?*"":, r-*
IC/88/359 INTERNAL REPORT (Limited distribution)
/ .:.-' /International Atonic Energy Agency The concerned region is north-trending in the centre of XALAGASCAR im- and mediately north tho volcanic mountains nass of ITA5T,about 100 km far froa the \ « \^ Uni^etf'Natiftns Educational Scientific and Cultural Organization capital AnTAHANARIVO.lt in about 13 bi long Mi 9 km wide, limited bj LABORDB coord nates j X > 422 X* - 431 ^___ "" INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS T - T9"2 I* - 805 aa indicated in SOAYlXANDRXANA geological nap (Fig, I )
STRUCTURAL STUDY IN "ITASY" VOLCANIC REGION (CENTRE OF MADAGASCAR) GENERAL GEOLOGY BY GEOPHYSICAL METHODS (MAGNETIC AND AUDIONAGNETOTELLURIC) *
The volcanic formations of nVUJT, north SQAVHrAKDRIAHA, are composed of mountains mass about 30 km long and 10 kn vide (from east to west).Its different eruptions so as those of the mountains mass of ANXARATRA ,mark oat recent xonea Jean Bruno Ratsimbazafy ** of instability of the nalagasy High Plain. International Centre for Theoretical Physics, Trieste, Italy. The different outflows of the region of TTAST ».r«,with the baealtio out- flows in the north extreme of the island,the most recent of KADAGASCAR.They are of quaternary age* The short eruptions provided multiple cones and domes separated eaoh other by craters and barrage lakes;between them is the adgmatitio and gneisaio underlying substratum.They gave 3 types of lava: - trachytes constitued by domes * ordanchitos -i lrt 1 overflows - basanitolds \ °«» "«'
After these eruptionstthere were S series which were emitted at different MIRAMARE - TRIESTE times : October 198B -the first serie with trachytes and basanitoi'ds
-the second aerie with trachytes,ordanohiteatbaBanitoIds The rocks of these 2 series were mixed with ultra-vulcanian projections at the end,and placed in the petrological point of view,on the magmatite variation diagram, tthey are ranged in the alcaline seria.
TECTONICS * To be submitted for publication. ** Permanent address: Geophysical Observatory, P.O. Box 3843, In general tectonical point of view,on Madagascar eoale,the voloania region of Antananarivo 101, Madagascar.
-1- The direction of reconnaissanoe magnetic profiles was ohoBen East-West Itasy is placed in the recant instability son* of the Malagasy High Plain.This for being perpendicular to the assumed H3 structures.ait because of the topography, instability con* is linked to the north-soutb-trending Bagmatic reactivation axis the effective direction was HE-SB in th« central ions of Xadagaaoar as described by HOTIHA (1972 ) and confirmed Sampling of 500 n was adopted,and measurements of total na&tetio field P . by seinological work of Professor RAKDTOKtlRAIWIBE (1977 ). The region of Itasy were enregistered with the sensor of the magnetometer at 2.5 m above ground level, la the Boat seiaMlogical son* after the Alaotra Lake (in the north east ) so the data were obtained with the minimum of journey: Three structural anomalies aonos (the first in the extreme north by . On the west part and east part of the Rj reconnaissance profile (west of the 3j the Aqdranomana Lake,Tsarazaza and Hahatsinjo,tha second in the west part on the Taia^fakalika station and east of the Se Ampaitimwiga station) the average top of Kaaigie mnmtainland the third on the south boundary of Ambohitriraanjaka ) value of F is about -5001 .This is to be related to the substratum which is eon- seen to indicate three eastwest—trending faults.These faults are confined by our oonstited by ndgmatlc gneiss.Between the stations 37 and Eg,the P values vary from results obtained by studying aerial photographs of the concerned xone with a -6ool£ to -iSOolfj these anomalies appear to be related to tha rtrombolian puys stereoscope. . On the reconnaissance profile R2 (Fig.l ),the P values vary from -IO95»*to +8551 between the milestone 125 and the entrance of Ana*4avory town.while on its east sad PREVIOUS GEOPHYSICAL WORK west parts its value is about -300If The two reconnaissance profiles define the sone to be studied which is Previous