Bulletin of the Seismological Society of America, Vol, 68, No. 4, pp , August, 1978

Transcription

1 Bulletin f the Seismlgical Sciety f America, Vl, 68, N. 4, pp , August, 1978 CRUSTAL THICKNESS IN NORTHERN NEVADA FROM SEISMIC REFRACTION PROFILES BY DOUGLAS A. STAUBER AND DAVID M. BOORE ABSTRACT P, arrival times were measured alng three nrth-suth prfiles in nrthern Nevada and sutheastern Oregn. Apparent arrival velcities f 8.0, 8.2, and 8.4 km/sec were bserved n the suthern ends f the prfiles and are explained by crustal thinning f up t 8 km alng the prfiles. This interpretatin is supprted by the shape f the Buguer gravity anmaly and a prminent secnd arrival interpreted t be a Mh reflectin bserved frm a lcal quarry blast alng ne f the lines. The Battle Muntain heat flw high as utlined by the 2.5 HFU cntur f Sass et al. (1976) des nt crrelate well with the area f thin crust in nrth central Nevada. Hwever, the suthern bundary f the heat flw high crrespnds t a decrease in crustal thickness n all prfiles-- the crust inside the heat flw high is 5 t 8 km thinner than the crust t the suth. The crustal thickness f 21 t 23 km in the Battle Muntain-Winnemucca area is cmpared t estimated depths t the wet and the dry basaltic slidus and n partial melting f the lwer crust is expected if it is dry, while partial melting is expected if it is wet. A prminent later phase at distances greater than 550 km is mdeled by an increase in P velcity f 0.2 t 0.3 km/sec at a depth between 70 and 90 km. A lw-velcity zne may exist in the mantle abve this velcity increase but is nt required by ur data. INTRODUCTION Frm Fall 1975 t Summer 1977 seismic bdy waves frm nuclear events frm the Nevada Test Site (NTS) were recrded in nrthern Nevada and suthern Oregn by Stanfrd University and the University f Nevada. The area is predminately in the Basin and Range prvince and includes several features which indicate current tectnic activity. The Battle Muntain heat flw anmaly, defined by Sass et al. (1976) as the western end f a NE-SW trending zne with heat flw f ver 2.5 HFU, extends int the studied area (see Figure 1). Heat flw measurements in this zne in the Battle Muntain-Winnemucca area are generally ver 3 HFU. A secnd feature is the nrthern Nevada seismic zne, which extends suth frm Battle Muntain and Winnemucca. The seismic zne has been the lcatin f 5 large (m > 7) earthquakes in the last 150 years (Ryall, 1977). Previus seismic refractin wrk in nrthern Nevada cnsists f tw reversed refractin lines (Figure 1), and a time-term analysis f NTS blasts and several earthquakes recrded at the University f Nevada netwrk statins. The refractin line reprted by Eatn (1963) runs east-west between Eureka and Falln, Nevada t the suth f the heat flw anmaly. The ther, reprted by Hill and Pakiser (1966), runs nrth-suth t the east f ur wrk. The time-term analysis f Batra (1970) includes tw statins in the heat flw anmaly, at Battle Muntain and at Lvelck. The data presented here were btained frm prfiles thrugh the heat flw anmaly and, in cnjunctin with ther data recrded by us frm mine blasts near Battle Muntain, prvide a lk at changes in crustal structure in the area. EXPERIMENT DESCRIPTION AND DATA REDUCTION A variety f recrding systems using 1 and 5 Hz vertical-cmpnent seismmeters were used t btain data. Mst f the recrdings made by Stanfrd University used 1049

