Formation of compound strike-slip fault zones, Mount Abbot quadrangle, California

13 Pages·2002·1.43 MB·English

lanruoJ of larutcurtS ,ygoloeG Vol. .21 .oN 7. pp. 968 to 882, 0991 09/1418-1910 + 0.00 $03.00
Printed in Great Britain Press Pergamon eip
Formation of compound strike-slip fault zones, Mount Abbot quadrangle,
California
S. J. MARTEL
Earth Sciences Division, Building 50E, Lawrence Berkeley Laboratory, University of California, Berkeley,
CA 94720, U.S.A.
(Received 21 March 1989; accepted in revised form 4 February 1990)
Abstract--Numerous strike-slip fault zones in granitic rocks of the Mount Abbot quadrangle, California,
developed from steeply-dipping, subparallel joints. These joints generally were less than 50 m long and were
spaced several centimeters to several meters apart. Some joints subsequently slipped and became small faults.
Simple fault zones formed as oblique dilatant fractures (splay fractures) linked non-coplanar faults side-to-side
and end-to-end. These simple fault zones are as much as 1 km long and accommodated displacements as great as
10 m. Compoundfauh zones formed as splay fractures linked small faults and simple fault zones. They are as
much as several kilometers long and accommodated displacements as great as 100 m. These zones are distinctly
different from 'Riedel shear zones' and the-mechanics of their formation are unlikely to be described well by
Mohr-Coulomb mechanisms. Simple and compound fault zones are composed of non-coplanar segments that
join at steps or bends; splay fracture length determines step widths. The longest splay fractures occur along the
longest fault zones, allowing step widths to increase as the length and displacement across the zones increase.
These findings are consistent with the structure of some active seismogenic faults, and they provide a
mechanically consistent, field-based conceptual model for fault zones that grow in basement rocks from a pre-
existing set of joints.
INTRODUCTION from their inception for three main reasons: (1) faulting
developed to different stages in different parts of the
F^ULr zones commonly serve as long-standing zones of quadrangle; (2) the effects of subsequent deformation
weakness in the Earth's crust, and the manner in which are minimal and (3) exhumed strike-slip faults and fault
they form is highly relevant to important problems in zones are well displayed. The smallest faults in the area
earthquake mechanics and subsurface fluid flow. It is are a few meters long, a few millimeters thick and
widely recognized that as fault zones develop in base- accommodated less than 1 cm of strike-slip displace-
ment rocks they take advantage of pre-existing fractures ment, whereas the largest compound fault zones are
(Hobbs et al. 1976, Price 1981, Davis 1984, Suppe 1985). several kilometers long, several meters thick and accom-
However, because fault zones can be reactivated in modated approximately 100 m of strike-slip displace-
different episodes of deformation under markedly ment. Two exposures of compound fault zones were
different environmental conditions (Sibson, 1977, 1986, mapped in detail, one at the Waterfall site and another
Sibson et al. 1981, McKee et al. 1984, Muehlberger at the Seven Gables site (Fig. 1); these fault zones are
1986), the manner in which fault zones form from pre- decidedly different from the structures that have been
existing fractures is commonly obscured. The scarcity of formed in many laboratory simple shear experiments.
relatively simple, extensive exposures of fault zones in The paper concludes with a discussion of how the Bear
basement rocks is a major impediment to developing Creek fault zones formed, contrasts that model with
field-based models for how fault zones form (Logan et al. the results of laboratory simple shear experiments,
1979). Indeed, the few well-defined conceptual models and addresses some of the seismic implications of the
in the literature of structural geology that describe how model.
fault zones grow are primarily based on laboratory Before proceeding further, the use of a few terms in
model studies (Syivester 1988). However, many of the the paper is clarified. Fractures are classified as either
laboratory simple shear experiments have been directed joints or faults in the manner suggested by Engelder
towards how fault zones develop in the sedimentary (1987). The term 'joint' is used where there is clear
sequences overlying a pre-existing basement fault (e.g. evidence for a lack of appreciable shear displacement
Nayior et al. 1986); how the basement faults themselves across a fracture, and the term 'fault' is used where there
develop remains an open question. Field observations is clear evidence of appreciable shear displacement. The
are essential to test to what extent laboratory simple more general term 'fracture' is used where there is not
shear models can be applied to large natural fault zones clear evidence on whether or not there was shear dis-
in basement rocks. placement. Adjacent segments of a fault zone may join
This paper presents a model for how compound strike- at steps or bends (Fig. 2c). Parallel but not coplanar
slip fault zones formed from pre-existing fractures within segments join at a 'step', and adjacent segments of
plutons of the Mount Abbot quadrangle in the Sierra different orientation join at 'bends'. Steps and bends are
Nevada of California (Fig. 1). This area provides a rare referred to collectively as systematic geometric disconti-
opportunity to track the development of fault zones nuities.
