JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 96, NO. B7, PAGES 11,895-11,910, JULY 10, 1991
Mechanicsof Emplacementand TectonicImplications
of the Ramon Dike Systems,Israel
GII)ONBAER
1
GeologicalSurveyof Israel, Jerusalem
ZE'EV RECHES
Departmentof Geology,Hebrew University,Jerusalem,Israel
A radial systemcomprisingmore than 200 basaltic and trachytic dikes and two minor systemsof parallel
dikes intrudedthe Ramon area, southemIsrael, duringthe Early Cretaceous.Field relationsbetween dikes
and fracturesin the radial systemindicatethat the dikes intrudedself-generatedfractures,and thus indicate
the directions of the tectonic stresses. Other field observationsindicate that the dikes propagated in
subhorizontaldirectionsup to distancesof 15 km from their source. Our analysisof the emplacement
mechanics of these dikes shows that the horizontal propagation is best explained by the density
differencesbetween the intruding magma and the host rocks. The measuredmean density for basement
rocks
atdepths
greater
than2.4kmis2.55+ 0.07g/cm
3,andit is2.36+ 0.21g/cm
3 forthesedimentary
cover above.ß For the magma to be
propagatinghorizontallyat its neutral buoyancylevel requiresa mean
3
magma densityof about 2.5 g/cm . The large distanceof horizontalpropagationrequiresa low viscous
pressuredi'op,below 0.1 MPa/krn within the dike, and an overpressure
of about 1 MPa in the magma
chamber. We computedstresstrajectoriesusing a two-dimensionalelasticmodel for a pressurizedhole in a
regional stress field and compared it to the Ramon radial system. The model reveals that the dikes
originatedat a central intrusionof about 3 km diameterand intrudedunder a predominantlyradial stateof
stresswith negligible regiomil stressfield. This period of weak tectonic stressesand intensivemagmatism
falls between the early Mesozoic extensional regime and the late Mesozoic-Cenozoic compressional
regimein Israel. The calculatedcenterof the radial systemis offsetfrom a large magneticanomalysouthof
the Ramon area, suggestinga 3 km right-lateral displacementalong the Ramon fault after the intrusion of
the radial system.
INTRODUCTION
The modelingof dike emplacementis basedon observations
and estimates of dike geometry, fracture patterns, and rock
Over 200 dikes of basalticand trachyticcompositionsdensity. The temporal and spatial relationsbetween dikes and
intrudedthe sedimentaryrocks in the Ramon area, southern
fractures in the host rocks make it possible to distinguish
Israel. Mostdikesarearranged
in a radialsystem,
anda smaller
between dikes which generatedtheir own fractures and dikes
numberof dikesform two systems
of NW-SE andNE-SW trends
which intrudedinto existing fractures. Dikes of the first group
(Figure1). The dikesareaccompanied
by manysillsof similar
trend perpendicular to the least compressive stress, whereas
composition. The dikes and the sills intruded Triassic and
dikes of the secondgroupmay be influencedby the weaknessof
Jurassic
layersduringa periodof about10 m.y. in Early
Cretaceoustimes (Table 1). The field observationsof the the preexisting fractures. The fracture-dike relations may be
Ramondike systems
raiseseveralquestions.The radialdikes utilized to evaluatethe relative magnitudesof the magmaticand
the tectonic stresses[Tsunakawa, 1983; Delaney et al., 1986;
propagated
in a subhorizontal
directionat depthsgreaterthan
0.5 km, andthereis no evidence
for theirreaching
thesurface
Reches and Fink, 1988].
Stressanalysesof vertical dike systemsare usuallybasedon
[Baer and Reches,1987; Baer, this issue];what were the
two-dimensional stress models that determine the principal
geologicaland mechanicalcontrolsupontheir propagation?
Someindividualdikes,whichare not morethan0.5 m thick, stressesin the horizontal plane. Using this approach,Ode'
[1957] analyzedthe stressstateduring the emplacementof the
radial
dikes in the Spanish Peaks area, Colorado. Muller and
propagate such long distances in cold, water-saturated
sedimentary rocks?
What stress state enabled the Pollard [1977] extendedOde's analysisto include the effect of
tectonic stressfields. Dikes may also be used to calculate the
contemporaneous
intrusionof threedike systemsand a set of
sills?Thepresent
studyis anattempt
to answer
thesequestionsdepth variations of the stress state from the flow directions
[Rubin and Pollard, 1987], or the shapeand extentof the dikes
by modelingthe emplacement
mechanicsof the Ramondikes.
[Reches and Fink, 1988].
In the presentstudy, we first determinethe depth variation
1Now
atInstitute
ofGeophysics
andPlanetary
Physics,
Universityof the dike's driving stress by calculating the tectonic and
of Califomia Los Angeles.
magmatic stressesrequired for horizontal propagation.Then,
the stress field in the horizontal plane is computed by
Copyright 1991 by the American GeophysicalUnion.
comparing the observedradial dike pattern with the predicted
stresstrajectoriesfor a pressurized,central intrusion, with and
Papernumber91JB00371.
propagated
asfar as 15 km fromtheirsource;
howcouldthey
0148-0227/91/91JB-00371
$05.00
without
11,895
tectonic
stress.
The
results
are used to constrain
the
11,896
BAERANDRECHES:
EMPLACEMENT
MECHANICS
OFDIKESYSTEMS,
ISRAEL
I
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i
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I
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olo
I
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oos
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½
.• Mi $hhor"'%.xJ
' x/ x %%\\,•\ •
½ ,
ooo
995
•
c<
-• Fau
Lt
intr'usion
•F-•-I(Ramon
Basic i/accoLith)
ntr•usion
99O
•
BasaLtic
FLow
I
I
I
I
I
I
I
1
115
120
125
130
135
140
t•5
150
Fig. 1. Locationmapof dikesandmajorigneousbodiesin the Ramonarea[afterNevoet al., 1958;Zak,
1968; Baer, 1989].
density and pressureof the intruding magma, and the regional
stressfield during the Early Cretaceous. Finally, we utilize
these calculations to evaluate the displacement along the
Ramon fault, which is one of the major tectonic elements in
southern
Israel.
lateral displacementwere measuredin Gebel Minshera, about
150 km west of the Ramon area [Bartoy, 1974].
