THE SYSTEM OF CONTINENTAL RIFTS BORDERING THESANTOS BASIN, BRAZIL

FEltNANDO F. ~1. DE AU\1EIOA

Jnsfi tuto de (~e('(têr.ctas. Universidade de São PauloSão Paulo, Brazil

ABSTRACT

Opposite vertical rnovernents exceeding elevenkilometers have occurred since lhe Lower Cre-taceous belwecn the Santos Baaín, localed on lhecontinental margin, and lhe neighboring con-tinental regíon. The geodynamic process waeaccompanied, between the Crelaceous and Eoce-ne, by íntense fissural basaltlc magmatism aswell as central type alkaline volcanism. Afterlhe volcanism ceased, when a great difference inleveI had been established between the tWQ tecto-me provinces, a syslem or rifts, whose depres-sicns were part.l y occupied by sedimenlary basins,developed tn lhe region. This tectoruc syslemevolved from an erosion surface of probableEocenic age, deformed since the Oligocene. Therlfts (the most notable of which ts lhe one lhatcontatns the mlddle valley of the Paraiba doSul Rivet- and lhe Taubaté Basin) are borderedby hlgh mountains, whíoh were elevated duringthe process of subaídence ín the areas betweenthem.

ln tte geographlc [ocatíon, the system ofrlfts wae Intiuenced by , the proxtmtty of theSantos Bnsln and the presence of a grea t systemor anctent trsnscurrent rauits. The Jntter, reacti-vated In the Cembro-Ordovíctnn. cuts obHquelythe precamhrlan metamorphlc structurea or theregion nnd WBS pa rtlally reactlvated slnce theMesozoíc. Thls system seems to have also ln-rtuencec the general ccntour of the coaet aswell as the northern border of lhe Santos Bastn.whtch is IImlted by 11 large tautt on the conti-nental shelf.

It is suggested tnat lhe late rifting of lheeleva.ted ccnt.inenta l reg-íon 15 due to a processof gravltational slidlng towards the Basl n. fi

result af the proximity ot the two provtnccs ondthe ímmense diffcrcnce in basement levei h(>-tween lhem.

lNTltOOUCTIOl':

The Santos Bastn is a mcso-cenoaoic. tcc-tonto de preaalon futl of basalts and scdímcuts,located on the continental margin between theStates of Rio de Janeiro and Santa Catnrtna.In front of the Basln, the Atlantic pla teau otsoutheas tem Brazil enda In I1ttoral scarps, 800to 2200 rneters high, .ín a mountain chaln strctch-ing 1200 kilometers, genericaJJy known as theSerra do Mar.

The complex or tectonic vnlleva, blockmountains and smal1er xedímcntary baains ortectonlc origino whlch form the Serra. du ~farRltt SY8tem, representa one of the most nota hletopographlc and structurat uspecta or the Attnn-ttc margtn of the Arnertcas. The opposlte verticalmovements whlch took place in thls region aincethe Late Jurasslc probably exceeded 11 krn ordlf!erence in level between lhe híghest mountnlnsand the Santos Bàeih basement. These move-mente were accompariíed In par t by Intensemegmattam "'h1ch produced trom ultrabasic andtholeiltic rocks to pernlkaline granites.

TIl•• eísmtc. actlvl~y or the reglon. af thougfreak, aeems to trtdlcate tbat the rift syetem i~

not entlrcly inactive.

16F'ERNANDO }o.,. M. DE ALMEIDA

Thc st udy or $0 great a differential verticalmov~ment. nccompunied by auch mngmatfsm,wlthm a. plate prescnts a suoject or specíaí Inte-rest, for whích an adequats explRnation has notvet becn found in medem geodynnrnic modela.

Although lhe Scna do Mar Rift System haanol y~t bee n fully xtudied, and has been mappedonl:: m a Jimiled urea. lhe a.vailable knowledgepel.lllt~ a charHctcri7.flllon or íta major corupo-nents. Mor-covcr. it pennits a tcntativc Jnter.pretat íon (")1" rts evotutton, with rcspect to the~cvelopment o( thc Santos Baaín. The Baainrtsetr has 'bcen inv(>f.!tigatcdvcry liltle as yet.Those arca.s least investigated, lhe States' orPnt-aná nnel Santa Cn tnrtna wlll not be describednot-e.

