Horts y Graben

8/9/2019 Horts y Graben

1/25

JOHN

H. STEWART

U.S. GeologicalSurvey

345Middlefield

Road Menlo

Park

California

94025

BasinandRangeStructure:ASystem ofHorsts

and

Grabens Produced

by

Deep-SeatedExtension

ABSTRACT

Basin and Range structure can be inter-

preted as a system of horsts and grabens pro-

duced by the fragmentation of a crustal slab

above a plastically extending substra tum . Ac-

cording to this view, the extension of the

substratum causes

the

basal part

of the

slab

to be pulled apart along narrow, systemati-

cally

spaced zones which

in

turn cause

the

downdropping of

complex horizontal prisms

(grabens) in the brittle upper crust. The

grabens form valleys at the surface; the inter-

vening areas are horsts, or tilted horsts.

Not all

geologists have agreed, however,

that Basin and Range structure consists of a

system of

horsts

and

grabens. Instead,

the

structure

is commonly considered to consist

of tilted blocks

in

which

the

upslope part

of

an individ ual block forms a mountain and

the downslope part avalley.Recent detailed

studies,

including geophysical work,suggest

that the horst and graben mo del may be more

generally applicable. Many of the valleys in

the Great Basin are bounded on both sides

by

faults that drop

the

valley block down;

thesefaults

are

exposed

at the

surface

or can

be

inferred

fromsteep

gravity gradients indic-

ative of

steep faulted subsurface bedrock

slopes. Some areas that were thought

to

represent

a typical series of tilted blocks may

be a series of highly asymmetrical grabens in

which

one

side

of a

valley

is

marked

by a

masterfault and the other sidebyvalleyward

tilt. With present knowledge,

most, or

per-

haps all, of the major valleys in the Great

Basin

can plausibly be considered to be

grabens,

and

most

or all of the

mountains

can be considered to be horsts or tilted horsts.

The

grabens,

and the

underlying inferred

deep zones of extension that cause them, are

systematically distributed in the Great Basin.

They

are

generally north-trending features

spaced

15 to 20 mi

apa rt. Locally,

the

pattern

is more

complex, and

individual

grabens

divide

and trend away from each other at

acute

or

high angles.

In a few

places,

the

pat-

tern may even be roughly polygonal. The

distribution pattern

of the

grabens

and the

related

deep zones of extension resemble

crack patterns in small-scale tensional sys-

tems, and

both

patterns m ay be mechanically

related. By analogy with the small-sca le sys-

tems,

the

areas

of

generally north-trending

and

parallel grabens require east-west exten-

sion, whereas the areas with a possible poly-

gonal pattern of grabens must extend radi-

ally.

The

geometry

of

block faulting related

to

Basin and Range structure requires sizable

east-west extension, estimated at about 1.5

mi on the

average

for

each major valley

and

at

about 30 to 60 miacross the entire Great

Basin. Most of this extension has taken place

in the last 17

m.y.,

or perhaps even in the

last 7 to 11

m.y.,

indicating a rate of exten-

sion in the range of 0.3 to 1.5 cm/yr.

INTRODUCTION

Many theories have been proposed to ex-

plain

Basin and Range structure; the histori-

cal

development of these ideas has been sum-

marized by

Nolan (1943,

p.

178-186)

and

more recently

by

Roberts (1968).

Most of

the

theories discussed

in the

last

15 yrs can

be grouped loosely into three main categor-

ies: (l )

Basin

an d

Range structure

is

similar

to that produced in landslides and related

either to

removal

of

lateral support

or to

slidingoff

of

regional highs (Mackin, 1960a,

1960b, 1969;

Moore,

I960);

(2)

Basin

and

Range structure

is

related

to

strike-slipdefor-

mation and, in part at least, to a conjugate

system

of

strike-slip

faults (Shawe,

1965;

Slemmons, 1957);

and (3)

Basin

and

Range

structure is

related

to

deep-seated extension

and

resulting fragmentationof theoverlying

crust

(Thompson, 1959, 1966; Hamilton

and

Myers, 1966; Cook, 1966; Roberts, 1968;

Hamilton,

1969). This paper considers the

last

theory. It relatesBasinand

Range

struc-

tureto the fragm entation of the brittle upper

Geological Society

of

America Bulletin,

v.

82,

p.

1019-1044,

13 figs.,

April 1971

1019


2/25

1020

J. STEWARTBASIN AND

RANGE STRUCTURE

crust

over a plastically extending substratum .

The upper crust can be considered to be a

slab

fragmenting along narrow

zones

at

its base. A structurally complex

horizontal

prism (graben) is downdropped over each of

these deep zones

of

extension, producing

valleysat the

surface.

The intervening moun-

tains are horsts.

Development

of

these concepts

is

depen-

dent on detailed knowledge of the surface

and subsurface structure of the basins and

ranges

of the

Great Ba sin. During

the

last 10

yrs, ne w geologic and geophysical data, in-

cludinggeologicmaps

a t a

scale

of

1:250,000

or larger, and detailed gravity and aeromag-

netic

surveys,

have been published

of

much

UT H

ARIZONA

Figure 1. Index map of Great Basin show-

ing mountains, major Basin an d Range faults,

and

localities

mentioned

in

text. Mountain areas

Generalized

and

slightly

modified

from Tectonic

Map of

United

States U S

Geol

Survey

and The American Association of

PetroleumGeologists 1961

are shaded. Hachures indicatedownthrownside

of

fault.

L ocalities: A) DixieValley, B) Sho-

shoneRange, and (C) Cortez Mountains.


3/25

BASIN

AND RANGE STRUCTURE

1021

of theGreat BasinofCalifornia,Nevada, and

Utah. These data provide the basis fo r most

of this article, which starts with adiscussion

of the geometry of Basin and Range struc-

tures

and

ends with more general interpre-

tations.

BASIN

AND RANGE

STRUCTURE

Gilbert (1874,

1875)

proposed that

the

ranges

of the

Great Basin (Fig.

l)

originated

by block fa ulting , a theory generally accepted

by

geologists

today.Thistheory relates ranges

to vertical movements along profou nd

faults

on one or

both sides

of the

mountain block

and has

been corroborated

by

detailed map-

ping

in

many parts

of the

Great Basin. Gil-

bert,

and

later geologists, recognized

tw o

distinct types of block faulting: (l) tilted

blocks in which the upslope part of an indi-

vidual block forms a mountain and the

downslope part avalley (Fig. 2A); (2) down-

dropped blocks (grabens), which form val-

leys,

an d

relatively upthrown blocks (horsts

or

tilted horsts), which form mountains (Fig.

2B). Most

geologists, although they have

recognized that valley blocks were in places

downdropped relative

to

mountain blocks,

have emphasized tilting as dominant in the

formation

of Basin and

Range

structure

(Gilbert, 1874, 1875, 1928;

I. C.

Russell,

1884; Louderback, 1904, 1923, 1924, 1926;

Da v i s ,

1903, 1905, 1925; Sharp, 1939;

Osmond,

I960).

A fewgeologists have im-

plied that

the

tilting

of

blocks

isvirtually the

only ma nner in which Basin and Range struc-

ture can be formed (Gilluly, 1928; Longwell,

1945;Eardley, 1951, Fig.

281;

Moore, I960;

Mackin, 1960b, 1969, Fig. 3; Gilluly and

Masursky,

1965;

Gilluly

and

Gates, 1965).

Other geologistshave stressed th e horst an d

graben concept

(R. J.

Russell, 1928; Fuller

and Waters, 1929; Cook and Berg, 1961;

Cook

and

others, 1964;Cook, 1966; Thomp-

son, 1959, 1966; Shawe, 1965,

p.

1362).

A

discussion

of the two

types

of

Basin

an d

Range stru ctu re is presented by describing

the geology of two areas. The first is Dixie

Valley,

wherethehorst an dgraben modelfits

well with

the

observed geology,

and the

other

is the Shoshone

Range

an d

Cortez

Mountains, where the tilted block model

agrees with the observed

surface

geology. A

modified horstandgrabenmodel also seems

to be

possible. Development

of the

idea that

Basin and Rangestructureis related to deep

zones

of

extension over

an

expanding sub-

stratum and collapse of the upper crust is

dependent

on

showing that

th e

horst

an d

graben type

o f

block faulting

is the

more

im -

portant

and

that

tiltingis

mostly

a

secondary

featurerelated

to the

formation

of the

horsts

and grabens.