2 1050 DOUGLAS A. STAUBER AND DAVID M. BOORE Sprengnether MEQ800 smked-paper recrders with a timing accuracy f 0.02 sec. Sme recrdings were btained with a prtable FM tape recrding system which is being used fr mre extensive refractin experiments in the area using mine blasts. The timing accuracy f this system is als abut _+0.02 sec. University f Nevada recrdings were made with a shrt-perid telemetered system with similar timing accuracy. The University f Nevada netwrk statin at Battle Muntain was used as a reference, s data frm different NTS events culd be cmbined, and is indicated n Figures 1 and t., + F 116 1:4L4. + a OREGON -'~ ~\ / \, / ', I', I i / i,e/ i I i..i\ II.t ) Q ",.." LVK ~ I IDAHO "~'x"d ~ ~" "f E i "' ' l, A NEVAOA SOURCES NTSA Km \ \ + 36 FI(L 1. Map shwing lcatin f refractin prfiles: A, Winnemucca prfile; B, Cpper Canyn prfile; C, Battle Muntain prf'fle; D, Hill and Pakiser (1966); E, Eatn (1963); F, Hill (1972). The shrt dashed line is the bundary f the Battle Muntain heat flw high frm Sass et al. (1976). The lng dashed line is an apprximate nrthern bundary fr the Basin and Range prvince. The abbreviatins are: BMT, Battle Muntain; CCM, Cpper Canyn Mine; LVK, Lvelck; NTS, Nevada Test Site; WIN, Winnemucca. The University f Nevada netwrk statin near Battle Muntain is the statin immediately suth f CCM n the Cpper Canyn prfile. The first-mtin times were read frm all the statins, and were crrected t a plane 1 km abve sea level using a near-surface velcity f 4.0 km/sec. This velcity is in the middle f the range f velcities fund in the area (3.5 t 4.5 km/sec), frm ur unpublished wrk and by the studies f Eatn (1963) and Hill and Pakiser {1966). The lcatins and rigin times f the NTS surces were supplied by the Lawrence Livermre Labratry. DATA PRESENTATION The first-arrival times fr the Winnemucca, Cpper Canyn, Battle Muntain prfiles and first-peak times frm Hill and Pakiser, line D, are pltted n reduced-

3 NORTHERN NEVADA REFRACTION PROFILES 1051 time plts in Figure 2. The high-frequency frerunners between 400 and 450 km discussed by Hill and Pakiser (1966), are nt pltted. The recrding statins near the Oregn brder and falling between prfiles A and B were arbitrarily assigned t the clser f the tw prfiles. It shuld be nted that there is a scatter f the pints f +_0.15 sec abut any line drawn thrugh them. This scatter is greater than the measuring accuracy f +_0.02 sec, and may be due c lcal gelgical structure. A significant feature n bth the Winnemucca and the Cpper Canyn lines is the high apparent arrival velcity f 8.2 t 8.4 km/sec n the suthern 100 t 200 km f the lines. The upper-mantle velcity determinatins in the nrthern Basin and Range Prvince frm ther studies range frm 7.3 t 8.0 km/sec (see Prdehl, 1970, Table 2). The area f high apparent velcity cincides with the suthern half f the I 1 1 T I BR 1 ~ UJ L/3 v r--: <3 I "Ld I-- 7 B C 55 % ~O 0 0 UN O0 0 0 " D 6 g O e BR~ ~ ~ ~ % eeo e I I l 1 I I SO DISTANCE (KM) FIG. 2. Reduced travel times: A, Winnemucca; B, Cpper Canyn; C, Battle Muntain; D, Hill and Pakiser (1966). University f Nevada netwrk statin near Battle Muntain is marked by UN n Cpper Canyn prfile. Nrthern bundary f the Basin and Range prvince indicated by arrws labeled BR. Errr bars are smaller than the symbls. Battle Muntain heat flw high. On ur Battle Muntain prfile, the maximum apparent velcity ver a cmparable distance range is abut 8.0 km/sec. On the nrth end f the Winnemucca prfile, the Pn branch dies ut and a largeamplitude, lw-frequency phase 1 t 2 sec later becmes prminent. A recrd sectin fr the nrthern end f the Winnemucca prfile is shwn in Figure 3. This sectin was cnstructed frm the smked-drum recrdings by measuring zer-crssing times and peak and trugh amplitudes. The wave frm was then sketched t fit these pints and t lk like the riginal recrds. Sme f the larger peaks are clipped. The recrdings shwn n the recrd sectin were made with tw types f gephnes, and n amplitude calibratin was perfrmed. The fur recrdings between 400 and 700 km were made with 4.5 Hz gephnes, and the ther tw statins used 1 Hz gephnes. A large-amplitude and lnger perid phase is evident at a reduced time