TIni: ANf'II':NT iõ;TltlJCTfJRF;S

,

. Wlth t rte excep ttnn or minor continental se-d'mentary bll!'lins Hnd Httora! sedimentation th

~\'hnlc const.m rcglon !Jonlcring lhe Santos ~aSi:1.':; compcsccí of prN':unbl'ian metamorphic rockspenet~ated h.:, granilc~ whose ages go up to th~Ordovictan he lnajol'ity or these rocks belongto the Ribe~r. folucd bett (Almeidn ct al., 1973J.c:,olved dur-j ng- the Late Precarnbria (BraziIianoCycleJ, 01' to its infrasttucture.

.Thc oldcst I'ocks Hre exposed in one of themeclHl~ nJ~tssifR or the belt, that of Joinvile(HnsuI e-t n.", 1975). Despite policyclic rework-mg. the maRsif sUl! rctnins R-Ar rlldlometric<l~(,.<:: exceeding ~lOOO!TI.y. (Mlnioli, 197.3). Tllehlghest elevations nf the Serra do Mar in Pa-~aná, ~Iose to 2200 m abov sea leveI, are found10 t.el'~l~ryfa.ult blocks supported by the cambro-or~ov,clan allcaJine granites that lntruded the!JomvUe massif. Also in the Ribeira belt Jnfra-l'ltructure, ror,k~ tIre found that belong to theTrnn!"l-.AIllDZOl1ic cycle. whose prinCipal mcta-morphlC and granitizntion episodes took Plac~between 1800 nnd 2000 m. y. These rocks formthe Pal'niba do Sul folded belt (Almcida et n)U~73), who~e Jnost developed areas lie east o'~Sall Paulo, SQuth af Minas Gerais and' thSt t f . . m e•. fi, e o RIO de Janeiro. The Paraiba do Sulbelt s rocks prcsent mctamorphic fneies varying~rom ampflibolite to gmnulite. and indicate an~nt~nse influence o( migmatization and graniti-za~lon also prodtlccd during' the Late Precam-bn:m.

The Ribeira belt is formed by metasedlmentsgeneral1y fOl.Hld tn metamorphlc fa(!les va'ryJn~from greenshist to alllphlbolite. the latter havtng

t1H. Acorl. bTrl.<f. CMnc.~ (1976). 48, (Suplemento)

bee~ partly migmatiz.ed and granttlzed. Thebelt fi late dcvelopment took place durtng theCambro-Ordovlcian. At that time. the mnlaaseaformed and the subsequent volcantsm occurredtogether with the intrusion of numeroun post-klnematic g,ranite plutons. After the fOlding~henomena or the Ribeira beIt, an intense fault-mg process occurred, possibly as a resuít of thereactivation or deep, anelent lineamcnts. Thtsfault system is 'composed of larg'e dextr-al, trans-current .fauIts, trending prlmarlly ENE. Theythus obhquely cross and inflect the precambrianstructures which gcneraJly trend NE to ENE(Flg. 1). lnformatlon on the Late Brazlllanofnult system is sUII incomplete, some of itsmajor components having only recently beenIdentifted. N~vertheless, many of Jts major faultsand mínor assoclated faults are already weü

:.ocumented ín field atudles by varlous Inves-,.lgators. Some of lhe faults present belta ormtensc eutaclasUc deformatton, hundreds ormetera thtck. The author .maintains that some otthese ancient fautts suffered local vertical mo-vements duríng- the Wealdian reactlvati~n (Al-meida, 1969), as did the documented case orthe CantareJra tault zone, north of tha ctt .Qf São Paulo. y

Magnetometrlc studies indicate that theabove-mentioned precarnbrian structures extendto ~e ba~ement of a large part of the SantosBasm which evolved on the continental crust.

The precambrian structures and especlalJythe late Brazll!ano faults had Rn appreclableInfluence on the dl.posltion of the major compo-nents of the Serra do Mar Rift Systemshawn In Fig. 1. A comparable phenornenon ~:;be o~served in other rneso-cenozoic continentalrift sl.stems of the world, such as: the grabensof Rhme and Oslo, influenced in lhelr deslgn bythe Hercynian faults (Bederke. 1965); the systemof East AfrJca rins, eonditioned by early precambri.n faults (McConnell, 1974); or 11;"Baikal rifts, also influenced by anclent f.g,uttsof the Bnikalldes and Early Caledonides .true-tures (Florensov, 1969). In South Ame,.icn,camparable examples are found ln the Braziii1.nNortheast (Almelda, 1972), Rnd In the grabellof Takutu in Gulana (McConnell, 1969).