869
870 S.J. LETRAM
118°50' A .TM HI
\ ~~':~, rmK
Kle
/ LoNe Ifal¥
FA '"~'~.WATE R LL
~\ SITE ::"
~'... _2c ..'
Kle 37°0-2 0
IK
Kmr
". "~ ,.~ : Kqm~ GEMINI
FAULT ZONE
,( ~ ..I,. STUDY \ i I~"'.
NEvEs'~ G,si!i..'~'.'...-,.-.-.
\\ .....
AINI ~ SITE "'"" "' "'.',";"
NEVADA ~ ~ /s/and I 0 I KILOMETERS
Lake
1'°5'su .:v"':v "'xN"x
Fig. l. Geologic map of the area of study in the Mount Abbot quadrangle. Geology generalized from Lockwood & Lydon
.)5791( IK = suoecaterC kcramaL ;etiroidodnarg elK = suoecaterC etiroidonarg of Edison; Lake rmK = ztrauq suoecaterC
etinoznom Mona of .sesseceR cigoloeG stcatnoc era where dashed snoitacol era .etamixorppa ehT nearly lacitrev ztrauq
etinoznom (Kqm) body si yletamixorppa decalpsid 07 m ssorca eht zone. fault Gemini wohs selgnairT snoitacol of kramdnal
.skaep
GEOLOGIC SETTING steeply-dipping subparallel joints (Segall & Pollard
1983a,b). Hydrothermal minerals fill the joints, epidote
The study area is in the Bear Creek region of the and chlorite being particularly common. Those joints
Mount Abbot quadrangle and includes three adjacent not involved in subsequent deformation contain unde-
NW-trending plutons (Fig. 1) mapped and described by formed minerals. Joint spacing and joint length range
Lockwood & Lydon (1975). Field relations show that from a few decimeters to a few tens of meters. The joints
the westernmost pluton (the Lamarck granodiorite) is typically are 0.1-10 mm thick.
the oldest, that the central pluton (the granodiorite of During the second stage of deformation some joints
Lake Edison) is of intermediate age, and that the east- slipped and became small left-lateral faults (Fig. 2b),
ernmost pluton (the quartz monzonite of Mona offsetting xenoliths and steeply-dipping aplite dikes by
Recesses) is youngest. Based on K-Ar dating of biotites, as much as 2 m (Segall & Pollard 1983b. Martel et at.
these plutons have ages of 79-85, 77-82 and 79--82 Ma. 1988). Not all joints in a given outcrop slipped, so
respectively (Stern et al. 1981). These ages provide unslipped joints parallel adjacent small faults. Slicken-
maximum ages for the fault zones. All three plutons lines on the fault surfaces typically plunge less than 20 , °
contain a weak, steeply-dipping foliation that strikes indicating that at least the most recent displacements on
NW. Mineralized joints, faults and fault zones that strike the faults were dominantly strike-slip (Segall & Pollard
ENE are abundant within the older two plutons. Miner- 1983b). As a result of slip, the material in these faults
alized joints that strike ENE are also abundant within acquired mylonitic fabrics, and NNE-striking fractures
the Mona Recesses pluton, but faults and fault zones (splay fractures) opened near fault ends. The splay
that strike ENE appear scarce. Considering the likely fractures are filled primarily by epidote, chlorite and
origin and evolution of the faults and fault zones (dis- quartz. Although some splay fractures are several
cussed below), their distribution suggests that the fault- meters long, they rarely extend more than a few meters
ing in the older two plutons may have occurred before or away from the faults. The splay fractures are inferred to
during emplacement of the Mona Recesses pluton. The strike in the direction of the maximum horizontal com-
faults would then have formed at the depth of emplace- pressive stress (Segall & Pollard 1983b). Some originally
ment. Work by Noyes et al. (1983) indicates that the unconnected left-stepping echelon faultsw ere linked by
nearby and nearly contemporaneous Eagle Peak and splay fractures, whereas some right-stepping echelon
Red Lake plutons were emplaced at a depth of approxi- faults were linked by ductile deformation of the inter-
mately 4 km. vening rock. Linked echelon faults are as much as
The first three stages of faulting in the Bear Creek several hundred meters in length and accommodated
area (Figs. 2a--c) have been discussed recently by Martel left-lateral displacements as great as several meters.