work about the aiaantation of the Itasy lavas indicates limited by Laborde coordinates as specified above. that almost of then hare a themoremanent aiaantatlon,the sense of which is that of the actual earth magnetic field.But a prelevement site in Itasy region,wMoh Detailed magnetio surveys is of Prepleistocene volcanls> has a reverse aimautation (its sense is opposite t When interest is focused on the above delimited tone,more detailed mag- of that of the actual earth magnetic field ).This particularity doesn't absolutely netic surveys were made.The points of nsasuraments are 50m spaced.More than I,J00 exclude the possibility of inversion of the earth magnetic field at this epoch, points of measurement have been carried out.They are distributed along: but it oust be confirmed by complementary research work. -.8 east-«est-trending profiles (KOV84,J»85P6,JH85P5,J»85P3|JM85P2,JH85PI, TJ,^) - i west-southeast-eastnortheast-trending profiles (MS85PI,JH85P4,T5)
GEOPHTSICAL 3RIDIES - 5 north-Bouth-trendlng profiles (Tj.TjfTyT^Tg ) FiB-2- To attenpt for placing in a prominent position a favoured direotion constituted
Reconnaissance magnetic surveys were made over the IPANJA region by northeast-southwest-trending strombolian pays,surveys were made on 5 profiles and its vicinity.Is a part of a geathernal program,more detailed .magnetic surveys (Il,l2,...,15)perpendicularly to the alignment of these puys were carried out and reconnaissance resistivity data were obtained from audio- These detailed aagnetio surveys.with sensor at 2.5" above the ground level, nagneWtelluric (AKF) surveys.Later,AIR1 soundings were made in the sones where define much greater detail of the magnetic field.Bi particular points,the Intensity great Magnetic anomalies (more than 1000 If) appear. of anomalies can reach +2000lf(such aa on JN85P5 and K335PI) and fall down at - 2872 0*(on JN85P3),-*7281f (on JN85P2),-228l)f (on J»85Pl).The elements of these Ground magnetic surveys anomalies well correlate with volcanic rocks.with highs and lows reflecting general-
Two reconnaissance magnetic profiles (Sj and R? ) oriented HNW-ESK, ly normal • and possible reversed remanent magnetiEation.To be assured that these values are not due to the action of lightning currents on superficial rooks,sensor long of about 17 kn each one,were carried out.They are separated by 1 km. Pron the. was elevated at 8,5m above the ground level. Soaronono village on the west to the Ampahiaanga village on the east,is the first profile.South of it,the Booond is from Amparaky village (on the west) to Antanetim- One can conolude that the sources of the aagnetio anomalies are relatively boahangy (on the east) Fig.2 and is longing the ancient main road Antananarivo-Tsi- deep. roanomandldy
-2- -3- Audiomagnetotelluric surveys boh anomaly of a profile, assumed to be due to a 2-D dyke.waa interpreted by; tha method of KDULOKZINE,L/U«BTAfflrE and NADBUJ.lt 1* tha suitable method because of the Audlomagnatotelluri range 4,IHE to 2t3OOHx.This survey la used to confirm the presence of a vain and PROFILE AHOHLLY NUMBER DEPTH H (Meter) THICKNESS E(Heter) its elta in tb» magnetic anomalies cones. NOV84 (I) 200 500 Instrumentation JH85P6 (2) 6 10 For tha nagnetlo surveys a proton magnetometer CBOK0THICS was uaed(Fig.l). It JK85P5 (4) 65 180 measures tha scalar total magnetio field F. Readings are done in If (gammas) .The JS85P5 (5) 110 65 sensibility of tha magnetometer is about IY for a measurement value about 35000f. KSS5PI (6) no 606 Its working principle 1B baaed on the nuclear Magnetic resonance. JH85P4 (10) 160 390 For the AMP survey,we handled a reaiativimetar ANT ECA 542.0 composed of JM85P3 (E) 80 330 two selected chains of measurements with an acquisition and microprocessor comput- JM85PI (14) 50 170 ing system. The realstlvimeter function can be nsad when handling magnetic sensor The values of E and H are determined approximately about I0m.No more accuracy is and telluric line at the entrance.Ths amplitudes of the signals are checked at ovary got because it is Halted by the plotting of the tangents (at tha maximum and sonant bytwo galvanometrio indicators. The signals are acquired and processed by the nininun of the anomaly) and of the symetrio and the antieymetrio curves.But In the aloroproceasor then the values of apparent resistivity ,electrical field t any case,an order of values of E and H is obtained. Because of the high negative - and magnetic field H (produced by tha telluric current) appear on a numerical indi- ananolies values(remanant magnetization certainly),tha slope and the magnetic suscepti- cator. l*e AKT equlpnent la constituted of: (Fig.4) bility of the dykes camot be estimated. - a magnetio field senBor.lt is an inductive sensor with flux counter-reaction The correlated profiles NOV84,JN85P6, JB85P5,»S95PI (Fig.5 ) show that positive - an electrical sensor with 2 electrodes and one or two telluric lines anomalies essentially occur on the north side of the work tone.Those on the south - a measurement box there detected signals are processed and digitised data picked up aide,JH85P4,JH85P3,JN85P2,JW85PI include each one a high negative signature(-200OY) The profiles Ij,13,13,I, on the wast of the previous profiles include positive lfflSULTS OBTAINED , anomalies and negative anomalies on the southeast part of Ij and Ij (Fig.6). Tha two major features of anomalies in tha work field are a positive anomalies Results obtained by magnetic method sone in the north wast part and a negative anoaallaa in tha southern part. These results are clearly shown on the ground total magnetio field and the First,all the magnetic data {the values of total magnetic field P) are reduced upward continued(at 200m) total magnetic field maps (Figs.7,8). Two essential fea- for being compared.Then the sero of local anomalies field has been determined by tures appear:_ p^t^ anomalies tones ( f^] ) in tha north and in tha three methods: west part aligna along a NT5-SH direction -the method by the normal field world nap - negative anomalies aones ( ]l'~'\ •) esoantially In the south -the method by ohooBing the nearest reference station which is not in- aligna along a N-3 direction fluenced by the volcanic formations It nay be pointed out that the magnetio anomalies values (observed and up-* •• -the statistical method ward continued) along these directions do not indicate relatively a great variation. Thus,the reference field in the workfield has been determined such 33O50X One can deduce that the anooalies sources are not shallow because upward continua- tion gives a smoother field than at the surface of measurement. -A- -5- ezceptional to find a layered space where a quantitative interpretation can be It la to be stressed that NS-5W direotlon of tha positive anomalies seems to have coincidence-with that corresponding to tha alignemsnt of stroobolian puys and carried out. that of a fault suggested by photogeology. J M 85 P g. io No differences between the apparent resistivities in this profile appear:there is Tho negative anonalles zones which align* along a K-S axis,certainly corres- no vertical fault(or at least an Important accident) between stations 21 and 29. pond to a structural direction,Tha strong values of the negative anomalies indicate Terrains are rather layered.Then,an averaged AM" sounding for the stations 21 to that the source rocks have an important renonent nagnetiiation which must be taken into 29 was established by computing for each frequency the average of the apparent account for the quantitative interpretation.The corrections will have an effect in the resistivities at each station.This sounding was interpreted by one-dimensional values of the slope and suceptibility of the 2-D dykea.Unfortunately,these corrections modelling.Than,an average section was obtained (see below) and one can get an cannot be made in this report because of tha lade of the remanent magnetisation values. idea of the investigation depth ftr the part of this profile.At the east of Jfce If one refers to tha geological map,the NE-Stf direction(related to alignment station 2I,a vertical inhomogeneity centered on station 19 is obaraotericed by of Btrombolian pays) can be associated to pleistocene volcanisai and the HS direction high resistivities (IOOOif).Between L. to the preplelstocene volcanisai . 