If the

configuration

of

basins

and

ranges

is

primarily

due to

tilting

of

blocks

along downward-flattening

faults, and not to

the formation of

horsts

and

grabens, then

some other theory, or a considerable mod i-

fication of the

present theory, would

be

necessary

to

explain

th e

distribution

an d

origin

of the

basins

and

ranges.

Dixie Valley

Dixie Valley,

the

site

of

large earthqua kes

and surface faulting in 1903 and 1954 (Slem-

mons and others, 1959; Slemmons, 1957;

Romney, 1957;

Whitten,

1957;

Byerly

and

others, 1956; Shawe, 1965), is in western

Nevada, about 75 mi east of Reno (Fig. 3) .

It trend s north- northe ast, is about 30 mi long

and 10 mi

wide,

and is

bounded

by the Still-

Figure 2. Tilted blockandhorstan d graben

models

of

Basin

and Range

structure.

Upper

illustration A) is tilted block model from

Moore, I960,

Fig. 188.1).

Lower

illustration

B)

is horst and grabenmodel from Thomp-

son, 1966, Fig.

3). Inmodel B, the

underlying

dike is hypothetical and e is horizontal ex-

tension on one

fault.


4/25

22

J.

STEWART-BASIN

AND RANGE

STRUCTURE

waterRangeon thewestand theClan Alpine

Mountains

on the

east.

It is of

particular

interest no tonly becauseofhistoric faulting

butbecause detailed informationisavailable

on the

surface geology Page, 1965; Burke,

1967;

Willden

and Speed, 1968) and on the

subsurface structurefrom gravity, aeromag-

netic,and

seismicrefraction

surveys

Thomp-

son, 1959, 1967; Meister, 1967; Herring,

1967a, 1967b; Smith, 1967).

The StillwaterRange to thewestofDixie

Valley

and the

Clan Alpine Mountains

to the

east consistofcomplexlyfolded and faulted

Triassicand

Jurassic siltstone, limestone,

and

volcanicclastic sediments overlainbyTertiary

rhyolitic to dacitic tuffs, welded

tuffs,

an d

Geology

from Page

1965),

Webb andWilson

1962),

Willden and Speed

1968)

and Stewart and

McKee

1970).Contours on

magnetic

basement from

Smith 1967,

fie 4)

Pre-Tertiarysedimentary and volcanic rocks

High-angle fault

Dashed

where approximately located;

dotted

where

concealed.

Ballon

downthrown side

Contours

on top of

magnetic rocks

Dashed where approximately located;

Hachures indicate closed basins.

Datum

is 1100 meters Contour

interval

300

meters

Figure 3. Generalized

geologic

map of the magnetic basement rocks

Dixie

Valley area with

contours

on top of


5/25

BASIN AND RANGE

STRUCTURE 1023

flows,

andesite

and

basalt

flows, and tuffa-

ceous sediments. The Stillwater Range is

bounded

onboththe

east

and

west

by

high-

anglefaultswith valley side down; the range

is clearly a horst (Page, 1965), and at one

place where the range is only 5 mi wide, the

crest is over 3000 ft above flanking alluvial

fans. Minor

normal faulting has sliced the

range

into

many narrow north-south-trending

blocks, some

of

which have been tilted. Page

(1965) suggests that many

of

these minor

blocks were sloughed off by gravity sliding

during or after the upliftof the range. Minor

valleywardfaulting on either side of the Clan

Alpine Mountains suggests that it too is a

horst.

The

subsurface

structure of

Dixie Valley

has

been clearly

defined as a

complex asym-

metrical graben (Fig. 4) on the basis of

gravity,

seismic refraction, and aeromagnetic

studies (Thompson, 1959, 1967; Herring,

1967a, 1967b; Meister, 1967; Smith, 1967).

Steep faults on each side of the valley drop

Tertiary and pre-Tertiary rocks down toward

a narrow trough

( graben-in-graben )

cen-

tered

under the west side of the

valley.

At its

narrowest,

this inner trough is only 5 mi wide

and contains a maximum thickness of 10,500

ft of

Cenozoic volcanic

and

sedimentary

rocks,on the basis of seismic refraction data

(Meister, 1967). The average thickness of

Cenozoic rocks in this inner trough is about

6500 ft, on the basis of aeromagnetic data

Figure

4.

Generalized

blockdiagramof

bed-

rock surface of

central

and

northern Dixie

Valley redrawn rom Burke,

1967,

Fig. 6) .

Alluvium

is

removed

and

eroded

bedrock is

restored.

(Herring,

1967a, 1967b).

To the

north,

the

width of Dixie Valley and the thickness of

Cenozoic fill decrease because of progres-

sivelyless displacementalong

faults

(Fig. 4).

Steep faults on the west side of Dixie

Valley dip 55 to 70 E., as determined by

side refraction studies by Herring (I967a).

At a

different

locality, Meister (1967) mea-

sured

dips

of 35 to

45.

(He

assumed

that

only one

fault

occurs and noted that if the

fault zone

is

actually composed

of

several

steepfaults,

the dip of

individual

faults

would

be greater.) The faults do not flatten at

depths of about 3000 ft, the attainable limit

of the method. The surfacetrace of the fault

along

the

west side

of

Dixie Valley

is

irregu-

larandlocallycurvesasmuchas90.Meister

(1967,

p. 68) found

from

seismic reflection

studies that these

irregularities

on the

fault

surfacesextend to depths of at least 2500 ft;

thus, large strike-slip movement could not

have occurred on these faults.

In 1954, a series of earthquakes that caused

surface

breakage occurred in and near the

DixieValley area. The first two were at Rain-

bow Mountain,

directly

southwest of the

Stillwater

Range, and consisted of shocks

with magnitudes of 6.6 and 6.8. They pro-

duced

several northerly aligned

fault

scarps,

with amaximumof about 1.5 ft of dip-slip

displacement. On December 16, a third earth-

quake (magnitude 7.4) produced an impres-

sive

series

of

scarps near

Fairview

Peak;

4

min later a fourth earthquake (magnitude

7.1) produced scarps along the west side of

Dixie Valley. First-motion studies and retri-

angulation in the area both indicate a con-

siderable right-lateral strike-slip component

offault

movement amounting

to a

maximum

of

nearly

10 ft on faults

near

Fairview

Peak.

Such

movement

is

commonly cited

as

evi-

dence

of

strike-slip control

of Basin and

Range structure (Shawe, 1965), although the

work of Meister (1967, p. 68), indicating

major

irregularities

onfaultson thewest side

of Dixie Valley, seems

to

preclude

a

large

component

of

strike-slip movement

on at

least some

of the faults in the

Dixie Valley

area.

Dixie Valley is, therefore, a complex asym-

metricalgraben downdropped

on a

complex

series of high-angle faults on both sides of

the valley. The width and subsurface depth

of the valley decreases to the north because

there is progressively less displacement on

the

faults

in

that direction.


6/25

1024

J. STEWART-BASIN AND

RANGE STRUCTURE

Shoshone

Range

and

Cortez

Mountains

The

Shoshone Range

an d

Cortez Moun-

tains are both considered typical Basin and

Range tilted blocks (Gilluly

and

Gates,

1965,

p. 126-127; Gillulyand Masursky, 1965, p.

95-97;

Muffler,

1964,p.

71-77;

Wallace,

1964, p. 37; Moore> I960, Table 188.1).

They are composed of highly faulted and

folded

lower Paleozoic sedimentary and vol-

canic rocks, less deformed upper Paleozoic

and Triassic sedimentary and volcanic rocks,

Jurassic and Tertiary granitic rocks, and Ter-

tiary volcanicand sedimen tary rocks, mostly

basaltic andesite

flows

dipping

5 to 8 SE.

