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1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 94, NO. B4, PAGES , APRL 10, 1989 A High-Reslutin Seismic Reflectin/Refractin Study f the Chugach-Peninsular Terrane Bundary, Suthern Alaska THOMAS M. BRaCHER, MCHAEL A. FSHER, AND ERC L. GEST U.S. Gelgical Survey, Menl Park, Califrnia NKOLAS. CHRSTENSEN Department f Gesciences, Purdue University, West Lafayette, lndiana We present results frm a high-reslutin seismic refractin analysis f the shallw (apprximately 2 km) crustal structure alng the 107-km-ng Trans-Alaska Crustal Transect Chugach reflectin line in suthern Alaska and a cmparisn with labratry measurements f field samples. The refractin analysis includes the tw-dimensinal interpretatin f several thusand first- and secndary-arrival travel times digitized frm 1024-channel split-spread cmmn sht gathers. The velcity mdel derived frm this analysis better defines the lcatin and gemetry f terrane bundaries than des the nrmal incidence reflectin sectin and agrees well with surface mapping f lithlgies. Furthermre, the mdel predicts travel times within 100 ms f the reflectin times recrded frm the base f the Quaternary n the Chugach reflectin sectin. Thicknesses f Quaternary depsits, with velcities between 1.1 and 2.0 km/s, crrelate inversely with the quantity f bserved lwer crustal reflectins n the Chugach sectin, suggesting that the presence r absence f these sediments in sufficient thickness exerted primary cntrl n the quality f the deeper prtin f the sectin. There is a significant velcity cntrast between crystalline rcks acrss the Brder Ranges fault (5.0 versus 5.6 km/s), the majr cntact between the Chugach and Peninsular terranes, in agreement with labratry measurements f field specimens. n the Peninsular terrane the mdeling indicates that an unnamed fault delimiting the suthern flank f the Cpper River Basin dips steeply nrthward at 50 and has abut 1300 m f vertical ffset. Labratry measurements dcument a maximum velcity anistrpy f 200;;, fr phyllitic schists f the Valdez Grup in the Chugach terrane. n agreement with the bserved E-W strike and near-vertical dip f the Valdez Grup, we determined a significant (14%) velcity anistrpy fr ray paths riented N-S versus NE-SW. NTRODUCTON Recent innvatins in seismic data acquisitin and in data prcessing and interpretatin have blurred the frmerly rigid distinctins between reflectin and refractin prfiling f the cntinental lithsphere. Fr example, the recrding f densely sampled wave fields t ranges f 10 km and mre during "reflectin" experiments allws the precise definitin f the velcity-depth structure f the upper 1-2 km f the crust. The analysis f refracted first arrivals recrded during reflectin prfiling has been used t examine the structure f the upper crust in the Ri Grande rift [Jurdy and Bracher, 1980; Bracher, 198a], the Canadian crdillera n Vancuver sland [Mayrand et al., 1987], and Death Valley, Califrnia [Geist and Bracher, 1987]. These studies allw better understanding f reflectin events bserved frm the middle and lwer cntinental crust [Bracher, 1981 b; de Vaagd et a., 1986]. n February 1986 the U.S. Gelgical Survey (USGS) acquired 107 km f deep crustal seismic reflectin data alng a N-S trending prfile which fllwed the Richardsn Highway in suth central Alaska. These data were cllected in supprt f the Trans-Alaska Crustal Transect (TACT), a cmprehensive gelgical and gephysical study f the crust within a crridr acrss Alaska [Page et al., 1986]. n this paper we discuss the Chugach reflectin line, which traversed nrthward acrss gelgical strike frm the center f the Chugach Muntains int the Cpper River Basin acrss a majr fault bund- Cpyright 1989 by the American Gephysical Unin. Paper number 7B /89/007B-5090$05.00 ing tectnstratigraphic terranes (Figure ). This terrane bundary, the Brder Ranges fault, is the main structural bundary between the Chugach terrane, suth f the fault, and the Peninsular terrane, nrth f the fault [Janes et al., 1984]. Recent hyptheses cncerning the evlutin f the cntinental crust f the Alaskan Crdillera and elsewhere emphasize the rle f accretin f tectnstratigraphic terranes [Cney et al., 1980; Hwell et al., 1985]. The main bjective f the 1986 seismic reflectin survey was t delineate the crustal structure beneath majr terrane bundaries. nasmuch as stacked sectins f the deep seismic reflectin data t date prvide little new infrmatin n the Brder Ranges fault itself, we examined the first arrivals n cmmn sht gathers t btain a high-reslutin velcity structure f the shallw crust acrss this fault system. n this reprt we present a high-reslutin velcity mdel alng the Chugach seismic reflectin line and interpret the mdel gelgically. The interpretatin is based n the crrelatin f refractin velcities with rck velcities measured in the labratry and with rck expsures. We relate aspects f the wave prpagatin exhibited by shallw guided waves t the quality f the deep crustal seismic reflectin data. The high-reslutin mdel is cmpared t a similar mdel derived frm reginal refractin data alng the same line. The interpretatin is further cnstrained by cmparing the nrmal incidence reflectin and refractin images f the shallw crust. Finally, the irregular gemetry f the reflectin line and the lng recrding array used t acquire the reflectin sectin als allw us t test the hypthesis that a measureable velcity anistrpy exists within the Chugach terrane. 4441

2 4442 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY c 0 0 0" 10 KLOMETERS O' ) C J " 62' 00' N Fig. 1. Lcatin map shwing the study regin as well as a mre detailed map shwing the gelgy in the area traversed by the Chugach seismic reflectin line. Numbered ( ) stars shw vibratr pints. Numbered (0-107) dashes shw ranges in kilmeters alng the Chugach reflectin line. Map features and map unit symbls used in this figure: BRFS. Brder Ranges fault system; He. Haley Creek terrane, which includes Upper Palezic metamrphic rcks and Upper Palezic and Late Jurassic metaplutnic rcks; Jt, Lwer Jurassic Talkeetna Frmatin, vlcanic and vlcaniclastic rcks; Jb, Nelchina River Gabbrnrite; Middle Jurassic schist f Liberty Creek; ltg, Tnsina ultramaficmafic assemblage; KJm, Jurassic and Cretaceus McHugh Cmplex, a subductin melange; Kv, Upper Cretaceus Valdez Grup, which includes mainly strngly defrmed turbidite sequences and ceanic mafic vlvanic rcks; Qs, Quaternary surficial depsits. The lcatin f Pump Statin 12 n the Trans-Alaska il pipeline is shwn as a large circle.