4 1052 DOUGLAS A. STAUBER AND DAVID M. BOORE f 7.7 see n all the recrds at distances greater than 500 km. The amplitude f the first arrival (Pn) decreases rapidly with distance and is nt clear beynd 600 km. Additinal recrdings were made alng a suthward extensin f the Cpper Canyn line using quarry blasts at Cpper Canyn Mine (CCM in Figure 1). This 9.0 I I I I I. L.; LU (3D 8.0 LLI,,.,, Z l-- Lrl I I l I I, I qo0 600 BOO DISTANCE CKDd) FIG. 3. Recrd sectin fr nrth end f Winnemucca prfile. Travel times fr Mdel A (with LVL) shwn with slid lines and travel times fr mdel B (withut LVL) are dashed. Mdels A and B are listed in Table 1. A CKM FROM CCM) I i r I I i i 30 V ug 29 E) r~ 97. 7,N8 KN/SEC I I I I I I I I I I I I I 7,BI KM/SEC ~---- ~ - - I i I I ] 2NO A ( KM FROM NTS) FIG. 4. Reversed refractin data at suth end f Cpper Canyn prfile. Tp is fr Cpper Canyn mine sht and are relative times nly. Bttm is fr an NTS surce. The tw halves are ffset hrizntally t the apprximate psitin frm which the Pn waves leave the crust-mantle bundary. prvides reversed refractin cverage f the suthern 50 km f the mantle path sampled by waves cming frm the Nevada Test Site and recrded n prfile B. The data in the regin f verlap are shwn in Figure 4. Least-square linear fits yield apparent velcities f 7.48 ± 0.17 km/sec fr the nrth t suth directin and

5 NORTHERN NEVADA REFRACTION PROFILES _ 0.13 km/sec fr the ppsite directin. The apparent velcities are cnsistent with an upper mantle velcity f 7.64 km/sec and a Mh dip f 0.8 t the suth. Because the recrdings fr the tw directins were nt made at the same surface lcatins, the pertubatins f the travel times due t near-surface gelgy d nt cancel when using the reversed cverage t determine the true Pn velcity. Systematic variatins in gelgical structure culd cause a variatin f +_0.30 km/sec in the cmputed "true" Pn velcity. INTERPRETATION Since mst f the travel-time data in Figure 2 are unreversed, the high apparent velcities n the suthern halves f the lines culd be explained in several ways, as fllws: (1) the average crustal velcities increase systematically t the nrth; (2) the crust thins frm suth t nrth; (3) they represent true, high, upper-mantle velcities. The first hypthesis requires a mean crustal velcity f 7.0 km/sec in the area f 20 0 S A TON ~, BRI 30 ~HF WIN WO j ~ & OOOO bj> 30 B ATO N 0 0 w 0% O, I, HF CCM < 40 l..d C ATON 30 O-LO LO --J T HI= '~BHT m 40 I,~'- 30 ~ ~L EIA~ ~1 srp _ D (P ~ HF T ELKO 40 T! I I I 1 1 i WOO DISTANCE FIG. 5. Crust-mantle bundary prfiles fr data in Figure 2. Arrws marked HF indicate apprximate suthern bundary f the heat flw high. Errr bars shwn when larger than symbls. earliest arrivals f Figure 2 if the crustal thickness remains cnstant and if we use the crustal velcities fund by Eatn (1963), with mean f 6.25 km/sec, at the suthern end. The refractin data frm chemical explsins presented by Hill and Pakiser (1966) rules this ut fr the prfile in Figure 2d. Als, preliminary interpretatin f an unreversed refractin line suth frm the Cpper Canyn Mine (see Figure 6) indicates that at least the upper 10 km f crust has a P velcity f 6.0 km/sec r less, ruling ut a systematic increase in crustal velcities f sufficient size t explain the high apparent velcity n the Cpper Canyn prfile. The secnd hypthesis, that f changing crustal thickness, is used in cnstructing the mdels shwn in Figure 5. The upper-mantle and crustal P velcities needed fr the cnstructin were based n the prfiles f Eatn (1963) and Hill and Pakiser (1966), described earlier. The upper-mantle velcity was taken as 7.8 km/sec, and a simple crustal mdel with P velcities f 6.0 and 6.7 km/sec in the upper tw-thirds and the lwer third f the crust, respectively, was used. Fr this mdel, the change in crustal thickness, in kilmeters, is apprximately ten times the change in reduced travel time in secnds. Other simple crustal mdels yield factrs f 9 t 13 depending (KM)

