6 to 4 0 km s the basement upper crust consists of four units 1 1 a 1 0 1 5 km thick layer of velocity 5 4 5 8 km s 2 2 a 3 4 km thick layer of velocity 6 0 6 3 km s 3 3 a 1 5 3 0 km thick layer of velocity 6 5 6 6 km s and 4 4 a laterally discontinuous 1 5 km thick layer of velocity 6 8 7 0 km s the mid crust lies at 11 14 km depth is 5 8 km thick and has a velocity of 6 6 6 7 km s on the northwest side of our profile the mid crust is a low velocity zone beneath the 6 8 7 0 km s lid the lower crust lies at 16 19 km depth is 7 13 km thick and has a velocity of 6 9 7 2 km s crustal thickness increases from 26 to 29 km from nw to se in the model although an unequivocal determination of crustal composition is not possible from p wave velocities alone our model has several geological and tectonic implications we interpret the upper 7 km of basement on the northwest side of the profile as an ophiolitic fragment since its thickness and velocity structure are consistent with that of oceanic crust this fragment which is not present 10 15 km to the west of the refraction profile is probably at least partially responsible for the great valley gravity and magnetic anomalies whose peaks lie about 10 km east of our profile the middle and lower crust are probably gabbroic and the product of magmatic or tectonic underplating or both the crustal structure of the great valley is dissimilar to that of the adjacent diablo range suggesting the existence of a fault or suture zone throughout the crust between these provinces

seismicity of the wrangell and aleutian wadati benioff zones and the north american plate along the trans alaska crustal transect chugach mountains and copper river basin southern alaska the configuration of the aleutian and wrangell wadati benioff zones in southern alaska between longitudes 142 w and 148 w is determined from regionally recorded earthquakes from 1971 to 1986 the earthquakes are located with a simple velocity model calibrated with seismic refraction measurements conducted for the trans alaska crustal transect tact west of the transect meridian approximately 145 20 w and beneath the chugach mountains shocks in the depth range 25 60 km compose the shallow part of the northwest to north dipping aleutian zone this zone can be traced updip and seaward beneath prince william sound east of the transect meridian and north of 61 n latitude the weakly active north northeast dipping wrangell zone extends to a depth of at least 100 km beneath the wrangell volcanoes and along strike for at least 150 km the two zones appear to be continuous at least in the depth range 20 45 km and may define adjacent limbs of a buckle in the subducted oceanic lithosphere north of 61 n focal mechanisms in the shallow aleutian zone are compatible with a uniform stress field characterized by in plane least compressive stress directed toward the steeply dipping deeper part of the aleutian zone north of cook inlet stress orientation in the wrangell zone is unresolved because of the low level of seismicity no focal mechanisms corresponding to slip on a gently northward dipping megathrust have been observed along the transect meridian a change of focal mechanism with depth near the top of the dipping seismic zone is inferred to reflect the interplate boundary beneath the chugach mountains this boundary coincides with a band of strong reflectors imaged by tact vertical seismic profiling the overriding plate appears to be aseismic beneath the northern chugach mountains but minor shallow seismicity occurs beneath the copper river basin first motion patterns for these shallow shocks suggest the most compressive stress within the overriding plate is oriented subhorizontally and approximately north south

the cenozoic denali fault system and the cretaceous accretionary development of southern alaska the juxtaposition of disparate geologic terranes in southern alaska has been previously interpreted to be mainly the result of several hundred kilometers of right lateral offset along the denali fault system in cenozoic time recent geologic investigations in the healy quadrangle strongly suggest that cenozoic horizontal displacements of such magnitude along the denali fault system do not exist in the healy quadrangle isograds and metamorphic facies boundaries of an early late cretaceous metamorphic belt trend across the cenozoic mckinley strand of the denali system without significant horizontal offsets the present geologic makeup of most of southern alaska is primarily the result of the talkeetna superterrane consisting of the previously assembled peninsular terrane and wrangellia colliding with and subsequently being thrust upon the yukon tanana and nixon fork terranes of the ancient north american continent in about middle cretaceous time the leading edge of the talkeetna superterrane faces a wide complexly deformed zone that contains numerous northwestward thrust miniterranes tectonically intermixed with jurassic and cretaceous flysch the flysch is interpreted to have been deposited mostly in the narrowing and subsequently collapsed oceanic basin between the converging continental blocks the postcollisional denali fault system developed in cenozoic time across the already accreted continental margin in eastern alaska along an older cretaceous suture

rupture process of the m 7 9 denali fault alaska earthquake subevents directivity and scaling of high frequency ground motions displacement waveforms and high frequency acceleration envelopes from stations at distances of 3 300 km were inverted to determine the source process of the m 7 9 denali fault earthquake fitting the initial portion of the displacement waveforms indicates that the earthquake started with an oblique thrust subevent subevent 1 with an east west striking north dipping nodal plane consistent with the observed surface rupture on the susitna glacier fault inversion of the remainder of the waveforms 0 02 0 5 hz for moment release along the denali and totschunda faults shows that rupture proceeded eastward on the denali fault with two strike slip subevents numbers 2 and 3 centered about 90 and 210 km east of the hypocenter subevent 2 was located across from the station at ps10 trans alaska pipeline pump station 10 and was very localized in space and time subevent 3 extended from 160 to 230 km east of the hypocenter and had the largest moment of the subevents based on the timing between subevent 2 and the east end of subevent 3 an average rupture velocity of 3 5 km sec close to the shear wave velocity at the average rupture depth was found however the portion of the rupture 130 220 km east of the epicenter appears to have an effective rupture velocity of about 5 0 km sec which is supershear these two subevents correspond approximately to areas of large surface offsets observed after the earthquake using waveforms of the m 6 7 nenana mountain earthquake as empirical green s functions the high frequency 1 10 hz envelopes of the m 7 9 earthquake were inverted to determine the location of high frequency energy release along the faults the initial thrust subevent produced the largest high frequency energy release per unit fault length the high frequency envelopes and acceleration spectra 0 5 hz of the m 7 9 earthquake can be simulated by chaining together rupture zones of the m 6 7 earthquake over distances from 30 to 180 km east of the hypocenter however the inversion indicates that there was relatively little high frequency energy generated along the 60 km portion of the totschunda fault on the east end of the rupture