Several tectonicphaseshave affected the Ramon area since
early Mesozoic [Zak, 1963; Garfunkel, 1964; Bartov, 1974;
Garfunkel and Derin, 1988; Baer and Reches,1989]. The main
phasesare the following:(1) Late Triassicnormalfaultingand
basinformation;(2) Early Jurassic
uplift of the southernpart of
the Ramon; (3) Late Jurassicto Early Cretaceousuplift and
GEOLOGIC SETrING
Makhtesh Ramon (the Ramon erosion cirque), southern
Israel, is deeply eroded into an elongatedN70øE trending
asymmetric anticline (Figure 1). Early Triassic to Recent
sedimentaryrocks and Early Cretaceousigneousrocks are
exposed within the cirque. The southeasternflank of the
structureis boundedby the Ramonfault, a dextraloblique-slip
fault, whichextendsfor 250 km in a generalENE-WSW to E-W
direction; this fault is part of the Sinai-Negev shear zone
[Bartoy,1974]. The maximumverticaldisplacement
alongthe
Ramon fault measuredin the Ramon area is 700 m; 2.5 km
intensive igneous activity; (4) Late Cretaceous to Eocene
foldingandreversefaulting;(5) Neogeneto Recentfoldingand
strike-slipfaulting (along the Ramon fault, the Sinai-Negev
shearzone, and the Dead Sea rift).
RAMON SHEET INTRUSIONS: FIELD RELATIONS
The Early Cretaceousmagmaticfeaturesin the Ramon area
include intrusive and extrusiveforms. The prominentbodies
are quartz-syenitic and basaltic laccolithlike intrusions,
basaltic and trachytic sheet intrusions, basaltic flows, and
TABLE 1. K-Ar Agesof SelectedBasalticSamplesFrom SheetIntrusionsin EastemRamon
Sample
Description/Location
Coordinates
Age, Ma
1
N-S trendingdike, Ardan
1455000268
130.5 + 2.7
2
sill, Afar area
1421206205
136.1_+2.8
3
sill, Afar area
1418400191
128.7 _+2.7
4
N30øW trendingdike,Afar
1415700239
130.8 _+2.7
5
N30øWtrendingdike,Mohila
1413000262
138.7 _+2.9
6
sill, Afar area
1405200074
134.3 + 2.8
Ages from Baer [1989]. Coordinatesare in Israel network.
BAER AND RECHES:EMPLACEMENTMECHANICS OF DIKE SYSTEMS,ISRAEL
11,897
basic to intermediate pyroclastic rocks [Bentor, 1952]. The
sheet intrusions in Ramon were previously described in detail
radiometric ages of dikes and sills (Table 1) and the indication
that sills originated from the same source as the radial dikes
[Baer and Reches, 1987; Baer, this issue], and their main
[Baer and Reches, 1987] suggestthat sills and dikes intruded
observations are outlined in this section.
contemporaneously.Furthermore,the total volume of magma
Three dike systems,a radial systemand two sets of parallel intruded into dikes is comparableto that intrudedinto sills. For
dikes, are found within the Triassic and Jurassicsedimentary the 600 m thick sequenceof Triassic and Jurassicrocks in the
layers. Basaltic sills intruded mainly into the Triassic layers Ramon
the volumes
are approximately
3x107m3/km
2 and
and are less abundant
in the Jurassic
units.
The
radial
dike
system comprisesabout 200 basaltic and trachytic dikes and
forms the dominant dike pattern [Zak, 1968; Baer, 1989].
Only the northernpart, which accountsfor less than half of the
inferred complete radial system, is exposed (Figure 1).
Individual dikes are 0.2-3 m thick and extendup to 15 km away
from the estimatedlocation of an unexposedcentral intrusion,
southof the Ramon. Trends of individual dikes changealong
their path and some dikes branch toward the north; commonly,
5x107m3/km
2 fordikesandsills,respectively.
Thesimilar
agesand volumessuggestthat therehas beenno preferredform
of intrusionduring that period.
Age Relations BetweenDikes, Joints, and Faults
Most joints in the Ramonarea are eitherparallel to adjacent
dikes or associatedwith the Ramon fault system[Baer, 1989].
Jointsof the radial systemare encounteredin the entire Ramon
area but are significantly more abundantclose to the dikes of
one of the northern branches is continuous to the southern
this
system(Figure 4). The occurrenceof thesejoints is much
segmentand the other is more westerlytrending(Figure 1).
lower
at distancesmore than 100 m away from the dikes. The
The NW-SE dike systemincludesseveralsubparallelbasaltic
and trachyticdikes which trend N50øW to E-W. In the eastem dike-parallel joints most likely formed during the dike
Ramon this system is clearly distinguished from the radial intrusion,probably in front of the propagatingdikes [Delaney
et al., 1986].
Joints which show no clear geometric
dikes, whereas in the western Ramon the two systemshave
relationshipto the dikes are presumedto have formed as part of
similar trendsand cannotbe separated(Figure 1). The NE-SW
the regional tectonic activity, unrelatedto dike emplacement.
dikes include a small number of dikes in the northeasternpart
Faults of various lengths are associatedwith some of the
of the Ramon. These dikes trend N60øE to N20øE
and are
radial
dikes [Zak, 1968]. The displacementalong the majority
typically shorter and thinner than dikes of the other systems
of thesefaultsdoesnot exceedseveralmetersandis usuallyless
(lessthan 2 km long and 0.5-1 m thick).
than 1 m. The faults are generally parallel to the adjacent
dikes;
in only a few locationsdo they deviate by 10ø or more
PropagationDirectionsand the Geometryof the Dikes
from the dike trend (Figure 5). Faults are found at the dike
Field observations
of dike-related
structures indicate that the
radial dikes propagated in a predominantly subhorizontal,
northward direction from an unexposedsource south of the
Ramon fault [Baer and Reches, 1987; Baer, this issue]. The
observations include bedding-parallel segment boundaries,
containmentof 1-20 m high segmentswithin distinct layers in
the well-stratified host rocks (Figure 2a), and subhorizontal
fingers, 1-10 cm wide and 10-100 cm long in the massive,
porousrocks (Figure 2b). Since there is no evidencefor dikes
feeding flows, and since most dikes become thinner in the
younger stratigraphic units, it is unlikely that these
horizontally propagating dikes reached the surface [Baer,
1989]. The geometry of the dikes at levels deeper than the
presentexposureshas not been studied;however, at the scale of
a single outcrop the horizontal dimensions of the dike
segmentssignificantly exceed their vertical dimensions. Thus
these dikes are regarded as blade-like dikes, similar to those
found in active volcanic rift zones [Rubin and Pollard, 1987].
contact, within the dike, or a few centimeters to a few meters
away from the dike contact(Figure 5). Displacedstructureson
oppositedike walls indicatethat someslip occurredalongthe
faults during the emplacementof the dikes as part of the
propagation process [Baer, this issue]. Small faults with
displacements less than 1 m are also common between
overlapping dike segments (Figure 6). These faults are
attributedto the interactionbetween the boundingsegments
[Baer, this issue]. There is no evidencefor the existenceof any
well-defined fracture systemprior to the intrusionof the dikes.
The few faults that show larger displacementsare primarily
strike-slip faults with horizontal slickensides that formed in
associationwith later activity along the Ramon fault [Baer and
Reches, 1989].