3 BRaCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY 4443 <J) <.0 X" Range. Fig. 2. Examples f cmmn sht gathers used fr the travel time analysis. Surce-receiver ranges were calculated using the surveyr's lcatins fr the gephne statins. As described in the text, the travel times have been linearly reduced using a velcity f 6 km/s, and the amplitudes have been crrected fr gemetrical spreading accrding t the square rt f range. Numbers n the gathers indicate the lcatins f the vibratr pint in kilmeters alng the Chugach reflectin line (Figure 1). Large-amplitude guided waves can be bserved as a tight cne nearest the surce. km 4 8 COLLECTON AND PROCESSNG OF SESMC REFLECTON DATA Seismic reflectin data alng the Chugach line were cllected fr the USGS by Gephysical Systems Crpratin, using a sign bit recrding system and an array f Vibrseis (trade and service mark f Cntinental Oil Cmpany) surces. Field acquisitin parameters are described by Fisher et al. [this issue] and include a sweep between 10 and 30 Hz and a symmetrical split spread f 1024 gephrie arrays spaced at 30-m intervals. Nminal sht-t-grup ffsets ranged frm t km. These maximum ffsets were nt always achieved, hwever, wing t curvature f the rad. N gap intervened between the surce and first active grups f the spread, s that refracted arrivals frm near-surface hrizns are generally well recrded. The gephnes were gruped int cmpact pint rather than linear distributed arrays; lw-velcity arrivals therefre were nt degraded by the recrding array. Surces were spaced at 120-m intervals alng the line. We pltted cmmn sht gathers after crrectin fr shtgephne range, using the surveyed gephne and surce 10- catins. These ranges are accurate t within a meter. The amplitude f each trace was crrected fr gemetrical spreading by multiplying by the square rt f the range. N ther prcessing was perfrmed n the gathers. Typical cmmn sht gathers frm alng the Chugach line are shwn in Figure 2. Over 6800 travel times f first arrivals were digitized frm cmmn sht gathers alng the line (Figure 3). Gathers were digitized fr surce pints separated by apprximately 3 km, which prvided six t eight reversed measurements f the travel time beneath every surface lcatin. Because the data were acquired with a crrelated Vibrseis surce, we chse the arrival time t be the time f the first amplitude maximum. Timing errrs inherent in the recrding instrumentatin are cnsidered negligible at ms. Travel times were digitized frm paper sectins pltted at a scale f cm/s; at this scale, ne half the predminate perid f the refracted energy is 0.5 cm. Since digitizing errrs are less than ne half a cycle, errrs in digitizing travel times are estimated t be less than 25 ms. The nnlinear gemetry f the reflectin line results in three-dimensinal ray paths. Our tw-dimensinal ray thery algrithm required the apprximatin f the crked reflec-

4 4444 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY s N u w (f)... 0 CD '- X ~ f LJO DSTRNCE (M Fig. 3. Reduced (a 6.0 km/s) travel times used fr vibratr pints between km 0 and km 83. Additinal travel time data (nt shwn) were used between km 83 and the nrthern end f the line. Nte the dramatic increase in the bserved travel times frm suth t nrth. tin line t a linear transect. nstead f prjecting the crked reflectin line nt the mean azimuth f the line, as is nrmally dne in refractin studies, the tw-dimensinal apprximatin was achieved by straightening the reflectin line by pulling the ends f the line taut. Specifically, this straightening was perfrmed by calculating the lcatin f each sht gather alng the velcity depth mdel using the statin number f the surce and the nminal 30-m statin spacing. n a similar manner, surveyed elevatins accurate t within m alng the prfile were incrprated int the velcity-depth mdel using the statin number as an estimate f the range alng the mdel. The advantage f this apprach is that the resulting velcity mdel is easily crrelated t the seismic reflectin sectin itself. The accuracy f this apprximatin was checked by examining the surface cnsistency f the travel time curves pltted alng the line as in Figure 3. As expected, the apprximatin was in greatest errr where the reflectin line is mst crked, near km 28 (Figure ). Errrs f a few hundred meters in these sht and elevatin lcatins within the mdel near nnlinear prtins f the reflectin line prbably explain sme f the minr misfits f the calculated travel times with the bserved travel times. Althugh the high spatial density f bservatins makes this data set ideal fr autmatic ne- and tw-dimensinal interpretive methds based n slant stacking [e.g., Bracher, 1981a; Milkereit et al., 1985], the strngly tw-dimensinal structure limits the usefulness f these techniques. Time term (delay time) methds f analysis were als cnsidered but were nt applied wing t the significant variatin in basement velcity alng the line. n rder t perfrm the least amunt f prcessing f the data we chse t frward mdel the bserved travel times with a tw-dimensinal velcity-depth structure using cmputer algrithms described by Cerveny et al. [1977] and Hill et al. [1985]. nitial velcity mdels were derived using tw independent algrithms based n the bservatin that, very rughly, the arrivals culd be characterized as riginating frm a 2.0 km/s layer ver a 6.0 km/s half-space. n the first algrithm, intercept times f the arrivals having velcities clse t 6 km/s were used t define the base f the 2 km/s surficial layer. n the secnd algrithm, tw-way travel time picks f the base f the 2 km/s layer made frm the stacked Chugach reflectin sectin were cnverted t depth. Althugh the secnd algrithm prvided an initial mdel yielding smaller travel time residuals than did the first algrithm, even that mdel required extensive revisin befre it prvided an adequate match t the bserved refracted first-arrival times. During mdeling, bserved first-arrival travel times can be matched by varying either layer thickness r velcity r by varying bth. Fr the uppermst layer f the mdel it was

5 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY 4445 necessary t vary the layer velcity as well as the layer thickness in rder t match the bserved first arrivals. Fr the deeper layers it was pssible t btain agreement between the bserved and calculated travel times by varying primarily the layer thicknesses rather than the velcity within layers. As discussed by many previus authrs [e.g., Hill et al., 1985J, the uniqueness and reslutin assciated with the twdimensinal velcity-depth mdels which result frm this frward, nnlinear mdeling prcedure are extrardinarily difficult t quantify. This is particularly true if the parameterizatin f the mdel has been t restrictive. Lacking rthgnally riented prfiles, fr example, it is impssible t estimate the degree t which the tw-dimensinal mdels have been cntaminated by ut-f-the-plane prpagatin f seismic energy. Frm trial and errr perturbatins in mdel parameters we believe that, n average, P wave velcities are reslved t within 5'Y and the depths t bundaries are reslved t within 10 /". Given a sweep frequency cntent between 10 and 30 Hz, the shrtest bservable wavelengths in the near-surface layers having velcities f 2.0 km/s are km and wuld increase t 0.2 km in rcks having P wave velcities f 6.0 km/s. f this is cupled with the average sensr spacing f 0.03 km, we may reslve, under ptimal cnditins, the psitin f lateral and vertical changes in near-surface structure t within several tens f meters. The psitin f lateral and vertical changes in structure at greater depths is prbably reslved t within a few hundred meters. Figure 4, shwing ray paths thrugh the velcity-depth mdel, indicates that the reslutin f the prcedure differs alng the mdel depending n the infrmatin density alng the prfile. Figure 4 illustrates that, n average, nly the upper 1.5 km f the structure is sampled by ray paths, that the deeper structure immediately beneath a surce pint is nt sampled by rays frm that surce, and that bth ends f the prfile lack reversed cverage, degrading the reslutin f the prcedure. The uniqueness f the resulting velcity-depth mdel depends mst imprtantly n the particular chices f phase crrelatin between arrivals and the chice f ray path and refracting hrizn used t mdel these arrivals. T minimize the ambiguities assciated with these chices, we analyzed cmmn sht gathers separated by sufficiently small intervals, abut 3 km, that the phase crrelatins culd be interplated frm ne surce pint t the next. Furthermre, the resulting velcity-depth mdel was required t cmpare favrably t the Chugach nrmal incidence reflectin sectin itself. T cmpare the refractin mdel t the seismic reflectin sectin, we calculated synthetic, nrmal incidence reflectin times frm the tw-dimensinal velcity-depth mdel. SURFACE GEOLOGY The fllwing discussin f the near-surface gelgy alng the Chugach