a high resolution seismic reflection refraction study of the chugach peninsular terrane boundary southern alaska we present results from a high resolution seismic refraction analysis of the shallow approximately 2 km crustal structure along the 107 km long trans alaska crustal transect chugach reflection line in southern alaska and a comparison with laboratory measurements of field samples the refraction analysis includes the two dimensional interpretation of several thousand first and secondary arrival travel times digitized from 1024 channel split spread common shot gathers the velocity model derived from this analysis better defines the location and geometry of terrane boundaries than does the normal incidence reflection section and agrees well with surface mapping of lithologies furthermore the model predicts travel times within 100 ms of the reflection times recorded from the base of the quaternary on the chugach reflection section thicknesses of quaternary deposits with velocities between 1 1 and 2 0 km s correlate inversely with the quantity of observed lower crustal reflections on the chugach section suggesting that the presence or absence of these sediments in sufficient thickness exerted primary control on the quality of the deeper portion of the section there is a significant velocity contrast between crystalline rocks across the border ranges fault 5 0 versus 5 6 km s the major contact between the chugach and peninsular terranes in agreement with laboratory measurements of field specimens in the peninsular terrane the modeling indicates that an unnamed fault delimiting the southern flank of the copper river basin dips steeply northward at 50 and has about 1300 m of vertical offset laboratory measurements document a maximum velocity anisotropy of 20 for phyllitic schists of the valdez group in the chugach terrane in agreement with the observed e w strike and near vertical dip of the valdez group we determined a significant 14 velocity anisotropy for ray paths oriented n s versus ne sw

structural style and kinematics of an underplated slate belt kodiak and adjacent islands alaska the kodiak formation composed of coherent maastrichtian turbidites is a slate belt whose dominant structures developed during underplating to an accretionary wedge in the latest cretaceous it consists of about 80 coherent landward dipping thrust packets zones of disrupted sandstone associated with a scaly argillite matrix constitute the remainder about half of these disrupted sandstone zones are related to pre accretion deformation and the rest formed along late stage strike slip faults that postdate development of slaty cleavage the formation is divided into three structural belts the landward and seaward belts include steeply dipping structures and the central belt contains shallowly dipping structures and rocks that have experienced the highest strain the central belt probably acted as a low angle southeast verging floor thrust zone beneath the landward belt the structural history of the kodiak formation includes 1 early soft sediment disruption 2 tectonic stratal disruption 3 thrust faulting slaty cleavage s 1 development and folding f 1 4 intrusion of granodioritic plutons dikes and sills and associated normal faulting 5 development of crenulations f 2 and crenulation cleavage s 2 6 thrust faulting and 7 development of right lateral strike slip faults event 3 produced the dominant northwest dipping structural grain and caused the greatest amount of shortening the timing of event 7 relative to events 5 and 6 is not certain the dominant structures of the kodiak formation are interpreted as developing during underthrusting underplating and intra wedge shortening during latest cretaceous time within an accretionary wedge the maximum time from deposition to emplacement was about 12 m y and the timing is constrained by the maximum depositional age 74 m y and the age of plutonic rocks 62 m y that crosscut the dominant fabric pre cleavage zones of stratal disruption reflect deformation on the lower plate during underthrusting slaty cleavage thrusts and f 1 folds developed during underplating that also resulted in the formation of duplexes crenulations are probably related to post underplating subhorizontal shortening of sediments within the accretionary wedge syn deformation dynamic recrystallization of quartz minimum syn accretion metamorphic temperatures between 205 and 250 c and pressures of at least 2 65 kb recorded during the earliest stages of deformation suggest underplating at 10 km

seismic reflection images of the crust of the northern part of the chugach terrane alaska results of a survey for the trans alaska crustal transect tact deep crustal seismic reflection data show strong reflections from the middle and lower crust of the convergent continental margin near the eastern end of the aleutian trench these data were collected across the border ranges fault system a major suture zone that separates the peninsular and chugach tectonostratigraphic terranes the chugach terrane consists of metamorphosed strongly deformed flysch subduction melange as well as blueschist and greenschist facies rocks even so highly reflective rocks lie at great depth within or beneath this terrane the shallow part of the seismic section 0 5 s reveals only a few reflections which undulate and interweave indicating that some of them originate outside of the plane of the seismic section other undulating events could reveal early cenozoic granitic plutons or antiformal stacks of rock imbricated along thrust faults in contrast to this shallow reflection pattern three reflection bands contain most of the events on the middle part of the seismic section 5 12 s or 12 34 km we interpret divergent reflections within the uppermost band 5 5 5 s as gently north dipping thrust faults and subparallel reflections truncated at the boundaries of the upper and middle bands may image a metamorphic foliation or thrust faults if the truncated reflections represent thrust faults then the reflection geometry suggests that subparallel roof and floor thrust faults bound imbricated rocks forming duplex structures the upper and middle bands are separated by an area on the seismic section that reveals few reflections this area correlates approximately in depth with a high velocity 7 5 km s layer evident in refraction models the poorly reflective high velocity unit might be volcanic rocks like those exposed in the structurally lowest part of the chugach terrane if so then the bottom of the poorly reflective rocks indicates the base of the chugach terrane rocks that cause the upper and middle reflection bands and the intervening poorly reflective volcanic rocks may form a midcrustal shear zone that is about 10 km thick the deepest band of subparallel reflections 30 35 km correlates closely in depth with the top of the wadati benioff zone associated with the underthrusting pacific plate the reflections probably stem from the decollement that separates lithospheric plates within the subduction zone this reflection band increases abruptly in thickness possibly indicating local underplating of subducted sediment no events from the oceanic moho have been recognized neither the border ranges fault system the suture between the chugach and peninsular terranes nor deep crustal layers of the peninsular terrane are evident in these seismic reflection data