et al. (1988). The faults and fault zones developed from Larger left-lateral displacements were accommodated
Compound strike-slip fault zones, California 178
/
a
MINIMUM HO~AL COMPRF.SSIVE STRE,~)S
I
JOINTS,,,
MAXIMUM HORIZONTAL
COMPRESSIVE STRESS
b
.4-- f UNKAGE
SPLAY FRACTURE ~ / ae"
""-- ,"6,///
• --- ////
i/i ---
--- /
.._-~Y~ ~. - ._. J
q
-/
f - ///
SMALL FAULTS
f
/ ,?'/ / _ /~
c ~ -_
f
q
-'- ~/// / ///////////if////
/ 1 I"// "-"
~/////-//////"
- ~ - - _ ~1IIIII IIIIIIII1~ ~'- I r "~ \~,..~
-- "~---."~-" - v~,
-- ~ ~//////////////" ////////~"
O~CnVAn~D DETAVnCA.FO LLAMS TLUAe I"// I / ~ I
./ / ~'~ -- /// /
s,~.~^o~ J / --w / / / -..../.... / / ,,/,"7', ............ ," /
I - - ~ ~.~i'Jlllfll llllllllX~ r / / / / I
/ / / o o
Fig. 2. The four stages of faulting in the Bear Creek area. (a) Opening of joints.( b) Development of small left-lateral faults.
(c) Development of simple fault zones. (d) Formation of compound fault zones.
872 S.J. LETRAM
EXPLANATION
,o **
I z..-_.
f O ~ g Q • Fault zone with left-lateral
SRETEM
• g 4 O I I component of displacement.
Small fault with left-lateral
component of displacement.
Dotted where concealed.
~- Aplite dike.
A~ \ Dotted where concealed.
o .-i
..~',..-" • :;::-:
/ John Mwr Trad
t
Fig. 3. Map showing displaced dikes and a displaced kink band at the Waterfall site. The dikes and the kink band are offset
similar amounts across the fault zones. Note the irregularities in the along-strike trace of the northern fault zone near dikes
A and D.
in the third stage of deformation (Fig. 2c) as abundant layering in the granodiorite by several tens of meters.
fractures linked faults side-to-side to form simple fault This network is a few tens of meters wide, is well
zones (Martel et al. 1988). The fractures within these exposed over a horizontal distance of 500 m, and crops
zones contain epidote, chlorite and quartz, with smaller out over a vertical interval of at least 300 m.
amounts of muscovite and calcite. The most prominent Two prominent sets of steeply-dipping mineralized
fractures strike obliquely to the sharply defined faults fractures occur at the site (Figs. 3 and 4). The first set
(the boundary faults) at the edges of the zones. The rock consists of joints, small faults and two fault zones. These
inside a zone is highly fractured, in sharp contrast to the strike 060-075 . ° The second set consists of mineral-filled
relatively unfractured rock immediately outside. Simple fractures that strike NNE. Fractures in the second set
fault zones typically are 0.5-3 m thick and up to 1 km are present across the site, but are most common near
long. Their traces consist of straight, non-colinear seg- the fault zones.
ments a few tens of meters long that join at steps or A kink band with axial planes that strike 010 ° deflects
bends. This indicates that non-coplanar small faults some of the joints and faults in a right-lateral sense (Figs.
linked end-to-end to form the boundary faults of simple 3 and 4). The spacing between the kinked joints and
fault zones. The maximum strike-slip displacement ob- faults generally is less than 20 cm, whereas away from
served across a simple fault zone is about 10 m. The the kink band the spacing generally ranges from about
displacement is concentrated on the boundary faults. 20 cm to 3 m. Similar kink bands occur elsewhere in the
Bear Creek region (Segall & Pollard 1983a, Davies &
Pollard 1986, Martei etal. 1988). The right-lateral defor-
mation along the kin k bands is interpreted as conjugate
STRUCTURE OF COMPOUND FAULT ZONES to the left-lateral displacement across the faults and fault
zones.