A

southeastern tilt of the ranges is suggested

by

the dip of the Tertiary volcanic rocks and

by

the shape of the ranges, which are dis-

tinctly asymm etrical with steep northwe st

flanks

2000 to 3000 ft high , with long, gentle

southeastern slopes. Important Basin and

Range high-angle

faults

bound

the

north-

western sidesof both ranges (Fig. 5), but no

such faulting is evident on the southeastern

Quaternary

alluvium

Tertiary

volcanic and sedimentary

rocks

Tertiary to Jurassic

granitic

rocks

Pre-Tertiary sedimentary and volcanic rocks

Geology from

Gilluly

and Masursky

1965);

Gilluly andGates

1965);

Muffler

1964);Roberts

and others

1967,

pi. 3); and Stewart and

McKee 1970).

ravity

contours

after Mabey 1964)

High-angle

fault

Dashed

where

approximately located;

dotted where concealed. Ball on

downthrown side

ravity

contours

Contour intervals 5 milligals

Hachuresindicateclosed basins

Figure 5. Generalized

geologic

andgravity area,

mapofShoshoneRange andCortez Mountains


7/25

BASIN

AN D

RANGE STRUCTURE

1025

sides, where

th e

mountains

are

embayed

by

long

tongues of alluvium. The surfaceex -

pressionof them ountain s clearlyfits amodel

of

tilted blocks like that illustrated

in

Figure

2A .

Analternate modelo f the Shoshone Range

and Cortez Mo untains structure seems equally

likely, however, and is more closely allied

with the

inferred

structure of Dixie Valley.

Cloos (1968, Figs. 16 and 18, reproduced

here as Figure 6) has produced highly asym-

metrical grabens in clay models in which one

side of the structure is bent downward with

synthetic and antithetic faults and the other

side is a master

fault

dipping toward the

graben.

The layers on the

downbent side

(left-hand

side of the models) have been

rotated ab out 20. This asym metrica l graben

produced in the clay model experiments has

many of the same structural features as the

Shoshone Range and Cortez Mountains, in-

cluding a master

fault

on one side of the

Figure 6.

Clay

modelso fhighly

asymmetri-

calgrabens.

Thicknessof

clayslababout

2 .5 to

3 in .Upper illustration A) is a

drawing

from

Coney (1969, Fig. 1) basedonmodelofCloos

(1968,Fig. 16, p. 428). Lowerillustration (B)

is a drawing

based

on model by

Cloos

(1968,

Fig. 18). Reprinted through the courtesy of

Ernst

Cloos (1968)

and the

American Associ-

ationof PetroleumGeologists Bulletin.

valley, and tilting, but no

major

faulting, on

th e other side. The graben structure in the

Shoshone Range

an d

Cortez Mountains area

may be an end member in a series of types

that range from symm etrical to highly asym-

metrical.

With available evidence, choosing which

model (tilted block or asym metrical graben)

is

th e best to apply to the Shoshone Range

andCortez Mountains area,isdifficult.Grav-

ityd ata is, however, perhaps more suggestive

of the graben model than th e tilted block.

The g ravity maps of both Crescent Valley and

Pine Valley show relatively steep gravity gra-

dients on each side of their respec tive valleys

(this relationship is more evident in Pine

Valley)and low gradients in the central part.

Th e

high gradients

may

represent steep slopes

on

pre-Tertiary rocks,

du e

either

to

down-

bending or dow nfaultin g of rocks. The grav-

ity data is thus suggestive of a graben struc-

ture with faulting or increased valleyward

slope

on e ither side of a relativ ely flat (bu t in

places na rrow ) central basin.

The

amount

of

Cenozoic

fill in

Crescent

Valley and Pine Valley is large and perhaps

more easily accounted for by agraben

struc-

ture than by simple tilting of range blocks.

Estimates

of the

amount

of

Cenozoic

fill in

Crescent Valley have ranged

from

7000 ft

(Mabey, 1964,

n

Gilluly and Masursky,

1965, p. 108) to 12,000 ft (Donald

Plouff, n

Gilluly

and

Gates,

1965, p. 129). A simple

tilted block model like that shown in Figure

would accountforonly abou t 4000to 5000

ft of fill,

assuming that

the

tilt

of the

range

is

5

(the slope

of a

large cuesta developed

on

Tertiary

lava flows in the Shoshone Range;

Gilluly

and

Gates, 1965,

p.

127).

In

Pine

Valley,

Cenozoic

fill may be

about

10,000 ft

(Mabey,

1964) and only

about

5000 ft of

this

can be accounted for in a simple tilted

block model, assuming, as indicated by Gil-

luly

and

Masursky (1965,

p. 97),

that

th e

tilt

of the

range

is 5 to 8.

Perhaps

the

tilt

of the

ranges

ha s

been underestimated;

a

higher

tilt

would account for a greater thick-

ness

of

Cenozoic

fill, but

simple tilt seems

inadequate

to

account

for all the

Cenozoic

fill indicated by the gravity data. A history

of

progressive tilting

in the

Cortez

an d

Sho-

shone Mountains, however, could account

for

the observed thicknesses of Cenozoic fill

in thevalleys. According to this idea,

tilting

adequate to account for the

deep

subsurface

trough could have occurred before the lava


8/25

1026

J. STEWART-BASIN AND RANGE STRUCTURE

flowscapping th e ranges were extruded. No

evidence, howe ver,

o f

progressive

tiltingha s

beennotedin the Shoshone Rangeor Cortez

Mountains.

Thus,

gravity dataissuggestive of agraben

structure below both Crescent Valley an d

Pine Valley, although other explanations of

th e

structure

are

possible

and

interpretation

of the gravity data itself is subject to con-

siderable uncertainty. The grabens may be

highly asymme trical and complex and the

observed

tilting

of the

ranges

m ay be

due,

in

part at least, to graben formation rather than

to

simpletilting

of an

entire range.

General Characteristics

of

Basin

an dRange

Structure

As

envisioned her e, Basin

and

R ange struc-

ture consists of mountain horsts and valley

grabens. Two examples of Basin andRange

structure have been described;

both

can ap-

parently be

explained

by the

theory.

The

problem remaining

is to see if the

horst

an d

graben interpretation can be applied more

generally.

A

B

2

Miles

Scale

Figure 7. Diagrammatic cross section com-

paring tilted block A) and asymmetrical gra-

ben

B)modelsofShoshoneRange an dCortez

Mountains area.

Stippledareas

indicate

Ceno-

zoic

valley fill. Small arrows indicate relative

movement

on

faults. Large opposed arrows

(modelB) indicate deep

zone

of extension.

The following discussion focuses mainly

on the

valley structure,

the

inferred graben.

If Basin and Range stru ctu re is related to

deep zones of extension, as proposed origi-

nally by Thompson (1959, 1966), then the

graben structures produced bythis extension

are

the active elemen ts in the system. Also, if

each

of the majorvalleys can be shown to be

a graben, the intervening mountains are ob-

viouslyhorsts.

Table 1lists areas w heregeologicand geo-

physicalevidence indicatesa

valley

underlain

by a graben. Evidenc e of the existence of the

graben consists mostly

of

maps which show

a valley bounded

by faults

that drop

the

valley ward block down, and gravity maps

that show relatively steep gradients and thus,

by inference, steep

subsurface

bedrock slopes,

on either side

of

a valley. In a few places,

other types of evidence also con tribu te to the

structural interpretation.The table illustrates

that m any of the valleys in the G reat Basin

can be considered grabens on the basis of

directgeologic and geophysical evidence.

The graben theor y of valley formation also

explains

some characteristics of Basin and

Range structure thataredifficult toexplainby

th e

tilted block theory.

The

mountains

on

either side

of a

valley,

for

example, com-

monly have "matched" shapes; an indenta-

tion in a mountain on one side of a

valley

corresponds

to a

promontory

on the

other

side.Them ountains appear to bepieces in a

giant jigsaw

puzzle

that has been pulled

slightly apart. Thus,

the

mountain

fronts on

either

side

of the

valley commonly have

a

similar curving

an d

irregular pattern,

an d

such a

pattern could

be

related

to

graben

formation

over similarly curving

an d

irregu-

lar zones of extension at depth. A further

characteristic more easily explained by the

graben theory is that the gravity trough of

some of the valleys is symme trical and the

low is at the mid line of the

valley;

the deepest

part

of the

bedrock

floor is

thus

probably

below the midline of the valley. Such a sym-

metrical bedrock trough

fits

better with

an

interpretation of a symm etrical graben than

with that

of

tilted blocks where

the

deepest

part

of the bedrock floor is depicted in most

illustrations

as distinc tly to o ne s ide of the

centerline.

In summary, many of the valleys in the

Great Basin appear to be underlain by a

graben, and such an origin appears to explain

some general chara cteristic s of Basin and


9/25

TILTING OF RANGES 1027

Range structure. With present knowledge, it

seems plausible that each of the majorvalleys

in

the Great Basin is agraben.

