120' Mooney & Weaver, GSA Memoir 72 · 134 Mooney and Weaver A cross section from the northern...

134 Mooney and Weaver

A cross section from the northern Borderland to the western Transverse Range and southern Coast Ranges shows a complex crustal structure with strong lateral variations (Keller and Prothero, 1987; Fig. 5). The crust thickens from about 23 km beneath the central Borderland to about 31 km beneath the west-ern Transverse Range and southern Coast Ranges. The dual pro-cesses of convergence and translation are reflected in this cross section. The lower crust is interpreted to consist of tectonically

underplated Mesozoic oceanic crust, which is in turn underlain by Neogene oceanic crust. Numerous faults, basins, and uplifted blocks provide evidence of the translational (wrench) tectonic setting that has controlled the late Cenozoic evolution of the Borderland and adjacent continental area.

Mojave Desert. The Mojave Desert forms a vast area of nearly uniform crustal thickness, and is the most featureless re-gion on the Moho contour map of California (Fig. 3). Beneath

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Figure 3. Contour map of crustal thickness for California and adjacent regions derived from seismic refraction, seismic reflection, seismic network, and gravity data (Table 1). Estimated error is 10 percent, or one to one and one-half contour intervals. Solid triangles mark volcanoes as identified in Figure 1. Southern California (south of the Garlock fault) has a crustal thickness of 30 ± 2 km, whereas central and northern California show a pronounced west-to-east crustal thickening.

on November 13, 2013memoirs.gsapubs.orgDownloaded from

Mooney & Weaver, GSA Memoir 72, 1989





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Mooney & Weaver, GSA Memoir 72, 1989 Frasetto et al., Geosphere, 2011

Big incompatibility here. What is the deal?





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Mooney & Weaver, GSA Memoir 72, 1989 Frasetto et al., Geosphere, 2011

2026 DISCUSSIONS

A study of the Eureka (California) earthquake2 of June 6, 1932 first indicated that the Pn wave of the seismologist, when it originates in northern California, is delayed in its arrival at the Owens Valley seis-

mographic station of Tinemaha. The Pn wave is a first preliminary (compression-rarefaction) wave which travels in the ultra basic rock below the layered crust of the earth. The epicenter of this earthquake

2 Perry Byerly and Neil R . S parks: The first ‘preliminary waves o f the California earthquake o f June 6, 1982, Am. Geophys. Union, T r., 14th Ann. M eeting (1933) p. 254-256.

4.5s

~3 s

~2 s

6.6s

Byerly, BSSA, 1937

no lag

no lag

Times are delays from expected arrival time. No lag from EQs near Eureka to Fresno. Also no lag in Pg.

SAVAGE ET AL.: SIERRA NEVADA ROOT 813

Asymmetry of the Root Figure 8 shows evidence for a sharp boundary on the eastern

edge of the Sierra, but Plate 1 and Figure 9 show a strong con- trast in residuals between the western and central Sierra, sug- gestive of a sharp boundary to the west as well. Arrivals from earthquakes on the western edge of the Sierra exhibit stronger variation in arrival times across the Sierra (Figures 4 and 7) than those from the eastern edge of the Sierra (Figures 5 and 6). Other studies have seen similar asymmetry; Carder et al. [1970] observed little variation in arrivals across the Sierra from NTS explosions to the east, while Byerly's [1938] observations suggesting a root were from events to the west. Oppenheimer and Eaton [1984] noted that the Moho beneath the Central Valley dipped westward beneath the Sierra but that arrivals from the east showed little to no dip beneath the Sierra. A study by one of the authors [Jones et al., 1994] has presented Pn arrival times for two earthquakes located well out- side the Sierra, but recorded at stations within the Sierra, along paths that are ~ 180 ø apart. They found arrivals with apparent velocity of 7.5 krn/s from the north and 7.6-7.8 km/s from the south, but arrivals with apparent velocities closer to the expected Moho velocities of 7.9-8.2 km/s were not observed within the Sierra. Such asymmetry cannot be explained by 1-D structures but requires geometrical effects from 2-D or 3-D structures. Jones et al. [1994] interpreted the results in terms of two models, the simplest model consisting of a single refractor dipping 1.5 ø south under the Sierra at a depth of 33 +5 km beneath the High Sierra with velocity 7.6 + 1 km/s. We have shown above that this simple model cannot explain a number of the arrivals from earthquakes crossing nearby paths.