Waterfall site The two fault zones at the site (Figs. 3 and 4) appear to
offset the kink band. giving a left-lateral strike separ-
The Waterfall site lies along the north edge of a ation of several meters. Prominent aplite dikes also have
network of faults and fault zones that left-laterally left-lateral strike separations of about 10 m across each
separates steeply-dipping aplite dikes and compositional fault zone. The fault zones are 2-3 m thick and are
C~ O O .-1 i° 7;" N. N O G G~ ,,,,I
of of of
shown. Some
EXPLANATION outside of fault zone, Most fractures are shown. unmineralized are not shown. Outcrop boundaries where approximately located. area. Includes some unmapped within fault zone. Only some most prominent fractures are zone with component left-lateral Border is dotted where with component left-lateral Dotted where concealed. fracture showing dip. be small faulls with up to several of left-lateral displacement. where concealed. dike more than a few centimeters showing dip. Dotted where dike less than a few centimeters showing dip. Dotted where and dip of composilional layering aplite dike. boulder.
)utcrop nineralized ~lorthwest-striking 3ints lashed Covered outcrops. )utcrop he :ault lisplacement. :onceated. Small fault Jisplacement. Vlineralized "nay ;entimeters ~)otted ~,plite vide ;oncealed. ~.plite vide "oncealed. ;trike vithin )rominent
.°
•
, '7 ~ 26

Jr" 1
.o (cid:127) Icxl.
lhc
i
.... Io in
KINK/ '
C
OF .....
/DIKE TRACE i I °~o°.°°" arc rot)erred
Ill

1
I ....... ""~ F tI~ i J I, |l and
.°.,
°"
.°° ll.26 ~ •DIKE B Silos
site.
.....
°°°°°°'°:
°
_ Walcrfall
~ITE U Ihc
/
.''" i at
°.o.-°"°°'~° °.°° • " •DIKE zones
. ° • ...'" faull
and
I0 I Q faults
O
joints,
)IKE
I showing
I
4ETERS Map
I
:ig. 4.
I
) i
-."
874 S.J. ~IARIEI.
spaced 7-14 m apart. The rock exposed inside each fault to the north. The eas~ and ~" t,-_e limbs strike NE. Near the
zone is highly fractured, whereas the rock immediately east end of the central limb the fault zone offsets a
adjacent to the zones is little-fractured (Fig. 5). The steeply-dipping quartz-monzonite body --7qt m in a left-
fractures contain epidote, chlorite, quartz, calcite and lateral sense (Fig. 1), Some steeply-dipping dikes are
white mica. The topographic expression of the fault offset left-laterally bv about 100 m near the west end of
zones as shallo trou,hs mdmates that internal fractures the central limb (Lockwood & Lydon 1975),
W o " "
are abundant along the length of the fault zones. Examinations of several outcrops along the Gemini
The boundaries of the northern fault zone do not form fault zone indicate that it contains a series of faults that
straight traces across the Waterfall site. Instead, the are subparallel to tile zone as a whole. These faults
fault zone is segmented along strike, making a left-step divide the zone into a series of fractured bands. The
at site I of Fig. 4 and a bend of ° ~ 10 near dike D of Fig. 3. intensity of fracturing and the degree of chemical alter-
The segment defined is ~50 m long. The northern ation varies from one band to another. Fractures striking
boundary fault of this zone has a maximum exposed oblique to the fault zone typically are scarce in the rock
thickness of 1 cm. Like all other boundary faults immediately adjacent to the zone.
