JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B5, PAGES 8645-8663, MAY 10, 2001
A new model for the evolution
of the volcanic
island
of R6union (Indian Ocean)
Jean-Francois
L•natandBarbara
Gibert-Malengreau
1
Universit6 Blaise Pascal-CNRS, Clermont-Ferrand, France
Armand
Galddano
Laboratoirede G6omagn6tisme,
Institutde Physiquedu Globe de Paris,Paris,France
Abstract. The islandof R6unionhasbeenstudiedusingdatafrom airborneand
shipborne
magneticsurveys.The subaerialhistoryof R6unionspansthe last2.1 million
years.The Brunhes-Matuyama
geomagnetic
reversalenablesdifferentiationof volcanic
rocksolderandyoungerthan0.78 Ma. The lowersubmarine
flanksarepoorlymagnetized
andare interpretedas landslidedeposits.The coreof the islandis composed
of highly
magnetizedrocks.PitondesNeigesvolcanois composed
mostlyof rocksolderthan0.78
Ma. Only its westernflank andcentralareaincludethickpilesof youngerrocks.Pitonde la
Fournaiseis a highlyandnormallymagnetizededifice,but its northernandeasternflanks
areunderlainat shallowdepthby reverselymagnetized
formations.
The latterareregarded
asremnantsof Les Aliz•s volcano,associated
with the Grand-BrOl•hypovolcanic
complex.
At the Matuyama-Brunhes
transitionthe islandwascomposed
of at leasttwo main
volcanoes(Piton desNeigesandLes Aliz•s) andperhapsalsoof a thirdold volcano
(Takamaka)at the center-noahof the island.Pitonde la Fournaisegrowson the flank of
Les Aliz•s andis a relativelyyoungfocusof volcanism.Thesevolcanoeshavehad
successive
phasesof construction
anddestruction.
The analysisof magneticanomaliesover
R6unionwasdecisivein defininga new coherentmodelfor theevolutionthe island.
1. Introduction
Oceanic
islands
and Gee et al. [1988] for studiesof JasperSeamountin the
and seamounts
are volcanic
constructions
that usually evolve over of a few million years and are
controlledby a complex interplay of geodynamicfactors (hot
spot displacement,regionaltectonicstresses,and variationsin
magma production and magma composition) and volcanic
processes(intrusion,eruption,formationof magma chambers,
calderasubsidence,and flank collapse).Usually, only the last
stagesof volcano developmentcan be observedand sampled
directly,leavingmostof their geologichistoryto be inferredby
Pacific Ocean).
We have studied the structure of R6union island, in the
Indian Ocean, using data from magnetic surveys. The
preponderanceof the thermoremanentcomponent of the
magnetizationof the volcanicrocksenablesto locateareasthai
are predominantlycomposedof formationseither youngeror
older than the Brunhes-Matuyamageomagneticreversal(0.78
Ma). First, from the analysisof negative anomaliesnear the
seashore,we demonstratethe presenceof reverselymagnetized
formations
indirect methods.
To overcomethis inherentdifficulty, two main approaches
are commonly used. The first involve study of a group of
edifices showing different stages of evolution. The best
illustrationis providedby the Hawaiian-EmperorChain, which
exhibits
a succession of more than 100 volcanoes at different
stagesof evolution, from a young,actively growing seamount
(Loihi) to old, extinct,eroded,and sunkenseamounts.
Syntheses
based on observationsat the different sites allow general
evolutionary hypothesesto be derived [e.g., Peterson and
Moore, 1987]. The secondapproachusesgeophysicalmethods
to image the internal structureof a given seamountor islandin
order to infer
its structural
evolution.
Numerous
and various
beneath the two subaerial volcanoes of the island.
Next, we delineate the main structurescorrespondingto
volcanicconstructions
before and after the Brunhes-Matuyama
geomagneticreversal. We then derive a coherentmodel for the
evolutionof the island by integratingthe magneticresultswith
other geologicaland geophysicaldata. The resultinggeologic
synthesisleads us to (1) reconsiderpreviousevolution models
for Piton des Neiges and Piton de la Fournaisevolcanoes,(2)
documentthe presenceof a third major volcano (Les Aliz6s)
and to reinforce the gravity and seismic evidence for the
presenceof a fourth volcano(Takamaka),(3) delineatemajor
sourceareasfor large-scalemass-wasting
eventsduringgrowth
of the island, (4) propose a possible explanation for the
occurrence of the shallow eastward-directed
geophysicalstudies have thus been carried out, mostly on
islandsbut also on seamounts(see,e.g., Hammer et al. [1994]
landslides of Piton
de la Fournaise,and (5) explainthe developmentof the presentday rift zones of Piton de la Fournaise and the westward
migrationof its center.
•NowatPyros,
Cournon
d'Auvergne,
France.
Copyright2001 by theAmericanGeophysical
Union.
2. Geologicaland Structural Setting
Papernumber2000B900448.
R6unionisland is locatedin the southernmost
part of the
MascareneBasin (Indian Ocean),800 km eastof Madagascar
0148-0227/01/2000B900448509.00
8645
LI•NATET AL.' NEWMODELFOREVOLUTIONOFRI•UNION
8646
45 ø
i
50 ø
!
55 ø
i
INDIAN
OCEAN
15,
MAURITlUS
O 20•
20
120-
40 km
LA REUNION
25 ø
45ø
50ø
5•o
100'
40-
-20 ÷
-4O
80
100
120
140
160
180
200
220
240
Easting (Km)
Figure1. Map showing
thetopography
of theislandandsurrounding
seafloor(courtesy
of Philippe
ßLabazuy).
Mapgridbased
onGauss-Laborde
R6union
projection.
Contour
interval
is 100m.Accuracy
of the
mapvaries,with zonescovered
by detailedconventional
or multibeam
bathymetry
showing
greaterdetail.
Inset is location of R6union in the southwest Indian Ocean.
(Figure 1). R6unionvolcanismmaybe associated
with eitherthe
hot spotthatgeneratedthe DeccanTrappsanda volcanicchain
whose youngerstructuresare Mauritius and R6union islands
[Duncanet al., 1989] or, as proposedmore recentlyby Burke
[ 1996], to a youngerhot spot.
The islandof R6unioncomprisesonly a fractionof a much
larger volcanic system.Rising above a seafloorof Paleocene
age, the R6union edifice is a flattenedcone having a basal
diameterof 200 to 240 km andheightof about7000 rn (Figure
1). Recentseismicstudies[Charvis et al., 1999; Gallart et al.,
1999; de Voogd et al., 1999] have revealed remarkable
structuresin the interior of the edifice and underlying
lithosphere.The most notableof thesefeaturesare the virtual
absenceof crustalflexurebeneaththe islandandthepresenceof
an intermediate-velocity
layerat the crust-mantleinterface.This
underplatedbody, 140 km wide and up to 3 km thick, is
centeredbeneaththe southwestern
part of R6union.Agesof the
oldestsurficialrocks are more than 2.08 Ma old [McDougall,
LI•NAT ET AL.: NEW MODEL FOR EVOLUTION OF RI•UNION
8647
La :Monta
Massif
PITON
70-
DE
Cap -La
LA
FOURNAISE
E 60
-'-' 50o
z
40--
3O
PITON
DES
NEIGES
130
140
150
160
170
Easting (Km)
18.0
190
Figure 2. Shadedrelief mapof R6,unionIsland(illuminationfrom the NW) with locationof the mainplaces
discussed
in text. Coordinatesin kilometers(Gauss-Laborde
R•union projection).
1971], but becausethe subaerialpartof R•union comprises
only
3% of the volcanicsystem[de Voogdet al., 1999] (an estimate
does not include the volume of the underplatedmaterial,
estimatedby Charviset al. [ 1999]to be less.thanhalf thatof the
materialemplacedabovethe plate),the presenteffusiverateof
Vatin-Perignon,
1982;Ran{zon,
1982;Rocher,1988;Kieffer,
1990a1990b;Kiefferet al., 1993;Gillotet al., 1994;Lecotntre,
1992],no cleargeneralmodelhasyet beenestablished.