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

hydrologic framework of a sabkha along arabian gulf during the last 4 000 to 5 000 years sedimentary offlap and a relative fall in sea level have resulted in the development of broad gently seaward sloping about 1 3 000 planar areas or sabkhas along much of the south shore of the arabian persian gulf the depth of the water table below the sabkha surface increases with distance from the sea until it reaches 1 0 to 1 5 m after which it remains constant when the water table depth is in the range of 1 0 to 1 5 m the sabkha surface has reached a state of deflational equilibrium the water table beneath the sabkha always slopes seaward indicating that the direction of ground water flow is toward the sea the subsurface flow system beneath the sabkha is part of a regional seaward flowing groundwater regime in the seaward part of the sabkha a downward flow component is superimposed on the general seaward ground water flow pattern as a result of the infiltration and percolation of water from shallow bodies of seawater that are propelled inland over the sabkha surface by strong onshore winds farther inland where marine flooding does not occur the vertical flow component is directed upward in response to evaporative losses at the sabkha surface the net rate of evaporation from the central parts of the sabkha is on the order of 6 cm factor of 2 of ground water per year a regional ground water flow system which causes the water table level to rise as the sabkha progrades appears to be a requirement for the development and the preservation of broad sabkhas fluctuations in sea level during sabkha development influence the rate of progradation and the thickness of sedimentary facies the refluxing of sabkha brines beneath the lagoons in the abu dhabi area indicates a mechanism for preserving anhydrite during marine transgressions thus permitting the development of stacked sabkha successions which are common in the rock record

crustal structure in the northwestern foothills of the sierra nevada from seismic refraction experiments abstract analysis of travel times from a reversed active seismic refraction experiment in the western foothills of the sierra nevada reveals a localized layer of high velocity material within 10 km of the surface this layer with a velocity of 7 0 km sec has a five fold increase in dip approximately 25 km south of the epicenter of the 1 august 1975 magnitude 5 7 earthquake near oroville california we interpret this material to be a remnant of buried ophiolite these data also suggest a 5 per cent anisotropy in the near surface p wave velocity the compressional velocity is 6 3 km sec parallel to the strike of the sierra range and 6 0 km sec normal to it a series of aftershocks to the magnitude 5 7 oroville earthquake recorded on a linear array of portable seismographs extending 300 km southward along the west edge of the sierra nevada foothills show an apparent p n velocity of 7 8 km sec the mohorovicic discontinuity is located between 30 and 39 km depth this wide range of depth reflects the facts that the profile is unreversed the receivers are widely spaced and the origin times and depths of the sources are not exactly known

paleogene metamorphism of an accretionary flysch terrane eastern gulf of alaska reconnaissance field studies have identified a regional metamorphic complex at least 200 km long and 25 to 50 km wide in the eastern chugach mountains southern alaska the metamorphic complex was developed within the cretaceous valdez group a flysch sequence that was accreted to the continental margin during the late cretaceous or early tertiary the metamorphic complex shows the progressive development from subgreenschist and green schist facies slate phyllite and semischist that is regionally characteristic of the valdez group to biotite plagioclase quartz schist muscovite cordierite andalusite staurolite garnet and sillimanite in an intermediate zone and amphibolite facies muscovite biotite plagioclase quartz schist and gneiss sillimanite and garnet in a higher grade central zone k feldspar is characteristically absent in the amphibolite facies schists but it is present in migmatitic rocks that are common to the central zone the mineral assemblages and petrographic relations indicate that metamorphism took place under low pressure intermediate facies series conditions partial melting took place at the highest grade of metamorphism melting produced granodioritic magmas in migma titic rocks of the central zone that were emplaced as discrete plutons in both the intermediate zone and peripheral rocks of the complex the timing of metamorphism as indicated by field relations and k ar data is about 50 to 65 m y ago and overlaps or is coeval with the emplacement throughout the gulf of alaska region of early tertiary plutons that are thought to be anatectic in origin the distribution of these plutons suggests that a high temperature low pressure metamorphic belt about 1 800 km long developed along the leading edge of the continental margin in late paleocene to early eocene time

two dimensional velocity models of the nazca plate subduction zone between 19 5 s and 25 s from wide angle seismic measurements during the cinca95 project the interpretation of seismic refraction wide angle reflection data from the 1995 crustal investigations off and on shore nazca central andes cinca95 project has resulted in the derivation of nine e w two dimensional 2 d velocity cross sections for the region between the peru chile trench and the coast between 19 5 s and 25 s with three of the cross sections extending a farther 100 200 km inland these sections define the major lithospheric structures of the upper south american plate and the lower nazca plate down to uppermost mantle depths of 30 60 km beneath this part of the present day forearc region in addition to showing the nazca plate subducting at an increasing angle of 9 25 down to 30 50 km depth near the coast these cross sections show a portion of the moho dipping eastward from 43 50 km near the coast to 55 64 km up to about 240 km inland owing to a gap of almost 50 km in the data coverage of the moho near the coast it is uncertain from this data set alone whether the moho defined east of the coast should be correlated with the lower oceanic nazca plate or the upper continental south american plate however a comparison with other seismological data suggests that the moho identified here east of the coast defines the base of the continental crust of the upper south american plate immediately behind the downgoing nazca plate thus the hypothesis of delouis et al 1996 that the zone of seismic coupling between the plates correlates with the region where continental crust is in contact with the plate boundary is supported the cross sections also show between 20 s and 22 5 s a boundary extending from upper crustal levels down to 25 30 km depth subparallel to and 5 10 km above the top surface of the subducting nazca plate the discussion focuses on either that the boundary is a fossil extensional structure or preferably that the wedge below the boundary represents gabbroic material from layer 3 of the oceanic crust tectonically underplated onto the lower part of the upper plate