Figure 10 shows record sections from the 1988 portable network in the southern Sierra for an earthquake near Salinas, Cal- ifornia, that is WNW of the stations and for an NTS blast that is ENE of the stations (Figure 1 and 10). They contain some of the best evidence to date of the asymmetry in arrivals for earthquakes on different sides of the Sierra. The Pn arrivals from the earthquakes in the west show the classic pattern of early arrivals in the foothills, late arrivals in the central Sierra, and earlier arrivals again in the Basin and Range. The arrivals from the NTS blast, however, show very little variation in arrival time across the Sierra, similar to the results of Carder et al. [1970] farther to the north.

A second, more complicated model considered by Jones et al. [1994] may explain more fully the results herein (Figure 11). In this model there is a wedge of 7.x-km/s material above the Moho, with a higher-velocity (Moho) refractor below it. The Moho dips to the east beneath the Sierra and has a relatively sharp boundary to the east. Arrivals from the west are refracted along the dipping boundary and show progressive delays as arrivals reach into the Sierra. As the boundary returns to shallower levels in the east, the delays decrease. Arrivals from the east, however, travel along the 7.8- to 7.9- km/s Basin and Range Moho at a depth of ~ 30 km. They arrive at the Sierran boundary with velocity close to that of the 7.x-krn/s wedge, and instead of being bent around the sharp comer of the refractor in that region they continue along the 7.x-km/s refractor. Figure 11 shows a sample crustal model with the features of the 7.x-km/s wedge that can explain some of the features of the Salinas and NTS earthquakes. We used a two-dimensional (2-D) ray-tracing code of Luetgert [1992] to

ESE ' i

ß I .

4OO

Distance (km)

Basin & Range I Sierra NE SW 8 I I ' I

I

I I I II I II I I ! I I I I I I

b 160 200 240 280 320 Distance from Blast (km)

Figure 10. Record section reduced to 7.8 km/s for stations described in the study by Jones et al. [1994] and shown in Figure 1 (diamonds); (a) Salinas earthquake, (b) Nuclear test from the Nevada Test Site.

calculate the rays and arrival times shown for the model. This particular model contains an 8.0-krn/s Moho beneath the Cen- tral Valley and the Sierra and 7.9 krn/s beneath the Basin and Range and a simple crest with constant velocity of 6.25 km/s, while the velocity in the 7.x-km/s wedge varies from 7.2 km/s in the west to 7.7 km/s in the east. The basic features of the arrivals in Figure 10 are explained, in that for the Salinas earthquake the increasing reduced arrival times with distance in the Sierra and the decrease in the Basin and Range are seen, although the placement of maximum delay is somewhat east of the observations. Similarly, the relatively little change in reduced arrival time with distance for the NTS event is repro- duced. Using a slower velocity of 7.8 km/s in the Basin and Range would have made the arrivals somewhat closer to the observed ones. However, we do not attempt to produce an exact fit to the observations, because the point is to show the type of model that will be needed to fit the asymmetry and

EQs on west of Sierra: Arrival

late

Savage et al., Tectonics, 1994

Arrivals at station just east of the Sierra late for events from west--this section shows what that looks like.

Byerly 1938

EQs on west of Sierra: Arrival

late

Savage et al., Tectonics, 1994

SAVAGE ET AL.: SIERRA NEVADA ROOT 813

Asymmetry of the Root Figure 8 shows evidence for a sharp boundary on the eastern

edge of the Sierra, but Plate 1 and Figure 9 show a strong con- trast in residuals between the western and central Sierra, sug- gestive of a sharp boundary to the west as well. Arrivals from earthquakes on the western edge of the Sierra exhibit stronger variation in arrival times across the Sierra (Figures 4 and 7) than those from the eastern edge of the Sierra (Figures 5 and 6). Other studies have seen similar asymmetry; Carder et al. [1970] observed little variation in arrivals across the Sierra from NTS explosions to the east, while Byerly's [1938] observations suggesting a root were from events to the west. Oppenheimer and Eaton [1984] noted that the Moho beneath the Central Valley dipped westward beneath the Sierra but that arrivals from the east showed little to no dip beneath the Sierra. A study by one of the authors [Jones et al., 1994] has presented Pn arrival times for two earthquakes located well out- side the Sierra, but recorded at stations within the Sierra, along paths that are ~ 180 ø apart. They found arrivals with apparent velocity of 7.5 krn/s from the north and 7.6-7.8 km/s from the south, but arrivals with apparent velocities closer to the expected Moho velocities of 7.9-8.2 km/s were not observed within the Sierra. Such asymmetry cannot be explained by 1-D structures but requires geometrical effects from 2-D or 3-D structures. Jones et al. [1994] interpreted the results in terms of two models, the simplest model consisting of a single refractor dipping 1.5 ø south under the Sierra at a depth of 33 +5 km beneath the High Sierra with velocity 7.6 + 1 km/s. We have shown above that this simple model cannot explain a number of the arrivals from earthquakes crossing nearby paths.