22m! 44 Jim the stations 17 and 13,further in the east,a cone of conductivity with There should, have been two dlffemt tectonic directions ( H-3 and HE-Sw) low resistivity, appears,but without associated to two voloanisms of different ages (prepleistocene and pleistocene).'Thus, 700m 3O0 Jl*> exceeding TOO Am. It leads to suggest tho hypothesis of a normal magnetisation agreeing to pleistocene At the station 10,on* can find again voloanisa and reverse magnetization to prepleistoone voldlism. aa inhonogeneity,wider than that centered at the irtation"^ with a high resistivity in the same order of Magnitude. Results obtained by etotelluric method PROFILE JM85P5 Pig. 10 Audiomagnototellurlo(AWT) soundings have been oarrlsd out in the magnetic The station 14 is the east boundary of a vertical inhoaoganeity which seen* rather anomalies cones to gain a better understanding of the structure of the volcanic wide, its influenoe has an effect until the station 20 on the west part of the rocks.At each location,JOT measurements were made for different frequencies.Tho computed profile. values of apparent resistivity are plotted on a hilogarithmio graph,apparent There is an other inhomogeneity centered at the station 26 but narrower and shal- resistivity being plotted as a function of the inverse of the square root of the lower. frequency.So,the apparent resistivities looateft'on the left hand of the field curve The sone between the two inhomogeneitles oan be considered as approximately a correspond to the shallow la^rs while those on the right to the deeper layers. tabular sone and a quantitative interpretation of the soundings at the stations To have an idea of the distribution of the apparent resistivity along a iOO Sim 5 and 3 gives the following section I aeotional elevation," semi-quantitative section * was used.It is a diagram where sound- 2OO SOOm ing locations are distributed along the abscissa axis,and the ^penetration depth' P H (whloh is a parameter,function of the frequency and of the apparent resistivity) along JO M5B5PI Fig. 10 the ordinate axis.At each location,the apparent resistivity,computed at a given 5 This profile offers rather high resistivities at the east frequency corresponds to a penetration depth.'This apparent resistivity is plotted at the of an other wne where the resistivity is lower (50OJ*4.Biit the variations of the vertical of the sounding location,and in front of the computed penetration depth(Fig.9). resistivity for a given penetration deptb are much lesa important than between But the penetration depth is not really a depthslsoresistivity lines must not absolutely t be associated to layers.Etanples are given in Fig. 10. 2 stations of the other profiles. Each part of the field curve is interpreted on the account of the data PROFILE JH85P4 Fig. 14 of several sounding locations In the vicinity.Host of the tins,the interpretation TMO vertical aonea of high resistivity (more than 1000C) appear on this profile 1 remains a qualitative interpretation because strueturn is complicated and it is one is rather wide and centered at the station 8,the other is narrower than that -6- at the station 48.Tha west sone is characterised by resistivitieB more than IOOOI" uaxiaua reaches the value of Iioot .On the other hand,the AWT section shows a rather but th« anomaly fom seems to show that It is longer along the horizontal direc- narrow and relatively deep vertical inhomogeneity with a high resistivity of 10001* tion This justifies the two-dimensional dyke interpretation by KCTJLOK23NE et al. On the AJTT section,the high reuifrtivitiea, rising 9OOH» ,c(m be associated to aeries of peaks (-20001 ,-13001* ,-900^ ,-700V).Their minima come nearer to the formations of basalt type, sero level of the magnetic field as you go towards the east.Ons of the previous situations (profile MS85PI) Is found again.lt can be assumed that there is a PROFILE JH85F4 Pig. 14 main source