crustal structure of the chugach mountains southern alaska a study of peg leg multiples from a low velocity zone a seismic refraction profile recorded along the geologic strike of the chugach mountains in southern alaska shows three upper crustal high velocity layers 6 9 7 2 and 7 6 km s and a unique pattern of strongly focussed echelon arrivals to a distance of 225 km the group velocity of the ensemble of echelon arrivals is 6 4 km s modeling of this profile with the reflectivity method reveals that the echelon pattern is due to peg leg multiples generated from within a low velocity zone between the second and third upper crustal high velocity layers the third high velocity layer 7 6 km s is underlain at 18 km depth by a pronounced low velocity zone that produces a seismic shadow zone wherein peg leg multiples are seen as echelon arrivals the interpretation of these echelon arrivals as multiples supersedes an earlier interpretation which attributed them to successive primary reflections arising from alternating high and low velocity layers synthetic seismogram modeling indicates that a low velocity zone with transitional upper and lower boundaries generates peg leg multiples as effectively as one with sharp boundaries no pmp or pn arrivals from the subducting oceanic moho at 30 km depth beneath the western part of the line are observed on the long offset 90 225 km data this may be due to a lower crustal waveguide whose top is the high velocity 7 6 km s layer and whose base is the moho a deep 54 km reflector is not affected by the waveguide and has been identified in the data although peg leg multiples have been interpreted on some long range refraction profiles that sound to upper mantle depths the chugach mountains profile is one of the few crustal refraction profiles where peg leg multiples are clearly observed this study indicates that multiple and converted phases may be more important in seismic refraction wide angle reflection profiles than previously recognized

two dimensional velocity structure along the synclinal axis of the great valley california abstract in 1981 and 1982 the u s geological survey conducted a seismic refraction study of the great valley central california in order to obtian a better understanding of the deep crustal structure and tectonic evolution of the region we present the interpretation of a 147 km long seismic refraction profile recorded along the synclinal axis of the valley the northern 98 km of the profile are reversed whereas the remaining southern portion is unreversed observed travel times and amplitudes of primary and secondary arrivals from three shot points have been modeled using two dimensional ray tracing techniques the resultant p wave velocity cross section consists of a 27 km thick crust whose isovelocity contours generally rise gently to the southeast the crust is described in six layers with average velocities average layer thickness in parentheses of 2 8 km sec 3 km 4 25 km sec 3 km 5 5 km sec 2 km 6 1 km sec 6 km 6 75 km sec 6 km and 7 2 km sec 7 km the second through fifth layers are separated by 1 km thick high gradient transition zone the lower crustal boundary 6 75 7 2 km sec and the moho 7 2 8 1 km sec are modeled with first order discontinuities the first layer cenozoic sedimentary deposits is characterized by a velocity gradient of 0 7 km sec km starting with a velocity of 1 7 km sec at the surface the second layer 4 25 km sec consists of the cretaceous great valley sequence and shows a very modest velocity gradient of 0 2 km sec km the third layer 5 5 km sec basement may consist of metasedimentary rocks of the franciscan assemblage fractured crystalline rocks or possibly serpentinized ophiolite the fourth layer 6 1 km sec apparently has no velocity equivalent 30 km to the west in the diablo range it and the layer beneath it 6 75 km sec may be correlated with the metamorphic and igneous rocks of the eastern foothills of the sierra nevada the basal crustal layer 7 2 km sec is evidently mafic igneous rock an independent interpretation of this profile using the reflectivity method produced a similar model an interpretation of a parallel profile 10 to 15 km east of our line shows a structure similar to our model below 15 km but a thinner sedimentary section and velocities as much as 0 9 km sec faster 6 to 15 km deep the crustal velocity structure of the central great valley is atypical of either continental or oceanic crust but is consistent with the geologic hypotheses that the valley is a thickened marginal basin formed in either a forearc or backarc setting located west of the mesozoic sierra nevadan arc

the border ranges ultramafic and mafic complex south central alaska cumulate fractionates of island arc volcanics a discontinuous elongate zone of mafic and ultramafic plutonic rock crops out in south central alaska for a distance of more than 1000 km intermediate and detailed scale geologic mapping petrographic study and compositional data suggest that the plutonic rocks are compositionally petrologically and mineralogically distinct from rocks in mid ocean ridge and back arc basin ophiolites the mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc the mafic ultramafic zone referred to as the border ranges ultramafic and mafic complex brumc is composed of ultramafic cumulates gabbronorite cumulates and massive gabbronorites a very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the brumc a thick sequence of andesitic volcanic rocks the talkeetna formation of early jurassic age lies to the north of and structurally above the mafic ultramafic zone voluminous calcalkaline plutons composed of quartz diorite

northward displacement and accretion of wrangellia new paleomagnetic evidence from alaska wrangellia is a vast terrane that accreted along the pacific margin of north america from oregon to central alaska previous paleomagnetic studies of the triassic volcanic rocks of wrangellia in south central alaska and british columbia have given paleolatitudes of 10 17 which are anomalously low in comparison to results from similar aged rocks of nuclear north america the discrepancy in paleolatitude implies substantial northward drift of the terrane relative to the craton we have extended paleomagnetic sampling to the northern boundary of the terrane in the mount hayes and healy quadrangles of the central alaska range this collection of 46 triassic lava flows yields a mean paleolatitude of 13 9 95 3 8 the polarity of the data and hence the hemisphere of origin are not resolved with certainty in contrast triassic paleomagnetic poles from the stable part of north america predict a paleolatitude of 42 1 n 95 4 6 for the region the northernmost fragment of wrangellia probably reached mainland alaska in the middle cretaceous when upper jurassic and lower cretaceous flysch within the intervening basin underwent severe deformation tertiary transcurrent faulting has modified the positions of wrangellian fragments in alaska but such displacements are probably minor as indicated by paleomagnetic data from early tertiary volcanic rocks that overlap the terrane

wrangell benioff zone southern alaska the first unequivocal evidence for the existence of a benioff zone that may be related to the quaternary wrangell volcanoes comes from a set of 86 well located hypocenters for earthquakes smaller than about magnitude 4 in the region between lat 61 and 62 5 n and between long 142 and 145 5 w about half of the earthquakes occur at depths of 25 km or less below 40 km a clearly defined north northeast dipping zone of seismicity here termed the wrangell benioff zone extends to a depth of about 85 km and continues for about 115 km along strike subparallel to the volcanic trend the western end of the zone may be offset from the northern end of the much more active aleutian benioff zone where the benioff zone shoals to 30 to 40 km it becomes nearly horizontal and cannot be clearly distinguished from upper plate seismicity it is uncertain whether the subducted plate segment that contains the wrangell benioff zone is structurally part of the pacific plate or the yakutat block