Figure 10 shows record sections from the 1988 portable network in the southern Sierra for an earthquake near Salinas, Cal- ifornia, that is WNW of the stations and for an NTS blast that is ENE of the stations (Figure 1 and 10). They contain some of the best evidence to date of the asymmetry in arrivals for earthquakes on different sides of the Sierra. The Pn arrivals from the earthquakes in the west show the classic pattern of early arrivals in the foothills, late arrivals in the central Sierra, and earlier arrivals again in the Basin and Range. The arrivals from the NTS blast, however, show very little variation in arrival time across the Sierra, similar to the results of Carder et al. [1970] farther to the north.

A second, more complicated model considered by Jones et al. [1994] may explain more fully the results herein (Figure 11). In this model there is a wedge of 7.x-km/s material above the Moho, with a higher-velocity (Moho) refractor below it. The Moho dips to the east beneath the Sierra and has a relatively sharp boundary to the east. Arrivals from the west are refracted along the dipping boundary and show progressive delays as arrivals reach into the Sierra. As the boundary returns to shallower levels in the east, the delays decrease. Arrivals from the east, however, travel along the 7.8- to 7.9- km/s Basin and Range Moho at a depth of ~ 30 km. They arrive at the Sierran boundary with velocity close to that of the 7.x-krn/s wedge, and instead of being bent around the sharp comer of the refractor in that region they continue along the 7.x-km/s refractor. Figure 11 shows a sample crustal model with the features of the 7.x-km/s wedge that can explain some of the features of the Salinas and NTS earthquakes. We used a two-dimensional (2-D) ray-tracing code of Luetgert [1992] to

ESE ' i

ß I .

4OO

Distance (km)

Basin & Range I Sierra NE SW 8 I I ' I

I

I I I II I II I I ! I I I I I I

b 160 200 240 280 320 Distance from Blast (km)

Figure 10. Record section reduced to 7.8 km/s for stations described in the study by Jones et al. [1994] and shown in Figure 1 (diamonds); (a) Salinas earthquake, (b) Nuclear test from the Nevada Test Site.

calculate the rays and arrival times shown for the model. This particular model contains an 8.0-krn/s Moho beneath the Cen- tral Valley and the Sierra and 7.9 krn/s beneath the Basin and Range and a simple crest with constant velocity of 6.25 km/s, while the velocity in the 7.x-km/s wedge varies from 7.2 km/s in the west to 7.7 km/s in the east. The basic features of the arrivals in Figure 10 are explained, in that for the Salinas earthquake the increasing reduced arrival times with distance in the Sierra and the decrease in the Basin and Range are seen, although the placement of maximum delay is somewhat east of the observations. Similarly, the relatively little change in reduced arrival time with distance for the NTS event is repro- duced. Using a slower velocity of 7.8 km/s in the Basin and Range would have made the arrivals somewhat closer to the observed ones. However, we do not attempt to produce an exact fit to the observations, because the point is to show the type of model that will be needed to fit the asymmetry and

That was the seismic observations…was there support elsewhere?

38 Simpson and Jachens

P L A T E A U A R E A O C E A N A R E A

9 8 0 , 0 0 0 r

O o Q o S? ' 9 7 9 , 0 0 0

i uu i— ° r -100

_ ISOSTATIC RESIDUAL

Figure 1. Observed gravity and various gravity anomalies calculated for a simple two-dimensional model. Although geometries are greatly simplified, the model does serve to demonstrate the intended effects of the various adjustments to observed gravity. The profile is assumed to extend in an east-west direction so that there is no latitudinal variation. The 980,000-mGaI level for observed gravity values at sea level was chosen arbitrarily. The bottom of the model was constructed assuming local Airy compensation for the broader topographic features, and ocean-ic crust was assumed to be 0.2 g/cm3 more dense than continental crust. The isostatic residual anomaly shown in the figure was calculated assuming equal densities for both types of crust; this incorrect assumption leads to the small low-high isostatic residual anomaly pair over the continental-oceanic transition. The high-low free-air anomaly pair in the same location is largely the result of change in water depth—the sedi-mentary wedge and the change in crustal densities contribute to the free-air anomaly at this location in only a minor way. Note that the vertical scale for the anomalies is twice that for the observed gravity.

