Post on 01-Jun-2018
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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
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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.
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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.
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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
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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.
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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
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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
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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
8/9/2019 Horts y Graben
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.
8/9/2019 Horts y Graben
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
Faults along part of both
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
Steep gravity gradients
indicate
faults
on
both
sides of valley dropping
valley block down
Steep gravity gradients
indicate
faults
on both
sides
of
valley dropping
valley block down
Steep gravity gradients
indicate faults on both
sides
of
valley dropping
valley
block down
Steepgravity gradients
indicate faults on both
sides of valley dropping
valley block down
Steep gravity gradients
indicate faults
on
both
sides of valley dropping
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
Walker and Repenning,
1965; Fuller
and
Waters,
1929; Russell, 1884
Walker and Repenning,
1965; Fuller
and
Waters,
1929
Walker and Repenning,
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
8/9/2019 Horts y Graben
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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
Faults along part of both
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
Steep gravity gradients
indicate faults on both
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.
Steep gravity gradients
indicate
faults on both
sides of valley dropping
valley block down
Steep gravity gradients
indicate faults on both
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
Kleinhampl and Ziony,
1967
Kleinhampl and Ziony,
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
sides of valley dropping
valley block down
Steep gravity gradients
indicate
faults on both
sides of valley dropping
valley block down
Steep gravity gradients
indicate
faults
on
both
sides of valley dropping
valley
block down
Steep gravity gradients
indicate faults on both
sides of valley dropping
valleyblock down
Steep gravity gradients
indicate
faults on both
sides
of
valley dropping
valley
block down
Steep
gravity
gradients
indicate faults on both
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
sides of valley dropping
valley block down
I960, Fig. 2
Healey
and
Miller, 1962
Healey and Miller, 1962
Cook and others, 1964, PI.
1,and Fig. 2
Cook and others, 1964, PI.
1, and
Fig.
8
Cook and others, 1964, PI.
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
Cook and others, 1964, PI.
1,
and
Fig.
6
8/9/2019 Horts y Graben
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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
sides of valley dropping
valley block down
Steep gra vity gradients
indicate
faults
on both
sides of valley dropping
valley
block down
Steep gravity gradients
indicate faults on both
sides of valley dropping
valley
block down
Steep
gravity gradients
indicate
faults
on
both
sides of valley dropping
valley block down
Steep gravity gradients
indicate faults
on both
sides of valley dropping
valley block down
Steep gravity gradients
indicate
faults on
both
sides
of
valley dropping
valley block down
Cookand
Berg, 1961,
PI.13 ,
and p.79-80
Cook and others, 1964, PI.
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
8/9/2019 Horts y Graben
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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-
8/9/2019 Horts y Graben
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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
8/9/2019 Horts y Graben
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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).
8/9/2019 Horts y Graben
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
8/9/2019 Horts y Graben
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-
8/9/2019 Horts y Graben
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-
8/9/2019 Horts y Graben
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
8/9/2019 Horts y Graben
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,
8/9/2019 Horts y Graben
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