The age of the radial faults is evaluated from their
displacementrelations. These faults displacethe sills and the
sedimentaryrocks by about equal amountsand are thus not
older than the sills.
As the sills and the dikes are of almost the
same age (Table 1), the radial faults are presumablyalso not
Age Relations Between the Sheet Intrusions
The dikes of the radial systemand the dikes of the NW-SE
system display clear crosscuttingand bending relations in a
few locations in eastern Ramon (Figure 3). These relations
indicateat least four successiveeventsof intrusion(Figure 3);
in some locations the radial dikes predated the NW-SE dike
(e.g., intersectionpoints 22 and 24 in Figure 3), whereasin
other locations the opposite relations prevail (intersection
points 5, 9, and 17). It thus appearsthat the dikes of these
systemsintruded intermittently. The field relations between
the NE-SW trending dikes and the other two systemsare not
clear, and their age relationshipscould not be established.
In the few sites with intersections between sills and dikes,
their age relations are not too clear. Nevertheless,the similar
older than the dikes.
The
above observations
lead to the conclusion
that the
radialjoints and faultsformedduringthe intrusionof the radial
dikes. Thus the radialdikesdid not entera preexistingfracture
systemand may be utilized as reliable stressindicators.
M•HA•½AL
ANALYSIS
General Approach
The intrusion of a radial dike system into a layered rock
sequenceunder regional tectonic stresseswas studied in three
steps: (1) the propagationmechanismsof an individual dike
intrudedinto a layered sequenceof sedimentaryrocks, (2) the
11,898
BAERANDRECHES:
EMPLACEMENT
MECHANICS
OFDIKESYSTEMS,
ISRAEL
Fig.2. Dike-related
structures
indicating
horizontal
dikepropagation
in eastern
Ramon[Baer,thisissue].
(a)
Bedding-parallel
segment
boundaries
in theJurassic
ArdonFormation.
(b) Fingerprintsandgroove.
molds
along quartziticwalls in the JurassicInmar Formation.
BAER AND RECHES:EMPLACF2VIENT
MECHANICS OF DIKE SYSTEMS,ISRAEL
11,899
BASALT
:i.•-:"..
:.,'.'•".'::TRACHYTE
(•)
1,2,3
INTERSECTION POINT
INTRUSIVE
PHASE
Fig. 3. Intrusivephasesin the Ardon area,easternRamon. Dike thicknessis not to scale;circlednumbers
mark someof the observedintersections
betweenradial dikesand the NW trending"dike crosser"[seeBaer,
1989].
mechanicsof the horizontal propagationof an individual dike
in the Ramon area under uniform tectonic stress, and (3) the
developmentof the radial dike systemunderuniform tectonic
stressfield, uniform driving pressurewithin the dike, and an
axisymmetricradial stressfield. The first step is discussedin
detail by Baer [this issue]and will not be repeatedhere. The
presentanalysisis devotedto the secondand third steps.
We
envision
that dikes
observed
in the field
initiated
as
small fracturesfilled with magma, that grew in length, height,
and thicknessdue to the internal magma pressure.A dike could
grow from initial to finite dimensions if the following
conditionsare satisfied:(1) the internalmagmapressurewithin
the dike exceedsthe remote stressesperpendicularto the dike
plane, (2) duringquasi-staticpropagation,the driving pressure
of the dike, which is the differencebetweenthe magma pressure
and the remotestresses,equalsthe elasticresistanceof the host
rock, and (3) the stressesat the tip of the dike exceed the
strength of the surrounding host rock. In the following
sectionswe will analyze these conditions which control the
dike growth and propagation.
Thedrivingpressure.
Thedriving
pressure
of a dike,Pd,is
differencebetween the lithostaticpressureat depth D (the top
of the magma chamber) and the pressurein the column of
magmabetweenthe depthD andZ:
Ph= PrgD- Prn
g(D- Z)
(2)
wherePrnis themagma
density.
Overpressure
of magma
chamber
Pro:Theamount
bywhich
the pressure in the magma chamber exceeds the lithostatic
pressure.This pressurecould be generated,for example, due to
differentiation within the magma chamber which leads to
oversaturationin water at the chambertop [Blake, 1984]. The
overpressure can be evaluated indirectly from the other
parameters of the analysis [Reches and Fink, 1988](also see
below).
Viscous
pressure
dropPvis: Thisparameter
reflects
the
viscous resistanceto flow of the molten magma within the
dike. At a horizontaldistancel from the magmachamberand at
depthZ (Figure7) the viscouspressuredropis
Pvis
=APvis
(/2+(D-Z)2)
•/2
(3)
defined as the amount by which the internal magma pressure
within the dike exceeds the remote compressive stress
whereAPvis is thepressure
dropperunitdistance.
APvis is
themagma
chamber,
Pvisistheviscous
pressure
dropandSh is
Tectonic
stress
perpendicular
tothedikewall,Sh: Since
the
more or lessuniform over the centralpart of the dike but varies
perpendicular
tothedikeplane.
?d canbepresented
asthesum significantlynear the tips. The viscouspressuredrop may be
of the stresscomponentswhich act on the dike wall:
calculateddirectly when dike thickness,magma viscosity, and
Pd= Ph+ Prn-Pvis-Sh
(1) flow velocity are known. The pressuredrop was previously
estimated indirectly for certain field cases to be up to 0.75
where
Ph isthehydrostatic
stress,
Pmis theoverpressure
of MPa/km [Rubin and Pollard, 1987; Rechesand Fink, 1988].
Ramon dikes intrudedself-generatedfractures[Baer, 1989], it is
Forhostrockwithuniform
density assumedthat they developed normal to the axis of the least
the tectonicstressperpendicularto the dike wall.
Hydrostatic
pressure
Ph:
Pr' thehydrostatic
pressure
at depthZ (Figure
7) equals
to the compressive
stress,
{•3'Theabsence
of regional
faulting
during
11,900
BAER AND RECHES:EMPLACEMENTMECHANICSOF DIKE SYSTEMS,ISRAEL
Pe= W/H[[.t/(1-v)]
(5)
where v is the Poisson ratio and I.t is the shear modulus
[Weertman, 1971; Pollard and Muller, 1976].
Dike propagation involves brittle fracturing of the host
rock at its tips. A dike propagateswhen the stressescloseto its
tip are high enoughto overcomethe fractureresistanceof the
countryrock. The stressfield near the tip of a dike is given by
the equation
c•
ij=Kt•j(
O)/(2rcr
)l/2
I
(6)
where
(•ij arethestress
components,
KI isthemode
I
(tensile)
stress
intensity
factor,
fij(O)aregeometric
functions
I
I
of the angle0 aroundthe tip, and r is the distancefrom the dike
!
I
tip.Thestress
intensity
factorKI depends
on thegeometry
of
I
the dike and on the distributionof the driving pressurealong it.