Geodesy, 1971). In closed form, where g represents the normal force of attraction in Gal on the surface of the reference ellipsoid:

1 + k sin2

g = ge Vi - e2 sin2 </>

where <f> is the latitude, and

ge = 978.031 845 58 Gal k = 0.001 931 663 383 21

e2 = 0.006 694 605 328 56.

The companion International Gravity Standardization Net 1971 (Morelli, 1974) consists of a set of worldwide gravity base-station values that should be used in conjunction with Gravity Formula 1967. Chovitz (1981) has given a lucid summary of the history and role of the various geodetic reference systems in gravity studies.

Although the reduction of gravity data seems like a straight-forward process, certain common misconceptions about the sig-nificance of the various reduction steps often lead to confusion at the interpretation stage. For example, the application of a free-air adjustment is often loosely referred to as a "reduction of the obser-vation to sea level.". This is not at all what has been accomplished, because the observation is necessarily fixed in space by its geo-metric relationship with neighboring massive bodies. A better way to view the free-air adjustment is as a refinement that allows the prediction of the attraction of gravity at the actual elevation above sea level at which the observation was made. The pre-dicted and observed effects are differenced to produce the free-air anomaly.

In general we have found the best conceptual framework for thinking about the gravity reduction process to be the "predic-tion" approach (Simpson and others, 1987). Each step in the reduction process is regarded as an adjustment to the predicted gravity (starting with the theoretical gravity predicted on the reference ellipsoid) and as a corresponding refinement to a con-ceptual density model that approximates the true Earth. Gravity anomalies are the differences between the actual gravity values and the values calculated from our Earth model. The goal of gravity reduction and interpretation is to make the model as close to the real Earth as possible. If the model ever finally became correct in every detail, no anomalies would remain.

The alternate conceptual approach to the reduction process, which in our experience has often led us astray, is to regard each reduction step as a correction to the observed gravity. The result is then compared to the theoretical gravity on the reference ellip-soid. The difference between the predictive and corrective ap-proaches seems to be nothing more than a rearrangement of terms to be added and subtracted. The real difference, however, lies in the utility of the predictive conceptual model in illuminating and motivating the reduction steps. Although a corresponding con-ceptual Earth model can be imagined for the corrective approach, it seems to us to be less in tune with the goals at hand—the explanation of gravity anomalies in terms of density contrasts within the Earth and the construction of an increasingly refined Earth model.

GRAVITY DATA BASES AND MAPS FOR THE UNITED STATES

Gravity data for the conterminous United States are availa-ble in several formats. The most up-to-date and comprehensive data set in map form is the Gravity Anomaly Map of the United States published by the Society of Exploration Geophysicists (1982). This colored contour map shows Bouguer gravity anom-aly values onland and free-air gravity anomaly values offshore.

Simpson & Jachens, GSA Memoir 72, 1989

First we need to have a basic idea of how gravity anomalies work.

38 Simpson and Jachens

P L A T E A U A R E A O C E A N A R E A

9 8 0 , 0 0 0 r

O o Q o S? ' 9 7 9 , 0 0 0

i uu i— ° r -100

_ ISOSTATIC RESIDUAL

Figure 1. Observed gravity and various gravity anomalies calculated for a simple two-dimensional model. Although geometries are greatly simplified, the model does serve to demonstrate the intended effects of the various adjustments to observed gravity. The profile is assumed to extend in an east-west direction so that there is no latitudinal variation. The 980,000-mGaI level for observed gravity values at sea level was chosen arbitrarily. The bottom of the model was constructed assuming local Airy compensation for the broader topographic features, and ocean-ic crust was assumed to be 0.2 g/cm3 more dense than continental crust. The isostatic residual anomaly shown in the figure was calculated assuming equal densities for both types of crust; this incorrect assumption leads to the small low-high isostatic residual anomaly pair over the continental-oceanic transition. The high-low free-air anomaly pair in the same location is largely the result of change in water depth—the sedi-mentary wedge and the change in crustal densities contribute to the free-air anomaly at this location in only a minor way. Note that the vertical scale for the anomalies is twice that for the observed gravity.