to which a dyke is due;this dyke cuts again the profile at the:station The magnetic profile and the JOT Beetion show that t 9 which is characterized by the most important magnetic anonaly and by high l)at the west of the station 46 to the station 16, a negative anomalies sons where resistivities. the intensities are falling to -2000If at the station 46 and to -IOOOfat the station Secondary dykes to which the Magnetic anomalies far in the east would be due, H.appears.In account of the Inclination of the earth Magnetic field I - -53*these are less and less important when becoming far of the Main dyke. negative anomalies carrot be only due to the magnetisation of the rocks,induced by The positive magnetic anomaly centered at the station 20 is certainly due. the earth magnetic field.Onoe again,the hypothesis of the existence of a rather to the strombolian puys in the vicinity.Without doubt,it is dated of pleistocene important raaanent magnetisation in this sone with reverse magnetisation can be volcanisB. advanced .These negative anonalies could be associated to the prepleistoeene vol- oanisn dated of nore than 1,8 Million years. PFPFTLE JH85P2 Pig. 16 This hypothesis of the inversion of the aagnetlo eartb'Tield in the pasttorlgin of Prom east to west,we have successively: a reverse magnetism of rocks is not inconsistent with the following resultst -a positive magnetic sons of anonalies between the stations 10 and 27.This a)GIHDLER R..W.and PETER 0. (i960) In "An example of the Importance of natural anomaly must be associated to the presence of a trachytio dome in the vicinity. resonant Magnetisation in the interpretation of Magnetic anomalies" point out that The corresponding AWT section shows a shallow una rather tabular (2000^300Am' many laboratory studies with different rocks dated of quartenary or uppsr tertiary 500-G«\.) which would be related to the alteration of trachytic formations, have a reverse remanent ramagnetiEatlon,corresponding to an Inversion of magnetic -a narrow sone of a negative magnetic anonaly (14) (peak at -2000 V) centered earth field at the station 8,to which is related a fault moderately resistant (5OO-6O0A^. b)the volcanic scale of inversions Bhows that the "inverse epoch" of HATUTAHA, Its magnetisation would be reverse (hypothesis previously suggested ).ThlB for which the periods with an inverse polarity are for sore important than the anomaly would correspond to the prepleistocene voloanisM. periods with normal polarity,corresponds to periods between 0.69 HA *"d 2.43 HA -a positive magnetic anomaly centered at the station 4,for which are associated c)ANDRIAMIHABO (I969),in his work results about the Magnetisation of the flows high resistivities but rather shallow.Its proximity with the previous negative of the WAST, pointed out that a sample site of the ITAST region dated of pre-plei- anomaly leads to suggest that the flows of the pleistocene voleanism circulated stooene volcanieM,has a Magnetization for which the sense is in the opposite of through the fractures of pre-pleistooene volcanisa.Then,all that could be related that of the actual field.This particularity leads to not absolutely, eiclude the to a (Jjfco of preponderant normal magnetisation. possibility of the inversion of the magnetic earth field at this epoch but it -a positive magnetic anomaly between 41 and 53 and to which corresponds a dyke should be confirmed by coBplenent research vorks. with normal magnetization associated to pleistocene volcanism. PHOFILE JM85PI Pig,17 -10- At the station 9 of this profile,!a a resistive dyke with reverse mag- -11- netiwtion related to the negative magnetic anomaly (l§).In *he BUM way,at the station 94,and corresponding to the negative magnetic anomaly (15),an other djka with the same previous nature ean be considered. this section is In particular obtained from the result* of the profile JN85P5. The substratum is shallow.lt is oovered by a thickness of 50 to IOOm volcanic for . Global view tions,essentially of flows and projections of prepleiBtocene volcanism in that case. A rising of pleistocene