sampled in the Bear Creek region, it has a cataclastic The largest identified exposure of the Gemini fault
texture, with angular fragments of quartz and feldspar zone occurs along its central limb (Fig. 1), The northern
set in a dark green-brown, fine-grained, isotropic part of the fault zone was mapped in detail at that
matrix. The cataclastic texture contrasts sharply with the outcrop (Fig. 6): the southern part of the zone is covered
mvlonitic fabrics of small faults (Segall & Pollard by talus. The fault zone does not have a straight trace
1983b). near this outcrop. Tile fault zone strikes approximately
Two subsets of mineral-filled dilatant fractures strike 080 ~' over a 40-50 m interval at the outcrop and dips
NNE at the Waterfall site. The first subset comprises approximateb 70 ° to the north, hnmediatelv east of the
fractures that are no more than several decimeters long exposure, the fault zone bends to a strike of --065 .°
and contain epidote, quartz and chlorite. They typically Several meters west of the outcrop, the fault zone steps
branch from small faults (site lI, Fig 4) and are referred several meters to the south. The fault zone thus consists
to as splay fractures. They are interpreted as forming in of segments that are not coplanar,
response to slip on the small faults (Segall & Pollard The rock north of the fault zone contains two major
1983b). Fractures of the second subset typically are sets of mineralized fractures (Fig. 6). The first set con-
1-4 m long and are bordered by halos of hydrothermally sists of fractures that parallel the fault zone and contain
altered rock (site III, Fig. 4). These long fractures are epidote and chlorite. The spacing between these frac-
common only between or alongside the two fault zones tures typically is half a meter to a few meters, similar to
and are interpreted as being splay fractures that formed the spacing between ENE-striking fractures at the
in response to slip on the fault zones. Some of the long Waterfall site. Some of the fractures appear to be joints,
splay fractures are continuous structures, whereas but others are faults \~ith slickenlines. The second set
others are bands of short echelon cracks. Some of these comprises fractures that strike 1)10-050: and dip steeply
fractures cross small faults; most are not offset by the to the east. Some arc several meters long. Fractures of
fauhs, although some appear to be offset slightly (less the second set appem: to bc dilatant cracks, for they cross
than 2 mm). These relationships support the interpre- fractures of the first set but do not offset them laterally.
tation that the long fractures formed during the episode This set has three subsets: continuous fractures, bands of
of faulting, but indicate that the long splay fractures short echelon cracks and ribs of hydrothermallv altered
formed after all or nearly all of the slip on the small faults granitic rock. These fractures are concentrated near the
had occurred. Because the long NNE-striking fractures fault zone and are interpreted to have formed in associ-
do not visibly offset the small faults they are interpreted ation with slip on the zone.
as dilatant fractures. Long NNE-striking fractures have The Gemini fault zone itself is divided imo fracture-
been observed along other fault zones in the Bear Creek bounded bands of markedl 3 different fracture intensity
area, but only along those that accommodated displace- (Figs. 6 and 7). The fractures bounding the bands almost
ments of at least 10 m. certainly are all faults, but t~ lack of markers prevented
the amount of displacement across them from being
The Gemini fauh zone and the Seven Gables site documented. Four bands are exposed, but additional
bands might exist beneath the talus that covers the
The Gemini fault zone (Fig. 1) is the largest fault zone southern part of the fault zone. The first and northern-
mapped by Lockwood & Lydon (1975) within the Mount most band is about 8!t cm thick, has an exposed length of
Abbot quadrangle. The topographic trough marking the 3 m, and contains highly fi-actured rock. Prominent
fault zone is generally about 10 m wide and extends over fractures within the band strike -35 ° counterclockwise
an elevation range of approximately 700 m. However, from the band boundaric~ and cut an aplite dike but do
neither the top nor the bottom of the fault zone is not offset it laterally. This band appears to be a relict
exposed, so the fault zone could have a vertical extent simple fault zone, The second band is about 1 m thick
much greater than 700 m. The zone is about 01 km long and is exposed over an interval about 01 m long. The
and has a sigmoidal shape in plan view. The 4 km hmg intensitx of fracturing within this band (Fig. 7b) is
central limb strikes approximately 075 ° and dips steeply markedly less than that of the thst band and si similar to
¢3 o "t3 (3 e~ i "5" E" I%1 O 5" 0"3 -,-J
..
show
Dotted
where
i ¸ ¸ marks zone. concealed, ZOllC,
:.i;i,,: Tick dashed fault
/ i
~: where tile
.... zone. approximate. in
~ / ~: Dotted are
• i marks fractures clearly lacking left-step
i:~,~bG :¸~:, EXPLANATION Area. concealed. "U" fillings. locations of tile
:i
Outcrop boundaries ellS|
/, // !~ ~, ~ 11~] Outcropw,,,,n,ao,,zone. Outcrop outside fault northwest-striking foliation. Covered ..-" Northern boundary fault of fault S where Fracture. .-,?i u mineral Note: I) is loci|ted just
i!!~ili
OLIICFO
! G I
This
~ ~i ~ ~iL~i~i~ii~ site.