seismic line is abstracted frm descriptins f the utcrp gelgy near the study regin presented by Nichls and Yehle [1969J, Winkler et al. [198J, Winkler and Plajker [198J, Jnes et al. [1984J, Plafker et al. [1985, this issuej, and Nkleberg et al. [this issue]. Frm suth t nrth the Chugach line traverses the Chugach and the Peninsular terranes, which are in fault cntact acrss the Brder Ranges fault system (Figure ). The Chugach terrane near the transect cnsists f a nrthward successin f three sequences: the Valdez Grup, the McHugh Cmplex, and the schist f Liberty Creek. All three sequences are bunded by faults and are strngly defrmed by asymmetric, suth verging flds and assciated faults [Winkler and Plafker, 1981; Plafker et a., this issue; N kleberg et a., this issue]. The schist f Liberty Creek may be faulted ut near the surface just east f the transect (Figure ). The sequences increase nrthward in age frm Late Cretaceus t Middle r Early Jurassic. The Upper Cretaceus Valdez Grup cnsists primarily f pelitic schist, metagraywacke, and metabasalt that have been metamrphsed t lwer greenschist facies [Winkler and Plafker, 1981; Winkler et al., 1981; Plafker et al., this issue]. Metabasalt predminates in the structurally lwest (suthernmst) part f the Valdez Grup suth f the area in Figure. The McHugh Cmplex is a subductin melange that includes mainly marine sedimentary and vlcanic rcks. The melange matrix ranges in age frm Late Triassic t mid-cretaceus [Winkler et al., 1981; Plajker et al., this issue]. The schist f Liberty Creek cnsists f blueschist and greenschist facies rcks that are inferred t have Middle Jurassic metamrphic ages n the basis f cmparisns f istpically dated schist t the west [Sissn and Onsttt, 1986; Plafker et al., this issue]. The Brder Ranges fault system bunds the nrthernmst utcrps f the Chugach terrane [MacKevett and Plajker, 1974; Phi/ker et al., this issue]. n the vicinity f the Chugach line this fault system juxtapses the McHugh Cmplex f the Chugach terrane against Jurassic ultramafic and mafic rcks that frm the suthernmst unit f the Peninsular terrane (Figure ). The Peninsular terrane cnsists mainly f the lwer t middle Jurassic Brder Ranges ultramafic-mafic assemblage [Bur/s, 1985; Cleman and Burns, 1985; Plafker et al., this issue]. as well as the Lwer Jurassic Talkeetna Frmatin. Ultramafic and mafic rcks f the Tnsina ultramafic-mafic assemblage that frm the lwer part f the Brder Ranges ultramafic-mafic assemblage are interpreted t have been emplaced at the base f an early t middle Jurassic ceanic island arc; they are verlain by a thick sequence f shallwer level gabbrs (Nelchina River Gabbrnrite) nrth f the Brder Ranges ultramafic-mafic assemblage [DeBari and Cleman, 1986, this issue]. The Lwer Jurassic Talkeetna Frmatin and assciated Middle and Late Jurassic intermediate plutnic rcks are reginally expsed [Burns, 1982, 1985]. The predminantly andesitic flws and vlcaniclastic rcks in the lwer part f this frmatin are verlain by sedimentary rcks and tuffs [Winkler et al., 1981; Burns et al., 1983]. The Talkeetna Frmatin is at least 2 km thick, is extensively faulted, and, in places, is bradly flded [Winkler et al., 1981; Plajker et al., this issue]. The Talkeetna Frmatin is in fault cntact t the suth with the Nelchina River Gabbrnrite. The Cpper River Basin, lcated n the nrthern end f the Chugach line (Figure ), is filled by a thick sequence f Middle Jurassic t Upper Cretaceus clastic rcks depsited in a subaerial t deep marine envirnment; these Meszic rcks are in turn verlain by a thin veneer f Tertiary, predminately cntinental, sedimentary rcks [Plajker et a., this issue]. A variety f uncnslidated Quaternary depsits are lcated alng the Chugach reflectin line. n the suthern third f the line. glacial, alluvial, and minr fan depsits predminate. whereas in the nrthern tw thirds f the line, glacilacustrine sediments predminate with minr expsures f alluvium, slpe depsits. and mraine sediments [Nichls and Yehle, 1969].

6 4446 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY s N 0.5 UJ "- X f :r: f- ~1 u ~0.5 s N 0.5 UJ "- X ~O s gal ~ :r: f- ~1 -~""',..;---""'~v 0 f- 20 ' 50 :r: f- ~1 20 DSTRNCE Fig. 4. Observed (circles) and calculated (crsses) travel times fr the cmmn sht gathers shwn in Figure 2. Als shwn beneath each travel time plt are ray paths thrugh the apprpriate velcity-depth structure. Such plts prvide an indicatin f the subsurface cverage and shw that nly the upper 1.5 km f the structure is cnstrained by the first-arrival data. 30 KM 40 Quaternary Depsits THE REFRACTON MODEL The velcity-depth mdel inferred frm the iterative frward mdeling f reversed and verlapping travel times indicates that a layer, generally less than 500 m thick and having velcities between. and 2. km/s, is cntinuus alng the entire Chugach reflectin line (Figure 5). Because these velcities crrelate t thse expected fr the Quaternary depsits described abve (Table ), we infer that this layer crrespnds t Quaternary sediments. The discntinuus, thin (100 m thick), and lw-velcity ( km/s) uppermst layer is an unusual but wellcnstrained feature f the mdel. Due t the deplyment f the reflectin gephne grups as pint, rather than linear, arrays, the arrivals traveling at velcities as lw as km/s were nt attenuated by the gephne grups. Arrivals frm this layer are bserved as direct waves, allwing the velcity f the layer t be determined withut ambiguity. The lateral distributin f the layer is knwn chiefly by bservatin f the arrivals frm the layer using surce pints distributed at 1- t 3-km intervals. Figure 5 indicates that this layer ccurs where fan and carse-grained alluvial depsits have been mapped.

7 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY 4447 S MAPPED QUA TERNARY DEPOSTS N -0.5 E 6.<:... a... 0 Alluvium 5.0 Lacustrine 1.9/2.1 clays \ Mraine f ~ ~depsits! Glacilacustrine i C. ~~.ff : arse-grame d V '" depsits fluvial depsits 1 1.9/2.1 ~ L--/ \~~3' ~ "/1'3 2.5 '" 1.7/ / Distance Ckm) Fig. 5. Cmparisn f mapped Quaternary sediments by Nichls and Yehle [1969] and G. Plafker et al. (manuscript in preparatin. 1988) with the uppermst prtin f the velcity-depth mdel derived frm the refracted first arrivals. Numbers n mdels indicate cmpressinal wave velcities in kilmeters per secnd. A crrelatin between the mapped Quaternary depsits alng the Chugach reflectin line and the velcity and thickness f the uppermst layers in the refractin mdel is presented in Figure 5. The fan and alluvial sediments lying between Stuart (km 0) and Bulder (km 8) creeks thin nrthward, tward the Chugach Muntains. Frm km 8 t km 30 the thin layer f alluvium is cnfined t a narrw (200 m wide) belt f sediments in a V-shaped glacial valley [Nichls and Yehle, 1969; G. Plafker et a., unpublished data, 1987]. Frm km 30 t km 58, lacustrine clays lie alng mst f the reflectin line in a mre extensive 2-km-wide belt which is rughly defined by the 656-m cntur n tpgraphic maps and may explain the thickening f the 2.0 km/s layer in this regin. Between km 58 and 61, mraine depsits utcrp and may underlie thin glacilacustrine depsits between km 55 and 58 and between km 61 and 63. Frm km 63 t km 85, glacilacustrine depsits within a brad basin underlie mst f the reflectin prfile [Nichls and Yehle, 1969]. Carse-grained fluvial depsits are expsed frm km 85 t the nrthern end f the seismic line. These depsits prbably accunt fr an in- TABLE 1. Cmparisn f Labratry and Refractin Measurements f Frmatin Cmpressinal Wave Velcities Unit Surficial depsits Till Frzen clay Valdez Grup Phyllite Metagraywacke Metatuff Brder Ranges Ultramafic-mafic assemblage Talkeetna Frmatin Andesite breccia Flw Previus Labratry r Refractin Velcities, * km/s t, :j: *Table 2. t Schn [1983]. :j:haeni [1986]. Christensen [1982, p. 141]. Refractin Velcities Frm This Study, km/s creased scatter f travel times nted in the Cpper River Basin. T investigate the influence f the surficial lw-velcity layer n the quality f upper crustal reflectins, we examined, as a functin f sht lcatin, the prpagatin f seismic wave energy guided within the 2.0 km/s layer. These arrivals, when present, nrmally cnsist f a series f multiple reflectins and multiply reflected refractins whse velcities are asympttic t that f the direct wave frm the 2.0 km/s layer. The arrivals are nt strngly dispersed and typically have frequencies f Hz, crrespnding t wavelengths f m. We fund significant variatins in the efficiency f the generatin and prpagatin f guided wave energy alng the Chugach reflectin line. Sht gathers selected at -km intervals shw that these variatins directly crrelate with fluctuatins in the thickness f the 2.0 km/s layer (Figure 6). Between km 0 and km 30 the guided waves are absent, whereas between km 30 and km 60 they have large amplitudes. Fr instance, at km 20.88, where the 2.0 km/s layer is abut 100 m