volcanism circulates in one of the tectonic aocidents but . . There are two categories of profile does not arrive at the surface,at least on tha profila.lt becomes a source of a strom- - the B-W-trending profilea JH85P bolian puy close to the profile and the strombolian projections mentioned at the sur- -the 3 B - S W -trending profilea I face of this aeotion are due to it. • Profilea JK85P Tha simultaneous examination of these profilea points out thatt Pleistocene and grepleistocene volcanism section - the profiles situated in the north include,in the aggregate,positive magnetio anoaalies.lt is about profiles JM85P6,JN85P5,KS85PI.The3O positive anomalies It is essentially suggested by the data of the profile JN85P6. are associated to pleistocene volcanism then nore recent. The substratum is always close to the eurfaoe and even crops out in the west - the profiles situated in tha south,JH85P4,JN85P3,JJl85P2,JK85PI principally in- part.A rising of prepleistocene voloanism produces the trachytio dome situated in tha olude in their east part,negative magnetic anomalies which aligns along a lf-3 middle of the profile,The flows and projections emitted from this dome cover again the axis sad coincide with the axis 427 of Laborde coordinates.On the other hand,it east part of the profile.A rising of pleistocene volcanism circulated later on along ' is to be atreased that the width of the anomalies decreases when going far away an other fracture of the substratum but did not arrive at the surface . The west part of in the south.These anomalies are associated to the prepleistocene volcanlsm.'niat the profile is very locally covered again by strombolian projections vMob have not • leads us to suggest a faulty zone,of B-S direction,continuously or discontlnuously reached the east part. filled by volcanic intrusions for which the magnetization would be inverse. •Profiles I Prepleistocene volcanism section To provide data which are necessary for a better correlation of the anoaaliss from a profile N-3 to another,the profilea I were established in a direction approximately It is suggested from, the interpretation of the volcanism JN85P3. perpendicular to that of previous profiles.Their direction is less suitable for detecting anomalies N-S and tha majority of these profiles are situated in the A system of voloanio multiple risings with common source produced trachytic domes in the -vicinity of the profile,at the pre-pleistocene period.The domes emitted mt part of the work cona.That is why:the examination of the magnetic profiles Iif flows and projections which oovered again tha east part of tha profile.The substra- 1 2' 3 »M, uniquely shows positive magnetic anomalies associated to pleistocene vol- tum and a part of previous formations was covered again by projections and atrombo- canism.The alignment of these anomalies along a KB-SW axis suggests the existence lian scoriea at the pleistocene of a fault system along this :.direction. Fro* thia analysis,a geological interpretation of geophysical anomalies can be deduced in the form of three sections where are summarised the encountered situations (Fig.18). Then,if these conclusions are assoolated to the examination of the total tic field maps (ground level,upward continued moe),ons concludes that 1 -12- -13- That period would correspond at that of the inverse period of KATOTAKA (scale of strombolian pwys easily aligne along several axes of HE-3H direction. Furthermore,tnia direction coincides with that of a fault detected fay photogeology, voloanic inversions ) These axes correspond to positive anomalies zone (orange-coloured on the magnetic Paleooagnetic studies are necessary,In particular,magnetization measure- •ftps).on the other hand,the negative cones aligns along a M-3 axis. ments (intensity and sense) of samples to be picked in the tones of very negative -the direction HE-SW can be associated to the pleistooene volcsnion and magnetic anomalies,would be very useful to oonfirm or infirm the hypothesis of the direction K-3 to the pre-pleistooene volcanisa,There are two tectonic