i ~iliiiiilC~i! WlltCl'filll
~ Gili~ii~ //// tile
ii!i ill
•
~C, ?:;
i~ ~ i • zone
~:~!~,i I'aull
nloo'tlnern
|lie
I ill
OtltCl-~.)p
/ small
/ a
/
METERS willlin
/// . LIICS
I'l';ICl
.~ of
O ilia: mill'~
il; . 5,
/ U'ig.
l ~ii~i:i!ii~:
.4 p, ,.4 "rl
..~ ' • • the
" of
. . . . ". BOULDER " " "r~LUS approximate, are planar; many showing
o,..,,o AREA , . • ' is fractures causes where concealed. of band
", , '\ is approximate location The outcrop Dotted dip granodiorite
:/~ k~--- where dip. of the showing altered concealed.
L_. location
I \ ". X ; '~ .. ,~.' ,v::,, • ; ".. . i - -...\~ :' -'". "'" " . : .: . ', "'" " - ~, '" .z. " " " : : ' ~, q" " " , 3ontact, dashed where :ault, showing dip. Dashed :lotted where concealed. vlineralized fracture showing he "staircase topography" raclure traces to be nonlinear. 3and of echelon fractures of hydrothermalty :laised rib tip. dike. Aptite Dotted where Prominent boulder. al the Seven O.'d~les site.
/::.-."" t. . " ~ ... :-; ' . . " BOUL.DER TALUS .. ". .... . . , .''"." ' " -" - : • i J .i- f 78 i f¸~'j j// S~ 68 I) ~..-~J ~i '7-" ~' ,..ii~, ) Gemini faull zone
OF " " i the
ZONE .' ". of
EDGE FAULT : ~'~ t-: "~'" " " ii :~XPLANATION mar~m
.."
~
" NORTH ''"IN' ~: " i with northern
• "~ , in the
- cm
#''' Northwest-striking in diameter• in diameter 20 pebble-sized ~dong
4 than
cm cm
less fractures
~ ~' fractures, shown. 5-100 5-100 and angular of
" not mostly mostly sand 6. Map
IO I are Fig.
I joints blocks mostly blocks surfaces. fragments granitic
chlorite-coated surfaces. by
/,i I-- I.__l showing mineralized of angular of angular of angular with covered
• 1 METERS )utcrop tnmineralized }and 3and ;hlorite-coated 3and Jiameter talus ~,rea :ragments.
I
I:III I
1
75 b - O [ 1 ......... ol m
O 3 G ¢,. I'I J, d~ N o ¢3
and
notch I
largest
the
showing bands
through
outcrop
passes
zone end of
west
fault of
view
Gemini
of
Close-up
(b)
view. Trace
boots.
Distant
(a) 6.
geologist's l:ig.
zone. the of
of 2
fault
right
Gemini the
to
the
block
along
light-colored
Gables outcrop the
on
is
Seven (b)
the of
of
,scale
cast
cm
the 15
to
The
Views
7.
skyline.
Fig. in
O0 ,.,d ---,I
Compound strike-slip fault zones, California 978
that in the rock outside the fault zone. The third band is fault zones alsor eflects the distribution of joints in local
1-4 m thick and consists of chlorite-coated fragments 5- unfaulted outcrops. The segment of the Gemini fault
52 cm long. Where exposed at the east end of the zone at the Seven Gables exposure and the segment in
outcrop, the contact betweent he thirda nd fourth bands the northern fault zone at the Waterfall site are both --04
is a fault with a cataclastic texture. This fault strikes m 50 long (Fig. 3). The principal fractures in the network
parallel to the zone. The fourth band is at least m 5 thick of faults east of the Waterfall site usually are no more
and consists of cobble- to boulder-size blocks that gener- than several tens of meters long. The longer joints in
ally lack chloriticco atings. local outcrops also have maximum lengths of several
tens of meters (Segall & Pollard 1983a). It seems that the
simple and compound fault zones preferentially devel-
A MODEL FOR THE GROWTH OF COMPOUND oped from joints several tens of meters long. Perhaps
FAULT ZONES these long joints were able to slip more readily than
shorter ones. The thickness of the fillings in the joints
The fault zones that extendt hrough theW aterfalaln d appears to scale approximately with joint length, so
Seven Gables sites are larger and have accommodated opposing faces of a joint are less likely to interlock the
more displacement than simple fault zones. For longer a joint is. Still, the formation of fault zones
example, most simple fault zones appear to be several several kilometers long apparently did not require the
hundred meters long, less than a few meters thick and presence of joints several kilometers long.