Thehorst and graben modelofBasinand

Range structure described here applies to the

gross structure of the major valleys and

mountain ranges, but is not intended as a

model of smaller scale block fault ing within

mountain masses. These smaller scale struc-

tures consist in places of a series of tilted

blocks bounded by high-angle faults,similar

to the model of Basin and Range structure

shown by Moore (i960, Fig. 188.1, shown

here asFig.2A).The tilting of these smaller

blocks as well as the tilting of the entire

mountain horst,

may be due to

rotational

gravitysliding related to the release of lateral

pressureduring the development of a graben.

TILTING

OF

RANGES

The ranges of the Great Basin classically

have

been consideredto betilted blocksand,

asMackin (I960b,p. 110)stated, any theory

of

Basin

and

Range structure must take tilt-

ing into account.

Does

thehorst andgraben

theory discussed here conflict with the ob-

served tilting?

The

clay model studies

of

Cloos (1968,

Figs.

16 and 18, reproduced

here

in Fig.6)

indicate that tilting goeshand

in

hand with

the formationofgrabens.The upper surface

of theclay modelin theupper partof Figure

6 hasrotated about20(Cloos,1968,p.424),

much

more than that required in such typical

tilted blocks as the Shoshone Range and

Cortez Mountains, where Tertiary volcanic

rocks

are tilted 5 to 8

(Gilluly

and

Gates,

1965, p.

127;

Gilluly and Masursky,

1965,

p.97).A seriesof tilted slices also occursin

thelower halfof the clay model in Figure6

(upper

illustration),

and theanalogous struc-

ture could beexposed in the Great Basin.

Some

of the

observed tilting

in the

Great

Basin, however, could

be due to

rotational

gravity sliding.

Page (1965),

for example,

suggested that large tilted blocks bounded

by

normal faultsslidof fthe Stillwater Range.

Mackin

(I960a,

1960b,

and

1969)

and

Moore

(I960) related tilting

to

rotation

of

entire

rangesalongdow nward-f la t tening

faults

and

suggested that this structureisanalogous to

that in rotat ional landslides.Moore inWal-

lace, 1964,

p. 37, and

1969, oral commun.)

suggested that manyof the blocks aretilted

toward regional topographic highs

and

that

they may have been tilted by sliding

of f

these

highs. Mackin (I960a; 1960b, p. 127-128)

suggestedthat this structure results from the

withdrawal

oflateral supportdue to eruption

of large volumes of volcanic material along

certain belts and slump-creep movement of

segments

of the

crust toward

the

free-side.

Rotational gravity sliding

of

large blocks

or entire rangesin themanner envisioned by

Mackin

and

Moore seems

to be a

likely

ex-

planation forsomeof the tiltingin the Great

Basin. The release of lateral pressure, as de-

scribed

by

Mackin (1960a, 196ob),could

be

relatedto the

development

of

grabens

above

deep zones of extension rather than to the

eruptive

process he suggests. Once lateral

pressure

hasbeen released, rotational gravity

sliding could develop off regional highs.

A model showing simple rotational tilting

of blocks, similar to that envisioned by

Mackin (I960b

and

1969)

and

Moore(I960),

is diagrammatically compared in Figure 8,

with amodel showing complex grabens and

rotational tilting.Inboth models, valleyb is

considered a graben. In model A, valleys a,

c,

and e are

considered

to be

simple tilted

blocks, whereas these valleys in model B are

complex asymmetrical grabens. In model A,

valleyd is considered a simple rotated block,

whereas

in

model

B it is a

complex

rotated

block considered to have originally formed

asanasymmetrical graben and laterto have

developed intoarotational block.Thestruc-

tures

in model A are related primarily to the

release of

lateral pressure

and are

similar

to

tilted blocks

in

landslides.

The

structures

in

model B are related to crustal fragmentation

alongnarrowdeep

zones

of

extension.

Model

Plastically extending substratum

Figure

8. Diagrammatic cross section com-

paring

tilted

block A) andhorstandgraben

B)modelsofBasinandRangestructure.Valley

d shows rotational tilting of the mountain block

in

both models. Stippled

areas

indicate

Ceno-

zoic valley fill. Small arrows indicaterelative

movement on

faults.

Large opposed arrows

model

B)

indicatedeep zones

of

extension.


10/25

TABLE

1.

S E L E C T E D G R A B E N S

IN THE

BASIN

AND

R A N G E P R O V I N C E

Area

Surprise

Valley,

California and

Nevada

Death

Valley,

California

Goose Lake,

Klamath Falls,

Oregon

Summer Lake,

Oregon

Warner Lake Valley,

Oregon

Guano

Valley,

Oregon

Alvord Lake Valley,

Oregon

Catlow Valley,

Oregon

McDermitt Valley,

Oregon

Long Valley,

Nevada

Northern part

Reese River Valley,

Nevada

Boulder Valley,

Nevada

Crescent Valley,

Nevada

Pine Valley,

Nevada

Diamond Valley,

Nevada

Ruby Valley,

Nevada

Humboldt Sink,

Nevada

Dixie Valley,

Nevada

Army Map

Service Sheet

Alturas and Vya

Death Valley

Klamath Falls

Klamath Falls

Adel

Adel

Adel

Adel

Jordan Valley

Vya

Winnemucca

Winnemucca

Winnemucca

Winnemucca

Winnemucca

Elko

Reno

an d

Lovelock

Reno

Evidence

of

graben

Faults along much of

both

sides of valley;

valley

block down

Faults along part

of

both

sides of valley;

valley block down

Faults

on both sides of

valley; valley block dow n

Faults

on both sides of

valley;

valley block down

Faults along much of

both sides

of

valley;

valley

block down

Faults

alongmuch

of

both sides

of

valley;

valley

block down

Faults along both sides of

valley;

valley block down

Faults along both sides of

southern part of valley;

valley

block down

Faults along part of both

sides

of

valley;

valley

block down


sides of valley;

valley

block down

Steep gravity gradients

on

both

sides of valley indicate

faults

dropping valley

block down

Faults

along both sides

of

valley;

valley block down


indicate

faults

on

both

sides of valley dropping

valley block down


indicate

faults

on both

sides

of

valley dropping

valley block down


indicate faults on both

sides

of

valley dropping

valley

block down

Steepgravity gradients



valley block down


indicate faults

on

both


valleyblock down

Faults

along both sides

of

valley; valley block down.

Seismic refraction, aero-

magnetic, gravity,

an d

surface

mapping indicate

"graben-in-graben"

structure

Source of information*

Gay

and Aune, 1958; H. F.

Bonham, 1968, written

commun.; Russell, 1928,

p.

486-487

Hunt and Mabey, 1966,

PI.1

Walker, 1963; Fuller and

Waters, 1929

Walker, 1963; Fuller and

Waters, 1929

Walker

an d

Repenning,

1965; Fuller and Waters,

1929; Russell, 1884

Walker and Repenning,

1965; Fuller

and

Waters,

1929; Russell, 1884


1965; Fuller

and

Waters,

1929; Russell, 1884


1965; Fuller

and

Waters,

1929


1966; Fuller

and

Waters,

1929

H. F. Bonham, 1968,

written commun.

Erwin, 1967

Stewart

an d McKee, 1970

Donald Plouff, Fig. 40,

in

Gilluly

and Gates, 1965

Mabey, 1964

Mabey, 1964

Gibbsand others, 1968

Wah , 1965

Thompson, 1967; Meister,

1967; Herring,

1967a;

Smith, 1967; Burke, 1967


11/25

TABLE 1. Continued)

Area

Smith Creek Valley,

Nevada

Big

Smoky Valley

(near Kingston

Canyon), Nevada

Steptoe Valley,

Nevada

lone Valley, Nevada

Big

Smoky Valley

(near Manhat tan),

Nevada

Little

Fish Lake

Valley, Nevada

Railroad Valley,

Nevada

Army M ap

Service S heet

Millet

Millet

Ely

Tonopah

Tonopah

Tonopah

Lund

Evidence of graben


sides of

valley;

valley

block down

Faults

on west side of

valley

have valley side

down. Steep gravity

gradient on east side

indicates

fault

with valley

side down



sides ofvalley dropping

valley block down

Quaternary faults on both

sides of valley;

valley block down

Faults on both sides of

valley; valley block down

Faults on both sides of

valley; valley block down.


indicate

faults on both


valley block down



Source

of Information*

Stewart and McKee, 1970;

Herring,

1967b, Fig.