One
reasonof this failurelies in the complexityof the geological
pattern
developed
overmorethan2 Myr.of volcanic,
volcano0.34 m3 s-• [Lgnatand Bachblery,
1988]suggests
thatthe tectonic
activity,
andhighrates
oferosion.
Anbther
mainre//son
ß
growthof R•unionbegan- 7 Ma (theassumed
rateis therateof
exogenousgrowth;it doesnot includeendogenous
growthor
possiblevariationsof magmaproductionover the time). The
island itself is composedof two volcanicmassifs:Piton des
NeigesandPitonde la Foumaise(Figures2 and3).
PitondesNeiges,whichoccupiesthenorthwestern
two thirds
was the lack of knowledgeregardingboth the submarineparts
of the volcanoand its interior.In recentyears,new bathymetric,
gravity, and seismicdata have providedmuch additional
information. The bathymetry (Figure 1) shows that the
submarineflanks of Piton des Neigesare mostlycomposedof
huge landslidedeposits[L•nat and Labazuy,1990]. This
of the island, is a dormant volcano.Its oldestdated lava flows impliesthatPitondesNeigeshasbeenrepeatedly
affectedby
are 2.08 Ma basalts.Followinga basalticshieldbuildingstage, landslidesin different directionsduring its growth. The new
et al., 1999]showsthat
from before 2.08 Ma to after 0.423 Ma, Piton des Neiges gravitymapof theisland[Malengreau
erupted
alkaline
differentiated
lavasbetween
330 and12 ka ' thecenter
of PitondesNeiges
isunderlain
bya large,shallow,
[McDougall,
1971;GillotandNativel,1982;Denielet al., anddeeplyrooted(several
kilometers)
complex
of intrusions,
1992](Figure
3). Although
thestructural
evolution
of Pitondes gabbros
andcumulates.
Thisindicates
thatthegrowth
of Piton
Neigeshasbeenthesubject
of several
works[Chevallier
and desNeiges
began
in theearlystages
of R6union
volcanism,
with
LI•NATET AL.' NEWMODELFOREVOLUTIONOFRI•UNION
8648
Ages
Polarity
(•
Epoch event
Piton des
Piton de la
Neiges
Fournaise
Differentiated
":::••
•
Series
I
1.0
Jaramillo
--
Oceanite
Series
Olduwai
2.0
R6union
R6union
--
i
i
i
Kaena
3.0
--
normal
polarity
Mammoth
reverse
polarity
PITON
des
PITON
de la
FOURNAISE
NEIGES
I
a
::::....':.....'•
I
Differentiated
Series
Grand-BrQl•
Complex
(buried)
Oceanite
Series
Drill
hole-•
buried dense
complexes ß
--PN'
P/ton
des
Neiges
•½i
::::::::::::::::::::
TK'Takamaka
l':•::':J•i•
...........
- NNE ßnorth-northeasl
P/ton de la Fournaise
A
ß 3-> 360 ka
G 1 426-> 578ka
E
$
•, 950-> 2080 ka
ß::::::::::::'
•;................
:'.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.'.
::::::::::::::::::::::::::::
:::::::::::::::::::::::::::::::::::::::::::
.............
:*:
..................
'::::::::::::'
'.....:ß:.:.:.
b
Figure 3. (a) Diagramshowingtime relationsbetweengeomagnetic
epochsand the evolutionof Pitonde la
Foumaiseand Piton des Neiges. Becauseof the preponderance
of the thermoremanent
componentin the
naturalmagnetizationof the volcanicrocks,the rockseruptedduringthe BrunhesEpochshouldhave a bulk
normal magnetization,whereasthe rocks older than the Brunhes-Matuyama
reversalshouldhave a bulk
reversemagnetization.(b) Geologicalsketchmap and geochronological
data.For PitondesNeiges,only the
Oceanitesand the DifferentiatedSeriesare distinguished.
Also shownis the extentof the densehypovolcanic
complexes.The available radiometricdates are clusteredin three groups:3 to 360 ka, a period that
encompasses
the DifferentiatedSeriesof PitondesNeigesvolcano;426 to 578 ka, a periodthatencompasses
the Oceanitesof Piton des Neigesemplacedduringthe Brunhesnormal-polarityepoch;and 950 to 2080 ka,
the Oceanitesof Piton des Neigeseruptedduringthe Matuyamareverse-polarity
epoch.(No rock agesfrom
PitondesNeigeshavebeenfoundbetween360 and426 ka, 578 and950 ka, or before2080 ka.)
LI•NAT ET AL.: NEW MODEL FOR EVOLUTION OF RI•UNION
8649
as an ancient volcanic center predatingPiton de la Fournaise
[Ranfon et al., 1989]. As in the caseof Piton des Neiges,the
fact that the densecomplexis deeplyrootedindicatesthat this
volcaniccenterstartedto developin the early stagesof R6union
volcanism.An alternativeinterpretationwould be to consider
al. , 1999].
that the Grand-Br•l• complex representsan early stage in
Piton de la Fournaise,at the southeastern
part of the island growthof Piton de la Fournaiseitself, havingbeendisplaced
(Figures2 and3) is a highlyactivebasalticshieldvolcano.Most laterallyin a largeproximalslumpblockof a landslidehavinga
of its knownactivityhas beenrestrictedto effusionsfrom its deep slip surface. Although this hypothesis cannot be
centralcone and northeastand southeastrift zones.Owing to categorically
rejected,the fact thatgravitymodelsshowthatthe
the presence
of deepvalleys,morethanhalf a millionyearsof complex extendsvertically down to, at least, the underlying
the history of Piton de la Fournaisecan be inferred from crust and that the seismic data do not reveal the presenceof
surficial evidence [Gillot and Nativel, 1989]. The oldest lava landslidesat the crust-edificeinterfacein this area [de Voogdet
flows (0.527 to 0.498 Ma) cropout at the bottomsof Remparts, al., 1999] stronglysuggestthat the complexis not displaced.
the complexof denserocksdevelopingupwardas the volcano
grew[Ryan,1987].Malengreauet al. [1999]alsosuggest
thata
secondary
gravityhigh,centeredon the east-northeast
flank of
PitondesNeiges,maybe associated
with an old, buriedvolcano
(Takamaka)whichis recognized
alsoin seismicdata[Gallart et
Langevin,and Est valleys(Figures2 and 3). Althoughthe Thus this eastern volcano, which we named Aliz6s volcano, and
structuralevolutionof this shieldvolcanoappearssimplerthan
Piton des Neigesappearto comprisethe two primaryand long-
thelonger,complicated
evolutionof PitondesNeiges,its study lasting volcanic loci of R•union. Malengreau [1995] and
hasyieldedcontrasting
andsomewhat
contradictory
hypotheses.Malengreauet al. [1999] also observea gravityhigh beneath
the northernflank of Piton de la Fournaiseand suggestthat it
could be associatedwith anotherseparateancientvolcanoor a
continuationof Takamaka volcano. By contrast,Piton de la
al., 1982; Gillot et al., 1994] and by otherworkersas partsof Fournaise,with virtually no underlyingdensecomplex,appears
actualcalderarims [Chevallier and Bachi•lery,1981; Bachblery as a young manifestationof R•union volcanism.In detail,
and Mairine, 1990; Bachblery,1995]. All of theseauthors, however, secondarygravity highs suggestthe presenceof a
however,agreethattheGrand-Brfi16
depression
is thescarof an small complex of intrusionsand cumulatesbeneathits old
center(west of the presentcenter) and of denserocks beneath
eastwardmovinglandslide.
Marineinvestigations
offshoreof Pitonde la Fournaisehave partof its southernflank.
A seriesof caldera-likerims and curvedrims of valleys have
been interpretedby some workers as the headwallsof
successively
smallereastwardmovinglandslides[Duffieldet
providedadditionalconstraints
to the structuralmodelof the
volcano
[L•natet al., 1989,1990;Ro•setet al., 1987,1989'
Labazuy,1991, 1996].About600 km'• of landslidematerial 3. Data Acquisition and Processing
have been recognizedon the submarineeasternflank of the
The magneticdata used here come from three sources:an
island.This materialis composed
almostentirelyof fragmented,
subaerially
eruptedlava flowswhoseagesrangebetweena few
thousandyears and 100 ka. These ages were obtainedon
samplesdredgedfrom the surfacesof the landslides,and
thereforethisdoesnot precludea largerrangeof agesin deeper
partsof thedeposits.