Geodesy, 1971). In closed form, where g represents the normal force of attraction in Gal on the surface of the reference ellipsoid:

1 + k sin2

g = ge Vi - e2 sin2 </>

where <f> is the latitude, and

ge = 978.031 845 58 Gal k = 0.001 931 663 383 21

e2 = 0.006 694 605 328 56.

The companion International Gravity Standardization Net 1971 (Morelli, 1974) consists of a set of worldwide gravity base-station values that should be used in conjunction with Gravity Formula 1967. Chovitz (1981) has given a lucid summary of the history and role of the various geodetic reference systems in gravity studies.

Although the reduction of gravity data seems like a straight-forward process, certain common misconceptions about the sig-nificance of the various reduction steps often lead to confusion at the interpretation stage. For example, the application of a free-air adjustment is often loosely referred to as a "reduction of the obser-vation to sea level.". This is not at all what has been accomplished, because the observation is necessarily fixed in space by its geo-metric relationship with neighboring massive bodies. A better way to view the free-air adjustment is as a refinement that allows the prediction of the attraction of gravity at the actual elevation above sea level at which the observation was made. The pre-dicted and observed effects are differenced to produce the free-air anomaly.

In general we have found the best conceptual framework for thinking about the gravity reduction process to be the "predic-tion" approach (Simpson and others, 1987). Each step in the reduction process is regarded as an adjustment to the predicted gravity (starting with the theoretical gravity predicted on the reference ellipsoid) and as a corresponding refinement to a con-ceptual density model that approximates the true Earth. Gravity anomalies are the differences between the actual gravity values and the values calculated from our Earth model. The goal of gravity reduction and interpretation is to make the model as close to the real Earth as possible. If the model ever finally became correct in every detail, no anomalies would remain.

The alternate conceptual approach to the reduction process, which in our experience has often led us astray, is to regard each reduction step as a correction to the observed gravity. The result is then compared to the theoretical gravity on the reference ellip-soid. The difference between the predictive and corrective ap-proaches seems to be nothing more than a rearrangement of terms to be added and subtracted. The real difference, however, lies in the utility of the predictive conceptual model in illuminating and motivating the reduction steps. Although a corresponding con-ceptual Earth model can be imagined for the corrective approach, it seems to us to be less in tune with the goals at hand—the explanation of gravity anomalies in terms of density contrasts within the Earth and the construction of an increasingly refined Earth model.

GRAVITY DATA BASES AND MAPS FOR THE UNITED STATES

Gravity data for the conterminous United States are availa-ble in several formats. The most up-to-date and comprehensive data set in map form is the Gravity Anomaly Map of the United States published by the Society of Exploration Geophysicists (1982). This colored contour map shows Bouguer gravity anom-aly values onland and free-air gravity anomaly values offshore.

…in essence, we find that gravity anomalies are smaller (more negative) the higher you go, indicating isostasy is active.

4268 OLIVER, PAKISER, AND KANE

-75 '•_....•_• A' -,oo r --.• ...

erveO • -125

_z Computed •' • •-•,,• .-• .... ---- ---' '"' '"' - • -175 •'•.•... Computed Effect of • . • .... Crustal Root• ...... o _ 200 •

o

- ;)50

- Z75

S,erro Gloss 5 )olton Boss Loke Crest29 Long Volley Mounto,n

LEVEL•'•;.;.I;.;.;.;.;,•. '.'.. i:;•i'i :'. i:i:i:i:i::::::::::::::::::•::::::::::•:::::•::•::•:::::•::i:i:i:i:i:i:i:i:i:i:i•i:i•i:i:!:i:i:i:i:!::::•:::::::::::::•:::::::::::::i:i:::::::::::::::::::::::i:i:•:i:!:::•::•:::;•:::::::::::::::::::::::::::::::::::::::::::::::::::::::