thick, there is essentially n guided wave energy. At km 43.92, where the 2.0 km/s layer is mre than 400 m thick and is underlain by 5.6 km/s material, a strng set f guided wave arrivals is evident. The latter arrivals appear t be similar t the dispersed acustic wave arrivals bserved during the prpagatin f sund in shallw water. This bservatin suggests that techniques fr investigating the acustical prperties f shallw water envirnments culd be applied t the near-surface structure alng the Chugach reflectin prfile [e.g., Brcher and Ewing, 1986]. n particular, the grup and phase velcity dispersin curves f these arrivals culd be inverted t place cnstraints n the velcity gradients within, and the thickness f, the 2.0 km/s layer. The crrelatin between the thickness f the 2.0 km/s layer and the efficiency f guided wave energy prpagatin suggests at least tw mre mechanisms fr explaining the prer quality areas f the deep crustal reflectin prfile. The first and preferred explanatin is that degradatin f the reflectin data by large-amplitude guided wave energy during stacking may have lwered the quality f the reflectins in the upper 4 s f the crust. Prcessing f the reflectin data t date has nt included the filtering r muting f the high-amplitude guided waves and has relied upn the stacking prcess itself t reduce this energy. The secnd explanatin is that regins f the line

8 4448 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY (J). q (J) l{) C\J. X"- X"- - - C\J Range, km Fig. 6. Cmparisn f tw cmmn sht gathers pltted using linear reductin velcities f 2.0 and 2.5 km/s in rder t better demnstrate the surface guided energy. Numbers n each gather prvide its lcatin in kilmeters alng the Chugach line. Nte that the sht at km is underlain by abut 100 m f surficial sediment cver, whereas the sht at km is underlain by mre than 400 m f surficial sediments. Range, km 8 having thicker 2.0 km/s layers crrespnd t thse having efficient guided wave energy. Thus mre seismic energy is reflected at the base f this layer than where the layer is thinner, allwing less energy t be transmitted thrugh the layer t deeper hrizns. The Deeper Structure n this sectin we describe imprtant features f the lwer prtin f the velcity-depth mdel inferred frm the frward mdeling. As previusly described, the travel time data generally permit the reslutin f refractin velcities t depths f abut 1.5 km belw the surface, althugh in the Cpper River Basin, subsurface depths as large as 2.0 km were sampled. The mst prnunced feature f the velcity-depth mdel shwn in Figure 7 is the steeply nrth dipping bundary at the suthern end f the Cpper River Basin, at km 73.5, well nrth f the Brder Ranges fault system. Mdeling f the dip f this bundary indicates that the best match t the bserved travel times is btained when the dip is clse t 50", althugh nrthward dips as lw as 39' and as high as 68 prduce less accurate but reasnable matches. n agreement with the reginal refractin results reprted by G. S. Fuis et al. (Crustal structure f the Chugach, Peninsular, and Wrangellia terranes, suthern Alaska, submitted t Jurnal f Gephysical Research, 1988, hereinafter referred t as submitted manuscript, 1988) the bundary at the suthern end f the Cpper River Basin represents the largest change in near-surface velcity structure alng the entire Chugach reflectin line. Suth f the bundary, there is nly a thin veneer f inferred sediments having velcities less than 5.0 km/s. Nrth f the bundary the thickness f material having velcities less than 5.0 km/s increases t 2 km. Significant differences in wave prpagatin nrth and suth f this bundary are illustrated in Figure 8, which shws a cmmn sht gather frm a surce lcated directly ver the bundary. Owing t the significant differences in the velcity mdel nrth and suth f this bundary, we discuss these regins separately in the fllwing discussin f the mdel. Velcity Structure Suth ()[ the Cpper River Basin Suth f the Brder Ranges fault a layer having velcities f km/s lies intermittently beneath the inferred Quaternary depsits (Figure 7). The km/s layer is thickest in the Valdez Grup where phase velcities f km/s are bserved in first-arrival times (Figures 2-3). This layer is als present near km 65.9, where phase velcities f km/s are bserved in the first arrivals fr surce-receiver ranges f 2-3 km (Figure 3). Althugh this intermediate-velcity layer was unreslved frm the bserved travel times fr ranges between 42 and 59 km, where the Brder Ranges ultramaficmafic rcks are prjected t ccur, trial and errr mdeling indicated that the layer may be present but undetected if it is less than 100 m thick. Lw-amplitude but spatially cherent refracted shear wave arrivals are bserved n several cmmn sht gathers acquired within the Valdez Grup. Shear wave velcities fr five reversed gathers lcated between km 13.2 and are apprximately 3.3 km/s. This shear wave velcity wuld be apprpriate, assuming a Pissn's rati f 0.25, fr the km/s layer in the Valdez Grup. Fr the mdel suth f the Cpper River Basin the halfspace underlying the 2.0 km/s and km/s layers has a velcity gradient f apprximately 0.3 km/s/km, which is sufficient t allw the rays t bttm and refract upward back t the surface. Althugh we used a few trial velcity gradients when mdeling this half-space, n particular significance shuld be attached t the magnitude f the velcity gradient assigned t the half-space. Velcity Structure Within the Cpper River Basin The velcity-depth mdel fr the Cpper River Basin indicates structural cmplexity within the basin (Figure 7). Be

9 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY ~ 0-3 ~l - 2 PEMNSULARTERRANE S t COPPER RVER BASN N - - 1,1/ /1.3 ~0 s 1.1/1.3 ~,~.8 ' VALDEZ CHUGACH GROUP 1.9/2.1 TERRANE 2.2 -~ 2.3 ~il Distance (km) MCHUGH COMPLEX / en PENNSULAR TERRANE s ~ BRUMA -+- TALKEETNA FM. N /5.7 ~ ~ : 2 ~ ~ Fig. 7. The velcity-depth mdel derived frm the fitting f firstarrival times with velcities in kilmeters per secnd. The distributin f gelgic frmatins as mapped at the surface is als indicated n the figure. tween km 73.5 and 89 the bserved travel times have been mdeled using a layer f km/s material ranging in thickness frm 250 t 375 m. An abrupt ffset in the base f this mdel layer crrelates in lcatin with an ffset in the N ~ - 2 base f the km/s layer, suggesting that the thicknesses f bth the 1.2 and km/s layers are fault cntrlled. Alternatively, the apparent ffset in the base f the km/s layer culd be explained by further thickening the 1.2 km/s layer, suggesting that nly the thickness f the uppermst layer is fault cntrlled. The layer beneath this km/s layer has velcities between 3.5 and 4.4 km/s and is nearly 2 km thick. The lwer layer is best cnstrained in the mdel range interval km 73.6 t 88.6 within the Cpper River Basin, where the verlying km/s layer is relatively thin. The minimum thickness f the km/s layer is cnstrained by the requirement that calculated wide-angle reflectins frm the base f the layer cannt precede the bserved first arrivals. The maximum depth t the base f this layer, hwever, is prly cnstrained by first-arrival data. Nrth f km 89, within the fldplain f the Cpper River, the bserved travel times reveal n evidence fr seismic velcities in the secnd layer less than 2.3 km/s. ndeed, the bserved travel times are best explained by a large linear velcity gradient within this layer between velcities f 2.3 and 3.2 km/s. Near km 97 the km/s layer is abut 1.8 km thick, and in this lcatin the gemetry f the layer resembles a V-shaped basin. At the nrthern end f the Chugach reflectin line the lack f reversed refractin cverage des nt allw the precise definitin f the base f this layer with cnfidence. The lwermst layer has a seismic velcity f km/s. GEOLOGCAL NTERPRET AnON Cmpressinal and shear wave velcities have been measured as functins f hydrstatic pressure fr several Chugach-Peninsular basement rcks cllected frm expsures alng the Richardsn Highway using a pulse transmissin technique described in detail by Christensen [1985]. Velcities were btained fr three mutually perpendicular directins frm each rck type. Bulk densities and velcities averaged ver the three directins are presented in Table 2 fr several. <D... X Range, Fig. 8. Cmmn sht gather with the vibratr pint at km 73.7, directly ver the majr velcity bundary at km As fr Figure 2, the travel times fr this gather have been linearly reduced using a velcity f 6.0 km/s. Nte that asymmetry in prpagatin nrth and suth f the surce. km 8.i~':': Km 73.7.~~~~ ~L:~.' "' ~:, j.:.:",:\h~f.~. 12