This volumeis about10 timeslargerthan
thepresenttopographic
scarof theGrand-Brfi16
slideandwould
representthe equivalentof about 50,000 years of magma
production
from Pitonde la Fournaise,
assuming
the historical
effusionrate [L•nat and Bach•lery, 1988]. Theseresultsled
Lgnat et al. [1990] and Labazuy [1996] to proposethat (1)
eastwarddirectedlandslideshave recurredduring at least the
last 100kyr (2) theEnclosdepression
couldbe theheadwallof
the Grand-Brfil• slide insteadof a caldera,and (3) the Plaine des
Sablesdepression
couldbe an earlieranalogof theEnclos(i.e.,
a slide headwall and not a caldera). However, in contrastto the
hypothesis
by Duffield et al. [1982] and Gillot et al. [1994],
these eastward directed landslides would have occurred inside a
aeromagneticsurvey, two detailed shipborne surveys, and
individual routes from oceanic cruises(Plate 1a).
3.1. Aeromagnetic Survey
An aeromagneticsurveyof R•union was flown at a constant
altitude of 3500 m above sea level (asl) [Gald•ano et al., 1988]
along south-northlines spaced3 km apart with west-easttielines spaced10 km apart. Over the centralzone of Piton de la
Fournaise,the flight line and tie line spacingswere changedto 1
and 3 km, respectively.Along the profilesthe distancebetween
successivemeasurementsis about 150 m. The surveyextended
10 to 30 km beyondthe coastof the island.Owing to technical
problemsduring the survey,large departuresfrom the planned
flight pattern occurred, but the intended coverage was
nevertheless achieved.
3.2. Sea Level Surveys
The eastern and southwestern submarine flanks of R6union
relativelynarrow(10-13 km) corridor,insteadof alongcurved
are
coveredby two detailedsurveys[L6natet al., 1990; Stoffers
faultscuttingthe entiremassif.The narrowness
of the inferred
and
SO87 Cruise Party, 1994] (Plate la) which allow detailed
corridoris supportedby the distributionof landslidedepositson
the submarineflank [Labazuy, 1996, Figures 1 and 3] and by
the presenceof constructionalfeaturesin the continuationof the
subaerialNE and SE rift zones(theseapparentlyold features
will be discussed
laterin the paper).
The discovery,by drilling (Figure 3b), of a large intrusive
and cumulatecomplex 1000 m beneaththe Grand-Br•l• area
[Ranfon et al., 1989] has added complexity to the
volcanologicalstory.This extensive(Figure 3b), deeplyrooted
(5-6 km accordingto gravity models [Roussetet al., 1989;
Malengreauet al., 1999]) complexhasnot beenidentifiedwith
anyknownvolcanicstructure,andit hasthereforebeenregarded
mappingof these areas. In the other areaswe used generally
isolatedprofiles,mostlyfrom oceaniccruiseroutesto andfrom
La R6union and Mauritius. Although the latter generally
provided only a low density of data coverage,they proved
useful for defining the regional magnetictrends outside the
boundariesof the aeromagneticand detailed marine surveys
[Malengreau, 1995]. The marine data also allowed us to
constructthe magnetic map that we used to describe the
geodynamiccontextof R6union(Plate lb). All magneticdata
were reducedto anomalyvaluesusingthe appropriatedefinitive
geomagnetic
referencefield model.
Ll•NATET AL.:NEWMODELFOREVOLUTIONOFR•UNION
865O
i
palco-oceanicrift
(presumed
wheredashed)
----
.....
transform fault
(presumed
wheredashed}
9
spreading
centerjump
m.agncfic
anomaly
-18 o
•
various
nT
cruise
...
.:::(.
ß
:
Mau
La R6umon
!
b
53"
54 ø
55ø
56ø
57"
58ø
52ø
Longitude
Longitude(degrees)
nT
100-
F--I- 800
- 700
600
500
80-'
- 400
300
- 200
- 100
0
--100
- -200
-300
-400
-500
- -600
- -700
- -800
- -900
- - 1000
--1100
- -1200
- -1300
60-'
__
I
t
!
!
1
I
1
I
I
80
100
120
140
160
180
200
220
240
c
Easting(Km)
Plate 1. (a) I,ocat•onsof the magneticdata usedin this study. (b) Regionalmagneticanomalymap, basedon the shipboarddataof Plate
la superilnposedon shadedbathymetry.The superimposedoceanictkatures(rills. li-actures.and anolnalynumbers)are t¾omDy•nent
[ 1991]. (c) Reducedto pole magneticanomalymapco•nputcdat sealevel t¾omSomme
andCharcotsurveys(seePlate l a), superimposed
on shadedbathymetry.Map coordinatesin kilmneters(Gauss-Laborde
Reunionproject). (d) Reducedto pole magneticanomalymap
computedat 3500 m asl superimposed
on shadedtopography.
The map is obtainedby merginglhe sea-level(upwardlycontinued)and
aeromagnetic
data. I,cttcrsreli:r to anomaliesdiscussed
in the text. (e) Reducedto polemagneticanomalymap computedat 7500 m asl
superimposed
on shadedtopography.This map is obtainedbv upwardcontinuationoft late !d. !.ottersreli2rto anomaliesdiscussedin
the text. The two straightlines showthe locationof profilesof Figure 7.
LI•NAT ET AL.' NEW MODEL l:OR EVOLUTION OF REUNION
8651
120
nT
[• 2800
'
2600
100'
240O
2200
2000
80
1800
1600
1400
1200
,60
I000
8O0
-600
• 40
-400
2OO
0
-200
20-
-400
--600
-20.
_40-•
80
RTP 3500 m
I
120
' j'
140
1•o
180
200
220
2 0
d
Easting(kin)
120
nT
-• 900
850
100-
- 800
750
700
650
600
80
550
- 500
- 450
- 400
'60-
- 350
- 300
- 250
-200
• 40
- 150
- 100
5O
-0
- -50
20-
-100
-150
- -200
-20
RTP 7500 m
-40-
80
I00
120
200
Easting(kin)
Plate 1. Continued
220
2,•o
e
8652
LI2NAT ET AL.: NEW MODEL FOR EVOLUTION OF RI•,UNION
3.3. Reductionto Pole (RTP) and Upward Continuationof
Anomaly Maps
At R6union's latitude, becauseof the inclination of the
between the measurementand the body. The amplitude
decreases
nonlinearlywith the distance(for a magneticdipole,
theamplitude
isa function
of 1/ds,where
d isthedistance
tothe
magnetic
field,theanomaly
induced
bya normally
orreversely dipole). Anomaliesfrom a surveycloseto the groundsurface
magnetized
bodywill havea strongdipolafappearance.
A express mainly topographicrelief and small-scaleshallow
normally
magnetized
bodywill giveriseto a magnetic
highto sources.On the other hand, a higher altitude survey will
the north,with a maximumlocatednearits northernedgeanda enhancedeeper and larger sources.A transformationcalled
magnetic
low to the southwith a minimum
located
nearits upwardcontinuationallows computationof anomalymapsat
southern
edge.The locationof the sources
of the magnetic altitudeshigherthan that of observation[Hendersonand Zietz,
anomaliesare thusdifficult to visualize,contraryto the caseof 1949]. We used this transformationto merge the marine and
aeromagneticsurveysand to computean anomalymap at 7500
gravitywherethe anomalies
aremonopolar
andalwaysoccur
m
asl. RTP and upwardcontinuationswere performedusingthe
above the causativebodies.To reducethe complexityof the
programFFTFIL from Hildenbrand[1983], availableas.partof
magnetic
anomaly
pattern,
thedatawerereduced
topole(RTP)
[Barahoy,1957].This transformation
cancels,
or significantly the U.S. Geological Survey Potential-Field Geophysical
SoftwarePackage[Cordelletal., 1992].
reduces,
the dipolafappearance
of the anomalies
andoffsets
themto their sources.A correctRTP transformation
requiresthe
directionsof both the Earth'smagneticfield and of the total
magnetization
of the rocksto be known.At R6anionthe
magnetic
fieldhasa declination
of-18ø andaninclination
of57ø. Thesevaluesare significantly
differentfrom thoseof a
geocentric
axialdipolefield(declination
of0øandinclination
of
3.4. Magnetic Anomaly Maps Constructed
for R6union
We have computeda regional anomalymap at sea level to
describethe geodynamic
frameworkof Rdunion(Plate 1b) anda
more detailedRTP anomalymap, also at sea level, in the areas
coveredby the denseCharcotand Sonnesurveys(Plate 1c). The
sealevel data were thenupwardcontinuedto the altitudeof the
aeromagneticsurvey (3500 m asl) and merged with the
aeromagnetic
data (usingthe programJMRG from Cordellet al.