Thc NNE trend of the Brazlltan caa!::tb~tween Salvador and lhe Paraiba do Sul Riveru(;lt'a retlects the trend of the oldest Trans--'~Imnzonicstructures, reworked .dunng the Br",.7.t

•.ano cycJe. The Paralba do Sul River. in thf'!arca where it borders the States of Minas Gere.i'i

THE SYSTEM DF CONTINENTAL RIFTS BDRDERING THE SANTOS BASIN, B.RAZfL

and Rio de Janeiro. has its rectiJinear eoursedetermJned largely by a vast dextral transcurrcnttnult of the late BraziUano systcrn (Almeida.Hasul and Carneiro, 1976). The Trans-Amazon~cetructures were markedly inflected ENE becauseof thís as well as other late Braziliano faults. lnaddltlon to the coastune, the fault Itself, whlchbordera tne northem part of the Santos Bastn,adopt the ENE trend of the large cambrc-orôcvícíen faults. In the States of Santa Ca-tarina and Paranã, the coastal region 19 devel-oped in the Jo1nvUe median masstr, Itaelf notarrected In Its Interior by that fault systcm;the coastline as well as the faulted flexurebordering the Santos Baain trend NNE, accomp-anled at thI8 potnt by tertlary inults or theSerra do Mar, Identltled by Maack In 1947.

P08T-PALEOZOIC STRUCTURES

From the beglnnlng of the Wealdlan reactí-vallon, the e!ellCl'lbedreglon became rrubjoct toan Intenso tectonlem and magmatllm. Two 01the most notable reSUlta ot these phenomena arelhe upUit oi the Ponta Gro... arch anel theovertlow oi lho Paraná Baatn'. b•••••lta.·

TIlo Ponta Gr_ arch I. 11 large upllftadstructure wttOM lÍXIlIInclln•• NW, normal to thecORBlIlneoi lhe 8tata Di Parl\M. Although It.orlgln seema to ·extend to Uppor Paleozolc, thestructure suttered Intanse uplltt, •• Umatad byBowen (l966) at 7.~ km. between lhe LateJurn.ol. ane! Neooomlan. AIlhough .uch a valueseems to be O\'8J'oe8tlmated, the uparchlngnctlvlty dlll'lD&'lhe extrullon oi lho blll8lUcmagro. li won confirme<! by lhe Immense quan-tlty Di vertical. dlabue dyko., parallellng theurch's ax18. Beeau.e the arch's pre8ence 18 notreflected In lhe Santos Bam (Leyden et aI.,1971), Il _ma· lha! the acUvlty 01 lhe PontaGrossa arch coued at the developmenl of apUan-albtlUtsedlmentatloo In lhe b•••in. To judge iromthe largo quaoUty Df dlab•••e dykes trendlngENE 00 the Berra do Mar and the coaBt, it.eemo that thúI reglon also particlpateel In 8

large uparching or nexure slmtlarly ortented,resultlng 10 the extruslon af the basaltic magmnthrough the tenslon tractures.

THE SANTOS BASIN

The Santo. Basln was identiiled In 1960 asa result of seismlc retraction studles developedby tnvesttgatora trom the Lamont-Doherty Geo~logtcal Observatory. In 1967, new researchunderlaken by these 8clent1st8, uslng sonobuoys.

17

perrnitted them to delineate 'the Bastn In partoand to determine its great thickness of scdlments(Ewlng et aI., 1969). Butler (1910) discuss e. thenature of the Baaln's sediments, basing hís con-clusíons on selsmlc reflectlon prorües. He poJntedout lhe existence ot salt domes and unatyzedthe conditIons in which the sedimentation devel-oped. Seismograph1c research by Petrobrás in1969 was pubUshed by Baccar (l970) , whlleLeyden and hlo collaborators descrlbed In 1971the resulta obtained with new setsrníc, gravl-rnetrlc and magnetometric proflles taken through-out the BaBln. Mlrand& (1970) nollfied theresulte of the setamtc rerractíon and ref1ectlonresearch undertaken by Petrobrás whtc h led tothe discovery or the large Santos fault. Shortlyatterw.rds,· Petrobrãs begnn the rtrst drilllngsIn the Basln ..