1

Kleinhampl and Ziony,

1967; Stewart and McKee,

1970; D. L. Healey, 1967,

written

commun.

Carlson and Mabey, 1963

Kleinhampl

and

Ziony,

1967


1967


1967; D. L. Healey, 1967,

written commun.

D. L.Healey, 1967,

written commun.; Osmond,

Yucca

Flat, Nevada Goldfield andDeath Valley

Kawich

Valley,

Nevada

Junct ion

Valley,

Utah

Lucin

graben

Pilot

Valley,

Utah

West Newfo undland

graben, Utah

EastNewfoundland

graben, Utah

Wasatcht rench,

Utah

Wendover graben,

Utah

Goldfield

Brigham City

Brigham City

Brigham City and

Tooele

Brigham City

Brigham City and

Tooele

Ogden

Tooele


valley block down


indicate

faults on both


valley block down


indicate

faults

on

both


valley

block down




valleyblock down


indicate

faults on both

sides

of

valley dropping

valley

block down

Steep

gravity

gradients


sides of valley

dropping

valley block down

Steep

gravity gradients

indicate

faults on both

sides

of

valley dropping

valley block down

Steep grav ity gradients

indicate faults onboth

sides

of

valley dropping

valley block down

Seismic

refraction

profiles

an d

surface mapping

indicate

valley block down

Steep

gravity gradient

indicates faults on both


valley block down

I960, Fig. 2

Healey

and

Miller, 1962

Healey and Miller, 1962

Cook and others, 1964, PI.

1,and Fig. 2


1, and

Fig.

8


1,and Fig. 7

Cook and others, 1964, PI .

1,and

Fig.

5

Cook

an d

others, 1964,

PI.

1,

and Fig. 5

Cook, 1966, Fig. 9


1,

and

Fig.

6


12/25

1030

J. STEWARTBASIN AND

RAN GE STRUCTURE

T ABL E

1.

Continued)

Area

Army Map

Service Sheet

Evidence of graben Source of Information*

Jor dan Valley, Uta h Salt Lake City

Utah Valley, Uta h Salt Lake City

Tintic Valley, Utah Delta

JuabValley, Utah Price

Upper Ra ft River Pocatello

Valley, Idaho

Curlew Valley, Pocatello

Idaho

Steepgravity

gradient

indicates

faults

on both


valley block down

Steep gra vity gradients

indicate

faults

on both


valley

block down




valley

block down

Steep

gravity gradients

indicate

faults

on

both


valley block down


indicate faults

on both


valley block down


indicate

faults on

both

sides

of

valley dropping

valley block down

Cookand

Berg, 1961,

PI.13 ,

and p.79-80


13, and p. 81-82

Mabey

and

Morris, 1967;

Cook and Berg, 1961, PI.

13 and p. 85

Cookand Berg, 1961, PI.13,

and p. 82

Cook

and

others, 1964,

PI.

1,

and

Fig.

3

Cook

and

others, 1964, PI.

1, and Fig. 4

*Graben interpretation

no t

necessarily that

of

authors indicated.

B seemstobestfitmuch of the information

about the deep structurein the valleysof the

Great Basin and, as will be discussed later,

leads

to the

conclusion that fragmentation

took

place along rather uniformly spaced

deep zones

of

extension analogous

in

some

respects

to

tension cracks

in

small-scale ten-

sional systems.

DISTRIBUTION

OF

GRABENS

IN THE GREAT BASIN

Figures

9 and 10

show

the

distribution

of

known and inferred major grabens in the

Great Basin. Each line is the inf erred struc-

turally lowest part

of a

graben. Relatively

small-scale

grabens which have been recog-

nized within mountain masses in a few areas

are

no t shown on thisfigure.

Th e position of the

major

grabens was

determined

from

detailed gravity surveys

where

the

results

of

such surveys, which

cover about a third of the Great Basin, are

available. Large negative anomalies extend

along the

trend

of

most

of the

valleys

in the

Great Basin, and alinealong the axis of the

gravity trough,

as

illustrated

in

Figure

11,

should approximate

the

position

of the

struc-

turally lowest part

of the

inferred graben.

Such astructural interpretationiscorrectpro-

vided that most of the gravity low is pro-

duced

by

downdropped blocks

of

low-density

Tertiary rocks

and by

thick deposits

of

low-

density

alluvial

fill in

topographic

an d

struc-

tural

depressions above the grabens. The

gravity anomalies associated with someof the

valleys

consist of a series of aligned gravity

basins and intervening saddles, rather than a

well-defined

trough. Gravity values

in

both

the

basins

and

saddles, however,

are signifi-

cantly lower than that of a djacent mountains,

and such valleys can be considered as com-

plex grabens with local deep sags.

Outside

of the areas of detailed gravity

surveys, grabens can be

inferred

to underlie

major

valleys, and the midline of the valley

can

beinferredto benearthe position of the

structurally lowest part

of the

underlying

graben.

In

parts

of the

Great Basin, such

as

in

the

region near

Winnemucca (A

inFig.

9)

in northwestern Nevada and near Dugway

Valley (BinFig. 9) in west-central Utah, the

mountain ranges are isolated, irregular, circu-

lar

or

elliptical masses, surrounded

by

allu-

vium.The shapeandspacing of these moun-

tains

mightbe

partially

due to

erosion which

destroyed more typical

elongate

ranges,

bu t


13/25

DISTRIBUTION

OF

GRABENS

IN THE

GREAT BASIN

1031

Structuralty lowest part ot a graben or the

mldline

of

a

graben based

on gravity

surveys

Dotted

where

uncertain

Structurally lowest part

of a

graben

or the

midline

of a

graben

inferred

from

topography. Corresponds approximately

with fnidline

of a

valley

Asymmetrical

graben.

Arrows

point

away

from side with master fault

Figure 9.

Distribution

and

symmetry

of

gra- north-central

Nevada; B)

Dugway Valley

re-

bens in Great Basin. A)

Winnemucca

region,

gion,west-central Utah.

I

favor

the view that much of this pattern

results from a

complex s tructu re setting that

has

broken

the

crust into irregularly shaped

and, in part, equidimensional blocks sepa-

rated by

struc tural sags.

A

structu ral, rather

than an erosional, interpretation of the shape

and distribution of these isolated ranges is

favored

because faulting clearly seems to be

responsible for the distribution of moun tains

of similar

relief,

but of more typical Basin

and Range shape, elsewhere

in the

Great

Basin. If the

structural interpretation

is ac-


14/25

1032

J. STEWARTBASIN AND

RANGE STRUCTURE

Figure 10 .

Sources

of information

used

in

Figure

9. 1)

Chapman

andBishop,

1968;

2)

Cook

andothers, 1964; 3) Gimlett, 1967; 4)

Thompson an d Sandberg, 1958; 5)

Wahl,

1965; 6) Erwin, 1967; 7)

Mabey,

1964; 8)

Gibbsan dothers, 1968; 9) Mabeyand

Morris,

1967; 10) Cooka ndBerg, 1961; 11 )

Pakiser

andothers, I960; 1 2) D. L.Peterson, 1968,

written

commun.;

13) Erwin, 1968; 14) D.

L.

Healey, 1967, written commun.; 15 ) Carl-

son and

Mabey, 1963; 16) Petersen, 1966;

17)

Pakiser

an d

others,

1964; 18) Mabey,

1963; 19) Healey andMiller, 1962; an d 20 )

Kane and

Carlson, 1964.

cepted, the distribution of sags, or

grabens,

between the mountains may be roughly

polygonal.

More detailed grav ity surveys, particula rly

in areas of seemingly unusual Basin and

Range structure, are needed for the accurate

location of grabens; nonetheless, if most or

all

of the m ajo r valleys in the Great B asin are

grabens,

as I

proposed earlier,

then the

dis-

tribution shown onFigure9mustbeapprox-

imately correct. Less certainly known, how-

ever, is the structure where two grabens

converge an d join. On this figure, the lines

showing the inferred s tructu rally lowest part

of a

graben

are

generally shown

as

intersect-

ing at a high angle. As discussed later see

"Similarityof the Great Basin graben system

to small-scale tensional cracks"), high-angle

(orthogonal)

intersectionsmightb e

expected

in

Basin and Range structure, although in

most places the nature of the intersection

cannot be determined from present gravity

orgeologicinformation.

The

distribution

of

grabens

in the

central

part of the Great Basin is

fairly

systematic.