different inversion.'Hie datation Rr-*A of the relative lavas would complement the measure- directions and two volcanisms of different ages. ments. •Urns,the hypothesis that the normal aagaetixatlon is associated to the Besidee.the measurements are limited to the north part of the volcanic pleistooene volcanisn and the reverse nagnetiiation to the pre-pleitrtocene volca- mountains of ITAST.An aeromagnetio survey of all the region would certainly give niam,ean be advanced. complementary resultstaeromagnetio survey is the most suitable,In account of the width of the area and the topography, the combination of other geophysical methods,in particular,the gravity and electrical methods would renforoe the know- CONCLUSIONS DRAWN ledge about these volcanic structures,and even give an evaluation of the geothermio potential of the region. In fact,the hypothesis and incertitudes of the magnetic interpretation show that to utilise only the magnetic interpretation Is a risk. The magnetic prospsotion showed the repartition of the magnetizations Moreover, it must be reminded that the AW method allowed to confirm the presence of in our work cone (magnetic nap). It remains the •oat suitable Method for the study a dyke and precise its site.In addition,this method gave the first informations of voloanio structures and hypostruotures. of the depth of the substratum,estimated about I00» below the voloanio formations. The detailed interpretation of the different aagnetic profiles in the north ; part of the volcanic region of the ITAST distinguished two well-separated sones of nagnetic anomalies, a aone of positive magnetic anomalies and a ions of negative magnetic anoaalies. The xone of positive Magnetic anomalies strengthens in the west part of ACKNOWLEDGMENTS the region along a direction DB-SW.Tnat direction detected as a tectonic direction by photogeology,corresponds to the direction of the alignment of stronbolian puys.The The author would like to thank Professor Abdus Salam, the International different rocks oonstituing the different formations of this aone are nonal mag- Atomic Energy Agency and UNESCO for hospitality at the International Centre for neti>ed,lhe corresponding volcanic eruptions are associated to the pleistocene Theoretical Physics, Trieste. voleanism. On the other hand, the HM of negative nagnetic anoaalies is east-trending along a N—3 dlrection,whlch constitutes an other tectonic direction. Hie correspond- ing formations would be constitued by reverse magnetised rooks.If the intensities of negative anomalies are very important on the profile JH85P4,JHS5P3,JK85F2, JH85rT,the collected data are insufficient to affirm surely and definitively the Inversion of the earth magnetic field at the period of pre-pleletooene voloanism. -14- -15- REFERENCES CENTRE REGIONAL DE DOCUMENT AT ION PeDAGOGIOUE 11965) 15, RUE D'ftHBOISE AttCRIAHIRADO R, 11967 : ^nif 1' jlricntiltian dBS IJUUTJ cje Datifo 63037 CLERMONT-FERRAND Antsirebj ct Jc l'ltasy (Centra dta ris- djgabcdr] - Cofriptc r»ndu du seminaira Le volcanismo geolugiquu MQdJE-scar 136? LEXiqUE Centre GRS10M - Antananarivo CQULC11B J. at JOBERT G. 1976 : BENDEfilTTER V, et OUPIS A. [1963) : Tralt6 de G£ophyslque Interne. Tome II PruspectlLn geaphysique par l tiagnetisme ot Geodynamique - m;.j;n§tc ta 11 urlque . niaeB A Jour, tome 4 Edit. I1ASS0N - Paria p. 2S1-274 GACHON A. et LENAT J.F. (1975) : BESAIRIE H. Ct JOQ J. MS72) ; Etudu d1 dncmalitis msgnetiques aitu^eb fcn Hlsa ou pclnt sur Je volccnisme ds Llmsgne et sur sea marges (These Ji Cyc- l'ltasy (MaaagrTscer) . Bull, Acad. Nalg. le - University tin Clermont-Ferrand) t. 49/2, 19?1 GIRDLER R.W. and PETfcR G. (1960) ! BHATTACHhRYYA B.H.. (1976) ; An example of the importance Df naturul Cunputar modeling in gravity and rtsmanent magnetizat ion in the lnterprt- tlc interpretation geophysicB, Vol. 43 tetlon of magnetic anomalies N'5 pp S12-S2S Geoph. Prosp. 