Despiu ali these Invcstigatlons the SantosBuln Is BUlionly barely known.

1'he Buln 18 open towards the ocenu whereIt bordara the São Paulo plateau. It.'l continentaledge ta bordered by lhe Santos fauJt wnose ellpOR the nortb exceeds 3000 m. However. It be-comes o. Jarge tnult flexure in the SW. F'ig. 1was ndapted irom Leyden et et. (1971), to lhearea correspond1ng to the ocean, and has lheSantos fault outllned. It shaws lhe Basin'sdeepest zone On lhe continental slopc in fmntot the Stn.te af São Paulo's coaslllnc. The ha~sement h!l tound to be over 8000 m under sen)evel. Bacéar (1970, Flg. 5) shows a depth ofup to 7tsOO m for the base of lhe layer (I[ ~ei8mlcveloelty 4.2 to 4.~ km/oec., whlch I. Interpreteda8 belng tormed by apttan evaporiteR. Underlhe salt beds, there are rocks Wh05C seismit'velocltte8 vary belween 5.1 and 5.6 krn/sec. rmdwho8e lhickncs8 reaches 3.8 km (Leyden et aI..1971). They 8eem to represent bnsaltíc rl)cksrelR.ted to those of Serra Geral.

Above the 'aalt beds, there are up to 5.6km or thlckness (Flg. 5, Baccar, 1970) of clnstlr:sedltnenls lntercalated with carbonnte~, repres-enting a. column from lhe Upper Creta('eou~ trl

Quaternary.· Butler recognLzed in lhose scdl·ments three unlts. while Baecar tnnintaihs thattherc are at lenst four. Thesc unit8 fornlprograded sedimentary wedges to which Butlertried to correIa te the coastal leclonic l1lovf'Jllentf>.Thc development of pedlments would cQrrc8pond

. to these wedges as well. The tcctonlc move-ments. in Butler's oplnion. would have thefoUowing ages: Late Cretaceous-EBrly Tertinry,Middle Tertiary nnd Late Tert1nry-Plcl~l()cenc.nceordlng to the ages attributed to ('nll~tnl pe-

AI~. Acad. arM. Ci6ftc., (1976), 48, (Ruplt'Jlwnt,'1

lR FERNANDO F. 1\1'. DE ALMEIDA

dtmenta by .Bigarel1a and hts collaborntors1.1965, 196ij n).

A selsmtc sect.íon (Baecar, 1970. Fig. 5)indicntes that lhe cr-yatalhne basement or thecontinental shclf of( Juattnga Poínt (Rio deJaneiro) constitutcs A surface inclined towardsthc Santos Ba etn. This surface extcnds up tonearly 65 km where í t shows a rapld nccentuat.íonof the slope towards the Santos faul t. The arcaalong the conat Irom the States of Rio de Ja-neiro to Pnraná has numerous islands of prec-ambrtan rock s, or senonian alkaline tntrustves.This crystntune surface is aupposedly a reaultof retreat due to lhe eroston ot the ancestralSerra do M~.1;', since its origin tn the UpperCretuceous nexl to the Santos fnult. The surfaceis covcred only by two younger parastratlgraphícunits (Baccar-. Hl701 with seísmic velociUes of2.4 and 1.85 km/~ec. The uníts contain cenozoicctnsuc eedtrnenta tton whose thickness reaches nmaxirnuru ot 800 m south of the Juatinga Point.

THE PARAlBA DO SUL RIFT VALLEY

also. These layers represent; the depostta of alarge lake dammed by the Cachoeira Paulistauplifted bloek. The upper formatíon of the Tau-baté Basin, separated from the lower by nnuncomformity, 19 apparently related to thatwhtch fiUs the Resende Bastn. This formatlon15 of a sand to clay nature, partly conglomerallc.and wea accumulated In flood plaina and ri verchannels. Its thtckneaa is approxímately 100 m.Ita age iR dubíoua slnce it lacks rossns, althoughtt ís generally presumed to be Pltocene.