Assuminga two-dimensionaldike, the stressintensityfactor at
its tip is
I
I
0
JO 20m
(a)
KI=Pd(El)l/2
(7)
where l is the dike half length [Lawn and Wilshaw, 1975]. A
dike will propagatewhen the stressintensity factor at its tip
300m
exceeds
thefracture
toughness
of thehostrock,Kic.Thestress
intensity factor at the tip of the dike could vary due to the
difference in the mechanical properties of the host rocks
intrudedby the dike or to the existenceof beddingplanes[Baer,
this issue].
The Horizontal Propagationof the RamonDikes
IIIV /
/
,
J
The propagation direction of a dike dependsupon the
magnitudeof the stressintensityfactor at the differentparts of
the dike. Rubin and Pollard [1987] studied horizontally
propagating bladelike dikes in volcanic rift zones. They
showed that several factors, such as the in-plane radius of
curvature of the dike tip and the proximity to the Earth's
surface,
yielda smaller
KI atthedownrift
tipthanatthetopor
V////////////A
bottom tips. Therefore they concluded that the horizontal
growth of these dikes requiresa nonuniformdriving pressure
Fig. 4. Field relations between dikes and joints, Ramat Saharonim,
easternRamon. Dike thicknessis not to scale. Notice the relatively
high abundanceof joints closeto the dike.
distribution
to compensate
thesegeometric
effects
onKI. In
this section we analyze the horizontal propagation of the
Ramon dikes by solving the driving pressureequation for
variousdepth levels and for different distancesfrom the central
intrusion.
the period of dike intrusion suggeststhat the stressstate was
not controlledby the rock strengthlimit [e.g., Rubin, 1990]
and that the deviatoric stresswas relatively small. Furthermore,
the equal abundanceand contemporaneous
intrusionof sills and
dikes imply that the least horizontal stressdoes not deviate
significantlyfrom the vertical stress,namely,
Sh=O3= prgZ
(4)
Depth of intrusion. The bedding-paralleldike segments
which are containedwithin distinctlayers in the well-stratified
Ardon Formation (Figure 2a) and the subhorizontalfingers in
the porousInmar Formation(Figure 2b) indicatethat the radial
dikes propagatedin a predominantlysubhorizontalnorthward
direction from an unexposedsourcesouth of the Ramon fault
[Baer and Reches, 1987]. The Ramon dikes intruded midTriassic to mid-Jurassic rocks; the dikes are less abundant and
The Ramonradial dike patternalso revealsthe natureof the becomenarrow in the early TriassicGevanimFormationand as
stressfield; for example, a distortedradial dike systemcould they climb into the mid-Jurassic Mahmal Formation [Baer,
manifest the superpositionof axisymmetric, radial stress and 1989]. It appearsthat most dikes did not continueupward
uniform tectonic stress state [Muller and Pollard, 1977]. The above the Mahrnal Formation. As neither the tops nor the
tectonic stress associated with such distortion will be discussed
bottomsof the Ramondikescouldbe observedandthereis yet
no geophysicalevidencefor the dike height, the dike height
Hostrockresistance.
Thedriving
pressure
of thedike,Pd, can only be estimatedfrom the propagationdirectionsand dike
is the pressure available to push the dike walls apart, thicknesschangesto be between1 km and 3 km.
separately.
Stratigraphic evidence in the Ramon area indicate that a
overcoming
the elasticresistance
of the hostrock,Pe'
Assuming a uniform resistancealong the entire dike with an Jurassicsequenceof approximately450 m was eroded from
averagethicknessW and heightH, the elasticresistanceis
above the Mahmal Formation after the Early Cretaceous
BAER AND RECHES:EMPLACF3/IENTMECHANICS OF DIKE SYSTEMS,ISRAEL
+
'
11,901
•
.-A
R'DO
N F"?M...,
.......'
DO'L;O•..
MI'T'":E:
';).
!"A..
'"S&"/iH..-'.A'
:L.
E.
L':tM:I'T
Fig. 5. Field relationsbetweena dike and faults, coordinates 1408/0032, easternRamon. The faults are
either parallel or at a low angle to the dike.
intrusivephase[Garfunkeland Derin, 1988]. Thus, as the roofs densityis 2.36 + 0.21 g/cm
•. The densityof basement
rocks
of most
dikes
are within
the Mahmal
Formation
and the
underlying Inmar Formation, the dikes were 450 m or more
below the paleosurfaceduringtheir intrusion.
The depth of the magma chamber is more difficult to
determine. The subhorizontalpropagationdirection of the
dikes and the relatively high abundanceof sills in the mid-
varies
from2.45g/cm
3 to2.66g/cm
3,withmean
valueof2.55
+ 0.07g/cm
3. Depth
corrections
forcompressibility
areminor
and were ignored.
Magma density Pm may vary considerably
with
composition, water content, vesiculation, and temperature
(Table2) [Muraseand McBirney,1973]. The densityof magma
Triassicunits suggestthat the top of the magmachamberwas in the Ramon dikes cannot be measured directly, and it is
alsorelatively shallow,and a depthof 1000 m is chosenfor the estimatedby the procedurepresentedbelow.
present analysis. However, since there is no evidence for the
The driving pressure-depth dependence. Consider a dike
nature of magma flow close to the inferred source, the which propagates from a source at a given depth under a
possibility of a deeper chamber with vertical flow at its lithostaticstressstate. The dike would propagatehorizontally
proximity,becomingprogressivelyless steepwith decreasing and would attain a bladelike shapeif the driving pressurenear
depth,cannotbe ruled out.
its centeris larger then the driving pressureat its roof and base.
Densityof host rocksand magma. The densitiesof the host The required depth distributionof the driving pressurewhich
rocks,
Pr' weremeasured
in thelaboratory.
Fifteen
samples
of could generate such horizontal propagation will now be
the Mesozoicsedimentary
coverandthe Precambrian
sequence evaluatedby using the parametersof Table 2 and the driving
(also sedimentary)were collectedon the surfaceand from core pressureequation(1).
samples from depths of 0-3.7 km in the Ramon 1 borehole
The drivingpressureis calculatedfor threedifferentmagma
(Figure 8). The densityof the sedimentarycover rocks varies densities
(Figure
8a):Pm=2.65g/cm
3,abasaltic
magma
with
from1.88g/cm
3 in theunconsolidated
sandstone
of theInmar densitylarger than the mean densityof the entire stratigraphic
Formation
to2.58g/cm
3 inTriassic
limestone
(Figure
8);mean section
atRamon,
Prn= 2.5g/cm
3,anandesitic
magma
heavier
11,902
BAER AND RECHES:EMPLA•NT
/
.../
[
•
MECHANICSOF DIKE SYSTEMS,ISRAEL
Weil, 1970].
/
/
/!
-
The Ramon intrusions could well fit into this
category [Role', 1987].