They are generally spaced 15 to 20 mi apart

and are aligned north-south. Locally, the

graben pattern is more complex, and indi-

vidual

grabens divide

an d

trend away from

each other at acute or high angles and, as

mentioned above, the pattern may even be

polygonal in places.

In a

belt about

50 to 100 mi

widealong

the

western border of the Great Basin in eastern

California

an d

western Nevada, major gra-

bensare

widely

spaceda ndm any trend north-

west, in contrast to the general north-south

trend elsewhere. The uniqueness of thisarea

was first

pointed

out by

Gianella

an d

Cal-

laghan (1934, p. 21), who noted that the

ranges in this western border area of the

Great Ba sin have a general northwest trend,

in contrast to the general north or north-

northeast trend of the ranges in its central

part.

The topographic lineament between the

two areas was called "Walker Lane" by Locke

and others (1940) who, along with Gianella

an d

Callaghan (1934), suggested that the

lineament might be the physiographic ex-

pression of a structural line characterizedby

r igh t- la te ra l d i splacement . Recent work

(Longwell,I960;

Nielsen,

1965;Albe rs, 1967;

Stewart, 1967; Stewart and others, 1968) has

outlined evidence of right-lateral displace-

ment along severalfault zones

in the

western

part

of the Great Basin, and Albers (1967)

has outlined evidence that a sizable amount

of right-lateral offset ha s occurred in this

region by apervasive right-lateral drag (oro-

flexuralbending),in addition to offset along

the faults themselves. The different graben

pattern

in the western part of the Great B asin

seems to be due to the interaction of the

right-lateral

strain and the more general east-

west extension

that

has led to the develop-

mento fBasinandRange structu re elsewhere.

The

spacing

an d

distribution

of

grabens

in

the G reat Basin are not entirelyuniform,even

in

areas outside of the Walker Lane,

These

local irregularitiesmay result from slightly

different stresses, or the same stresses pro-

ducingdifferent results because of buttressing

effects, volcanism, different time-sequences

of stress application, or other factors.These

less typical or mo difie d stress fields proba bly

are most important along themargins of the


15/25

116-00'

40 30

39-30

EXPL N TION

2

Gravity contour

Contour interval 5 milligals

Figure

11.

Gravi ty data , d is t r ibut ion

of

lowest part

of

grabens

in

Eureka

County,

north-

mountains,

an d

inferred

positionof

structurally central Nevada (gravitydata/row/Mabey,1964).


16/25

1034

J. STEWARTBASIN AND RAN GE STRUCTURE

province, where

the

effects

of

tectonism

in

adjoining areasare felt.

SYMMET RY OF GR BENS

The symmetryof thegrabens in the Great

Basin is shown on Figure 9 by arrows that

point away from the steep, highly faulted

side

of a graben,

toward

the

gentlysloping,

lessfaulted side. Such

a

symbol

w as

adopted

because

it

suggests

a

slope; perhaps

the

asymmetryof the grabens is related to a slope

in

thecrustal slab.Th e partof the graben on

th e

upslope side

of the

crustal slab might

be

expected

to be defined by a

more conspicu-

ous

fault than that part

on the

downslope

side. Thus, the arrow points in the inferred

slope direction

of the

crustal slab.

The symmetry of grabens was determined

from

both gravity

and

geologic

data. Asym-

metricalgrabens

are

shown where

(l)

a

grav-

ity trough

is

distinctly

on one

side

of a

valley

(as

in

Crescent Valley

and

Pine Valley, Fig.

5), and (2) the

mountains

on one

side

of a

valleyhave

a

steep valleyward front a nd/or

a

masterrange-frontfault that drops th evalley

block down,and themountainson theother

sideof the valleyaretiltedandslopegently

toward the

valley.

In a few

areas,

an

asym-

metricalgraben is shown where the character

of faulting and tilting is consistent with such

an interpretation, even though gravity data

suggest

a

symm etrical

or

nearly symmetrical

graben.

Over half

the

grabens

in the

Great Basin

show no conspicuous asymmetry. Of those

that areasymmetrical,

2.5

times asmanyare

asymmetrical toward

the

east (the trough

is

on the east side of the valley,and the arrows

on

Figure

9

point west)

as

toward

the

west.

In Utah, all but two of the grabens shown as

asymmetrical are

asymmetrical toward

th e

east.

In

Nevada, however,

the

direction

of

asymmetryis

less consis tent, although groups

of grabens within certain parts of Nevada

commonly have the same direction of asym-

metry. For example,in the southern part of

central Nevada,four side-by-side grabensare

allasym metrical toward

the

west, whereas

in

the northern part of central Nevada a series

of

grabens

are all

asymmetrical toward

the

east. Where the steep sides of two adjacent

grabens are toward each other,

both

flanks

of the

intervening mountain should

be

steep

an d

characterized

by

conspicuous master

faults. The

Stillwater Range,

the

Toiyabe

Range

south ofAustin,and the Ruby Range

are

such ridge-like mountains.

DEEP ZON S

OF

XT NSION

Grabens in clay model studiescan be pro-

duced

by a

pulling apart

of

material

at

depth

and downdropping

of the

overlying blocks

and

slices

along steeply

dipping

and

con-

verging fau lts. Several slightly different meth-

ods were used by Hans

Cloos

(1936) and

Ernst

Cloos

(1968, Figs. 12-18; also, n

Badgley, 1965, Figs. 4-17

and

5-17)

to

pro-

duce grabens in clay, but the simplest is to

place one side of a clay slab directly on a

table and the other sideon asheet of metal.

Whenthesheetofmetalispulledto oneside,

the part of the clay slab resting on it is also

pulled aside and a wedge-shaped graben

forms.

The

area

of

extension

at the

bottom

of

the

slab

is

small

in

relation

to the

surface

width of the graben. A similar narrow basal

zone

o f

extension also

can be

seen

in the

clay

mode ls of highly asym metric al grabens (Fig.

6

)-

The relationship ofgrabens to deep zones

of extension can beshowninanother typeof

model (Fig. 12). In this model, a pieceof

tissue paper was cut into segments along

lines that had a pattern similar to the axial

traces of some of the grabens shown on

Figure 9. The segmented pieces of tissue

paper

were then laidon aone-eighth-in. sheet

of rubber, and a

2-inch.-thick,

13.5 by 16-in.

rectangleof drymortarwasshaped on top of

it. Therubberwasassembled sothat it could

be stretched in one direction using hydraulic

jacks. After stretching,

the dry

mortar slab

had

dimensions

of

13.5

by

17.5

in. (a

10.7

percent extension),

and a

series

of

prism-

shaped grabens had developed directly over

th e

cuts

in the

tissue paper.

In

this model,

th e cutsin thetissue paper correspond to the

deep zones of extension.

Grabens in model studies can thus be re-

lated

to a

pulling apart

of

material

along

narrowzones ofextensionatdepthanddown-

dropping of an overlying wedge of material.

Th e grabens of the Great Basin probably

formed in a similar way, as has already been

suggested by Thompson (1959,

1966).

The

deep zones of extension sho uld, if the analogy

to small-scale models is correct, be located

approximately below

the

structurally lowest

part of the grabens. Thus, Figure 9, which

shows the positionof the structurally lowest


17/25

DEEP ZONES

OF

EXTENSION

1035

igure 12 Dry

mortar

models

of

systems

of

horsts and grabens See text for explanation of

methods used in

making

models Scale is in

inches

part of the grabens, is also a distribution map

ofthe deep zonesof extension.

Th e depth at which the converging faults

on thesidesof agraben intersectis the depth

of

the

deep zones

of

extension,

if the

analogy

to

small-scale models

is

correct.

The

bound-

ing

faults

of Basin and Range structure dip

40 to 80 basin-ward, according to Gilluly

(1928). Thompson (1967,p. 9) and Hamilton

andMyers (1966, p. 527) have used 60 as an

average

figure of the dip of

bounding faults.

Th eaverage widthof thevalleysin the Great

Basin

may be

about

10 mi, in

which case

the

bounding faults would intersect at a depth of

about

9 mi (14

km). Hamilton

an d

Myers

(1966,

p.

527) came

to

about

the

same con-

clusion,noting that the

faults

should inter-

sect at a depth of less than 9.5 mi (15

km);

Thompson (1967,

p. 9)

suggested that

the

outer bounding

faults

of Dixie Valley should

intersect

at

about

mi (17 km ).