z 30

•o !"-'-• "' '-' •"'•'•• '••••-.j• •;J•il "'•' ' •'•'"' ' •' '•' ••' ' ••' '"•'•} :;.•ii:.'..• ii i•:.:':i•;i•'•-'-;•'"•,;";';;'i'•;;'.'•.a;¾ :•:i:i:!: :i:i:!:i:• ................ •:•:•:i:!:i:!:! ................................................................... . ........... ' ••;••• .•..•..'..'.....'••!•_'.;".•{i .... w'•'•'•" '•- • '••••'.;.-• .• .......... ß ................................................................................................................................................ : .............................. "-•'•-'-'. ••. ß .•,•, - '•••••.•.,_'• '.'-•,:..• """ •' '" '•'' ' '•"''' c•' '• ' '••.. '. •'.....'•._ ':•.<:. '• •!•::•:: •::•i•::•i•::•i•::•i•::•i•i•::•::•?:::i::::ii::!::i :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::il i::::::i:::::: :::::::: :::: •!:• •:. '.'.: ::::::: ::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::: ........ 5o ' '•'•' "'•'"'•- ß •v'•'..- • ••.••;t '•j'.•a '-•'• '"'• •'' ' ''' •' •,'••, ß. ß..•. ß ,. - ;•••lt _,•:•;'• '"'• •' "'• :'•':. ::.•:: ................ : ............................................................................ "••'-

Cenozo,c sed•mentory and Cretaceous and Upper Juross•c Pre-Cretoceous volcanic rocks gronlhc rocks of the S,erro metamorphic rocks

Nevodo bothohth

Unknown rocks m the lower IX•t Unknown rocks ,n the upper part of the earth's crust of the eorth's montle

0

0 S ltO 15 20 25 S?. 75 ß • i f f l

•ig. $. Tnterpret, ation of Bouguer grav•t,¾ proSle •]ong •-•'. •umber• wit, Mn geologic units •r• t,•e •v'erage den•if.,ie• of t,•e unit• in g/era • u•ed in. f.,• computation of th• gravity pro81e. •v'erage den•if.,ie• of f.,• granH, i½ ro½]• •re ba•ed on 20 t,o 7,5 mea•urem•nt• in ea½• block (t, aken int,•rva]• of 0.02 g/½mo), and t• •tandard d•viat, ion• range from 0.02 t,o 0.0• g/½mo.

-150

-Z75

5

SEA LEVEL

5

]0

15

zo

Z5

30 ,

35

4O

45

5O

the earth's interior below the central Sierra Nevada that yields a 'computed' gravity profile that is in close agreement with observed Bouguer gravity. The mass anomalies employed in the model are, in decreasing order of gravitational effect, (1) a thickening of the earth's crust from 35 to 52 km beneath the Sierra Nevada, (2) a horizontal variation in the density of batholithic rocks based on density measurements of surface samples collected in the vicinity of the profile, (3) low-density Cenozoic rocks in Long Valley, (4) high-density metamorphic rocks, and (5) a deep, faulted structure beneath Long Valley consistent with near-surface faults. The computed

effect of the crustal root (1) is shown separately in Figure 3; the difference between the 'Com- puted' and 'Computed Effect of Crustal Root' curves represents the sums of the effects (2) through (5). The value for the crustal thickness of 35 •5 km taken at the western edge of the central Sierra Nevada is based on an interpola- tion of available seismic data made by Frank Press (written communication, 1959). The results of geophysical studies of crustal thick- nesses in California have been summarized by Gutenberg [1959, Figure 3.11a].

Gravity interpretation in terms of these five mass anomalies is not unique. For example, if

Oliver et al., GSA Bull, 1961

Gravity Profile across Sierra

…but there are multiple ways of interpreting this.

Eaton, CDMG Bull 190, 1966


Time, t

up-dip down-dipslope = sin(α+ic)/v1 = 1/v2dslope = sin(ic-α)/v1 = 1/v2u

slope = 1/v1intercept= t2d= 2zd cos(ic)/v1

intercept= t2u= 2zu cos(ic)/v1

x

zd

hd zuhu

ic

ic

α

distance

ic = sin-1 (v1/v2)


If we approximate crust as 6.55 km/s, then:From intercepts:Moho at Mono Lake end 60 km bslMoho at China Lake end 50 km bslDip of 2.0°

From apparent velocities:Dip to north of 2.2°Velocity of 7.9 km/s

Note that Pn only a first arrival from north at very south end of profile





1 2 0 '



Mooney & Weaver, GSA Memoir 72, 1989

Refraction line across the southern Sierra, 1993, clearly eliminated a thick root.

37°

36°

35°119° 118°

-200

-150

-100

-50

BouguerGravity(mgal)

Figure 17 (color)

Jones et al., JGR, 1994

Airy Root Pratt “Crustal” Root

• Constant Pn velocity• Large dips on Moho

• Constant Pn velocity• No dips on Moho• Variations in surface

geology ?