have accommodated less than 01 m of lateral displace- The evolution of faulting in the Bear Creek area (Fig.
ment. In contrast, the Gemini fault zone is several 2) largely reflects the linkage of non-coplanar structures
kilometers long, approximately 01 m thick and has by fractures. As a result, faults and fault zones have
accommodated -100 m of lateral displacement; its systematic discontinuities along strike. The scale of
dimensions are approximately an order of magnitude these discontinuities increased as the faulting pro-
greater than a simple fault zone. The Gemini fault zone gressed. Small faults evolved from joints. Detailed
and the network of faults at the Waterfall site are also examinations (Segail & Pollard 1983a) show that the
structurally more complex than simple fault zones. joints themselves are not perfectly planar structures.
Along a simple fault zone (Fig. 2c), two boundary faults Instead, they consist of numerous parallel echelon
are linked side-to-side. Internal faults that parallel the strands spaced less than a few centimeters apart. When a
boundaries of a simple fault zone are rare. In contrast, joint subsequently slipped and became a small fault,
the Gemini fault zone incorporates many internal faults these strands were linked. The discontinuities along the
that parallel its boundaries, and for that reason is strike of a small fault thush ave amplitudes of less than a
referredt o here as a compound fault zone (Fig. 2d). The few centimeters. Simple fault zones formed as small
fault network at the Waterfall site is interpreted to be of faults linked side-to-side and end-to-end. As a result,
an intermediate nature, for small faults and simple fault these zones are segmented along strike. Adjacent seg-
zones are distinct, but long oblique fractures connect ments join at steps as wide as a few meterso r at bends of
some of the simple fault zones side-to-side. The fault as much as ° 20 (Martel etal. 1988). Discontinuities along
network at the Waterfall site can therefore be regarded the strike of simple fauzlotn es thus have amplitudes of a
as a compound fault zone deactivated at an earlier stage few meters at spacings of several tens of meters. The
of development than the Gemini zone. formation of compound fault zones involves the linkage
Despite their differences in size, the across-strike of simple fault zones by fracturing. Because splay frac-
structure of both simple and compound fault zones tures along compound faulzto nes can be several meters
reflects a distribution of joints similar to that in local long, steps along the strike of compound fault zones can
unfaulted outcrops. Both kinds of zones are paralleled have widths of several meters.
by steeply-dipping small faults and joints. ENE-striking The distribution of fractures along the faults and fault
fractures inside and outside the Gemini fault zone (Fig. zones of the Bear Creek area thus can reveal the extent
6), as well as those in the vicinity of the Waterfall site to which geometric discontinuities affected slip as the
(Figs. 3 and 4), are generally spaced several decimeters faulting progressed. Experimental work (Erdogan Sih &
to a few meters apart. This is comparable to the spacing 1963, Brace & Bombolakis 1963, Horii & Nemat-Nasser
of joints and the thicknesses of simple fault zones 1985) and theoretical studies (Erdogan & Sih 1963,
throughout the Bear Creek region (Segall & Pollard Pollard & Segall 1987) indicate that splay fractures
1983a,b, Martel etal. 1988) and indicates that compound would form along faults where displacement gradients
fault zones developed from arrays of joints. The com- (i.e. strains) and the associated stress concentrations
pound fault zones apparently did not develop from joints become sufficiently large. Splay fractures along small
with either an usually close or unusually wide spacing, faults are concentrated only near the fault ends, indi-
nor did the growth of the zones involve the generation of cating that the discontinuities along the small faults did
new fractures parallel to the zone. The across-strike not act to produce large displacement gradients. Along
structure of compound fault zones thus indicates that simple fault zones, fractures are abundant in the adjac-
many already existing fractures were incorporated into ent rock only near the ends of segments (Martei el al.
the faulzto nes as they grew in thickness. 1988). The geometric discontinuities at segment ends
The segmented along-strike structure of compound were of sufficient size to cause steep displacement gradi-