[1992]). The resultinganomalymap was reducedto pole to
obtainthe map of Plate l d. Finally,we computeda mapat the
altitude of 7500 m asl (Plate le) from which we extractedthe
-36ø) whichis assumed
to represent
an average
fielddirection
over long periodsof time. The directionof the total
magnetization
wasassumed
to be thatof a geocentric
axial
dipolefieldbecause
a highKoenigsberger
ratio(theratioof the
remanentmagnetization
over the inducedmagnetization)
is
usuallycharacteristic
of volcanic
rocks(seeTable1).
The amplitudeand wavelength
of the magneticanomaly
causedby a givenmagneticbodydependuponthe distance two profilesinterpreted
in Figure7. The reasonfor computing
an anomalymap at 7500 m was to reducethe topographic
effectsandenhancedeepermagneticstructures
of theisland.
Table 1. MagneticProperties
of SomeVolcanicRocks
From R6union
4. Magnetic Interpretation
Age,
Ma
n
NRM,
Z,
A m-1
10-2SI
Q
Reference
Piton de la Fournaise"
13
8.07
1.75
16.2
0.052
11
5.63
1.44
13.7 A. Chauvin(personal
0.06
7
13.08
1.88
0.0047
Chauvin et al. [ 1991]
to
0.011
communication,1994)
24.5 A. Chauvin(personal
communication, 1994)
0.082
17
5.21
2.16
8.5
Chauvin et al. [1991]
13
3.95
1.86
7.5
A. Chauvin(personal
to
0.18
0.082
communication,1994)
to
0.18
4.1. Magnetic Properties of Volcanic Rocks
Magnetizationcontrastsbetweenrock massesat Rdunioncan
arise from two different causes:differencesin magnetization
directionor differencesin magnetizationintensity.The apparent
magnetizationis the vector sum of the inducedmagnetization
(Mi) and of the natural remanentmagnetization(NRM). Mi is
the product of the magnetic susceptibility(k) and the local
Earth'smagneticinductionfield (Be). NRM is the vectorsumof
severaltypesof remanentmagnetization.
In the caseof volcanic
rocks, thermoremanent
magnetization(TRM) is usuallyby far
the most importantcomponent.This magnetizationis acquired
as rockscool below Curie temperatureand it usuallyhasa large
intensity and a high stability over time unless chemically
altered. TRM also has the direction of the ambient BE at the time
of the cooling of the lava. Another type of remanent
magnetization
thatcanhavesomeimportance
at thescaleof our
work is viscous remanent magnetization (VRM). VRM is
producedby long exposureto an externalfield, is orientedwith
the ambientBE, and its buildup is a logarithmicfunctionof
0.5
time. VRM can representone fifth of the NRM in volcanic
Piton
desNeiges
andPiton
delaFournais?
rocks [Prdvot, 1975]. Table 1 showsaveragevaluesof NRM
0
>100
3.6
0.25
5.7 Chamalaun[1968]
and k determinedat different sites by several workers.The
Koenigsbergerratios (NRM/Mi) for volcanicrocks show the
typicaldominanceof NRM overinducedmagnetization.
At the scaleof the interpretationof the magneticstructureof
aAbbreviationsare n, number of samplesconsidered;NMR,
naturalremanent
magnetization
intensity;
Z, magnetic
susceptibility;
Q, Rdunion, we consider large volumes of rocks instead of
individual lava flows. Thus, like in paleomagnetism,
we shall
Koenigsberger
ratio.
bMean
NRM andZ valuegivenbyChamalaun
[ 1968]forsamples assumethat the averagedirectionof theTRM for a sequenceof
rocksspanningtensto hundredsof thousands
of yearsis thatof
fromPitondesNeigesandPitonde la Fournaise.
0.25
to
27
6.45
2.99
7.6
A. Chauvin(personal
communication,1994)
LI•NAT ET AL.' NEW MODEL FOR EVOLUTION OF R•,UNION
8653
a geocentric
axialdipole(declination
= 0ø,inclination
= -36øfor havingno, or weak,magneticsignature.AnomalyC, associated
normallymagnetized
rocks,and+ 36ø for reversely
magnetized with La Montagnemassifcan be interpretedstraightforwardly
rocks)and not that of the present-day
Earth'smagneticfield
(declination= -18ø, inclination=-57ø). Conversely,the VRM
and the Mi will be orientedin the directionof the presentfield.
ashavingmostlyreversemagnetization,
owingto theagesof its
rocks (Figure 3b). NegativeanomalyD also coincideswith
surficialrocksolder than 0.78 Ma (Figure3b) but extendsover
Note that for the same value of TRM a normally magnetized a broader area, eastwardoverlappingslightly the southwest
rock will havea largerapparentmagnetization
thana reversely flank of Piton de la Fournaise,suggestinga relatively small
thickness of the < 0.78 Ma formations from Piton de la
magnetizedrock.
Fournaise
and Piton des Neiges. Offshore,anomaliesE and F
The magnitudes
of NRM and k dependnot only uponthe
amountand natureof titanomagnetite
contentof the rock but forma largenegativezonecoalescent
with anomalyD.
NegativeanomalyG coincideswith outcropsof > 0.78 Ma
alsoon its grainsize,composition,
and thermalhistory(seea
but
discussion
by Hildenbrandet al. [1993]).Generallyspeaking, rocksat the northeastof Belouveand Bebourdepressions
basaltswill showhighervaluesthan will differentiatedlavas. extendsover a larger area, merging to the northwestwith
Two other main factors may act on the magnetizationin anomalyC andto the southeast
with negativeanomalyH along
volcanic contexts:hydrothermalalterationand temperature. the north and northeast flanks of Piton de la Fournaise.
Hydrothermalalterationwill alter the highly magnetic Anomalies H and I to the northeast,east and southeastof Piton
titanomagnetite
to nonmagnetic
or poorlymagneticminerals. de la Fournaise,are partly coincidentwith bathymetrichighs
High temperatures
will alsotendto decrease
TRM as they extendingfrom Pitonde la Fournaiserift zones.
A final qualitativeobservationis the generalweaknessor
approach
Curietemperature.
Thusthe volumeencompassing
a
absenceof magneticfeatureson the submarineflanks, except
magmareservoirmayhavea veryweakTRM.
4.2. GeneralTrendsin MagnetizationDirections
of the Volcanic Rocks of R6union
for anomaliesC, E, F, H, and I. This suggeststhat mostof the
flanks either have a homogeneousmagnetization or, more
likely,arepoorlymagnetized.
The latterinterpretation
wouldbe
consistentwith flanks composedmostly of landslidedebris
The rangeof agesof thedatedrocksof PitondesNeiges which may have accumulatedrepeatedlythroughoutthe
spansthe Brunhesand Matuyama
paleomagnetic
epochs,building of R6union, as suggestedby Holcomb [1990],
whereas the oldest observable rocks of Piton de la Fournaise
stillbelongto theBmnhes
epoch(Figure3a).Ona general
level
we caninferfromthe geomagnetic
timescale
thatthelavapiles
younger
thantheBmnhes-Matuyama
reversal
(0.78Ma [Spell
andMcDougall,1992;Izettand Obradovich,
1994;Bassinot
et
al., 1994])shouldhavea normalmagnetization,
whereasolder
formations
will havea bulkreversed
magnetization,
because
the
periods
of reversed
geomagnetic
fieldweredominant
duringthe
Matuyama.