reflected and mode converted seismic waves within the shallow aleutian subduction zone southern kenai peninsula alaska pronounced secondary phases observed in local recordings of quarry shots and earthquakes on the southern kenai peninsula alaska are identified as reflected p and s and converted s to p phases originating within four depth ranges in the upper few kilometers of the cook inlet tertiary basin at midcrustal depths within the overthrust north american plate at about 35 km depth near the top of the wadati benioff seismic zone in proximity to the inferred interplate megathrust and at about 5 10 km below the megathrust in the subducted pacific plate beneath the landward margin of the accreted chugach terrane the mid upper plate mup discontinuity dips as steeply as 20 degree 30 degree to the west northwest over the depth range 12 18 km at shallower depths it flattens and possibly arches with the crest at about 10 km depth similar midcrustal reflectors have been imaged about 125 km to the southwest and about 350 km to the northeast along the structural trend of the chugach terrane the extensive reflectors may have a common origin possibly caused by the presence of underplated rocks relative amplitudes and polarities of the secondary phases originating at mup depths provide few constraints on the nature of the discontinuity it appears that the mupmore discontinuity is seismically inactive and does not represent a brittle ductile transition zone within the upper plate the two converted s to p phases generated near the top of the subducted plate could indicate a low velocity zone associated with subducted oceanic crust less

the 1987 1992 gulf of alaska earthquakes abstract we present a study of the 1987 1992 gulf of alaska earthquake sequence using relocated seismicity data together with body wave analysis of selected larger events most of the sequence is located on a n s trending fault directly south of the yakataga seismic gap and consists of four main events of mw 7 2 7 8 7 7 and 6 8 and associated aftershocks the first earthquake is of left lateral strike slip type on an ene wsw trending fault the second event is a right lateral strike slip earthquake on the n s trending fault the fault plane of the third strike slip event was identified as an ene wsw trending fault located to the south of that for the first event in a previous study using body wave analysis we show that though the body wave study cannot unambiguously identify the fault plane the temporal development of the seismicity together with the pattern of aftershock distribution on the conjugate fault suggests that this event also occurred on the n s trending fault and is of right lateral type the seismicity on the conjugate fault is interpreted as being triggered by the increase of the shear stress in the direction of the normal to the fault plane due to the main shock the occurrence of the fourth main shock a right lateral strike slip event in 1992 which itself can be considered an aftershock of the 1987 1988 sequence with epicentres distributed along a n s trending fault favours this conclusion the first three events of the sequence have been described in earlier studies as having too short a rupture length for their seismic moment if the rupture lengths inferred from the aftershock zones in this study are used we find that this is not the case the events in the 1987 1992 sequence lie along magnetic anomaly 13 and related conjugate fracture zones indicating that the oceanic crust is rupturing along pre existing zones of weakness in response to plate boundary stresses at the southern end the seismic activity terminates near a seamount which rises 1 km above an otherwise relatively low relief ocean floor and close to a fracture zone perpendicular to the n s trend of the earthquake sequence suggesting that the n s striking rupture along the n s trending fault is terminated here by a barrier at the northern end the sequence terminates at the alaska trench near the yakataga seismic gap where there is a right lateral offset in the bathymetry along the transition fault this offset also being visible on the gravity map of the region thus this offset has the same right lateral sense as the n s trending strike slip fault planes of three of the main shocks and suggests that though there was no significant seismic activity in this region of the gulf of alaska since 1964 faulting here has been active in the past

p wave velocity and anisotropy of lawsonite and epidote blueschists constraints on water transportation along subducting oceanic crust p wave velocity vp and the anisotropy of lawsonite and epidote blueschists were measured up to 1 0 gpa and 400 c using the ultrasonic pulse transmission technique the slowest vp in the direction normal to foliation is similar between lawsonite and epidote blueschists 7 0 7 2 km s at 1 0 gpa and room temperature while the fastest vp in the direction parallel to lineation markedly differs between lawsonite blueschists 7 4 7 6 km s at 1 0 gpa and room temperature and epidote blueschist 7 9 km s at 1 0 gpa and room temperature crystallographic orientation measurements for main constituent min erals revealed that both epidote 0 1 0 axes fastest vp direction in epidote single crystal and amphibole 0 0 1 axes fastest vp direction in amphibole single crystal are preferentially oriented parallel to lin eation to enhance vp anisotropy of the epidote blueschist in contrast lawsonite 0 0 1 axes fastest vp direction in lawsonite single crystal are oriented subnormal to foliation whereas amphibole 0 0 1 axes are oriented subparallel to lineation so that relatively weak vp anisotropy was observed in the lawsonite blueschist our experimental results in conjunction with recent seismological observations suggest that the vp of the subducting oceanic crust at 50 km has been observed in several subducting slabs however the high vp values at 50 km depth are difficult to be explained by blueschists this indicates that the blueschist would be at least partially transformed to hydrous mineral bearing eclogite at 50 km depth in subducting oceanic crusts

deep crustal seismology of continental margins this review discusses results from recent seismic experiments at continental margins conducted by u s investigators as well as those by non u s investigators necessary to place the results of u s workers into context continental margins may be characterized as either rifted transform or convergent rifted or passive margins form where continents are torn apart during continental drift transform margins accommodate transform plate motions along strike slip faults such as the san andreas fault at convergent margins oceanic crust is being thrust beneath or subducted under the continental crust all three types of margins are found in north america