The othercomponents
of the drivingpressure(equation(1))
are not depthdependentand can now be constrained
by someof
the field observations.For a dike propagatingto a distanceof 1
km away from the source,any viscouspressuredropbetween0
I
and 0.75 MPa/km would allow horizontalpropagationand
bladelikeshapeif the magmaoverpressure
wouldbe lessthan 2
MPa (stippledareaof Figure8a). Highervaluesof overpressure
would yield dike eruption at that distance.However, the dikes
continuedto propagateas far as 15 km away from their source,
where viscous drops of above 0.1 MPa/km would result in a
negative driving pressureat all depths (Figure 8b). This
-----3
difficulty cannotbe resolvedby chosinga highervalue of
overpressure,
sincethat would yield near-sourceeruption.It
couldbe resolvedif the viscouspressure
dropis lowerthan0.1
I
z
o
MPa/km,enablinghorizontaldikepropagation
to 15 km away
from the sourcewithoutnear-source
eruption.In computing
this valueit hasbeenassumed
that dike propagation
wasnot
limitedby solidificationalongits course.Thus the valueof 0.1
MPa/kmplacesan upperlimit on the viscouspressure
drop.
The estimatedmagma overpressure
for the Ramondikes is 1
MPa,yet it hasnotbeencalculated
norverifiedindependently.
The tectonicstresses.The horizontalpropagation
of the
Ramon dikes has been shown to be sensitive to the vertical
gradientsof the lithostaticstress,causedby the density
stratification
of the hostrocks.The maximumdrivingpressure
dueto thiseffectis between1 MPa and2 MPa at a depthof 2.4
km (Figure 8). It has been assumedthat the least horizontal
stressdid not deviate significantly from the vertical stress
(equation (4)). A reasonable tectonic deviatoric stress, if
LEGEND
existed, could have completely dominated the pressure
Dolomite
Shale
Sandstone
L•mestone
For example,if normal
(pJsolHJc) gradientsdue to the densitycontrasts.
faults were active, the stress state would be near the rock
sill j• Basaltic
trachytlc
d,ke JJ
II Inferred
dike •JFault
Basaltic
or
strengthlimit, and the least horizontal stresscould be as low as
50% of the verticalstress[e.g.,Zobackand Healy, 1984].In
Fig.6. A schematic
cross
section
in theArdonFormation
showing
two that case, magma pressurewould significantlyexceedthe
of thesegmented
dikeswhichareexposed
in NahalArdon,eastempressurein the host rocksboth in the cover and in the
Ramon. In the two dikes, small faults extendfrom the segmenttips
into the overlappingzoneswith adjacentsegments[Baer, this issue].
basementyielding downwardpropagationof the dikes [see
Rubin, 1990]. Thus the field evidence for horizontal dike
propagation together with the measured host rock densities
alsosupportthe lack of significanttectonicstresses.
than the averagesedimentarycover but lighter than the average
basement
rocks,
andPrn
=2.3g/cm
3, arhyolitic
magma
which
is less dense than all host rocks at Ramon. For each density
the driving pressure is calculated for two levels of lateral
viscous pressure drop in the dike: no pressure drop and
relatively high viscouspressuredrop of 0.75 MPa/km [Reches
and Fink, 1988]. The driving pressure is calculated for
horizontal
distances
of
1 km
and
15 km
from
the
central
COUNTRY
D
ROCK
Pr
intrusion(Figure 8b). All curvesof the drivingpressureshow a
"kink"
at2.4kmdepth
which
corresponds
tothedifferent
densities
inthebasement
andinthesedimentary
cover.
Figure8a indicates
thatthelessdensemagma(Pro 2.3
g/cm
3)would
rise
upward,
whereas
the
most
dense
magma
will
propagatedownwardregardless
of the viscouspressure
drop.
Themagma
with
intermediate
density
willpropagate
horizontally
(the
stippled
zone
inFigure
8a),
and
form
a
bladelikedike. Thus in order to allow a horizontalpropagation
for the Ramon dikes the magma densityshouldhave been about
2.5g/cm
3. Thisdensity
istypical
forandesitic
magma
[MuraseFig. 7.
A schematicrepresentation
of a dike in the Ramon area. The
and McBirney, 1973], but water-rich or slightly differentiated parametersshownhere are used for the calculationof the driving
basaltic magma could also attain such density [Bottinga and pressureand are describedin the text.
BAER AND RECHES:EMPI•CFA/IENT MECHANICS OF DIKE SYSTEMS,ISRAEL
-JO Pa]eosurface -5
0
11,903
•o
5
Density
I
I
•Assumed top of dikes
•
I
\
t. 88
I
I
2 56
2.32
/
0 MPa/km
,
I
AP,.=
0.75
MPa/km
I
2.2t
I
2.51
2.58
• ooo
Assumed top
of magma source
2.49
/
J500
/
2.50
/
2.22
2 65
iI
iI
/
//
2500
?
I
I
/
/
/
%/
2.45
)00
Z (m)
I
1
I
I
I
I
1
I
I
4000
I
1
1
1
1
1
/
I
/
/
/
I
300
/
•
/
Fig. 8. The drivingpressure-depth
relationship
(equation(1)). The reconstructed
stratigraphic
sectionandthe
measureddensitiesare shownin the left columns. Precambrian:PCz, Zenifim Formation. Triassic:Trmr, Raaf
Formation,Trmg,GevanimFormation,Trms,Saharonim
Formation,Trmm,Mohila Formation.Jurassic:
Jla,
Ardon Formation,Jli, Inmar Formation,Jmm, Mahmal Formation,Juz, Zohar Formation,Jub, Be'er Sheva
Formation.
(a)Fordifferent
ty!•es
ofmagma
withdifferent
densities,
1kmaway
from
thesource.
(b)Fora
magmawith densityof 2.5 g/cm , at a lateraldistanceof 15 km from the source.
Estimatesof the mechanicalproperties of the host rocks m and 2 km, respectively,and hostrockswith Poissonratio of
from dike geometry. Figure 8 showsdriving pressurevalues 0.25 and shear modulus of 25 GPa [Birch, 1966], the calculated
between 1 MPa and 2 MPa for a typical horizontally elasticresistanceis approximately16 MPa.
This value is significantlyhigher than the valuesof driving
propagatingdike, which is 15 km long. This pressureequals
pressure
of 1-2 MPa which are calculated above. A similar
the elasticresistance
of the hostrocks,Pe' thatcouldbe
was foundby Rubin and Pollard [1987] for the rift
calculatedfrom the dike geometry(equation(5)). If a uniform discrepancy
resistance and uniform driving pressure along the dike is zone dikes in Hawaii. They showedthat in order to accountfor
assumed,then for a dike with averagethicknessand height of 1 the observedgeometryof the dikes,the in situ shearmodulusof
11,904
BAERAND RECHES:EMPLACEMENTMECHANICSOF DIKE SYSTEMS,ISRAEL
tE)
d
Paleosunface
Density
•
I '
1.88
f• 2.56
2.32
el.