The surnmlts of the Atlantlc hlghlands ofsoutheastern Brazil are leveled by the Japierosin surrace, whose altitude reaches close to

1200 m where not Iocaüy deformed. The Japisurface's age 18 controverstat since it lacksdatable eoverings. Becnuse tt extstec before the

begtnnlng of the subsidence or the Paraiba doSul graben, and because it leveled the senoninnalkaltne instrusions. this surface seems to have

evolved especlally during the Eocene. The peaksot the Serra da Mantiqueira are ostensibly leveled

to close to 2000-2100 m above sea level by anerosíon surface that E. de Martonne (1940)

Iabeled Campo's surface. He considered tt to beolder lhan Japi which he called Cristas MédiasCAverage Crests) surface. The aulhor in 1964dernonstrated that the two surfaces proposed byMartonne are actually only one which wae elev-ated tectanicaUy in the Serra da Mantiquelraregion.

The middlc cour-se of the Parafbu do SulRí ver devetop s in H tectontc depreasion 220 krnlong. g'enerüll y trending gNg of lhe precnmbrians tructures. Oeotogtcar. g-eomorphotogícal andgecphyaical etudtes. nlthough still incomplete,11ttest to the tectontc nature aí the depression,rccogrriaed by Maull (1930) as a graben. Thenverag'e width of lhe Puraiba do Sul Rift Valley

Is ctoec to 25 km. T'he difference in level, A geomorphological analysis seems to indi-between thc eroston surrace which levels the cale that the Paratba do Sul graben eollapsedbordertng mountains of Serra da Mantiqueira ín the axis of a large arched uplift, possiblynnd Ser-ra du Bocai nn , and lhe base of the faulted In lhe flanks, where the Japi surfacescdiments trmt. pfll'tially.fill the depresston, ts waa 'elevated to almost 900 m (Almeida, 1964).HIOO m. T'b e Itoor of the grabcn ts irregular: This interpretation is based on field observations,fi. medtum elevntton. the Cnchoeira Paulista the examlnatlon of lopographical maps and onuplifled hloc k. r-ises up to 800 m above sea the analysis of lhe drainage system of the upperlcvel. It is c rosseô by lhe Paralba do Sul River Rio Grande hydrographic basin. The basin'sin a deep v a.lley. weet of this elevatton, the major rtvers are dispersed radia1Jy in the

Tn.uba.té Bu s in. partially full of sedtrnents. has highest zone of the SeITa da Mantiqueira.í ts ct-ys taf lm e baeement a littlc above sea leveI. maintaining notable tndependence ln relat íon to

To lhe east of thi$ block, -the -Resende Baein, the NE - ENE trends of the precambrtanlesa deep anel extenstve, is ulso full or sediments. "'structures (see Fig,'·l,." This fact denotes- a

su~crimpOSjtion of the dratnage eyatem on theThe 'rauoate Bastn contains at lenta 400 m trends of the reslstant structures. The aouthet-n

of thickneas of continental sedtrnents. The Iower- rim or the arch was almost enttrety destroyedpa rt, cnucd the T'r'crnembé Forma tion, is fo rmed by ercsíon, favored by the subsequent character01' peltttc material of lacustrine origino Mar»- ot the draináge system, particularly of therna.lian Jcsaí Is of Olf goceue. or nt the rncst Lowe r major Paraitinga nnd Paraibuna Rlvers. WithinMiocene age 'Me7.7.:'llil'él and Couto. 1911), are this system, the levelect summlts of the Serrafnund in it. The lF.l('u~trine laycrs have fi~h. da Bocaina rose elos e to 2000-2100 m above seRcheloT1i~n nnd chil'npteJ' I'cmnins. which Mezzallm leveI, as did the SenEl da Mantiqueira acros~Hnd Cnuto (O)). dt.l a<lmittcd to have th8t ng~ the valley. This demanstrates that at one time

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20 FERNANDO F. M. DE ALMEIDA

THE SÃO PAULO BASINa. mountafn existed on íts southern síde. thoughonly the Serra da. Bocainn and its prolongation.tbe Serra de Quebra Cangalha rematn today.

The upwarping or the mountaíns nelghboringlhe Par-afbn. do Sul rlft ehould have occurredmore or Ices aímultaneous to the suoetõence oflhe middle regton. However, when the largeoligocene lake was formed, the marginal eleva-tions should have had Httle topographic premi-nence, as Rfch (1!l!'>3) observed. Thts explatnathe maturity and the esscntiaUy pelitic characterof the Tremembé Formation, contrastíng wlththe granule.r and mineraloglcally Immaturenature or the upper formation (Suguio, 1968).