• Varying Pn velocity• No dips on Moho• No variations in surface

geology

∆ ρ

∆ρCrust

Mantle

∆ρ =−ρc ∆h(hc + ∆h)

Pratt “Mantle” Root

d = ρc ∆h(ρm - ρc)

h

hc

d

ρc

ρmhm

ρc

∆ρ =−ρc ∆hhm

ρc

∆h ∆h





geology ?


geology

∆ ρ

∆ρCrust

Mantle

∆ρ =−ρc ∆h(hc + ∆h)



h

hc

d

ρc

ρmhm

ρc


ρc

∆h ∆h




geology ?


geology

∆ ρ

∆ρCrust

Mantle

∆ρ =−ρc ∆h(hc + ∆h)



h

hc

d

ρc

ρmhm

ρc


ρc

∆h ∆h

-250

-200

-150

-100

-50

0

50

-100 -50 0 50 100

Airy RootPratt Crustal RootPratt Mantle Root

mga

l

km

Bouguer anomaly

Isostatic anomaly

East-West Profilethrough the Sierra Nevada

Comparing Different Compensation Styles

Note the large isostatic anomaly (blue line) for an Airy root but much smaller anomaly if using lateral variations in the crust





geology ?


geology

∆ ρ

∆ρCrust

Mantle

∆ρ =−ρc ∆h(hc + ∆h)



h

hc

d

ρc

ρmhm

ρc


ρc

∆h ∆h

-250

-200

-150

-100

-50

0

50

-100 -50 0 50 100

Airy RootPratt Crustal RootPratt Mantle Root

mga

l

km

Bouguer anomaly

Isostatic anomaly

East-West Profilethrough the Sierra Nevada

Comparing Different Compensation Styles

-0.35

-0.25

-0.15

-0.05

0.05

0.15

0.25

0.35

2.5

2.6

2.7

2.8

2.9

3

3.1

3.2

-100 -60 -20 20 60 100

Comparison of density variations across the Sierra at 36.5°N

observed surface densities (Sikora et al., 1991)

density variation in top 30km to support Sierra

density

km E of 118°15'

Jones, unpublished

Red line is a smoothed fit to observed densities.

332 Saltus and Lachenbruch: Sierra Nevada Heat Flow

5O

-50 2.5

b

o -50

Gravity - density .

. . N Slope = 6.8 km ':'.1• '

2.6 2.7 2.8 2.9 Dmsity (g/cm 3)

3.0

120 ø 119 ø 118

•'•/ri• •.:.:.:...:.:.:,:.:.:•:.:.:...i.:.:...:.:•

Grid cell locations

o

37 ø

36 ø

, ß

•',' ' Elevation - gravity

ß ß

e" ß

3-

ß

ß ß ß

ß

ee e ß

ß .-

ß ß

ß e e ß

ß

Elevation- density

ß ß

ß ß ß e• ß ß ß

ß ß

ß

ß ß ß el e e ß ß lee ß ß ß ß

.... •) .... 50 2.5 i.6 1.7 •.8 •.9 Isostatic residual gravity (reGal) Dex•ity (g/cm 3)

Fig. 4. Gravity, density, and elevation relations in the central Sierra Nevada. Each data point is the average of values within one of the 20-km-square grid cells shown in the location map. (a) Isostatic residual gravity versus surface rock density. The linear correlation implies that the gravity anomalies can be explained by extending density contrasts measured at the surface to depths of about 6.8 km. (b) Elevation versus residual gravity. The scatter and mean of residual gravity decrease with increasing elevation, but there is not a good correlation. (c) Elevation versus surface rock density. For elevations lower than I km, density decreases somewhat with increasing elevation. For elevations greater than I km, there is no systematic trend.

3.0

scale D is a measure of the depth of the residual gravity anomaly sources and is not an artifact of the Bouguer reduction process.

Wollenberg and Smith [1987, p.298] concluded from a study of radiogenic heat production in crustal rocks that, "radiogenic heat production of igneous rocks generally varies with the silica content of the rocks". Comparison of density and bulk chemical composition for granitic rock samples in the central Sierra Nevada shows a systematic linear relationship between density and SiO9. content [Bateman et al., 1984; Moore and Sisson, 1987]. These correlations suggest the possibility of a general connection between the linear heat flow-heat production and the linear gravity-density relations. However, the difference in slope between the two linear correlations, 6.8 versus 10.1 km, and variations in heat production independent of whole rock silica content [Sawka and Chappell, 1988] point out the complexities in any possible connection. Characteristic depths from heat flow-heat production

relations have been associated with depths inferred from gravity models before [e.g., Birch et al., 1968; Roy et al., 1968; Decker et al., 1988].