EZ
r._.
2,21
[-
o
2. õ8
-iO
%
AP,•,= 0 MPa/km
%
%
•Assumed
top
If dikes
500
A/•,=
O. ] MPa/km
%
AP,•,= O. 4 MPa/km
%
A/•,=
O. 75 MPa/km
%
%
Assumed
•ooo
•
Pro=2.5 gr/cm'
%
of magma Poe
%
%
%
%
•
%
2.49
500
'•
2.50
•
2.22
r._.
2.65
7000
0
2.45
z (m)
2.51
2..50
I
2.66
2.53
i
i
i
i
'
i
I
•5000
Fio
•.
8
.
I continued•
the host rocks shouldbe about one tenth of the laboratory derived a two-dimensionalmodel for an elastic, homogeneous
values; such low shear modulus values were measured in field
and linear isotropic medium, with a circular pressurizedhole
and a rigid boundary. This boundary accountsfor a major
Thus for these low values of shear modulus the estimated elastic
lithologic inhomogeneity in the host rocks west of the
resistance in Ramon is 1-2 MPa, in agreementwith the SpanishPeaks. The model also includes a biaxial regional
calculationsof the dike drivingpressurepresentedabove.
stressorthogonalto the rigid boundary. Muller and Pollard
[1977] modified the solution for regional stresseswhich are
not necessarilyorthogonalto the rigid boundary. The stateof
EMPLACEMENTOFTIlE RADIAL DIKE SYSTEM
stressduring the emplacementof the Ramon radial dike system
is resolvedfollowing the derivationsof Ode' [1957] andMuller
and Pollard [1977]. The rigid boundaryis omitted from the
Model
analysisas it doesnot fit the structurein the Ramonarea.
Ode' [1957] presentedan analysisof the stresses
associated
The solutionsare based on the following assumptions:(1)
with the radial dike systemof SpanishPeaks,Colorado. He The radial dikes propagatedhorizontally from a pressurized,
tests of large fracturedrock masses[e.g., Bieniawski, 1978].
BAER AND RECHES:EMPLACEMENTMECHANICS OF DIKE SYSTEMS,ISRAEL
11,905
TABLE 2. ParametersUsedfor Driving PressureCalculations
Parameter
Source
Magnitude
1 km
field
Length
15 km
field
observations
Thickness
1m
field
observations
Height
1-3 km
field
observations
1.8-2.55
g/cm
3
presentwork
2.6-2.83
g/cm
3
2.4-2.5g/cm
3
Muraseand McBirney [1973]
Depth of intrusion
Dike
observations
dimensions
Density
ofhost
rocks
Density of magma
Basaltic
Andesitic
Rhyolitic
2.2-2.3
g/cm
3
Elastic properties
Poisson ratio v
0.25
Birch [ 1966]
Shearmodulusg
25 GPa
Birch [ 1966]
Viscouspressure
drop
0-0.75 MPa/km
Reches and Fink [ 1988]
Mag•na overpressure
1 MPa
Rubin and Pollard [1987]
estimated
vertical, cylindricintrusion. This assumption
is basedon the
field observationsof dike propagationdirections[Baer and
Reches, 1987]. (2) The dikes trend perpendicularto the local
least compressivestress. This is supportedby the dikesjoints-faultsrelationsdiscussed
above. (3) The regionalstress
field in the Ramon area did not changesignificantlyduring the
intrusion
of the radial dikes.
were evaluatedin steps. First, stresstrajectorieswere computed
for a widerangeof parameters.
Thetrendsof (51 in these
solutions indicate the theoretical trends of dikes predicted for
the given parameters. Second, the trends predicted by the
model were compared with the trends of all dike segments
digitized from the geological map. The mean mismatch angle
betweenthe observeddike trendsand their predictedtrendswas
calculated. Using this mismatch angle and additional field
observations
StressTrajectory Maps
We determine
the principalstresses
c•1 andc•3 resulting
from the superposition
of two stressfields: a radial stressfield
associated
witha cylindrical
intrusion
of radiusr0 andinternal
pressureP, and a uniformstressfield at an angleqbto the E-W
here
discussed
below
we
searched
for
the
better
solutionsto evaluate the unknown parameters.
Results
The mismatch angles were calculated for the following
axis,withmaximum
andminimum
compressive
stresses,
S1 rangesof the unknownparameters:(1) The normalizedtectonic
andS3 respectively
(Figure9). The calculated
principal shearstress
A = (S1 - S3)/P,0 <A < 0.1 in increments
of 0.01;
stressesform an angle o• with the radial and the tangential (2) Theradius
ofthecentral
intrusion
0.5km< r0< 5 kmin0.5
directionsin a polar coordinatesystem[Muller and Pollard,
1977; Figure 9] which is expressedby
tan2a = 2(5
r0/((500-(Srr)
(8)
km increments; (3) The regional stressorientations0ø < qb<
180ø in 10ø increments;
(4) An areaof 100km2 southof
Ramon
was scanned for the location
of the central intrusion.
Th• mismatch
angles
between
(51andthediketrends
vary
between 12.5ø and40ø in the differenttrajectorymaps. Many
where
solutionsyielded averagemismatch angleslower than 13ø, but
no single solution could be selected on the basis of the
mismatch angle alone. Figure 10 shows the possible
((500
- S3)/P= -(r0/r)
2+Asin
2(0-qb)
(Orr- S3)/P= (r0/r)2
+Acos
2(0- qb)
(9)
combinations
of thestress
ratioA, andtheintrusion
radiusr0,
that provide solutionswith mismatch anglessmaller and larger
than
13 ø.
Location and diameter of the central intrusion. In all
solutions with mismatch angles smaller then 13ø, the central
andA = (S1 -S3)/P,indicating
theintensity
of thetectonic intrusion is positionedin approximatelythe same location at
shear stressnormalized'by the pressurein the central magma coordinates145/997 (Israel network) (Figure 11). This site is
body. P is the sum of the hydrostatic pressure and the south of the Ramon fault in the downthrownblock. As only
overpressure
of the magmachamber.