8, pp 474-463 BRENCN P. IH57) Traits gtnirou*. du volcanlBine da Madagas- GREEN R. (19B0) Btminent ugnuilzatlun and the interpre- car dona la cadre de I1Ocfean-Indien (As- tation uf magnutlc anomalies aciclation Sclentiflque des pays de Gaaph. Prosp, B, pp 98-110 1'OcSan Indian - Antonanarlvo 1957) HERISSUN C. (1975) MiilhodE moj;n6tcite llurique dans la recun- CAGNIARD L, (1953) Principe da Is m^thode ncissancu du socle sous recuuvrement 1 ncuKilK mfithcde da prcspiectlon g6cjphysi sodimsnttire : qu^trc exeaiplcs dans la que, Extiait dss Annalda de G^ciphyalquE, Hnbbif Uuntral - These de specialite - toms 9. (1953) fascicule 2, P, 9S h 12S t iitv-rul JI re dt PiJ t rt. logi-j , i-i' 0 r 1 :> o n s -16- -17- ...»••• fe ..~Hr,o-n—' MJELT S.E. Bt HALLILA £. 11573) MUDN P. (1983) Utilisation du probleme inverse pour la Remanertce Correction in Hognetis Ir rvcherchd d'une Interface en gravimetric tatlon with tho Infinitely Dutp Platt- Me- et en magnStlsme. Application • a un bossin dal - Pure and .applied geophysics Vul 110 tertlaira en Laiguodoc et i une structure (1373/1X) aimantee dans lo Go He du Lion - (Th&ae de 3e Cycle) J00 J. (1966] Geolagla et prospection des fsullles Arabohlmiandra - Fenoarivo - Soavinendriana Une methods d'analyse fine des profils (1145 - 114fa - 1147) . Rapport annual du ser- NAUOV H. 11970) aeromag. Geophysical Prospecting. Vol. vice g6ol. Had. 1965 pp 7-17 10 p SB-63 K.OEF0EC 0. (1973) G&Daoundinj; principles, 1 Resistivity sounding Measurements P*>TRA H.P. at MALLICH H. (1980) ; Elsavler Edit . 276 p Geosounding principles, 2 Time varying geoelectric soundings LACROIX A. (19Z2-19Z3)tHineralogiB da nadflgoscar El3ewier Edit. 119 p RAHOTONDBMHIBE (197?): Con trlbution a l'ituda de la seismicite LAPLAIME L. (1951) ; Etude geolcglque des fsullles Tslroana- de Modarascor mandidy - Soawinandrlana T.B.G. N*20 - Service Gaol. Had. REtilOT H. (1966) Contribution 3 la conndisssnea Qe la 3Ui- ceptibilltiS magnetique de quelques rcchus LASfAft&UCS P. (1966) : nagnetlsme on geologie at prospection malgaches (Instltut de la Recherche Scien- magnetique au sol tifique du Madagascar) LENAT J.F. et TRICOT B. (1979) t Etude structurale par gccphyslque dans lu Premieres mosur^* de prospoctlon magnetl- RD81N G. (1984 1 bassln hydrcgeologique du Nohain (Nlevre) quG a la nfeunlon (Ocean Indian) Application IJB la methode magn^totellurl- Interpretation et perspectives - Nate que d ID detection d" ectMilements scuter- I.O.P.G. N'46 raina an militu carbonate' fissura LENOBLE A. (1940) Etudes sur la geologie de Madagascar : r la massif de l'ltasy. fifinoire de l'Aca- J.:1. [ 1 l 7 7 ) ; :-. inplifi-'J n-i, .. Ki;li: cufit.-ut inu thin d^mle rialgachs Fasc. XXXII Notes II et £ o 1S7/J Guuphuslca. Uol. 42 N°6 III -18- -19- SOAVINANDRIANA M.O Fi>.X Map of &e IWSy rcjioo Jie aMrtcficU and die fa -20- PROTON MT MAGNETOMETER magnetic electrode MAfflCTO-TELLURtC SMTJOV magnetometer Fig. ri8.3 -23- / N0VB4 50cJ "wes* Cost' 200m west (15) CORRELATED PROFILES -24- 8.6 \ \ JN8SP6 •JN85PS •JN85P5 - 200m 200m Fig.8 field map g.7 Ground totaL magnetic freU -27- AUDIO-MAGNETO-TaUURlC. PENETRATION B&1H FREQUENCE & © @©(3> © of staft .100: curve.of I resistivity .of 100r •UJ Cl Fig. SEMI-QUANTITATIVE SECTION -28- interpretotion scheme InherpreFahon scheme •u"1j 4«Am Fig: DETAILED VilW-PROFIL JN85P5 « «**« . -31- Fir.11 tiCT* 11 rtt. jii 1260 Geological secKon Moo Geological section C^-V ]s>raml>oliai ^rejections M06O -jiifrombotiun pnjvch'aivt 3 Plowt and frmjtctitns W -S-W 17) (8) , (9) E -N- E Magnetic vie -u. Audio-MT sech'on 30OW mf ^ SOOi* (m) fig.13 DETAILED VIEW OF PROFILE MS8SP4 FiK-1* VIEW OF PROFILE -32- -33- 4- '•'.< , x\--.Vmv-' ffagneh'c <16O **»**«*»*»*, WVWVVN*^!^ \ UOB Geological Section 40m Fig.is D£TAlLEii VIEW OF PROFILE JMS5P3 FiB-16 DETAILED VIEW OP PROFILE -34- -35- •Pleistocene &eo log Teal section »i Prtple.sh.cene and pltiilbecne WB&T {fi1 MAGNETIC fROFiCE Isoor Audio-MT section Coo- West- WOrnl r £00 Ceo CDOmt penetration projections f Pie/sbcene sfviKing n>agn*h'c dyKes wi "depHi volcanfsm diVecKon normal hh Cm) J . we - SW and high AOO"' wit noo < volcgn.sm Hinetiin reverse 1. j, j ^- S and htgfi OF fie doomes PROFILE rig.18 T^pes of volcan.c Udicapes deduced •frorn -36- -37- Ea* II8S3S.S68SS