the elasticity of lawsonite at high pressure and the origin of low velocity layers in subduction zones abstract subduction zones exhibit faster seismic wave velocities compared to the surrounding mantle due to the recycling of relatively cold oceanic lithosphere in certain subduction zones however a 5 10 km thick low velocity layer lvl has been inferred to exist along the top surface of the subducting slab at depths of up to 250 km shear wave velocities in particular within these layers have been estimated as up to 10 slower than the surrounding mantle we have conducted high pressure ultrasonic interferometric measurements to gain insight into the elastic properties of lawsonite caal 2 si 2 o 7 oh 2 h 2 o a hydrous mineral phase stabilized under cold subduction zone conditions in addition we have computed the full elastic constant tensor at elevated pressures and temperature using static electronic structure calculations and molecular dynamics simulations the bulk and shear modulus obtained from theory and experiments are in good agreement we find that lawsonite has an unusually low shear modulus at high pressure and its formation in subducted oceanic crust can explain some seismic evidence for lvl at depths exceeding 100 km to approach estimated lvl velocities requires lawsonite to form in the subducting crust as a result of a fluid influx due to the breakdown of other hydrous minerals such as serpentine the formation of lawsonite additionally lowers seismic velocities because it forms at the expense of garnet a mineral with relatively fast seismic velocities lvl observations may therefore be used to place constraints on the amount of h 2 o subducted into the deep mantle

possible seismic reflections from the downgoing pacific plate 275 kilometers arcward from the eastern aleutian trench multichannel seismic reflection data collected near the southern kenai peninsula alaska show deep events that could stem either from a linear diffractor or from a curved reflector whichever is the cause it must be at a minimum depth of 12 km if a linear diffractor causes the most obvious events it strikes at a 60 angle across the surface trend of mesozoic rocks thus the diffractor could be associated with a transverse tectonic boundary that on the basis of other evidence is thought to strike through the area of this study alternatively these deep events could be reflections from rocks associated either with a detached piece of oceanic crust or with the downgoing pacific plate these associated rocks could be porous and overpressured sedimentary rocks that are squeezed between indurated or metamorphosed sedimentary rocks in the overthrust plate and igneous rocks in the downgoing oceanic crust

crustal structure beneath exposed accreted terranes of southern alaska summary the crustal structure beneath the exposed terranes of southern alaska has been explored using coincident seismic refraction and reflection profiling a wide angle reflector at 8 9 km depth at the base of an inferred low velocity zone underlies the peninsular and chugach terranes appears to truncate their boundary and may represent a horizontal decollement beneath the terranes the crust beneath the chugach terrane is characterized by a series of north dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle this layered series may continue northward under the peninsular terrane earthquake locations in the wrangell benioff zone indicate that at least the upper two low high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust the refraction data suggest that the contact fault between two similar terranes the chugach and prince william terranes is a deeply penetrating feature that separates lower crust deeper than 10 km with paired dipping reflectors from crust without such reflectors

relation between the subducting plate and seismicity associated with the great 1964 alaska earthquake tectonic studies of the great 1964 alaska earthquake have underappreciated the nature of the subducted plate in influencing seismicity we compare seismological observations in the prince william and kodiak areas that ruptured during this earthquake with the corresponding morphology and structure of the subducting plate the upper plate geology prince william terrane and velocity structure are the same in both areas in the prince william area where the yakutat terrane subducted the energy released and coupling were stronger than above the kodiak subduction zone where thick trench sediment subducts the conjecture that lower plate character or the amount of subducted sediment affects coupling helps explain variability in seismology geodetic inversions and the horizontal velocity of gps stations

a comprehensive study of the seismicity of the kenai peninsula cook inlet region south central alaska abstract we compare 30 yr of background seismicity of the kenai peninsula cook inlet region extending from eastern prince william sound kayak island to west of the cook inlet volcanic arc to potential field gravity magnetics tomographic global positioning system geodesy and geologic information to better determine factors controlling seismicity we also compare the occurrence of m w 5 events over the past 80 100 yr to background seismicity in an effort to determine how reflective background seismicity is of the regional seismic hazard our results indicate that contrasts in upper plate rheology leading to changes in density magnetic susceptibility and velocity of the upper plate serve to concentrate seismicity the southwestern edge of the subducting yakutat microplate has a strong influence on both upper and lower plate seismicity density and velocity contrasts also indicate changes in lower plate rheology that influence the location of deeper 30 km seismicity and the occurrence of a double seismic zone within the subducting pacific plate

the crustal structure of the wrangellia terrane along the east glenn highway eastern southern alaska recently acquired seismic refraction data from eastern southern alaska provide new information on the structure and composition of the wrangellia and adjacent terranes the data comprise a 160 km long refraction profile along the east glenn tok cutoff highway that was collected as part of the u s geological survey s multidisciplinary trans alaska crustal transect program the upper 3 km of the wrangellia terrane and associated rocks is characterized by low compressional wave velocities vp 1 9 3 3 4 6 5 6 km s 1 and high velocity gradients common to most onshore seismic refraction profiles there is also clear seismic expression of the west fork fault system as a steep down to the southwest fault that separates the peninsular terrane to the southwest and the metamorphic complex of gulkana river to the northeast in contrast no seismic expression occurs for the paxson lake fault which separates the wrangellia terrane from the metamorphic complex of gulkana river adjacent to the denali fault within the wrangellia terrane two high velocity bodies vp 6 6 km s 1 occur in the upper crust one of these extends to 10 km depth and correlates with a late paleozoic dioritic complex suggesting that the wrangellia terrane is at least 10 km thick in this part of alaska from 5 to 23 km depth the crust appears seismically homogeneous with velocity increasing from vp 6 2 to vp 6 6 km s 1 beneath this level the crust is less well resolved although evidence exists for a low velocity zone between 23 and 26 km and a possible southwest dipping interface at 35 km no identifiable mantle refraction or reflection is observed possibly indicating a crust as thick as 55 km the relatively low seismic velocities in the upper 23 km of the crust compare favorably with laboratory measured velocities on pelitic schists and intermediate composition plutonic rocks granites and granodiorites both of which are recognized in wrangellia we interpret the seismic velocities to indicate that silicic to intermediate composition rocks are important constituents of the basement of this part of wrangellia geologic evidence indicates that the alaskan part of the wrangellia terrane is a paleozoic and mesozoic island arc our seismic evidence indicates it may have been built mostly on continental crust as opposed to the fragment of wrangellia from vancouver island which was probably built on oceanic crust