4.3. Qualitative Interpretation of the Anomalies
of R6union
The anomalymap (Plates l d and l e) is dominated by the
presenceof two main positiveanomalies:one over Piton de la
Fournaiseand one over the westernflank of Piton des Neiges.
Anomaly A, over Piton de la Fournaise,is a featurethat was
expected, but the actual shape of the anomaly presents
noteworthyfeatures.Its extentis narrowerthanthat of Pitonde
la Foumaiseformationsat the surface(Figure3b). This can be
observedon its northernflank. In addition,the anomalydoes
not extend offshore. The maximum of the anomalycoincides
with the central area of the volcano. This can be related to the
effect of the relief as well as to a greater thicknessof the
normallymagnetizedproductshere and/or to higher valuesof
magnetization.
The anomaly pattern over Piton des Neiges is more
surprising.From the available ages (Figures3a and 3b) it is
known that rocksolder than the Brunhes-Matuyama
reversalare
presentin a few places:La Montagnemassif,valleysnortheast
of Belouveand Bebourdepressions
and southof CilaosCirque.
Apart from these localizedzones the whole surfaceof the
massifis coveredby rocksyoungerthan0.78 Ma. The fact that
a main positivezone (B on Plates1d and 1e) is restrictedto the
westernflank and summitarea suggeststhat in other areasthe
productsemitted during the Brunhesepoch representonly a
relativelythin coatingoverlargervolumesof olderrocks.
In additionto the two main positiveanomaliesof the island,
we note six negativeanomalies(Plates lb and l e) and zones
Holcomb and Searle [1991], and Moore et al. [1994] for other
theoceanicislands.Longerwavelength
anomalies
t•, 13,and¾of
the distal submarineflanks probablyoriginateprincipallyfrom
the oceaniccrust beneathR6union (see the regionalmagnetic
map on Plate 1b) and thereforewill not be consideredhere.
4.4. Evidence of ReverselyMagnetized Formations
at the Base of R6union
Volcanic
Massifs
On the sea level map (Plate l c), intensenegativeanomalies
are observed at several places near the seashore. As
demonstrated
below, their intensityimpliesthat they are created
by reversely magnetizedrocks. The sea level data were thus
instrumentalin establishingthe presenceof large volumesof
rocks older than 0.78 Ma at the base of Piton de la Fournaise
and Piton desNeiges.
4.4.1. Eastern zone of the island (Figure 4). Anomalies
H1 and I coincide partly with bathymetric highs offshore
beneaththe present-dayNE and SW rift zonesof Piton de la
Fournaise(Figure 1). The analysisof those bathymetrichighs
led Labazuy [ 1991] and Ldnat et al. [ 1989, 1990] to infer that
they are remnantsfrom old volcanicconstructions,unrelatedto
the growthof the recent(< 0.5 Ma) Piton de la Fournaise.
The signaturesof the negative anomaliesdiffer strikingly
from thoseof positiveonesin this area.The negativeanomalies
are spatiallywell definedand have high amplitudes.Two twodimensional(2-D) models along two lines (locatedon Figure
4a) are shownon Figures4b and 4c. Those models,as well as
the other modelscomputedfor sea level data, have been made
as simple as possible,having simple shapes,with only one
magnetization contrast considered. Deriving more complex
modelswould be meaninglessin the absenceof supplementary
constraints.Therefore these simple 2-D modelsshouldnot be
consideredas accuratemodelsof the internalstructurebut only
as models establishingthe presenceof major magnetization
contrastswithin the upper submarineflanks of R6union. The
models show that high negative magnetizationcontrastsare
required to account for the amplitudes of the negative
anomalies.This implies that the anomaliesarise from reversely
8654
L15.NATET AL.: NEW MODEL FOR EVOLUTION OF RI•UNION
I
a
180
200
220
Easting (Km)
2OO
-200
-400
s
-600
-800
N
-0.5
-1000
-1.0
-1.5
P1
-13 A/m
-2.0
b
-2.5
-3.0
0
5
10
15
2o
25
3o
35
40
km
2OO
w
0
-200
-400
-
-600
0.0
- -0.5
-800
- -1.0
-1000
- -1.5
- 6.45A/m•
•
6.9Aim
- -2.0
P2
- -2.5
- -3.0
- -3.5
-5
0
5
10
15
20
25
30
35
40
45
50
km
Figure 4. Interpretation
of theRTP sealevelmagnetic
anomalies
neartheeasternseashore.
(a) Anomalymap
superimposed
on shadedbathymetry.
H 1, H2 andI areanomalies
discussed
in thetext.P1 andP2 arelinesof
profilesfor Figures4b and4c. Magneticcontours
are 100 nT. Blackdotsshowlocationof the magnetic
measurements
along the ship track.(b) Bathymetricand magneticprofilesof anomalyH1 alongline P1;
dashedline, 2-D modelprofile.(c) Bathymetricand•nagneticprofilesof anomalyH2 alongline P2; dashed
line, 2-D modelprofile.
LI•NAT ET AL.: NEW MODEL FOR EVOLUTION OF RI•UNION
8655
50
a
100
Easting
(Km)
150
200
5OO
SE
4O0
NW
3OO
E
2OO
!-c::
•
.
regional-corrected
anomaly
100
0
-100
raw anomaly
•E -200
-300
<• -400
-500
-600
-700
-800
-900
/'""
b
0
5
10
'•"'-/ -'•--•_....]
-14A/m
m -2
10A/m
-2.5
15
20
25
30
35
40
45
50
55
60
65
70
75
80
Figure5. Interpretation
of theRTPsealevelmagnetic
anomaly
nearthesouthwestern
seashore.
(a)Anomaly
mapsuperimposed
onshaded
bathymetry.
E andF areanomalies
discussed
in thetext.P3isthelineof profile
forFigure5b.Magnetic
contours
are100nT.Blackdotsshowlocation
of themagnetic
measurements
along
theshiptrack.(b) Bathymetric
andmagnetic
profiles
of anomaly
E alonglineP3;dashed
line,2-D model
profile.
8656
Ll•NAT ET AL.' NEW MODEL FOR EVOLUTION OF R•UNION
of the anomalyrequiresa largenegativecontrast
magnetized
bodiesratherthanby contrasts
betweennormally interpretation
althoughlowerthanin theprevious
casesof
magnetized
formations.
Thisis furthersupported
by thefactthat in magnetization,
the easternsubmarineflank is mostlyformedby disorganized theeastandsoutheast
zonesof theisland.The important
point
products
fromslides[Ldnatet al., 1989,1990;Labazuy,1996; hereis thatthe anomalycanbe confidentlyrelatedto subaerial
de Voogdet al., 1999]. Theselandslideformations
probably formations having ages compatible with a reversed
have no bulk TRM magnetization,becausethe blocks are
randomly rotated, and thereforecannot producenegative
contrasts
ashighasrequiredby themodels.
The modelingof anomalyH1 (Figure4b) showsthat it
cannotbe explainedby the topographic
effect of the sharp
magnetization.On the aeromagneticmap (Plate l d), the
existence
of thenormally
magnetized
structure
(6.9A m'l),
Across the Island
subaerialLa Montagnemassifcorresponds
to a negative
anomalythat continuesoffshore,in agreementwith sea level
observations.