6 1054 DOUGLAS A. STAUBER AND DAVID M. BOORE n the behavir f the thickness f the lwer crustal layer. In the absence f ther infrmatin, the factr f 10 was used. The errr bars n the depths shwn n Figure 5 reflect this uncertainty. The base f the crust is fixed at the depths determined by Eatn (1963) where his line crsses ur lines. Errr in this depth is nt included in the errr bars, and all depths may be changed by a cnstant amunt, retaining the shape f the crust-mantle bundary. The data f Hill and Pakiser (1966) were reinterpreted by us using the abve methd, and the resulting crustal thickness prfile is pltted in Figure 5d fr cmparisn with ur lines. On ur western tw prfiles the base f the crust rises frm 30 km belw sea level at the suth end t 22 km in the Battle Muntain- Winnemucca area. Our results agree with thse f Batra fr his estimate f crustal thickness at a pint abut 30 km suth f the University f Nevada netwrk statin near Battle Muntain. The shallwest depth t the crust-mantle bundary increases eastward t 25 km n ur Battle Muntain line and t 29 km n the line f Hill and Pakiser thrugh Elk (Figure 5). Us uj ZO DELTA[ KI'I ] FI. 6. Recrd sectin frm Cpper Canyn Mine shts recrded suth alng the Cpper Canyn prfile. Mdel Mh reflectin times shwn as circles. The critically reflected ray is at 64 km. Althugh the third hypthesis, that the upper-mantle velcity in the suthern prtin f ur lines is 8.0 t 8.4 km/sec, cannt be excluded by the unreversed traveltime data alne (we lack surces nrth f Cpper Canyn Mine t prvide reversed cverage f the high apparent velcities), there are ther data which supprt the crustal thinning case. The first is an estimate f crustal thickness in the Battle Muntain area frm a reflectin seen n the unreversed refractin line running suth alng the Cpper Canyn line. This reflectin begins at a distance f 64 km, abut 2 sec after the first arrival (see Figure 6). Using a preliminary interpretatin f upper-crustal velcity frm this data, a lwer-crustal velcity f 6.7 km/sec beginning at a depth f 11 km in the Battle Muntain area, and the Mh depth and dip calculated frm the NTS data, the calculated Mh reflectin times are shwn n Figure 6 (circles). The increase in amplitude expected at the critical reflectin distance is seen in the data, and it matches the calculated critical reflectin distance. The calculated travel times als fit the bserved times. Variatins in Mh depth change the travel times at a given distance by rughly 0.15 sec/km. A change f greater than 3 km in Mh depth wuld nt fit the travel-time data. The secnd piece f evidence supprting the changing crustal-thickness mdel ver the high-velcity upper-mantle mdel is the Buguer gravity. In the high-

7 NORTHERN NEVADA REFRACTION PROFILES 1055 velcity upper-mantle case, ne wuld expect a gravity anmaly t be centered ver the area exhibiting high velcities, while in the crustal-thinning case a gravity high is expected ver the area f thin crust. A Buguer gravity prfile is pltted in Figure 7 fr the Winnemucca line. Each pint f the gravity prfile represents the mean f a perpendicular prfile extending 35 km n each side f the refractin line. This prcedure remves the effects f individual basins and ranges by averaging ver 3 r 4 basin and range pairs. The gravity data is taken frm Erwin (1974 and 1977) and Wllard and Jesting (1964). The gravity high bserved n the Winnemucca line extends well nrth f the regin f high apparent Pn velcities, supprting the crustal-thinning mdel. An apparent incnsistency with this argument is the gravity high ver the western Snake River Plain, where Hill and Pakiser (1966) fund a thicker crust than in the ~ 7 bq I I I I I ).%0 I ~ 160 L9 ~z 180 >.- F-- 2OO <[ ~ 220 ~SED 0 O& 0 ee 8.. 