10 4450 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY TABLE 2. Cmpressinal (V,,) and Shear Wave Velcities (V.) as a Functin f Pressure (P) P Density, Rck Type Unit kg m-3 Mde 10 MPa 50 MPa 100 MPa 200 MPa 400 MPa 600 MPa 800 MPa Phyllite Valdez Grup 2723 V' V, Metagraywacke Valdez Grup 2716 V' V, Metatuff Valdez Grup 2779 V' V, n Serpentinized Tnsina Cmplex 2984 V' dunite V, Andesite Talkeetna Frmatin 2686 V' breccia V, Andesite flw Talkeetna Frmatin 2886 V' V, Velcities are in kilmeters per secnd. majr lithlgies underlying the seismic traverse. n the fllwing discussin, the labratry measurements are cmpared with ur velcity mdels fr the Chugach and Peninsular terranes. Suth f the Brder Ranges fault, basement rcks underlying the 2.0 km/s layer have velcities f km/s (Figure 7). The majr expsed lithlgies are phyllite, metagraywacke, and minr (less than 10(10) metatuff. Cmpressinal wave velcities f phyllite frm the Valdez Grup measured in the labratry at lw pressure and rm temperature average 5.1 km/s. Similarly, a sample f Valdez Grup metagraywacke has a mean cmpressinal wave velcity f 5.7 km/s (Table 2). The metatuff is significantly faster, with cmpressinal wave velcities averaging 6.3 km/s. Labratry measurements f the shear wave velcities f these rcks ranged frm 3.0 t 3.6 km/s (Table 2). Thus the refractin velcities determined fr this layer are cnsistent with the predminant lithlgies within the Valdez Grup. Bedrck velcities ranging frm 5.0 t 5.6 km/s under the prjected lcatin f the McHugh Cmplex alng the mdel are nt significantly different frm thse f the Valdez Grup (Figure 7). The prjected lcatin f the McHugh Cmplex, hwever, prved t be ne f the mst difficult areas in which t btain agreement with bserved travel times using the velcity-depth mdel shwn in Figure 7, wing t the sharp bend f the reflectin line at krri 28. The assumptin f a cnstant distance f 30 m between statins is least apprpriate fr this prtin f the mdel, and there is strng evidence fr a significant velcity anistrpy within the basement rcks in this lcatin (see belw). The prjected lcatin f the Brder Ranges fault crrespnds t the nrthern truncatin f the km/s layer at km 42. Nrth f the Brder Ranges fault the mdel shws that the 2.0 km/s layer rests directly n rcks having velcities greater than 5.6 km/s. A dunite cntaining apprximately 25'Y., serpentinite frm near the base f the Tnsina ultramafic-mafic assemblage has a labratry cmpressinal wave velcity f 6.6 km/s at lw pressure and rm temperature (Table 2). With increasing serpentinizatin, velcities in ultramafic rcks decrease t apprximately 5.0 km/s fr cmpletely serpentinized peridtite [Christensen, 1982, pp ]. Thus the bserved velcities in the Tnsina ultramaficmafic assemblage suggest appreciable serpentinizatin at depth. n additin, the velcities will be dependent upn the rati f gabbr t ultramafic rcks in the cmplex. Prminent vertical ffsets f the base f the 2.0 km/s layer in the Tnsina ultramafic-mafic assemblage and the Nelchina River Gabbrnrite may either represent reactivated splay faults f the Brder Ranges fault system r unrelated later faulting. We believe these ffsets dcument reactivatin f faults rather than the riginal mtin, since the Brder Ranges fault system is thught t have been mst active in the late Meszic. The ffset at km 57 is particularly prminent, shwing 350 m f apparent vertical displacement n the base f the 2.0 km/s layer. This ffset prduces a large ne-way travel time anmaly f clse t 0.1 s, which exceeds the s uncertainty f the travel times by a factr f 4. Althugh n labratry rck velcities are available frm the Nechina River Gabbrnrite, the high velcities in the mdel are cnsistent with thse f ther gabbrs [Christensen, 1982, pp ]. Where the Talkeetna Frmatin utcrps, the velcity mdel indicates a shallw layer having velcities between 5.1 and 5.7 km/s. Measured cmpressinal wave velcities f an andesite breccia and a flw frm the Talkeetna Frmatin at 10 MPa are 5.7 and 6.1 km/s, respectively (Table 2). Because the andesite breccia is the mre cmmn lithlgy within the Talkeetna Frmatin, the labratry measurements are cmpatible with the refractin velcities. The mst prminent structural bundary alng the entire Chugach prfile is the nrthward dipping ffset at km Frm its lcatin we infer that this bundary represents the suth mst large-scale fault defining the Cpper River Basin. On the basis f snic and ther gephysical lgging results frm within the Cpper River Basin nrth f Glennallen (E. L. Ambs et a!., Seismic refractin measurements within the Wrangellia-Peninsular (Cmpsite) Terrane, suth central Alaska, submitted t Jurnal f Gephysical Research, 1988, hereinafter referred t as submitted manuscript, 1988), the entire sectin within the Cpper River Basin having velcities less than 4.4 km/s represents Upper Jurassic and Lwer Cretaceus and yunger sedimentary rcks. n the Ahtna wells, just nrth f Glenallen, abut 300 m f Tertiary nnmarine Micene( 7) strata verlie 1600 m f Cretaceus marine strata (c. E. Kirschner, unpublished data, 1987). The Lwer and Upper Cretaceus Matanuska Frmatin, cnsisting f mderately indurated marine siltstnes and sandstnes, has snic well velcities between 2 and 3 km/s (E. L. Ambs et a!., submitted manuscript, 1988). With higher degrees f induratin, snic velcities within the Matanuska Frmatin are

11 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY 4451 CHUGACH TERRANE BRFS PENNSULAR TERRANE 'E 2 ~.. "'- S '" "'" Valdez Grup 4.3/ McHugh Cmplex /5.5 " BRUMA " " ' ~,,\ Talkeetna Frmatin Cpper River Basin N "" / Fig. 9. Cmparisn f velcity-depth mdels fr the Chugach reflectin line based n (tp) a reginal refractin line (G. S. Fuis et a., submitted manuscript, 1988), and (bttm) the Chugach reflectin line. Numbers n the mdel indicate velcities in kilmeters per secnd. Symbls used in this figure: BRFS, Brder Ranges fault system; BRUMA, Brder Ranges ultramafic-mafic assemblage. slightly in excess f 3 km/s. The Matanuska Frmatin is underlain by mre indurated rcks having essentially the same lithlgy, which may explain the bserved refractin velcity increase frm 3.2 t 3.5 km/s within the basin. The interpretatin f this entire sectin as mderately well lithified sedimentary rcks is cnsistent with the relatively pr prpagatin f cmpressinal wave energy within the Cpper River Basin. Directly nrth f the ffset at km 73.5, the km/s layer may represent Palecene and Ecene sediments shed nrthward during the uplift f the Chugach Muntains. These Palecene and Ecene sediments, knwn as the Chickaln Frmatin (an uncnslidated nnmarine cnglmerate having shale and minr cal units), wuld have velcities apprpriate fr this layer f the mdel. COMPARSO~ TO A REGO~AL REFRACTON MODEL A primary bjective f the TACT study is the determinatin f the crustal structure beneath the accreted terranes t Mh depths. Critical t this bjective is the acquisitin f reginal refractin lines up t 240 km lng. These lines, acquired using explsive surces at 20- t 30-km intervals and recrded by receivers spaced at l-km intervals, prvide nly a sparse sampling f the near-surface hrizns. A cmparisn f the velcity mdels derived frm the reginal refractin line and the first-arrival analysis f the reflectin data is presented in Figure 9. This cmparisn is achieved by prjecting the mdel f G. S. Fuis et al. (submitted manuscript, 1988) nt the crked rad fllwed by the Chugach reflectin line. The tw different velcity mdels presented in Figure 9 exhibit many similarities. The reginal refractin and reflectin velcity mdels lcate the suthern bundary marking the thickest accumulatin f sediments within the Cpper River Basin at km 70 and 73.5, respectively. The inferred Quaternary cver, while prly cnstrained by the reginal refractin survey, is thin fr bth mdels within the Chugach terrane, especially between km 8 and 30. The velcities f the basement rcks in the Valdez Grup and the McHugh Cmplex range between 5.6 and 5.9 km/s fr bth mdels. Bth mdels shw a thin sediment cver ver the Talkeetna Frmatin frm km 65 t 69. n bth mdels the velcities in the basement f the Talkeetna Frmatin are higher than thse f the Valdez Grup. Bth velcity mdels indicate that the Cpper River Basin is flred by a thick sectin f rcks having velcities less than 5 km/s. These mdels are cnsistent with previus interpretatins f gravity and magnetic anmaly data frm the Cpper River Basin [Andreasen el al., 1964]. The near-surface velcities in the Cpper River Basin vary laterally in bth mdels. There is, hwever, n structure in the reginal refractin mdel resembling the nt fully cnstrained V-shaped basin at km 97 in the reflectin mdel. We cnclude that while the velcity mdel based n highreslutin refractin data shws greater structural detail than des the mdel based n the reginal refractin lines, the mdels shw brad similarities. This agreement demnstrates that the reginal refractin prfiling prvides useful cnstraints n the structure within the first few kilmeters in depth and lends further supprt fr the mdels fr the lwer crust derived frm these reginal refractin data. We plan t incrprate the mre detailed mdel presented here int the reginal refractin mdels t gain greater cnfidence in the lwer crustal mdels.