Thus,

if the

analogy to

clay

models is correct, a slab

about 9 to 11 mithick isbeing pulled apart,

forming grabens.

AMOUNT OF

EXTENSION

The importance of regional extension in

the formation of Basin and Range structure

hasbeen em phasize d by Carey (1958), Tho mp-

son

(1959,1966),

Ham ilton

and

Myers (1966,

p. 527-528), and Wright and Troxel (1968).

A

fault

that dips 60, which is perhaps an

average

figure for the

faults

bounding many

of the ranges in the Great Basin, requires1

mi of lateral extension for each 2 mi of dip

slip.

From

the

number

of

major faults

along

the 40th Parallel across the Great Basin, and

an

estimate of the average displacement on

these faults, Hamilton and Myers estimated

that the total extension amounted to 30 to

60 mi (50 to 100 km) in the late Tertiary.

Thompson (1959) estimated 1.5 mi of ex-

tension across Dixie Valley to account for

theobserved structure and, using that areaas

a

sa mple of the Great B asin, suggested a total

extension of about 30 mi (48 km).

A similar figure can be obtained for the

total extension across the Great Basin by

using

the

graben

rule devised

by

Hansen

(1965, p. A4l) for grabens developed by

translatory slides during the Alaskan earth-

quake of 1964. This rule relates the lateral

displacement prod ucing the graben, 1, to the

cross sectional area of the

surface

trough of

the graben, A, and the depth of

failure,

D, by

th e following

formula:

1 = _

D

This relationship follows because the cross

sectional area

of the

surface trough

of the

graben

approximates the cross sectional area

voided behind

the

block

as the

block moves

outward.An average area of a graben trough

in the Great Basin, including that buried

under

alluvium, may be about 15 sq mi (a

trapezohedron averaging 10 mi across and

1.5 mi high), and the depth of

failure

(the

depth of the deep zones of extension), as

described

above , may be abou t 10 mi. If these

figures are correct, the graben rule indi-

cates that an average Great Basin graben re-


18/25

1036

J.

STEWART-BASIN

AND RANGE STRUCTURE

quires 1.5 mi of extension. About 30 such

grabens occur across the width of the Great

Basin

at the

40th Parallel, thus indicating

about

45 mi of

total extension.

TIME

ND

RATE

OF

EXTENSION

Most of the extension related to forming

Basin and Range struc ture has occurred in

the late Cenozoic, starting no more than 17

m.y. ago and culminating in the last7 to 11

m.y. Dating is based primarily on the rela-

tionship of Basin and Range faulting to

radiometrically dated silicic

ash-flow

sheets

that cover large parts

o f

central

and

southern

Nevada and adjacent parts of Utah.

Ekren

and

others

(1968)

have concluded

thatnorth-trending

faults

related

to the

pres-

en t

north-trending basins

an d

ranges began

to

form between

14 and 17

m.y.

ago in

southern Nevada. They noted tw o systems

of

faults

in the area: an older one of both

northeast- and northwest-striking faults, an d

a younger system

o f

north-striking

faults, the

latter being relatedtoBasina ndRange struc-

ture.Th e older setoccurs in rocks as young

as 17 m.y., but not in 14 m.y. old rocks,

whichare cutonlyby the younger set. Rhyo-

lite,intruded intoth eyounger north-trending

faults

and

truncating

the

older set,

can

also

be

dated

as 14 to 17

m.y. old. Ekren

and

others

(1968)

also noted that

an 11

m.y.

old

tuff,

w hich must hav e been deposited on a

fairly

flat

surface,

occurs

high

on mountains,

in

places over 4000

ft

above valleys.

A 7

m.y.

old tuff, on the other hand, seems to have

been extrude d into

an

area with

a

topographic

grain similar to that of today. They con-

cluded, therefore, that although Basin and

Range structure started to form from 14 to

17 m.y. ago in the southern Great Basin,

most of the structural movement ha soccur-

red in the last 11m.y.

Volcanic rocks

17 to 34

m.y.

old are ex-

tensively faulted

in

much

of

central Nevada

(Kleinhampland

Ziony, 1967; Anderson

and

Ekren, 1968; Stewart

and McKee,

1970).

Most

of

these volcanic units

are

sheet-like

ash-flow

tuffs,an dindividu al units commonly

occur in several ranges and at many

different

elevations along

the flanks and

tops

of

indi-

vidual ranges. As these units formed as

highly mobile ash flows which tend to fill

troughs much like water, theirposition high

on moun tains and at diverse structura l levels

can

only

be

explained

by

faulting. Thus,

most Basin

and

Range structure

in

central

Nevada

is

also late Cenozoic

in age and

probably younger than

17

m.y.

Much of the Basinand Range structurein

the Great Basin may therefore have formed

in 17

m.y.

or

less. This date

and a

total

ex-

tension

of 50 to 100 km

across

the

entire

Great Basin see section on amount of exten-

sion), give a rate of extension of about 0.3

to 0.6 cm/y r

across

the

region.

If

most

of the

movement has occurred in the last 7 to 11

m.y., as suggested by Ekren and others

(1968), the rate of extension would be on

the order of 0.5 to 1.5 cm/yr.

SIMILARITY

OF THE

GREAT BASIN

GRABEN SYSTEM

TO

SMALL-SCALE

TENSION L CR CKS

As

envisioned here, Basin and Range struc-

ture is produc ed by frag men tation of a crustal

slababoveaplastically extending subs tratu m.

The pattern and spacing of the zones of ex-

tension may be related in some respects to

th e mechanisms that control th e patternand

spacing of cracks in small-scale tensional

systems.In both systems, widespread tensile

stress is relieved by

failure

and pulling apart

ofmaterial along narrow zones. In the Basin

an dRange province, this pulling apart occurs

along deep zones

of

extension

and

results

in

graben formation;

in

small-scale tensional

features

it resultsinvertical cracks.

Although

th e

mechanisms that control

th e

pattern

and

spacing

of the

zones

offailurein

the

crustal slab

and in the

small-scale ten-

sional systems

may be

similar,

th e

manner

of

failure

along these zones is different in the

tw o cases. In the crustal slab, extension oc-

curs along narrow zones

at the

base

of the

slab, and

failure

of the overlying material

occurs along normal faults. These normal

faults

are shears with a vertical axis of maxi-

mum principal stress (maintained by gravity)

and a horizontal axis of least principal stress

perpendicular

to the

strike

of the

normal

fault.

In

s mall-sca le tensional s ystems,failure

is along vertical cracks. In spite of these

different

details

of failure,

both systems fail

along narrow zones and the

failure

is the

result of widespread tensile stress. Study of

the

characteristics

of

small-scale tensional

cracksystems, therefore, may provide insight

into what controls thepatternandspacingof

the zonesofextension in thecrustal slab.

The pattern of failure in small-scale ten-

sional systems depends

on the

stress

distribu-


19/25

GREAT

BASIN

SYSTEM AN D TENSIONAL

CRACKS

1037

tion. In a system in wh ich the stress is vir tu-

ally

radial,

a

roughly polygonal pattern form s

(Fig. 13), such as in mud cracks and in con-

traction cracks in permafrost (Lachenbruch,

1961, 1962,

1966).

Polygonal patterns also

were

seen in ground cracks related to the

Alaskan earthquake of 1964 where a

surface

layerwasunder stressdue to dilationof the

underlyingmaterial (McCullochand Bonilla,

1967, p.98-99,and Fig. 96). The sizeof the

polygons within a particular stress field tends,

to be similar, and the cracks join at right

angles (orthogonal intersections o

Lachen-

bruch,

1962).

In a system in which the stress is

virtually

unidirectional, the cracks formed ar eevenly

spaced, generally parallel,

an d

straight

an d

gently curved (Fig. 13). Crack intersections,

although sparse in this

sytem,

are also orthog -

onal. This system of generally parallel cracks

was

seen

in

ground cracks produced

by the

Alaskan earthquake where brittle

failure of a

surface

layer occurred in response to stress

created by the downslope displacement of

more plastic underlying sediment (McCul-

lochandBonilla, 1967,p.98-99andFig.96).

The

straight

and

slightly

curved inferred

deep zones of extension (similar to the pat-

tern of graben axes shown in Figure 9),

typical of Basin and Range struc ture in the

Great Basin, correspond

to

small-scale ten-

sion cracksproducedby unidirectional mov e-

ment. L ocally, in the G reat Basin, roughly

polygonal patterns appear to occur, perhaps

as

a

response

to

radial movements.