The existence of a linear residual gravity-density relation for the Sierra Nevada implies that the isostatic residual gravity anomalies have upper crustal sources [Oliver et al., 1987]. This conclusion is important because it is an argument against flexural models for the Sierra Nevada [Chase and Wallace, 1986; 1988] that require isostatic gravity lows over the high Sierra Nevada to result from deep isostatic "overcompensation" of a buoyant crustal root [Kenelley and Chase, 1989].

Previous Heat Flow Studies and the Thermal Transition Between the Sierra Nevada and the Basin and Range Province

The eastern front of the Sierra Nevada bounds the Basin and Range Province (Figure 1), an area of Cenozoic

Saltus & Lachenbruch, Tectonics, 1991

Δg =2πGρtslope = 2πGt

rhoc is x, delta g is y, so slope is

-50

-30

-10

10

30

2.6 2.65 2.7 2.75 2.8 2.85 2.9

Isostatic anomaly vs. density, southern and central Sierra

35 - 36°; D = 2.0 ± 0.6 km36.3 - 38°; D = 6.9 ± 0.32 km

Isos

tatic

gra

vity

ano

mal

y (m

gal)

density (g/cc)

y = m1*m0 + m2 ErrorValue

23.768182.474385197m1 64.0589-235.01448482m2

NA5423.1952041ChisqNA0.36367013661R

y = m1*m0 + m2 ErrorValue

13.6698290.89264523m1 36.8526-800.5510207m2

NA14415.248315ChisqNA0.83800376583R

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-200

-150

-100

-50

0

2.6 2.65 2.7 2.75 2.8 2.85 2.9

35 - 36°; D = 8.0 ± 1.6 km

36.3 - 38°; D = 22.6 ± 1.2 km

Bouguer anomaly vs. density, central and southern Sierra

Bou

guer

gra

vity

ano

mal

y (m

gal)

density (g/cc)

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-150

-100

-50

00 1000 2000 3000 4000 5000

Southern Sierra: Individual Gravity MeasurementsBedrock Localities

com

plet

e Bo

ugue

r ano

mal

y (m

Gal

)

Station elevation (m)

Saltus and Lachenbruch' Sierra Nevada Heat Flow 331

150

125

100

75

50

25

Sierra Nevada heat flow vs. heat production

Explanation Distance from

Data Basin and Range source

>40 <40 km

usos A A other I l--l this study ß O

...... ............................. ................... ............................ .. at' ß ....................... Sierra Nevada linear relation'

........................ D = 10 km, qr - 17 mW/m 2

CNB .................... "•'HMN O ............. K_NDY ,-. TAHO .............. ........... RKY ................... JAC .-''''

O THW ................. H_C•' '

RVN .................. BL LK.. - ' ..... A ............ DOME_.'' ß .......... THE ........... 0. - • ......................

0 1 2 3 4 5

Heat production (•W/m 3) Fig. 3. Heat flow versus heat production for sites within the physiographic Sierra Nevada. See Figure 2 for site locations and Tables 1 and 3 for data values. The regression line defined by the early work of Roy et al. [1968] and Lachenbruch [1968; Lachenbruch et al., 1976] is dashed. Solid symbols denote sites more than 40 km from the eastern Sierra Nevada boundary. As in Figure 2, the different shapes represent different data sources: triangle = USGS, square = other labs, and circle = this study. A series of constant reduced heat flow lines (for D = 10 km) are plotted for reference.

of densities for elevations from sea level to about 1 km (Figure 4c), but there is no systematic relationship of density and elevation for the higher elevations. Most of the range in density that allows the definition of the slope

in Figure 4a comes from the wide range in Ap and g observed at low elevations in the Sierra Nevada, the regions least affected by the choice of Bouguer reduction density. This confirms the interpretation that the depth

Saltus & Lachenbruch, Tectonics, 1991

q0 = DA + qr

Saltus & Lachenbruch,

Tectonics, 1991

House et al., Nature, 1998

Jones et al., Geosphere, 2004

We know there is a big variation in heat production (left)--what is the impact on shallow geotherms

Jones et al., Geosphere, 2004

120' Mooney & Weaver, GSA Memoir 72 · 134 Mooney and Weaver A cross section from the northern...

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