12 4452 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY DSTANCE (KM) L J L..L 2000 ' l- - ~ j l l Fig. 10. Cmparisn f synthetic travel times (shwn as slid lines) calculated frm the velcity-depth mdel shwn in Figure 7 with the uppermst prtin f the Chugach seismic reflectin sectin. Numbers alng the sectin are the vibratr pint lcatins prvided in Figure. 2 COMPARSON OF THE VELOCTY MODEL TO VERTCAL NCDENCE REFLECTON PROFLES The Chugach reflectin sectin exhibits reflectins frm the shallw crust (upper hundred meters). Because the base f the 1.2, 2.0, and 3.2 km/s layers in the velcity mdel may be strng reflectrs, we have cmpared the Chugach reflectin prfile t a synthetic reflectin sectin calculated frm ur velcity mdel. Cmparisn f the times predicted frm a synthetic reflectin sectin with the unmigrated Chugach seismic reflectin sectin (Figure 10) shws that the reflectin times predicted frm the mdel in Figure 7, assuming vertical incidence (nt vertical prpagatin), generally agree with the bserved reflectin time t within 100 ms tw-way travel time (twtt), which crrespnds t a discrepancy in depth f less than 50 m fr the 2.0 km/s layer. Since we did nt attempt t mdel tpgraphy less than 50 m at the base f the 2.0 km/s layer, the agreement between the predicted and bserved reflectin times is within reasnable accuracy. The bserved shaling f reflectins between km 8 and 29 (VP 3550 t 4300) is well matched by the synthetic travel times, as they are als between km 67 and 69 (VP 2150 and VP 2255). The vertical ffset f reflectin times bserved in the Chuga~h reflectin sectin at km 57 (VP 2650) is seen in the refractin mdel, but the details f the bserved prfile are nt reslved by the subset f the refractin data analyzed. Reflectins frm the base f the 1.2 km/s layer in the Cpper River Basin (VP 1001 t VP 1650) are als fit by the refractin-based mdel. The transitin within the Cpper River Basin between the base f the km/s layer and the tp f the km/s layer is als reflective (VP 1300 t VP 1650) and matches the predicted reflectin times. Generally, hwever, few reflectins stem frm within the basin, suggesting that the sedimentary fill is either prly bedded, prly srted, highly indurated, r highly defrmed. Anther mtivatin fr this study was t search fr gelgical explanatins fr the lateral variatins in the quality f

13 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY 4453 :e.; 0.00 '".; ọ..,;.0 :!!"' ::~ ~ta 'OJ.. -' "' >0... =' '" =' PRESSURE (GP), 0.2~ ~ 0.00 '" '".; '" =' N >- ~g 0- DO. U>'" z a: 0-'; :z - "' ffig L~.; PRESSURE (GP).~ ~ ~.20 ~.80 5.~ PRESSURE k8) Fig. 11. Labratry measurements f velcities (circles with tickmarks) in a phyllite frm the Valdez Grup measured nrmal (slid curve) and parallel (dashed curves) t fliatin. =' ~.OO PRESSURE kb Fig. 12. Percent anistrpy f a phyllite frm the Valdez Grup as a functin f pressure. midcrustal t lwer crustal reflectins in the Chugach sectin. The largest-amplitude near-vertical reflectins frm mid crustal depths f 5-9 s twtt n the Chugach seismic reflectin sectin are lcated between km 8 and 38 [Fisher et al., this issuej, where the thickness f the verlying 2.0 km/s layer is generally less than 200 m (Figure 5). T the nrth ',nd suth f this range interval the amplitudes f deep near-vertical reflectins decay mntnically ver a distance f abut 8 km until they can n lnger be bserved. The thickening f the km/s layer suth f km 8 (Figure 5) crrelates with reduced amplitudes f midcrustal reflectins. Hwever, in this area, we believe that the amplitude reductin is mst plausibly explained by the gradual lss f fld f the stacked sectin suthward f km 8. Because the reflectin spread was 15 km lng n a side, we believe that the thickening f the 2.0 km/s layer nrth f km 30 best explains the apparent lss f deep crustal reflectin events n the Chugach sectin nrth f km 38. Crrelatins f midcrustal t lwer crustal reflectr quality with variatins in near-surface sediments in ther lcalities [Stewart et al., 1986; E. R. Flueh, persnal cmmunicatin, 1987J suggest that the thickness f the 1.9~2.1 km/s layer cntrls the data quality f the near-vertical reflectin sectin. VELOCTY ANSOTROPY High-reslutin refractin analysis f the LTHOPROBE reflectin lines n Vancuver sland indicate a prnunced velcity anistrpy in the Leech River Cmplex [Mayrand et a., 1987], a Meszic accretinary melange [Fairchild and Cwan, 1982]. Labratry measurements at rm pressure and temperature f phyllitic rcks suggest that significant velcity anistrpy is present within the Valdez Grup in the Chugach Muntains. Cmpressinal wave velcities are shwn as a functin f pressure in Figure 11 fr three mutually perpendicular prpagatin directins in a sample f Chugach phyllite. The lwer velcities were measured nrmal t fliatin, whereas the tw dashed curves shw velcities measured within the fliatin plane. n Figure 12 the percent anistrpy has been calculated frm the velcity curves f Figure 11. At pressures abve 0.20 GPa the anistrpy primarily riginates frm preferred mineral rientatin within the phyllite. At lwer pressures, micrcracks riented sub- parallel t the fliatin als cntribute t the anistrpy [Christensen, 1965]. Thus the initial rapid decrease in phyllite anistrpy with increasing pressure shwn in Figure 12 riginates frm decreasing crack prsity. The Valdez Grup between km 18 and 35 includes strngly flded rcks, whse fld axes strike E-W. nasmuch as the slwest directin in the labratry measurements is perpendicular t the bedding plane, the fastest velcities shuld be riented E- W, and the slwest directin shuld be N-S. A frtuitus cmbinatin f the crked gemetry f the reflectin line and the 15-km length f the reflectin spread allwed us t determine whether a measureable velcity anistrpy exists within the near-surface Valdez Grup rcks. As can be seen in Figure 1, the reflectin line bends sharply within the Valdez Grup at km 28, where segments f the line have an rientatin f ::t45 frm nrth. Thus sme surces n ne side f the bend have sme receivers n the ther side. We therefre cmpared the velcities btained alng the rad segments having a NE-SW rientatin t thse determined fr purely N-S riented paths. N-S paths were btained by selectin f apprpriate surce and receiver pairs fr surces between km 19 and 35. Reduced travel times fr these picks are pltted in Figure 13. The travel times have been crrected fr variatins in the thickness f the 2.0 km/s layer, using thicknesses determined frm Figure 7. There als exists abut 100 m tpgraphic relief alng this prtin f the prfile. An additinal apprximate crrectin fr the OO-m difference in elevatin f the data frm km 19 t km 24 was estimated assuming vertical prpagatin f the seismic energy at 5.0 km/s. The velcity btained frm these picks is 5.18 ::t 0.09 km/s withut the crrectin fr elevatin variatins and is 5.11 ::t 0.10 km/s with this crrectin. Thus elevatin crrectins lwer the apparent velcity f energy prpagating N-S. The range f in situ refractin velcity estimates fr N-S azimuths is cnsistent with a significant velcity anistrpy within the Chugach terrane. The N-S velcity, determined with crrectins nly fr variatins in the thickness f the uppermst sediments, is significantly lwer than that determined fr the NE-SW azimuth at ranges greater than 5 km. The NE-SW velcity at ranges greater than 5 km is essentially 6.0 km/s (Figure 13), crrespnding t an anistrpy f 14%, which is reasnable cnsidering an anistrpy less than 20%