0"8 z OBSERVED ", 2-D CALCULATION (, 0.6) 0 55 "" IO 1",- 20 EL IaJ 30 {~q~'. ~e I L I i l l DISTANCE (KM) FIG. 7. Reduced travel times, bserved Buguer gravity and crustal sectin fr Winnemucca prfile, pen circles; Cpper Canyn prfile reduced travel times and crustal prfile shwn with clsed circles fr cmparisn. The calculated tw-dimensinal gravity anmaly fr the Winnemucca crustal prfile has been multiplied by 0.6 and is shwn with triangles fr cmparisn f shapes. Vertical errr bars are shwn when larger than the symbls. Basin and Range t the suth. Our argument assumes that the mean crustal density des nt have a significant lateral variatin. Seismic velcity structures frm refractin prfiles in the nrthern Basin and Range prvince shw n large changes in mean crustal velcities, suggesting that the densities are similarly well behaved. The crustal velcity structure determined by Hill and Pakiser (1966) fr the western Snake River Plain is very different than that fund in the Basin and Range t the suth, and therefre, ne wuld nt expect the same relatin between crustal thickness and Buguer gravity values t hld in the western Snake River Plain as in the Basin and Range prvince. A tw-dimensinal gravity prfile was calculated fr the Winnemucca crustal prfile frm 200 t 650 km, using a density f 2.7 gm/cm a fr the upper tw-thirds f the crust, 2.9 gm/cm 3 fr the lwer third, and an upper-mantle density f 3.20 gm/cm a. The calculated anmaly has a maximum amplitude f 98 mgal instead f

8 DOUGLAS A. STAUBER AND DAVID M. BOORE the bserved 60 mgal. The shape f the calculated tw-dimensinal anmaly, hwever, matches the bserved anmaly well as can be seen in Figure 7 where the calculated gravity anmaly values have been multiplied by 0.6 fr ease f cmparisn. Small mdificatins f the calculated shape are expected when three-dimensinal effects are included. The Winnemucca prfile is lcated near the eastern edge f a large, psitive gravity anmaly in nrthwestern Nevada, with an amplitude f 70 mgal. Apprximate three-dimensinal gravity calculatins shw that the shape f the crust-mantle bundary t the east f the Winnemucca and Cpper Canyn prfiles can accunt fr abut ne-third f the discrepancy between the twdimensinal calculatin and the bserved gravity. The amplitude f the calculated gravity anmaly als depends strngly n the amunt f lwer crustal rcks (p = 2.9 gm/cm a) which are included in the crust in the Battle Muntain-Winnemucca area. Analysis f mre detailed refractin lines frm mines in the Battle Muntain area and use f the Battle Muntain refractin prfile shuld allw mre quantitative gravity interpretatin. The large-amplitude, lng-perid, secnd arrival at the nrth end f the Winnemucca prfile may be a reflectin frm a velcity increase in the mantle at a depth TABLE 1 VELOCITY MODELS SHOWN IN FIGURE 3 Layer Thickness (kin) V~ {km/sec) Velcity Mdel A, with lw-velcity zne Velcity Mdel B, withut lw-velcity zne O f 70 t 90 km. A velcity increase f 0.2 t 0.3 km/sec at this bundary prduce~ a critical reflectin distance f 550 km as bserved. Travel-time curves fr tw mdels, ne with a lw-velcity zne abve the discntinuity, and ne withut, are shwn n Figure 3. The velcity mdels used are listed in Table 1. The rapid amplitude decay f the Pn branch suggests a negative velcity gradient near the tp f the mantle, but we cannt be quantitative because f lack f amplitude calibratin. The lngperid nature f the secnd arrival indicates an increase in attenuatin with depth in the mantle, which is expected fr a LVZ caused by partial melting. A similar arrival was reprted by Hill (1972) n a nrth-suth prfile frm a surce in suthern Canada, line F n Figure 1. His prfile runs suth int central Oregn and verlaps ur Winnemucca prfile. One mdel presented by Hill (CP1) t explain this phase was a velcity increase frm 8.0 t 8.4 km/sec at a depth f abut 100 km. Julian (1970) fund velcity increases f 0.3 km/sec at 100 km depth n a prfile nrtheast and east frm NTS but nt n a prfile nrth frm NTS. A velcity increase at a depth f 100 km is apparently a cmmn feature, but is nt evident n all prfiles in the western U.S., as it is in the eastern U.S. (see Hales, 1969).