ray equation migration of wide angle reflections from southern alaska a kirchhoff style prestack depth migration algorithm is implemented for wide angle reflections in two dimensional variable velocity acoustic media extrapolation of both source and recorder wavefields is performed by ray tracing the imaging condition at each point in the image space is the total source to recorder travel time since the migration algorithm is based on ray tracing migration can be performed for any medium through which rays can be traced there is no inherent limitation on the size of the region to be imaged regions up to 3 6 103 km2 are utilized in this study the algorithm is illustrated by migration of two synthetic and three real data sets from the u s geological survey s trans alaska crustal transect program images are produced of inferred subduction assemblages in southern alaska emphasis in this paper is on obtaining reflector positions and shapes rather than detailed structural interpretations

catastrophic landslides glacier behaviour and moraine formation a view from an active plate margin the influence of large bedrock landslides rock avalanches on the behaviour of glaciers is incompletely recognised here we present an example from an active tectonic margin in south island new zealand where large earthquakes leave a significant imprint on glacial records we demonstrate that terminal moraines on the western side of the southern alps record both ordinary i e climate driven and landslide initiated glacial advances following consideration of the processes involved in rock avalanche initiated moraine construction we suggest ways of determining the nature of the advance that built the terminal moraine the implications of these observations are important in breaking the conventional linkage of individual terminal moraines with climate forcing

west margin of north america a synthesis of recent seismic transects abstract a comparison of the deep structure along nine recent transects of the west margin of north america shows many important similarities and differences common tectonic elements identified in the deep structure along these transects include actively subducting oceanic crust accreted oceanic arc or oceanic like lithosphere of mesozoic through cenozoic ages cenozoic accretionary prisms mesozoic accretionary prisms backstops to the mesozoic prisms and undivided lower crust not all of these elements are present along all transects in this study nine transects including four crossing subduction zones and five crossing transform faults are plotted at the same scale and vertical exaggeration v e 1 1 using the above scheme for identifying tectonic elements the four subduction zone transects contain actively subducting oceanic crust cenozoic accretionary prisms and bodies of basaltic rocks accreted in the cenozoic including remnants of a large oceanic plateau in the oregon and vancouver island transects rocks of age and composition eocene basalt similar to the oceanic plateau are currently subducting in southern alaska where they are doubled up on top of pacific oceanic crust and have apparently created a giant asperity or impediment to subduction most of the subduction zone transects also contain mesozoic accretionary prisms and two of them vancouver island and alaska also contain thick tectonically underplated bodies of late mesozoic early cenozoic oceanic lithosphere interpreted as fragments of the extinct kula plate in the upper crust most of the five transform fault transects all in california reflect 1 tectonic wedging of a mesozoic accretionary prism into a backstop which includes mesozoic early cenozoic forearc rocks and mesozoic ophiolitic arc basement rocks and 2 shuffling of the subduction margin of california by strike slip faulting in the lower crust they may reflect migration of the mendocino triple junction northward seen in rocks east of the san andreas fault and cessation of farallon plate subduction seen in rocks west of the san andreas fault in northern california lower crustal rocks east of the san andreas fault have oceanic crustal velocity and thickness and contain patches of high reflectivity they may represent basaltic rocks magmatically underplated in the wake of the migration of the mendocino triple junction or they may represent stalled subducted fragments of the farallon gorda plate the latter alternative does not fit the accepted slabless window model for the migration of the triple junction this lower crustal layer and the moho are offset at the san andreas and maacama faults in central california a similar lower crustal layer is observed west of the san andreas fault west of the continental slope it is pacific oceanic crust but beneath the continent it may represent either pacific oceanic crust stalled subducted fragments microplates of the farallon plate or basaltic rocks magmatically underplated during subduction of the pacific farallon ridge or during breakup of the subducted farallon plate the transect in southern california is only partly representative of regional structure as the structure here is 3 dimensional in the upper crust a mesozoic prism has been thrust beneath crystalline basement rocks of the san gabriel mountains and mojave desert in the mid crust a bright reflective zone is interpreted as a possible master decollement that can be traced from the fold and thrust belt of the los angeles basin northward to at least the san andreas fault a moho depression beneath the san gabriel mountains is consistent with downwelling of lithospheric mantle beneath the transverse ranges that appears to be driving the compression across the transverse ranges and los angeles basin

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

character distribution and tectonic significance of accretionary terranes in the central alaska range the central part of the alaska range near mount mckinley is composed of nine separate tectonostratigraphic terranes that were accreted in southern alaska during late mesozoic time these terranes now form long linear fault bounded belts that are subparallel to the denali fault on the north but oblique to the fault on the south the postaccretion right lateral offset along the denali fault system is about 200 km from north to south the major terranes are 1 yukon tanana terrane metasedimentary and metavolcanic rocks mostly undated but including rocks of known late paleozoic age polymetamorphosed with terminal events in late mesozoic 2 pingston terrane isoclinally folded upper triassic deep water silty limestone quartzite and carbonaceous slate folded with upper paleozoic phyllite chert tuff and minor limestone 3 mckinley terrane upper paleozoic flysch chert and minor limestone intruded by large gabbro sills and dikes and overlain by thick piles of triassic porphyritic pillow lava the top of the section is thick sequence of upper mesozoic conglomerate flysch chert and phyllite 4 dillinger terrane very thick sequence of strongly folded lower paleozoic micaceous sandstone turbidites graptolitic shale and deep water limestone locally overlain unconformably by jurassic fossiliferous sandstone or triassic pillow basalt 5 windy terrane heterogeneous assemblage of serpentinite basalt tuff and chert ophiolite with paleozoic and mesozoic flysch and blocks of mid paleozoic fossiliferous limestone 6 mystic terrane predominantly upper paleozoic flysch and conglomerate but also includes lower paleozoic graptolitic shale pillow basalt and shallow water limestone and upper paleozoic fossiliferous limestone sandstone chert and undated pillow basalt 7 chulitna terrane upper devonian ophiolite overlain by upper paleozoic chert volcanic conglomerate limestone and flysch capped by lower triassic limestone and upper triassic red beds basalt and interbedded basalt and limestone later mesozoic rocks are sandstone chert and argillite 8 westfork terrane jurassic chert sandstone conglomerate and triassic and jurassic crystal tuff 9 broad pass terrane upper paleozoic chert tuff and argillite with blocks of devonian and older limestone locally associated with serpentinite these diverse terranes of mixed oceanic and continental affinities are now juxtaposed to form a complex sequence of folded and faulted rootless nappes major suture zones between them are occupied by intensely deformed upper mesozoic flysch all of the terranes differ markedly from wrangellia the southermost coherent accreted terrane in alaska paleomagnetic measurements from triassic rocks prove that wrangellia originated near the equator probably in the southern heimsphere paleolatitudes for terranes in the central alaska range are not yet determined but other lines of evidence biogeographic and lithologic suggest large scale northern transport for some terranes