14 4454 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY 0.4 N-S. NE-SW "w "- X ~ 0.2 [Plajker et al., this issue]. The up-t-the-nrth sense f mtin n ffset agrees with that f the Brder Ranges fault in this area and elsewhere [Nkleberg et al., this issue]. The Brder Ranges fault zne requires n significant velcity anmaly t fit the bserved travel times. This finding culd mean either that the fault zne is thin r that the zne lacks a sizeable velcity cntrast with the surrunding cuntry rck. A wide (1-2 km) lw-velcity fault zne, hwever, is incnsistent with the trqvel time mdeling Range. km Fig. 13. Reduced travel times fr N-S and NE-SW riented paths arund the bend in the reflectin line in the Chugach Muntains, pltted as a functin f surce-receiver ffset. As described in the text, the N-S travel times have been crrected fr variatins in the thickness f the 2.0 km/s layer. A linear regressin f these travel times indicates that the apparent velcity alng these paths is 5.18 :t 0.09 km/s, significantly lwer than the 6.0 km/s bserved n NE-SW azimuths, indicating an anistrpy f 14 /". wuld be expected fr this rientatin. The data in Figure 13 and ur velcity mdel suggest that this anistrpy begins at a depth f 1 km within the Valdez Grup, where the rcks are mre cmpetent and less fractured. Unlike the labratry measurements f hand samples, the in situ measurements reprted here shw n anistrpy f the near-surface rcks, suggesting that these rcks cntain large-scale fractures that bscure the anistrpy. As reginal-scale refractin prfiling within the Chugach terrane has als determined a velcity anistrpy with the identical rientatin (G. S. Fuis et a!., submitted manuscript, 1988), seismic experiments specifically designed t examine the anistrpy within the Valdez Grup may be useful in refining these azimuthal and depth variatins. DSCUSSON The mst significant structural bundary interpreted frm the velcity-depth mdel is the nrth dipping interface delineating the suthern limit f the Cpper River Basin at km 73.5 (Figure 7). The absence f mapped nrmal faults in this prtin f the basin [Nichls and Yehle, 1969] suggests that vertical defrmatin alng this interface is nt active. Fr this reasn, the interface at km 73.5, whse gemetry suggests a thrw f at least 1300 m, is inferred t be a relict (buried) nrmal fault juxtapsing sedimentary rcks in the Cpper River Basin and the Talkeetna Frmatin. A 0.3-km vertical ffset in the ppsite sense lcated 15 km suth at km 57 wuld appear t make a hrst blck ut f the Talkeetna Frmatin (Figure 7). The Brder Ranges fault is expressed in at least tw ways. First, the Brder Ranges fault zne is expressed at km 57 by a vertical ffset. Secnd, the lcatin f the Brder Ranges fault is marked at km 42 by the juxtapsitin f slwer basement rcks within the McHugh Cmplex t the suth against faster basement rcks f the Tnsina ultramafic-mafic assemblage t the nrth. The subsurface lcatin f the Brder Ranges fault clsely matches the prjectin f the fault at km 42 shwn in Figure but nt the ffset at km 57. The ffset at km 57 may thus reflect reactivatin f a Brder Ranges fault r an unrelated stage f later defrmatin. The ffset at km 57 lies abut 2 km t the nrth f the prjected lcatin f the cntact between the Tnsina ultramafic-mafic assemblage and the Nechina River Gabbrnrite, which is based n islated utcrps lcated 4 t 5 km frm the Chugach reflectin prfile SUMMARY A high-reslutin refractin study f several thusand first arrivals n large-aperture reflectin field recrds was undertaken t help place cnstraints n the structural relatins f accreted terranes in suthern Alaska. Travel times f the first arrivals were mdeled using iterative tw-dimensinal ray tracing and resulted in a velcity depth mdel alng the Chugach reflectin line. Evidence f anistrpy in seismic velcity f 0.8 ::!::0.1 km/s in the Valdez Grup was determined (14%) with the slw directin riented N-S and the fast directin riented NE-SW, in agreement with labratry measurements f seismic velcities and field relatins within the Valdez Grup. The actual anistrpy within the Valdez Grup is likely t be higher in the E-W directin. An inverse relatin was nted between the quality f the deep seismic reflectins and the thickness f the uppermst layer cmpsed f alluvium and ther uncnslidated Quaternary sediments. A thicker (mre than 200 m thick) surficial layer was accmpanied by larger-amplitude guided waves and lwer-amplitude midcrustal reflectins. While we are currently attempting t understand better the rigin f this crrelatin, a pssible explanatin is that the cmbined effects f higher intrinsic attenuatin, increased guided wave energy nise within the thicker surficial sediments, and randm time shifts (statics) degraded the deep crustal reflectin data relative t regins with less extensive sediment cver. Finally, lcatins f cntrasting basement velcities within the mdel crrelate t prjected cntacts between distinct lithlgies and/r frmatins. One such cntact may mark the subsurface lcatin f the Brder Ranges fault, which has n expressin in the Chugach reflectin sectin. T the nrth, a reactivated splay f the Brder Ranges fault system may be represented by a prminent 350-m vertical ffset f the basement surface. The suthern bundary between the sedimentary strata filling the Cpper River Basin is defined by a nrth dipping interface between these lw-velcity sedimentary units and the higher-velcity Lwer Jurassic Talkeetna Frmatin. We infer at least 1300 m f dip-slip mtin alng this interface. We plan t extend this mdeling t include reflectins frm the midcrust in rder t learn whether accurate cnstraints n the gemetry and velcity f the upper crust can be btained in this manner. We als plan t reprcess the Chugach reflectin prfile in rder t test the hypthesis that the unmuted guided wave energy significantly degraded the reflected energy frm upper crustal depths. The field parameters used fr the acquisitin f the Chugach reflectin line allw us t develp a well-cnstrained velcity-depth mdel fr the upper 1-2 km f the crust using first -arri val travel times. The clse receiver spacing, large number f receivers, gruping f the receiver grup arrays int pint receivers, and sampling f the wave field near the surce array permit us t reslve the structure f the Quater-