In summary,
the combined
observations
provide
robustevidencethat the negativeanomaliesobservedoffshore
andnorthcoastof theislandarecaused
by
topographic
ridgewhichnearlycoincides
withtheminimum
of theeast,southwest,
H1 on the profile.A broader,andpresumably
deeper,structure volcanic formationshaving reversedmagnetization.These
also strengthenthe interpretation
of the on-land
mustbe considered
to explainthe anomaly.The interpretation observations
of anomalyH2 (Figure4c) requiresa verticaldiscontinuity
(or negativeanomaliesD and G as causedby largevolumesof
variationof magnetization)
at about5 km fromthecoast,about reverselymagnetizedrocks.
wherethe easternboundaryof the Grand-Brill6densecomplex
is inferred from gravity data [Roussetet al., 1989]. The 4.5. MagneticInterpretation Along Two Profiles
addedto the model in order to improvethe fit betweenthe
The well-established
interpretation
of negativeanomaliesC
observed
andcomputed
curves,is highlyquestionable,
because to I as causedby largevolumesof reverselymagnetized
rocks
thegenerallevelof about200 nT to theeastof H2 is probably allowsus to derivea more generalmodelof the magnetic
relatedto the long-wavelength
positiveanomaly visibleon structureof the island.A 3-D modelingprobablywould be
Plates 1d and 1e.
unrealistic considering the poor knowledge of the
4.4.2. Southwestern zone of the island (Figure 5). Two magnetizations
at depth.We calculatedinsteadsimple2.5-D
minima (E and F) indicatea negativezone offshoreof the modelsto establish
andrefinethecoherence
of thequalitative
southwestcoast of the island. Anomaly E is conspicuously interpretationand of the sea-levelmodels.Two characteristic
elongate
towardthesouth-southwest
andpartlycoincides
witha profileswereselected
(Platel e): oneacrossPitondesNeiges
nearshoresubmarinepromontory(Figure 1) that continues and one across Piton de la Foumaise. In order to minimize the
fartheroffshoreasa prominent
submarine
ridge.A 2-D model, topographiceffects,a uniform 3-D modelof the islandwas first
alonga line roughlyperpendicular
to theelongation
of E, is calculated
usingtheprogram
PFMAG3DfromBlakely[1981].
shownonFigure5b.Theresultis similarto whatwasobserved Thetopof thisoverallmodelis definedby thetopography
and
in theinterpretation
of anomaly
H1 (Figure4b):(1) anomaly
E the bottomis a deephorizontal
surface.
Usingthisoverall
doesnotcorrespond
to a topographic
effectof theridge,butto a model,
a simple
topographic
anomaly
wascalculated
first,using
broaderstructure,
and(2) themagnetization
contrast
requiredto a constanttotalmagnetization
havingthe moderatevalueof 1.5
account
for theamplitude
of theanomaly
is largeandshould
be A m-•. The effectsof the 2.5-D structures
were then
associatedwith a reverselymagnetizedbody, for the same superimposed
on thissimpletopographic
anomaly.
The 2.5-D
reasonsas for anomalyH1. The small,normallymagnetized modelswerecalculated
usingthe programSAKI [Webring,
structure
(10 A m-l) models
a short-wavelength,
localanomaly 1985].
visible on the maps on Figures5a and Plate l c. The
4.5.1. Piton de la Fournaiseprofile (Figure7a). This
southwestern
negative
zoneis therefore
similarto theoneat the profileis influenced
by long-wavelength
anomalies
(a, 13,and
east of the island and suggeststhat reverselymagnetized 7) whichare attributedto the regionalmagneticfabricof the
formations
arepresent
beneath
theyounger
formations
of Piton oceaniccrust.We havenot attempted
to modelthesedeep
desNeigesin thisarea.Theelongation
of anomaly
E suggests
a structures,becauseour purposeis to study the volcanic
rift zone-like
structure.
4.4.3. Northern zone of the island (Figure 6). This areais
an offshore continuationof the La Montagne massif, which
constitutes
the largestsubaerialoutcropof rocksolderthanthe
Brunhes-Matuyama
reversal.La Montagnemassifis a pile of
lava flows intersectedby a high concentrationof dikes that
trend N20øW and have been interpretedas an old rift zone
[Chevallier and Vatin-Pdrignon,1982]. Offshore,a submarine
superstructureabove the seafloor. The curve obtained for the
island'sinterior model matchesthe shorter-wavelength
anomaliesin location, amplitude,and horizontalextent, but
becausethe deeperoceanicstructureis not modeled,it is
slightlyshiftedfrom the observedcurve.
Theobserved
profilerequires
themodelto include
(1) highly
magnetized
upperformations
of normalpolarity,
corresponding
to the subaerialedificeof Piton de la Fournaiseand (2)
promontory
andridges(Figures1 and6a) markthecontinuation underlyingreverselymagnetizedformationsculminating
of the La Montagnemassif.
beneaththe northernpart of the edifice.The gradientof
This area is incompletelycoveredby the marinemagnetic anomaly
A, abovethesummit,
impliesveryhighmagnetization
surveys(Figure6a). A 2-D modelis presented
on Figure6b. contrasts.
In orderto remaini_na realistic,
though
high,rangeof
The sourceof the anomalyseemswider thanthe ridgedirectly magnetization values, we allow a stratification of the
beneath
the magnetic
profile.The reasonof thisapparentmagnetization
intensitydistribution.
The high superficial
inconsistency
of the modelarisesfromthelimitation
of the magnetization
represents
the youngest
pile of lava flows,
methodused(2-D modeland no variationof magnetization
whereasthe deeper(i.e., older) formations
have a lower
considered).
In particular,
because
the topography
deepensmagnetization
intensity.
The presence
of a shallow
magma
dramatically
to the northin this area(as shownby the chamber
andof a hothydrothermal
zonewhichwould
produce
comparison
of thetopographic
profiles
T1 andT2) andthe lowmagnetization
arenotsupported
bythemagnetic
dataorat
source
is shallow,
the 2-D approach
is clearlynotadapted.leastdonothavea signal
perceptible
atthisscale.
Although
the
However,
this simplemodelis sufficient
to showthatthe presence
of thesefeatures
is strongly
supported
by a hostof
L•NAT ET AL.' NEW MODEL FOR EVOLUTION OF R•UNION
8657
100--
a
140
Easting(Km)
160
800
_
600
400
--
200
--
_
c
_
o
_
-200
--
-400
--
_
-600
_
-800
--
.lOOO
--
_
0
-7
5
10
15
20
A/m
b
Figure 6. Interpretation
of the sealevelRTP magneticanomalyneartke northernseashore.
(a) anomalymap
superimposed
on shadedbathymetry.C is the anomalydiscussedin the text. T1 and T2 are the locationsof
the bathymetricprofiles for Figure 6b. Magnetic contoursare 100 nT. Black dots show location of the
magneticmeasurements
alongthe shiptrack.(b) Bathymetric
andmaf•:•etic
profilesof anomalyC alonglines
T1 andT2; dashedline, 2-D modelprofile.
LI2NATETAL.'NEWMODELFOREVOLUTION
OFRI2UNION
8658
000 --
observed profile
9OO-800 --700 -600 -500 --
de la Fournaise
uniformly
magnetized (1.5 A/m)
relief (3D model)
400-
300 --200
Piton
--
100 -0-•
-1 O0 ---
ß
A
_
2S
• model
profile
G
(dashed
line)
18.6 Aim
.........
•,•,•-..••••
Aim
....................
•":"5....
.-. 1
E0
N
i '.•'
•::•::::::
-5
-6
0
10
20
30
40
50
60
70
80
90
100 110 120 130 140 150 160 170 180 190 200 210 220
km
500
-1 b
Piton des Neiges
400
uniformly
magnetized (1.5 A/m)
relief (3D model)
300
200
observed profile
100
0
-100
model profile
(dashed line)
-200
E
2 SSW
5.6
A•m
1
•-1
'•-2
C-G
B
NNE
2_ 3
-5
-6
I • I • I • I • I • I • I • I • I I I • I • I • I • I • I • I • I • I • I • I • I • I • I I I I I I I
0
10
20
30
40
50
60
70
80
90
100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250
km
Figure 7. Interpretation
of the RTP anomalyprofilesacross
the island(seePlatele) at 7500 in asl.The
effectsof the2.5-D structures
(extending
10km on bothsidesof theprofiles)aresuperimposed
on therelief
anomaly
estimated
in threedimensions
(seetext).(a) A 2.5-Dmodelacross
Pitondela Fournaise.
(b)A 2.5-D
modelacrossPiton desNeiges.
4.5.2. Piton des Neigesprofile (Figure 7b). This profile is
other geologicaland geophysicalobservations
[Ldnat and
Bachglery,1990; Ldnat et al., 2000], it is clear that their also influencedby long-wavelengthanomalies(c•and ¾),and
as above(Pitonde la Fournaiseprofile)
dimensionsare not large enoughto affect the magneticdata. the sameconsiderations
apply
to
the
calculated
curve.