seismological evidence for a sub volcanic arc mantle wedge beneath the denali volcanic gap alaska 1 arc volcanism in alaska is strongly correlated with the 100 km depth contour of the western aluetian wadati benioff zone above the eastern portion of the wadati benioff zone however there is a distinct lack of volcanism the denali volcanic gap we observe high poisson s ratio values 0 29 0 33 over the entire length of the alaskan subduction zone mantle wedge based on regional variations of pn and sn velocities high poisson s ratios at this depth 40 70 km adjacent to the subducting slab are attributed to melting of mantle wedge peridotites caused by fluids liberated from the subducting oceanic crust and sediments observations of high values of poisson s ratio beneath the denali volcanic gap suggest that the mantle wedge contains melted material that is unable to reach the surface we suggest that its inability to migrate through the overlying crust is due to increased compression in the crust at the northern apex of the curved denali fault

an 1800 km cross section of the lithosphere through the northwestern north american plate lessons from 4 0 billion years of earth s history the lithoprobe slave northern cordillera lithospheric evolution snorcle study across northwestern north america in combination with related crustal studies has been synthesized into an 1800 km long cross section of the lithosphere that is constrained by high resolution geophysical data seismic reflection refraction electromagnetic potential fields and detailed bedrock geology the cross section offers one of the longest continuous profiles of the continental lithosphere anywhere in the world that is constrained by combined geophysical measurements and electromagnetic properties and exposed bedrock geological relationships the primary conclusion of the study is that during all major orogenic episodes recorded from archean to present in that part of earth s lithosphere the crust and perhaps much of the mantle was reorganized and redistributed rather than being differentiated from the mantle at the time of orogenesis the observed subsurface geometries of relict subduction zones accretion

petrologic and age constraints on the origin of a low pressure high temperature metamorphic complex southern alaska the interrelationships between metamorphism deformation magma intrusion and 40ar 39ar geochronology were determined for a low pressure high temperature metamorphic complex which formed from an accretionary prism in the chugach mountains southern alaska compressional deformation which first produced south verging folds and associated thrusts was followed by magma intrusion and development of north verging folds synmetamorphic southward directed thrusting of metamorphosed flysch over flysch produced increased load in the footwall as documented by the distribution of mineral assemblages and by pressure temperature modeling of garnet growth the initial heating to the greenschist facies may have been accomplished by a combination of advective heating from aqueous fluids and of conductive heating from subducted young oceanic crust regionally developed amphibolite facies metamorphism followed intrusion of felsic sills the peak metamorphic conditions derived from geothermobarometry mineral assemblages and fluid inclusions ranged from 400 to 600 c at a depth of 10 km the increased heat from associated synmetamorphic concordant felsic sills raised the ambient temperatures to produce a regional distribution of andalusite and cordierite with a core zone of sillimanite bearing migmatites subsequent cooling was initially rapid 55 c ma to 350 c based on 40ar 39ar dates of 53 ma for hornblende and 50 ma for biotite and may have slowed to 11 c ma until 200 c based on an 40ar 39ar date of 35 ma for plagioclase intrusions of felsic sills and at least one pluton were along the initially north dipping foliations associated with south verging folds this suggests the source region for the melts may have been downdip in the subduction zone rather than from directly below within the accretionary prism

seismic refraction measurements within the peninsular terrane south central alaska we present an interpretation of crustal seismic refraction data from the peninsular terrane one of the many exotic terranes that have been accreted to the continental margin of southern alaska in the past 200 m y a seismic refraction line was collected along the glenn highway in the copper river basin of south central alaska in 1984 and 1985 as part of the u s geological survey trans alaska crustal transect tact program p wave velocities of 2 7 3 5 km s and thicknesses of 1 2 km characterize post lower jurassic sedimentary rocks that underlie most of the seismic refraction line an average crustal velocity structure includes the following five velocity divisions beneath the sedimentary rocks lie 1 2 km of 4 0 4 6 km s materials correlating with andesitic volcaniclastic sedimentary rocks and lava flows of the lower jurassic talkeetna formation below these rocks seismic velocity increases rapidly from 5 0 to 6 1 km s in 2 3 km at 7 8 km depth velocity jumps to 6 3 km s and increments to 6 6 km s by 10 12 km depth velocities increase from 6 8 to 7 0 km s between 12 to 20 km depth at about 22 km depth a jump in velocity from 7 0 to 7 4 km s is inferred but is poorly resolved depth to the moho discontinuity could not be determined from our data the absence of clear pmp reflections may indicate that moho is deeper than 40 km data from two offset shotpoints northeast of the line and within the wrangellia terrane constrain the deep structure transition between peninsular and wrangellia terranes the 6 3 6 6 km s material thickens to the northeast toward the suture between peninsular and wrangellia terranes but southwest of its mapped trace at the west fork fault peninsular terrane crustal structure appears dissimilar to that of continental interiors it is similar to velocity structures determined for accreted island arc fragments in california such as the basement of the great valley and the klamath mountains