Upper Cretaceousunits are exposedthere, we searchedfor a
The calculated stress trajectory maps depend on four geophysicalevidenceof a large igneousbody at depth of 1-3
unknown
parameters'
thenormalized
tectonic
shearstress,
(S1 km. The best indication for such rocks is an aeromagnetic
-S3)/P,theorientation
of thetectonic
stress,
qb,theradius
of anomaly, interpretedby Domzalski [1967] as a dipole body 1
thecentral
intrusion,
r0, andits location.Theseparameterskm deep and 3 km in diameter. This anomalywas recently
(SrO/P= -A sin(0- •) cos(0 -
11,906
BAER AND RECHES:EMPI•CEME•
MECHANICSOF DIKE SYSTEMS,ISRAEL
Fig. 9. The directionsof the principal stressesarounda cylindric body in a regional stressfield [after
Johnson, 1970]. See text for further details.
reexaminedutilizing both a portablegroundmagnetometer
and
namely, the regional tectonic stress generated only a
an airborne magnetometer,and Domzalski'sobservationsand negligible distortion of the radial stress field. Further,
interpretations
wereconfirmed(Gvirtzmanet al., manuscript
in solutionswith small mismatchanglesare obtainedfor almost
preparation,1991). However,the magneticanomalyis located any direction of the regional stresses,and from the model it is
3 km southwest of the inferred central intrusion and no
impossibleto unequivocallyselecta direction(Figure 11).
anomaly was detected at the site of the inferred intrusion
The trend of the tectonic stress field can be best estimated
(Figure 11). This 3 km offset doesnot necessarilyindicatea from theNW-SE dike system,whichwascontemporaneous
with
secondbody. On the contrary,it is possiblethat the observed the radial system(see above). The NW-SE systemindicates
anomaly is indeed associatedwith the central intrusion of the thatthetrendof S1 wasabout
NW duringtheemplacement
of
radialsystembut wasdisplaced
3 km aspartof thefight-lateral thesedikes. The propagationdirectionsof thesedikes are not
slip alongthe Ramonfault (seeBartov [1974] and discussion). clear, and their source could be a different, more distant one.
If the aeromagnetic anomaly represents the central Thesedikesmayhaveintrudedin periodsof temporary
decrease
intrusion,then the diameterof the intrusionis approximately
3 of the magmaticpressurein the sourceof the radialsystemand
km. This is a reasonablesize when comparingthe Ramon thus could indicate the prevailing regional stressfield. We
radial system to other radial systemsin which the central therefore
prefera solution
in whichS1 trends
N50øW.This
intrusions
areexposed.For example,
in the Spanish
Peaks stressdirection also conformswith the commonalong-strike
dike system,whichis largerthanthe Ramonradialsystem,the trend changesobserved in several dikes, from the radial to a
diameterof thecentralintrusionis approximately
5 km [Muller morenorthwesterlytrend(Figures1 and 11).
and Pollard, 1977], whereas in 'the Dike Mountain radial
The comparison between the dike trends and the stress
system [Johnson, 1968], which is slightlysmallerthan the trajectorymaps is only approximate,becauseit predictsthe
Ramon system,the centralintmsion'sdiameteris about2 kan.
dike trend without accountingfor the existenceof the dike
itself. A radial dike will probably tend to maintain its trend
The tectonicstressfield. Stresstrajectorymaps with small
furtherawayfrom the sourcethanpredictedby themodeldueto
mismatchanglesare obtainedfor a wide rangeof stressratios
the influenceof dike-inducedstresses
at its tip (equations(6)
(Figure 10); however, for a 3 km diameter of the central
and (7)). Thus the comparisonbetweendike trendsand stress
intrusion, the stressratio must be very low,
trajectoriesprobably leads to underestimationof the effect of
0 < (S1 - S3)/P< 0.02
(10) the regional stress.
BAER AND RECI-IF•:EMPLACF2VIENT
MECHANICSOF DIKE SYSTEMS,ISRAEL
11,907
Tectonic
Implications
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
- + Of<J3 ¸
-
+
+
-
+
+
+
_
The "reversal" of the tectonicstressregime. Freund et al.
[1975] suggestedthat a transitionbetweentwo different stress
regimesin the region of Israel occurredduring the Mesozoic.
The early Mesozoic is characterizedby NE-SW trendingnormal
faults and fault-bounded basins, indicating NW-SE extension.
The Late Cretaceous is characterized by NE-SW trending
flexures and open folds, indicating NW-SE compression.
Freund et al. [1975] identified this transition in subsurface
stratigraphyand seismicprofiles and regardedit as "reversal"
of the tectonic
O. 02
O. 04
O. 06
O. 08
O. J
structures.
The details of this reversal
until as late as the end of the Turonian.
Fig. 10. Possiblecombinationsof stressratiosand radii of the central
intrusion, which yield trajectory maps with mismatch angles higher
and lower than 13 o
DISCUSSION
Distortion of the Radial Dike System
The radial dike systemin Ramon appearsin generalto fit a
radialpattern,andthe derivedstresstrajectorymaps(Figure10)
yield averagemismatchanglesas small as 12.7ø. However,it
seems that the main source for the mismatch are two restricted
locations:Afor and Bab-Ramon areas(Figure 11). These local
deviationscould be generatedby severalmechanisms.
One possibility is the operationof two (or more) magma
sources. The radial systemincludesat least two differentrock
types, basalts and trachytes, and possibly an additional
intermediate type [Baer, 1989]. Different sourcesfor the
basaltic and for the trachytic magmasmay have been closely
spaced, as in the Spanish Peaks [Johnson, 1968; Smith,
1987]; in such case the contributionsof the different sources
couldhavebeen affectedby differenteffectiveradii or magmatic
pressures.
Another possibility is temporalvariations in the tectonic
stressfields. The dike intrusion period in Ramon lasted 10
m.y. or less (Table 1), during which either the tectonicor the
magmaticstressescould change. As the shearstresseswere low
(see above), even a slight changein the relative magnitudesof
the principal stressescould have also resulted in changesin
their trends. The directions of the regional principal stresses
could also change, as inferred for the Spanish Peaks area
[Muller, 1986]. Changes in the magmatic pressure in the
central intrusion would also lead to variations in the geometry
of the radial system.
A third possibility is the inhomogeneityof the host rocks.
It was assumed that the host rocks behave as a linear elastic,
isotropic, and homogeneousmedium. There is no apparent
evidence for inhomogeneity of the exposed host rocks;
however, more than half of the system, including the central
intrusion,is not exposed. Thus different material propertiesin
the southernhalf of the system could have affected the stress
field.
are not
fully understood. Reches et al. [1981] suggested that the
displacementsalong deep-seatedreverse faults in southern
Israel agree with a reversal model. Folkman and Brunner
[1987] analyzedseismicdata in southernIsrael and claimedthat
the senseof displacementalong severalfaults was not reversed
On the other hand,
Druckman [1981] argued that the early Mesozoic structures
differ from the Cenozoic ones only by the amplitudes;he could
not find
clear
evidence
for tectonic
reversal.
As the Ramon
dike systems reflect the stress conditions during the Early
Cretaceous,they may illuminate some featuresof the transition
period.