The simplestinterpretation
of this
Theycouldbe resolvedonlyby finer-scale
analysis
of magnetic
dataover the centralareaof Piton de la Fournaise.Althoughthe profile requiresthreemagneticbodies:(1) a pile of relatively
interpretation
of themagneticlow G on thenorthflankof Piton highly magnetizedformationshavingnormalpolarity(anomaly
de la Fournaiserequiresthe presence
of a reverselymagnetized B), (2) a reverselymagnetizedstructure(anomaliesC andG) to
the north, and (3) another reversely magnetized structure
structure,
its shapeis not well constrained
in thismodel.
L•NAT ET AL.: NEW MODEL FOR EVOLUTION OF R•UN1ON
8659
(anomaly E) to the south. Like the case for Piton de la
Fournaise,the modeledstructures
herearepoorlyconstrained
at
depth.This modelagreeswell with the qualitativeinterpretation
and the sea level modelsdevelopedabove.We note that the
densehypovolcaniccomplexrevealedby gravity (Figure 3b)
doesnot haveanyobviousmagneticsignature.
Neigeshas remaineda stablelocusof activitysincethe early
stagesof R6union volcanism.The densecomplexdeveloped
upwardasPitondesNeigesgrew.
The morphologyof submarineflanks surrounding
R6union
5. Discussion
Labazuy,1996] andby seismicsurveys[de Voogdet al., 1999].
Virtually all the dredgedsamplesin this areaare fragmentsof
subaeriallyeruptedlavas,indicatingthat the sourceareasof the
landslideswere on land. Four or five other large bathymetric
bulgeson the submarineflanks may be associated
tentatively
with sourceareason land (Figure8). Piton desNeigestherefore
musthave undergonemajor episodesof destructionduringits
evolution.The recurrenceof theselateralcollapsesmay account
for the apparentlycomplicatedgeologyof PitondesNeiges.For
example,the apparent700 ka (from 1.9 to 1.2 Ma) and 400 ka
(from 950 to 578 ka) interruptionsof volcanismof the shield
[McDougall, 1971] could be artifacts of accumulationin
restrictedsectorsduringtheseintervals.The lava flows erupted
at these times may have accumulated within landslide
depressions(like now in the Enclos-Grand-Brfi16
systemof
Piton de la Foumaise) and are now concealed by later
suggest
thatthoseflanksareaccumulations
of landslide
debris
instead of lava flows. This has been confirmed by detailed
studies of the eastern submarine flank [Ldnat et al., 1990;
5.1. Distribution of Predominantly Normally
and Reversely Magnetized Formations
The strongnegativeanomaliesnearthe eastern,southwestern
andnorthernshoresof theislandprovideevidencethatreversely
magnetizedformationsare presentat the baseof the subaerial
formationsin theseareas.On land,negativeanomaliesC, D and
G are coincidentwith outcropsof rocksolder than0.78 Ma but
alsoextendover areaslargerthanthoseoutcrops.We infer that
the capof rocksyoungerthan0.78 Ma in the areasC, D, andG
is minor above a larger volume of reversely magnetized
formations.This interpretationis illustratedby the modelsin
Figure7. The two main positiveanomaliesA and B correspond
to a largepartof Pitonde la Foumaiseedificeandto a thickpile
of normallymagnetizedrockson the westernflank of Pitondes
accumulations.
Neiges(Figure7), respectively.
None of the landslidesfrom Piton des Neiges suggestedon
From theseobservations
we can constructa map showingthe
Figure
8 have yet been recognized on land, but subaerial
distribution of predominantly normally and reversely
magnetizedformationswithin R6union.The resultingpattern landslide products have been recognizedin the Cap La
(Figure 8) differs strikingly from the one expected from Houssayearea [Bachbleryet al., 1996] and at the bottom of
previousmodelsof the island'sevolution.In thosemodelsthe Cilaos Cirque [Maillot, 1999]. Anomaly B tells us that a thick
evolution of R6union was regarded as the growth of two pile of Brunhes-agevolcanicproductshas accumulatedin the
volcanoes,Piton des Neigesfirst (from > 2 Ma to 12 ka) and westernpart of Piton des Neiges and has remainedvirtually
Piton de la Fournaiselater (~ 0.5 Ma to present)on the east- undisturbed.This pile may have accumulatedin a depression
left by westward-directed
landslides.The smallerthicknesses
of
southeastflank of Piton des Neiges.But accordingto Figure8,
Brunhes-age
products
on
other
parts
of
Piton
des
Neiges
could
prior to 0.78 Ma the islandwould have had a surfacearea at
least comparableto the presentone, with anomaliesC, E, and have resultedfrom channelinginto the westernstructureor that
suchaccumulations
havebeenerodedby recentflank landslides.
H1 possiblymarkingelongatedvolcanicrift zones.
5.2. Piton des Neiges
5.3.
Takamaka
Volcano
Inferenceof a concealedvolcaniccenterbeneathanomalyG
stems from combining gravity and magnetic observations.
Modeling of gravity data from the area of anomaly G
[Malengreauet al., 1999] showsthe presenceof a densebody
rising to less than 1 km beneath sea level and extending
downwarda few kilometers,thoughto smallerdepthsthan the
densebody beneathPiton des Neiges. Magnetic anomalyG
reflects the presence of an underlying large volume of
predominantlyreverselymagnetizedrocks.This is compatible
with pre-Brunhesaccumulationof volcanicproductsaroundan
northern,eastern,and southernareas.This difference cannotbe hypovolcaniccomplex.In addition,seismicstudies[Charviset
explained solely by the difference in rainfall between the al., 1999] reveal a high-velocityzone in agreementwith the
Althoughno geologicaldata are presently
windward (ENE) and leeward (WSW) sides of the island, gravityobservations.
available to supportthe existenceof Takamakavolcano, the
because in that case the south flank of the massif would not
look much different than the west flank. This large-scale various geophysicalobservationsprovide enoughevidenceto
infer the former existence of a volcanic center in this area.
contrast in topographyis certainly related primarily to its
However,
becauseof the presumeddepth of this system,it is
volcano-tectonic
history,which has led to the preservation
of a
probablethat only drilling couldprovidea definitiveanswerfor
moreregularpile on the westernflank thanin otherareas.
the existence of Takamaka volcano.
From gravity data [Malengreau et al., 1999] we know that
the centralpart of PitondesNeigesis underlainby a larged:nse
5.4. Piton de la Fournaise and Les A!iz6s Volcanoes
body, extendingfrom virtually the bottom of the "cirques"to
The existenceof the densecomplex beneathGrand-Brfi16
depthsof severalkilometers.This densemassrepresentsthe
core of the volcanoand is thoughtto consistof hypcvolcanic (Figure3b), near the easternshoreof the island,long stoodas
bodies(sheet intrusions,gabbros,and cumulates)which crop an enigma,sinceno volcanicsystemhadbeenrecognized
in this
out in a few places.Following Ryan [1987], we infer that the area. It could not be associated with Piton de la Fournaise
large vertical extent of the densebody implies that Piton des becausethereis a significantdistancebetweenthe two centers.
The generalmorphologyof Piton desNeigesis far from that
of a pristine, unmodified volcano (Figure 2). Erosion in a
tropicalenvironmentfor more than 2 Myr duringits subaerial
evolutionmustbe regardedasone of the main agentsleadingto
the present topography and landforms. The three main
"Cirques"(Mafate, Salazie,and Cilaos) are usuallyregardedas
erosional features, possibly influenced by tectonics(see a
summaryby Kieffer [i990b]). There is, however,an obvious
differencebetweenthe comparatively
pristinetopographyof the
westernflank of the massifand its highly dissectedandjagged
8660
L•NAT ET AL.' NEW MODEL FOR EVOLUTION OF R•UNION
riff:zonei
0
•.•
1•0'
Dominantly
normal
•
magnetization
Dominantly
reverse
magnetization
(Brunhes period)
(Matuyama period)
Submarine
Landslide
landslide
headwall
flow
Presumed
landslide
or inferred
headwall
Figure 8. Structuralmapof R6unionshowingtheinferreddistribution
of MatuyamaandBrunhesformations
and landslides.