9 NORTHERN NEVADA REFRACTION PROFILES 1057 SUMMARY AND DISCUSSION Nuclear events have been recrded alng three lines in Nevada and Oregn t the nrth f NTS, ne which passes thrugh Winnemucca, ne thrugh the Cpper Canyn Mine and the ther near Battle Muntain. First-arrival times give an apparent Pn velcity n the suthern part f the lines f 8.4, 8.2, and 8.0 km/sec, much higher than the nrmal values fr the Basin and Range prvince. Other reversed refractin lines in the area, including ne crssing ur lines, yield an uppermantle P velcity f 7.8 t 7.9 km/sec, suggesting that the high velcity fund by us may be due t a thinning f the crust nrthward frm central Nevada. Sme reversed cverage immediately suth f this area f high apparent velcity has been btained and is cnsistent with an upper-mantle velcity f 7.8 km/sec. Assuming an upper-mantle velcity f 7.8 km/sec and attributing the variatins in apparent velcities t changes in crustal thickness, a mantle upwarp with a relief f apprximately 8 km is present under the Battle Muntain-Winnemucca area in nrth-central Nevada. Reflectin times and amplitudes frm quarry blasts at Cpper Canyn Mine alng the. Cpper Canyn line and Buguer gravity prfiles are cnsistent with this interpretatin. On the nrthern end f the Winnemucca prfile evidence was fund fr a small, lw-velcity and high-attentuatin zne abve a depth f 70 t 90 km in nrthwest Nevada and sutheast Oregn; the amplitudes f P~ first arrivals decrease rapidly, and a large-amplitude, lw-frequency phase with an apparent velcity f 7.8 km/sec appears 1.5 sec later. The later phase may be reflected frm a velcity increase f 0.2 t 0.3 km/sec at a depth f 70 t 90 kin. The lwer frequency f the later phase can he explained by an increase in attenuatin with depth in the mantle. The decay f Pn amplitudes may indicate a negative velcity gradient in the upper mantle. If the 2.5 HFU cntur f Sass et al. (1976) is used t utline the heat flw high, there is nt a gd crrelatin with crustal thickness. Fr example, inside the bundary f the heat flw high the crust ranges in thickness frm 22 km in the Winnemucca area t 32 km near Elk. Furthermre, the thin crust fund n the Winnemucca prfile extends nrth ut f the heat flw high, althugh it shuld be nted that the bundary f the heat flw high is nt well cnstrained. Althugh there is little crrelatin between the mapped heat flw anmaly f Sass et al. (1976} and abslute values f crustal thickness, a change in crustal thickness f 5 t 8 km in all fur prfiles in Figure 5 ccurs near the suthern bundary f the heat flw high. In all the prfiles the crust-mantle bundary is 5 t 8 km higher inside than t the suth f the heat flw high bundary. The heat flw bundary frm Figure 1 is lcated n the crustal sectins f Figure 5 by arrws, and is rughly cincident with the change in crustal thickness. The mismatch in psitins n Figure 5 c, and d is within the uncertainties in the lcatin f the heat flw bundary. In the Battle Muntain-Winnemucca area, where the heat flw measurements are dense enugh t reliably shw a mean flw f 3.0 HFU, the ccurrence f thin crust has implicatins fr the ccurrence f crustal melting. Lachenbruch and Sass {1978, Figure 13) present getherms fr the Battle Muntain heat flw high fr a variety f cnductive and cnvective heat transfer mdels. A cnvective mdel with dike intrusin thrugh the lithsphere results in the lwest temperatures fr a given depth and the purely cnductive case gives the highest. The dry basalt slidus is reached at apprximately 60 km (1230 C) in the intrusive case and 26 km (1130 C) in the cnductive case. The wet gabbr slidus frm Wyllie (1971) is reached at depths f 20.5 km (725 C) and 16 km (760 C), respectively. Ttal crustal thickness