It was found here that the stressfield during the intrusionof
the radial systemwas predominantlyaxisymmetric,as expected
for a central intrusion with no strong tectonic stress (Figure
11). However, the radial systemis not the only one to intrude
the Ramon rocks during the Early Cretaceous.NW-SE dikes are
relatively widespread, and NE-SW dikes are restricted to the
northeasternpart of the Ramon (Figure 1). The dominanceof
the radial dike system suggeststhat the other two systems
indicate
minor
disturbance
of the local stress field rather
than
strong regional tectonic stresses.This interpretation implies
that the NW-SE extensionalstressesof the early Mesozoic were
not active during the Early Cretaceous,while the younger,
compressionalstressfield was too weak to significantly effect
the radial dike system. Thus the 10 m.y. (or less) period of
intrusive magmatismof Early Cretaceousoccurredbetween the
extensionaland the compressionalstressfields.
A slightly different interpretation could be that the actual
transitionbetween the two tectonicphasesoccurredduring the
Early Cretaceous. During this transition the effects of both
phasesmay be recognized: The older extensionalphase is
representedby the poorly developed NE-SW dikes, and the
younger compressionalphase is representedby the better
developedNW-SE dikes. Even in suchcase,the regionalshear
stressesduring the intrusionof both of thesedike systemswere
too weak to significantly distort the radial one. This
interpretation, in contrast to the first one, implies that the
transitionof tectonic style occurredover the relatively short
period of magmatic activity.
Lateral displacementalong the Ramonfault. A 3 km offset
appearsbetweenthe inferred locationof the central intrusionof
the radial system and the magnetic anomaly south of the
Ramon [Domzalski, 1967] (Figure 11). This offset may be
interpretedin several ways:
1. The magnetic anomaly representsthe central intrusion,
but our model predictionsdo not indicatethe correctlocationof
the intrusion. This seemsunlikely becausethe location of the
central intrusion appearsto be well constrainedby the model
solutions.
2. The magnetic anomaly representsa different igneous
body, intrusive or extrusive,rather than the central intrusion of
11,908
BAERANDRECHES:
EMPLACEMENT
MECHANICS
OFDIKESYSTEMS,
ISRAEL
r'0-
7.5
km
• ,'5
/
/
,\ ,t
r'o- 2.5
•$1-53)/P
20ø' •
/
,,\'•
krn i
/
Hoe At/don
/
= o.
!
I
!
,
lab
Ramon
•omon
F A'jL T
Fig.11. Stress
trajectory
maps
compared
totheRamon
radial
dikes,
fortwostates
ofstress:
(a)r0= 1.5km;A
= 0.01'q)= N40øW
and(b)r0= 2.5km;A= 0.03;
q)= S70øW.
the radial system. For example,this could be a buriedvolcanic
structuresimilar to those exposedin Giv'at Ga'ashor in Karne
Ramon (Figure 1), which are composedof several Early
Cretaceousbasalticflows and are youngerin age than the dikes
[Lang et al., 1988]. This interpretationis rejecteddue to the
modeling of the magnetic anomaly which indicates 1000 m
depth to the top of the igneousbody [Domzalski, 1967;
Gvirtzman et al., manuscriptin preparation,1991], whereas
the anticipated depth to an extrusive basalt flow should not
exceed500 m [Zak, 1968].
3. The magnetic anomaly indeed marks the sourceof the
radial dikes, and the modelpredictionsare alsocorrect. In this
BAERANDRECHF_3:
EMPLACEMENT
MECHANICSOFDIKE SYSTEMS,
ISRAEL
case, the offset between the anomaly and the calculatedcentral
intrusion indicates3 km of right-lateral displacementalong the
Ramon
fault that occurred
after the intrusion
of the radial dikes.
11,909
REFERENCES
Baer, G., Igneous intmsions in Makhtesh Ramon, Israel: Mechanics of
emplacement and structural implications, Ph.D. dissertation (in
Hebrew, with English abstract), 93 pp., Hebrew Univ. Jerusalem,
If correct, this explanationprovidesnew independentevidence
1989.
for the sense and amount of displacement along the RamonBaer,
G., Mechanisms of dike propagation in layered rocks and in
Minshera fault in the Ramon area. The present estimate of 3
massive,porous sedimentaryrocks,J. Geophy.Res., this issue.
km of displacementis in good agreementwith the observations Baer, G., and Z. Reches,Flow patternsof magma in dikes, Makhtesh
of Bartoy [1974] in Gebel Minshera area, along the western
Ramon, Israel, Geology, 15, 569-572, 1987.
extension of the Ramon fault, about 150 km west of the Ramon
Baer, G., and Z. Reches, Doming mechanisms and structural
development of two domes in Ramon, southern Israel,
area. About 2.5 km of right-lateral slip were determinedthere
Tectonophysics,166, 293-315, 1989.
from the displacement of the Iktepa dike of Miocene age
Bartov, J., A structural and paleogeographicalstudy of the Central
[Bartoy, 1974]. Our model contradictsthe previous structural
Sinai faults and domes.Ph.D. dissertation,(in Hebrew with English
estimates,which suggesteda significantlysmallervalue (< 100
abstract)143 pp., Hebrew Univ. Jerusalem,1974.
m) of horizontal displacement along the Ramon fault in the Bentor, Y.K., Magmatic intrusionsand lava-sheetsin the Raman area
in the Negev (SouthernIsrael), Geol. Mag., 89, 129-140, 1952.
Ramon area [Garfunkel, 1964].
Bieniawski, Z.T., Determining rock mass deformability: experience
from case histories (abs.), lnt. J. Rock Mech. Min. Sci. Geomech.
SLIMMARY
Abstr. 15, 237-247, 1978.
Dike emplacementin the Ramon area, southernIsrael, has
been studied using three different techniques: (1) Dike
propagation directions were determined by outcrop-scale
observationsof dike segmentsand fingers. (2) The mechanics
of dike propagation was evaluated in terms of the driving
pressureat various depths for a typical Ramon dike. (3) The
state of stressduring the intrusion of the predominantradial
dikes in Ramon was analyzed by comparing the radial dike
pattern to a two-dimensional elastic model for a pressurized
cylindrichole in a regionalstressfield [Ode', 1957; Muller and
Pollard, 1977].
The Ramon dikes are exposed within a well-stratified
sequenceof Triassic and Jurassicsedimentaryrocks and have
most likely intruded underlying rocks of Precambrian to
Triassic age as well. Detailed examination of dike-related
structures in two Jurassic units [Baer and Reches, 1987]
indicated horizontal propagationfrom an unexposedsource at
the south. This propagationdirectionis best explainedby the
density stratification of the host rocks in the upper few
Birch, F., Compressibility;elastic constants,in Handbookof Physical
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Acknowledgments. The authorsare grateful to Z. Garfunkel, Y.
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duringthe course
of this study.A. Rubin and an anonymousreviewerare thankedfor their
thorough reviews which significantly improved this paper. The
technical assistanceand the graphic work of S. Levy are greatly
appreciated.Z.R. was partially supportedby the U.S.-Israel BSF grant
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(Received June 25, 1990;
revisedJanuary29, 1991;
acceptedJanuary31, 1991.)