15 BROCHER ET AL.: HGH-RESOLUTON REFLECTON/REFRACTON STUDY 4455 nary sediment cver in detail. This detail allws the pre- Quaternary crustal structure t be reslved with greater cnfidence than if the near-surce prtin f the wave field is nt recrded. The field parameters emplyed thus prvide an unusual capability t image crustal structure and shuld be cnsidered fr ther study areas. Acknwledgments. We thank J. Luetgert fr writing and maintaining the tw-dimensinal ray thery sftware, W. Nkleberg and G. Plafker fr sharing their gelgical insights, E. Ambs fr many helpful discussins, and R. Catchings, J. Cllins, G. Fuis, R. Page, and G. Plafker fr reading early drafts f the manuscript. Critical cmments by L. Mayrand, J. Orcutt, and an annymus reviewer greatly imprved sectins f the manuscript. We acknwledge funding frm the Deep Crustal Studies prgram f the USGS. The labratry velcity measurements were supprted by ONR cntract N K REFERENCES Andreasen, G. E., A. Grantz,. Zietz, and D. F. Barnes, Gelgic interpretatin f magnetic and gravity data in the Cpper River Basin, Alaska, U.S. Gel. Surv. Prf Pap., 316-H, , Brcher, T. M., Shallw velcity structure f the Ri Grande rift nrth f Scrr. New Mexic: A reinterpretatin, J. Gephys. Res., 86, , 1981a. Brcher, T. M., Gemetry and physical prperties f the Scrr, New Mexic, magma bdies, J. Gephys. Res., 86, , 1981b. Brcher, T. M., and J.. Ewing, A cmparisn f high reslutin seismic methds fr determining seabed velcities in shallw water, J. Acust. Sc. Am., 79, , Burns, L. E., Gravity and aermagnetic mdeling f a large gabbric bdy near the Brder Ranges fault, suthern Alaska, u.s. Gel. Sun:. Open File Rep., , Burns, L. E.. The Brder Ranges ultramafic and mafic cmplex, suth-central Alaska: Cumulate fractinates f island-arc vlcanics, Can. J. Earth Sci., 22, , Burns, L. E., T. A. Little, R. J. Newberry, J. E. Decker, and G. H. Pessel, Preliminary gelgic map f parts f the Anchrage C-2, C-3, D-2, and D-3 quadrangles, Alaska, scale 1: 250,000, Alaska DiL'. Gel. Gephys. Sun'. Rep. nvest , 3 sheets, Cerveny, V.,. A. Mltkv, and. Psencik, Ray Methd in Seismlgy, 214 pp., Univerzita Karlva, Prague, Czechslvakia, Christensen, N.., Cmpressinal wave velcities in metamrphic rcks at pressures t 10 kbar, J. Gephys. Res., 70, , Christensen, N.., Seismic velcities, in Handbk f Physical Prperties f Rcks, vl., edited by R. S. Carmichael, pp , CRC Press, Bca Ratn, Fla., Christensen, N.., Measurements f dynamic prperties f rck at elevated pressures and temperatures, in Measurements j Rck Prperties at Elerated Pressures and Temperatures, edited by H. J. Pincus and E. R. Hskins, pp , American Sciety fr Testing and Materials, Philadelphia, Pa., Cleman, R. G., and L. E. Burns, The Tnsina high-pressure maficultramafic cumulate sequence, Chugach Muntains, Alaska, Gel. Sc. Am. Ahstr. Prgrams, 17(6), 348, Cney, P. L D. L. Jnes, and J. W. H. Mnger, Crdilleran suspect terranes, Nature, 288, ,1980. DeBari, S., and R. G. Cleman, Petrlgic aspects f gabbrs frm he Tnsina cmplex, Chugach Muntains, Alaska: Evidence fr deep magma chambers under an island arc, Gel. Sc. Am. Abstr. Prgrams, 18,99, DeBari, S., and R. G. Cleman, Examinatin f the deep levels f an island arc: Evidence frm the Tnsina mafic-ultramafic assemblage, Tnsina, Alaska, J. Gephys. Res., this issue. devgd, B., L. D. Brwn, and C. Merey, Nature f the eastern bundary f the Ri Grande rift frm COCORP surveys in the Albuquerque Basin, New Mexic, J. Ge()phys. Res., 91, , Fairchild, L. H., and D. S. Cwan, Structure, petrlgy, and tectnic histry f the Leech River Cmplex, nrthwest f Victria, Vancuver sland, Can. J. Earth Sci., 19, , Fisher, M. A., T. M. Brcher, W. J. Nkleberg, G. Plafker, and G. Smith, Seismic reflectin images f the crust f the nrthern part f the Chugach terrane, Alaska: Results f a survey fr the Trans- Alaska Crustal Transect, J. Gephys. Res., this issue. Geist, E. L., and T. M. Brcher, Gemetry and subsurface lithlgy f Suthern Death Valley basin, Califrnia based n refractin analysis f multichannel seismic data, Gelgy, 15, , Haeni, F. P., Applicatin f seismic refractin methds in grundwater mdeling studies in New England, Gephysics, 51, , Hill, D. P., E. Kissling, J. H. Luetgert, and U. Kradlfer, Cnstraints n the upper crustal structure f the Lng Valley-Mn Craters vlcanic cmplex, eastern Califrnia, frm seismic refractin measurements, J. Gephys. Res., 90, , Hwell, D. W., D. L. Jnes, and E. R. Schermer, Tectnstratigraphic terranes f the circum-pacific regin, in Tectnstratigraphic Terranes (the Circum-Pacific Regin, Earth Sci. Ser., vl. 1, edited by D. W. Hwell, pp. 3-30, Circum-Pacific Cuncil fr Energy and Mineral Resurces, Hustn, Tex., Jnes, D. L., N. J. Silberling, P. J. Cney, and G. Plafker, Lithtectnic terrane map f Alaska (west f the 141st meridian) part A, Lithtectnic Terrane Map f the Nrth American Crdillera, edited by N. J. Silberling and D. L. Jnes, U.S. Gel. Surv. Open File Rep., , A-A2, Jurdy, D. M., and T. M. Brcher, Shallw velcity mdel f the Ri Grande rift near Scrr, New Mexic, Gelgy, 8, ,1980. MacKevett, E. M., and G. Plafker, The Brder Ranges fault in suth central Alaska, J. Res. U.s. Gel. Surv., 2, ,1974. Mayrand, L. J., A. G. Green, and B. Milkereit, A quantitative apprach t bedrck velcity reslutin and precisin: The LTHO- PROBE Vancuver sland experiment, J. Gephys. Res., 92, , Milkereit, B., W. D. Mney, and W. M. Khler, nversin f seismic refractin data in planar dipping structure, Gephys. J. R. Astrn. Sc., 82, , Nichls, D. R., and L. A. Yehle, Engineering gelgic map f the sutheastern Cpper River Basin, Alaska, scale 1: 125,000, U.S. Gel. SUT.Map, 1-524, Nkleberg, W. J., G. Plafker, J. S. Lull, W. K. Wallace, and G. R. Winkler, Structural analysis f the suthern Peninsula, suthern Wrangellia, and nrthern Chugach terranes alng the Trans-Alaska Crustal Transect, nrthern Chugach Muntains, Alaska, J. Geph's. Res.. this issue. Page, R. A., G. Plafker, G. S. Fuis, W. J. Nkleberg, E. L. Ambs, W. D. Mney, and D. L. Campbell, Accretin and subductin tectnics in the Chugach Muntains and Cpper River Basin, Alaska: nitial results f the Trans-Alaska Crustal Transect, Gelgy, /4, , Plafker, G., W. J. Nkleberg, and J. S. Lull, Summary f 1984 TACT gelgical studies in the nrthern Chugach Muntains and suthern Cpper River Basin, U.S. Gel. Surv. Circ., 967, 76-79,1985. Plafker, G., W. J. Nkleberg, and J. S. Lull, Bedrck gelgy and tectnic evlutin f the Wrangellia, Peninsular, and Chugach terranes alng the Trans-Alaskan Crustal Transect Alaska, J. Gephys. Res., this issue. Schn, J., Petr physik, p. 112, F. Enke, Stuttgart, Federal Republic f Germany, Sissn, V. B., and T. C. Onsttt, Dating blueschist metamrphism: A cmbined 4" Ar!3" Ar and electrn micrprbe apprach, Gechim. Csmchim. Acta, 50, ,1986. Stewart. D. B., J. D. Unger, J. D. Phillips, R. Gldsmith, W. H. Ple, C. P. Spencer, A. G. Green, M. C. Liselle, and P. St.-Julian, The Quebec-western Maine seismic reflectin prfile: Setting and first year results, in Reflectin Seismlgy: The Cntinental Crust, Gedyn. Ser., vl. 14, edited by M. Barazangi and L. Brwn, pp , AGU, Washingtn, D. c., Winkler, G. R., and G. Plafker, Gelgical map and crss sectins f the Crdva and Middletn sland quadrangles, suthern Alaska, scale : 250,000, U.S. Gel. Sur!'. Open File Rep., , 25 pp., 1 sheet Winkler, G. R., M. L. Silberman, A Grantz, R. J. Miller, and E. M. MacKevett, Jr., Gelgical map and summary gechrnlgy f the Valdez quadrangle, suthern Alaska, scale 1: 250,000, U.S. Gel. SUT. Open File Rep., A, 2 sheets, T. M. Brcher, M. A. Fisher, and E. L. Geist, U.S. Gelgical Survey, 345 Middlefield Rad, MS 977, Menl Park, CA N.. Christensen, Department f Gesciences, Purdue University, West Lafayette, N (Received Nvember 23,1987; revised February 16, 1988; accepted February 17, 1988.)