The deep geothermalexploration drill hole [Ranqonet al.,
1989] penetratedthe densebodyfrom a depthof 1000m to the
bottomof the well at 3000 m and establishedits hypovolcanic
nature.Gravity modelssuggestits verticalextentto be similarto
that of the densecomplexof Piton des Neiges [Roussetet al.,
1989; Malengreau et al., 1999]. Accordingly,usingthe same
rationaleas for Piton des Neiges Complex, we infer that the
Grand-Brill6complexrepresentsa volcaniccenterthat beganto
grow in the early stagesof R6union volcanism.Studies of
similarly active basalticshields[e.g., Ryan, 1987] indicatethat
the tops of intrusivecomplexesare locatedat depthsof 2 to 4
km beneaththe surface.Thereforewith the top of Grand-Brill6
complex at about 1000 m beneath the present surface, the
corresponding
volcanicedifice shouldhave been emergedby
1000 m or more when it was active.Our magneticdataindicate
for the first time the presenceof old (i.e., < 0.78 Ma)
constructional
volcanicfeaturesat shallowdepthin the vicinity
of the Grand-Brill6 complex.We will now attemptto draw a
model commensurate with the available observations.
Magnetic anomaliesH and I, associatedto pre-Brunhes
formationsat shallowdepth,surroundtheGrand-Brill6complex
to the east.We suggestthat theyare the remnantsof an edifice
associatedwith the hypovolcaniccomplex. We name this
inferred edifice Les Aliz6s volcano (note that Ran•on et al.
LI•NAT ET AL.' NEW MODEL FOR EVOLUTION OF RI•UN1ON
[1989] speculatedpreviouslythat the Grand-Brfil• complex
belongedto a "proto-Fournaise"
volcano,but the submarine
topography
which they interpretedas the flank of their "proFournaise"volcanohasnow beenprovedto consistof slumped
material). In our interpretationthe context of Piton de la
Fournaiseis very differentfrom the previousview of it as a
youngvolcanogrowingon the flank of PitondesNeiges.Piton
de la Fournaiseprot•z.
bly is a younglocusof activity,sincethe
gravity data reveal that it has not yet developeda large
hypovolcanic
complex.But it beganto growon theflankof Les
Aliz6s volcanoinsteadof Piton des Neiges.With the flanks of
Takamakaand Piton des Neigesvolcanoesto the north and the
NW, respectively,Piton de la Fournaisewas buttressed
on all
8661
contributed
someof thesedeposits,
because
themorphology
of
its subaerialsouthflank (Figure2) suggests
the presence
of
buriedlandslidescars.The subaerial
southflankis lessregular
thanthe northflank, beingerodedby largevalleys,a difference
thatis anticorrelated
with therainfallon theopposing
flank.But
the large volume of landslidedepositsrequiresadditional
sourcesof south-flankdebris.This suggests
Les Aliz•s volcano
hadalreadyproducedvoluminous
landslides
priorto growthof
Fournaise. If so, the edifice of Les Aliz•s volcano must have
lostmuchof its volumeto masswasting,andperhapsthiscan
explain the disappearanceof most of that edifice above the
Grand-Brfil• complex.
With its flanksbuttressed
in all directionsexceptthe south,
sidesexceptto the south.The questionsthat arisefrom this Piton de la Fournaiseshouldhave been prone to landslides
model are (1) What happenedto the edifice of Les Aliz•s directedonly to the south.The eastwarddirectionof its recent
volcano? and (2) Why are Piton de la Fournaiselandslides landslides
canonlybeexplainedif a formerstructure
hasplayed
an influence. The width of the landslide scar on the subaerial
directed toward •he east ?
Fromseismicstudies,de Voogdet al. [1999] foundthatthick east flank of Piton de la Fournaise matches that of the
complexof Les Aliz•s volcano(Figure3b), and
layersof landslidedebrisform the bulk of the easternand hypovolcanic
southern submarine flanks of Piton de la Fournaise. This is in
the top of the complexin the drill hole is thoughtto coincide
agreement
with morphological
studiesof thoseareas[Ldnatet with the base of the landslides[Labazuy, 1991; Courteaud,
mi., 1990; Labazuy, 1996] (Figure 1). To the east, surficial 1996].It thereforeseemslikelythata mechanical
discontinuity
deposits
arerelatedunambiguously
to Pitonde la Fournaise,
but coincideswith the top of the hypovolcaniccomplex.That
the presenceof oldermaterial(i.e., from Les Aliz•s) deeperin discontinuityis likely to be a hydrothermallyaltered zone,
the thick pile of landslidematerialis probable.To the south, similar to those typically developedabove hypovolcanic
landslide-derived
formations
extend
down
to the
complexes. Such zones have low cohesion and are often
oceanic
basement.The provenanceof the debris in this area is not consideredas potentialruptureloci for landslideson volcanoes
established.We suspectthat Piton de la Fournaisehas [Siebert, 1984]. We hypothesizethereforethat the eastward
Piton de la Fournaise
recent lava pile
present-day
.........
IIIC•l•Cl
S
•oc•
(h•h
ly
magnetized)
/
......
rvoir
_••'
., •r
oeeper
levelsOTPitondeia•-ournaise
(possibly
alteredandpartlybrecciated)
and
conduit
..•..f'••
•y•,•/ 1
I
":::.:i•
Flank
landslides
• .._••,?'
[
,...'.•,•,•..'•
0
%.,
I
10
20
30
40
':'•.,••
X
50
60
N
70
LesAliz•s&Takamaka
volcanoes
(predominantlyreversely magnetized)
80
90
100 110 120
km
Piton des Neiges
thick
ß' lava
.. pile
' • .
weakly magnetized or
thin Brunhes period products
. /
taccumuiated during urunnes perioo
onthewestern
-art'
(predominantly
formed
during
Matuyama
period)
Etang-Sa16
••/•[III--•
rift zone
E 0
•-1
-
(1)
-
-0-2
LaMontagne
massif
and
continuation
/
/'"_••"•-'•'
I II
(Matuyama period)
I •-
r•:.%__.._
Flank
iLlIll
landslides
.•--3
_
<-4
_
-5
_
SSW
hypovolcanic
complex
NNE
-6
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
km
Figure 9. Interpretativecrosssections
of Piton de la Fournaiseand Piton des Neiges summarizingthe
interpretationsdiscussedin text.
8662
LI•NAT ET AL.: NEW MODEL FOR EVOLUTION OF RI•UNION
all consequences
of thenewmodelof theisland,butit provides
a basisfor reinterpretation
of previousdataanddefiningfuture
studies.This studyalsoillustrates
thepotentialof magneticdata
movinglandslidesof Pitonde la Foumaisewerecausedby a
rupture in the hydrothermally
altered zone of Les Alizts
volcano. These events could have further denuded the Les
for deciphering the structure of similar volcanoeswhose
evolutionspansreversalsof geomagnetic
polarity.
Alizts edifice and contributedto instabilityof the easternflank
of Piton de la Foumaise.
We suggestthat the horseshoe-shaped
depression
produced Acknowledgments. This work was supportedby CNRS-INSU.
by eastwarddirectedlandslides
influenced
thedevelopment
of Franqois-Xavier
Lalannewasin chargefor thetechnicalaspects
of the
the NE and SE rift zones of Piton de la Fournaise, as a
landslides directed toward the south.
aeromagneticsurvey of Rtunion. We thank Philippe Labazuy for
sharinghis bathymetricdata. Benjamin van Wyk de Vries kindly
acceptedto read the early versionof the manuscriptand madehelpful
suggestions.Thomas Hildenbrand deservesspecial thanks for his
encouragments
and advice. Finally, reviewsby RobinHolcomb,Mark
Pilkington,andMaurice Tivey haveconsiderably
helpedto improvethe
manuscript.
6. Summary and Conclusion
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du Globe de Paris, 4, place Jussieu, 75252 Paris, France.
(galdeano@ipgp.jussieu.fr)
B. Gibert-Malengreau,
sarl PYROS, 27, Routedu Cendre,F- 63800
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J.-F. L6nat, Universit6 Blaise Pascal-CNRS, OPGC-UMR6524CRV,
5,
rue
Kessler, 63038
Clermont-Ferrand, France.
(lenat@opgc.univ-bpclermont.fr)
(ReceivedJuly5, 2000; revisedOctober27, 2000;
L6nat,J.-F., andP. Labazuy,Morphologies
et structures
sous-marines acceptedNovember27, 2000.)