In

these

Figure 13. Crack

patterns

o f

t yp i c a l

small-scaletensional

systems.

Upper

illus-

t r a t i o n A ) sh ow s

cracks developedby

radialdilation.

Lower

illustration B)shows

cracks

developed by

east-westextension.

areas,the mountains are irregular, circular, or

elliptical masses surrounded

by

alluvium.

Alternately,

these

irregular, circular, or ellip-

tical mountains could berelated to a change

in

the

direction

of

extension with time

or to

th e

buttressing effect

of

plutons

or

rigid

blocks, factors that w ould com plicate a simple

pattern related

to

east-west

extension.

A

characteristic of both Basin and Range

structure

and of sm all-scale tensional featur es

is an even spacing of zones of failure. In the

Great Basin, the deep zones of extension are

spaced

generally

15 to 20 mi

apart.

In

small-

scaletensional features, the spacing ofcrac ks

is similar throughout a particular system

(Fig. 13). As

discussed

by

Lachenbruch

(1961,

1962, 1966,

p. 67-68),

this even

spacing is

related

to a zone of stress relief near a crack

that inhibits the formation of another crack

close by. Outward

from

the crack the stress

increases, and at some distance away from it

the stressis large enough to exceedacertain

threshold value of

failure

and a new crack

forms.

Thus,

a

crack tends

to

occur

at a

specific

distance from another,

and

have

a

uniform spacing.

The uniform spacing of the zones of ex-

tension

in the

Great Basin

may be

related

to

stress

relief associated with each zone, in a

manner similar to that described fo r small-

scale tensional features.

T he

nature

of

stress

relief,

however, may be

different

in the two

systems because, as Hubbert (1951) and

Lachenbruch

(1961,

p. 4286) h ave discussed,

tensional failure similar

to

that

in the

small-

scale systems

can

occur only within

about

1000 ft of the surface, unless the rock issub-

jected

to

high

fluid

pressures.

If the concept of stress relief adjacent to

each of the zones of extension isvalid in the

Great Basin,thenintersections of these zones

should

be orthogonal.

Lachenbruch

(1961,

1962, 1966,

p. 68) has

described

the

mecha-

nism that produces orthogonal intersections

of

sm all-scale tensiona l cracks

an d

relates

it

to an anisotropy of tensional stress in the

zone of stress relief. Tensional stress is least

in the direction perpendicular to the crack,

greatest in the direction parallel to the crack.

A

second crack entering

the field of

stress

release of the first wou ld, as stated by Lach -

enbruch (1966,p. 68) . . . tend to alterits

path in such a way that it trended perpen-

dicular

to the greatest tension, hence, would

tend

to

intersect

the first

crack

at

right angles.''

Field evidence that intersections

of the

zones


20/25

1038

J. STEWART-BASIN AND

RANGE STRUCTURE

of

extension

are orthogonal is

equivocal;

locally, this type of

intersection

is

suggested

by a

high-angle intersection

of

converging

gravity anomalies under valleys,

but in

most

places

the

nature

of the

intersection cannot

be

determined exactly

from

present

informa-

tion.

RELATIONSHIP

OF BASIN AND

RANGE STRUCTURE TO

PLATE

TECTONICS

Oneof the firstattempts torelate Basinand

Range structure

to

large oceanic

an d

con-

tinental c rustal features

was by

Menard(1964).

He

suggested that Basin

and

Range structure

was

related to convection currents and lateral

spreading

on the flanks of the

East Pacific

Rise which

he ,

among others, suggested

ex-

tended in to the Basin an d Range province.

In

support

of

this view,

the

crustal structure

of the

Basin

and

Range province

is different

from

that of other parts of the conterminous

United States

and

similar

to

that

of the

East

PacificRise. Bothhave

thin

crust,lowupper-

mantle ve loci t ies , a nd high heat flow

(Menard, 1964; Pakiser and Zietz, 1965;

James and Steinhart, 1966; Hill and Pakiser,

1966; Pakiser and Robinson, 1966; Wool-

lard, 1966;

Lee and

Uyeda, 1965; Blackwell,

1967).

In

additi on, Menard (1964)

has

pointed

out thatridgesand troughs analogous to the

basins

and ranges also occur on the ocean

bottom

on th e flanks of the EastPacificRise.

More recent interpretations, however, sug-

gest

that the East Pacific Rise extends into

the

Gulf

ofCalifornia,

where

it is

offset along

many transform faults,and finally

along

the

San

Andreas fault, and does not reappear

again until

off the

northern coast

of

Cali-

fornia (Morgan, 1968;Menard , 1969, p. 134).

According

to

these interpretations,

the

Basin

and Range province lies entirely within the

North American plate and not along the ex-

tension of the EastPacific Rise.

More

recent ideas relate

Basin

and Range

structure

to oblique tensional fragmentation

within a

broad belt

of

right-lateral movemen t

along

the west side of the North American

crustal plate. This theory is based on con-

cepts

developed

byCarey

(1958), Wise (1963),

and Hamilton and Myers (1966), and has

been

put in

terms

of

plate tectonics theory

by

Atwater

(1970). According to this view,

westernNorth America is within a broad belt

of

right lateral movement related

to

differ-

ential motion between the NorthAmerican

and Pacific plates. Some of the right lateral

movem ent is taken up on the S an Andreas

and related faults. The movement is also

thought

toproduce distributedextensionand

tensional crustal fragmentation (including

basin and range structu re) along tren ds orien-

ted obliquely to the trend of the San Andreas

fault.

Evidencesupporting a relationship between

Basin and

Range structure

and

oceanic struc-

tureshas

been described

by

Christiansen

an d

Lipman (1970) and Lipman, Prostka, and

Christiansen (1970). They have indicated

that

the

initiation

of

extension faulting

in the

western United States corresponds with a

change in the dominant type of volcanism

from largely an intermediate-composition

calc-alkaline

to

alkali-calcic suite

to a bi-

modal

basalt-rhyolite suite. They

suggest

that this change coincides with the intersec-

tion of North America with the East Pacific

Rise

(McKenzie and Morgan, 1969; Atwater,

1970) which apparently is the time of the

initiation of transform faulting and right

lateral

movem ent in the western United States.

CONCLUSIONS

The

interpretation

of

Basin

and Range

structure

presented here emphasizes a com-

plex

basin stru cture consisting of many dow n-

droppedslices

and

smallblocks

( graben-in-

graben

structure). This type of structure

seems

to be in

accord with what

is

known

from geophysical studies

of

subsurface bed-

rock co nfiguration and is similar to structures

produce d in some small-scale clay models of

grabens.

The

data permit

the

interpretation

that

the

grabens underlying valleys

are

com-

plex

collapse zones over narrow zones of

extension

at

depth

and

that these zones

of

extension

are

related

to

fragmentation

of a

crustal

slab above

a

plastically extended sub-

stratum.T he zones of extension are generally

systematicallyspaced ab out 15 to 20 mi apart

and have a regional pattern similar to crack

patterns in small-scale tensional systems.Ex-

tension across indiv idua l grabens may aver-

age about 1.5 mi, and the total extension

across the Great Basin may be 30 to 60 mi.

The rate of extension is proba bly in the range

of 0.3 to 1.5 cm/yr

over

the

last

17 m.y.

or less.

In

most parts

of the

Great Basin,

the

north-south trend of the zone of extension

indicates an

east-west pulling apart

of the

crust. A more complex pattern of failure,


21/25

REFERENCES

CITED

1039

polygonal

pattern

of

fracturing

issuggested

by the

distribution

of

mountains

and

valleys.

These

areas,

ifanalogoustopolygonal

crack-

ing in

small-scale features, would require

a

local radial

spreading. In the

western

part of

the GreatBasin,the grabens commonlytrend

northwest and are

more

widely spaced an d

lesssystematically distributedthanelsewhere

inthe Great Basin.

This

pattern seems to be

due to the

interaction

of

right-lateral dis-

placement

and the

more general east-west

extension.

As envisioned here, considerable varietyof

movement

is

possible

in the

plastically

de-

forming layer oelow

the

brittle upper crust.

The

dominant

east-west

extension could have

been interrupted

at

times

by

local radial

spreading

and at

other times

by

strike-slip

displacement. Such a variety of movement

could account

for the

local complexity

of

Basin

an d Range

s

Documents

Horts y Graben