10 1058 DOUGLAS A. STAUBER AND DAVID M. BOORE f 21 t 24 km in the Battle Muntain-Winnemucca area is less than the depth t the dry basalt slidus, and is greater than the depth t the wet gabbr slidus fr the limiting cases f bth the cnductive and cnvective getherms. N melting f basaltic material is required if the lwer crust is dry, while basaltic partial melting is expected near the base f the crust if it is water saturated. ACKNOWLEDGMENTS We wish t thank Rbert Rhrer f Lawrence Livermre Labratry fr prviding rigin times and lcatins fr the NTS events used, Dr. Alan Ryall fr the pprtunity t read the data cllected by the University f Nevada, Prf. Tm McEvilly f the University f Califrnia fr the lan f seismic recrders, and Gerge Thmpsn fr critically reading the manuscript. The research was supprted by the Divisin f Earth Sciences, Natinal Science Fundatin, Grant EAR A02. REFERENCES Batra, R. (1970). Travel-times and crustal structure in the Neveda regin frm earthquakes, nuclear explsins, and mine blasts, M.S. Thesis, University f Nevada, Ren. Eatn, J. P. (1963). Crustal structure frm San Francisc, Califrnia, t Eureka, Nevada, frm seismicrefractin measurements. J. Gephys. Res. 68, Erwin, J. W. (1974). Buguer gravity map f Nevada, Winnemucca sheet, Nevada Bureau f Mines and Gelgy Map 47. Erwin, J. W. (1977). Buguer gravity map f Nevada, Millet sheet, Nevada Bureau f Mines and Gelgy Map53. Hales, A. L. (1969), A seismic discntinuity in the lithsphere, Earth Planet. Sci. Lett. 7, Hill, D. P. (1972). Crustal and upper mantle structure f the Clumbia Plateau frm lng range seismic refractin measurement. Bull. Gel. Sc. Am. 83, Hill, D. P., and L. C. Pakiser.(1966). Crustal structure between the Nevada Test Site and Bise, Idah, frm seismic-refractin measurements, in Earth Beneath the Cntinents, J. S. Steinhart, T. J. Smith, Editrs, Gephys. Mngr. 10, Am. Gephys. Unin, Washingtn, D.C., pp Julian, B. R. (1970). Reginal variatins in upper mantle structure beneath Nrth America, Ph.D. Thesis, Califrnia Institute f Technlgy. Lachenbruch, A. H. and J. H. Sass {1978). Mdels f an extending lithsphere and heat flw in the Basin and Range prvince, Gephys. Sc. Am. Memir 152, (in press). Prdehl, C. (1970). Seismic refractin study f crustal structure in the western United States, Bull. Gel. Sc. Am. 81, RyaU, A. (1977). Earthquake hazard in the Nevada regin, Bull. Seism. Sc. Am. 67, Sass, J. H., W. H. Diment, A. H. Lachenbrnch, B. V. Marshall, R. J. Munre, T. H. Mses, Jr., and T. C. Urban (1976). A new heat flw cntur map f the cnterminus United States, USGS Open File Rept Wllard, G. P., and H. R. Jesting (1964). Buguer gravity anmaly map f the United States, Am. Gephys. Unin, Special Cmmitee fr Gephysical and Gelgical Study f the Cntinents, G. P. Wllard, Chairman, and the US Gelgical Survey, H. R. Jesting, Crdinatr. Wyllie, P. J. (1971). Experimental limits fr melting in the earth's crust and upper mantle, in The Structure and Physical Prperties f the Earth's Crust, J. G. Heacck, Editr, Am. Gephys. Unin. Gephys. Mngr. 14, pp GEOPHYSICS DEPARTMENT STANFORD UNIVERSITY STANFORD, CALIFORNIA Manuscript received Nvember 22, 1977