Geophysical studies are sUll acai-ce on theParntba do Sul rift. In a recent work. Davlnoand Harnlyi (1973) present results of gravi-metrtc and rnagnetometr-íc surveys or the graben,the adjacent mountains and the uplifted blockof the CachoeJra Paulista. Thc Bougucr g'rnvl-metrlc map indlcatcs lsoanornaly lines nlong thegeuern! dlrection of the rift, wlth decreaslngregional anornaues towards the interior of thebasln nnd 'wtth negntivc anomnlies near theNNW edge, wnere lhey reach -10 mgals. As

a reeult. fi reduced deficlcncy of mass compatiblewith the srrrall lh1ckness of sedtments retalnedin the 'I'nubaté Bnaiu ts observed. 'I'hls con-traste wlth lhe posttíve nnomaües of up to 30mgals which are observed on its elevated edges.1n additlon the magnetometric map (verticalcomponent) preaents ísoanomaly Unes trendingin lhe general dlrectlon of the valley, witllvatues decreaslng from lhe el~vated southernborde r of the valjey to the north, varying Irem

-/-14 to _10 gumrnas.

Very IIttle is known still about the modern

rautts eaeocteted wf th the downthrow of thegrnben, lt eeems that It8 marginal scarps arenot n result of unique, large fa.ults, but rather(lf a aystem of smnll, normal faults, eeveral ofwhich have already been identitled ln the fleldrPrang+pant and Pannuti, 1966; Sugulo, 1968;Cordnni et aJ., 1974l. Transverse faults alongthe rift cut the layers of the pllocene formation,as noted by Suguio (ep. (~It.).Mlnor ondulationsof the layers accompany those faults (Campos,1952. 1952a).

C. Wushburne (1930) noted a regional jncli~naUon of the Taubnté Bnsin's layers towards theSerra da Mantiqueira, which later observattons'confirmed, indicnting a structural assymelry,

A tI. ACRrl. bral'l. eMltc., (]978), 48. (Suplemento)

The tectonlc ortgln of the Slo Paulo Baslnwhere the c1ty of the same name Ia Jocated,waa conjectured by Washburne (1930) andFreltes (1951) and later conflrmed by the authorIn 1955. e Basín'a northem border touches aarge, late Brazlllano transourreet fault, tren-dlng E-W. Dur\ng the fonnaUon of the Basín,thls fault snffered a vertical movement.

The São Paulo Basín 10 close to 70 kmlong accompanled lengthwise by the TI.tê River.lt doesn't seems to be bordered by faults tothe south, 90 !t may be consldei'ed a half-gTllben.At the northem edge, cIose to the f'ault zonethat limita the Basin, the tnrckneee of the sedt-ments of the São Paulo Formatlon ortglnallyreached elose to 300 m. The Formation appearato be an interflngertng of psammltlc and pellUcbeds, the latter predominating In the upper part.The Basin contatns deposits of flood plainsand fluvial channels, accumulated ln a tropical'climate. The proceaa was not accompanled byupwarping of lhe bordertng fault block on thenorth. The latter !s called the Serra da Cants-relra, wnose altitude 113close to 1200 m, similarto the Jap! surrace, where not deformed.

The sedlmentary São Paulo Ba81n 18 sur-rounded by preeambrínn rock elevatlons (Cou-tlnho, 1972; Sadowskl, 1974) wh1ch the AltoTletê er0810n surface leveled to nearly 850 m.The author maíntaíns that sueh a .urtace, ofpresumable Neogene age was depressed ln thefault zone, thus orlginating the Basln. 'fheBasin's basement reveala acctdents which in partshould be due to faults trending NE, (Kollertnnd Davlno, 1963).

'l'he age of the Sli.o Paulo Fonnatlon 18dlttlcult to determine sinee It doeIII!.'tba"., fOllSiIs.lts probable correlatlon with the upper !ayersof the Taubatê Damn mnkes the generaLattrlbu-tion of Plfõcene age acceptable.

THE GUANABARA RIF'T

The topographícally depressed and elongatedregion on which the Guannbara Bay resta W8S

Interpreted In 1944 by Ruellan to be a grabenor a tectonic depreesion siluated between f8ultblocks t!lted NNW. F'reltas (1951, 1956) labeledIt «1ft valley Campo Grande-Guanabara-Río Bo-nIto>, notlng Its extenslon to the bay of llhaGrande. The reglon has not been tectonlcallystudled In deta!! 8u1flclent1y to exclude the alter-natlve proposed by Ruellan: that It 10an Inten-sely eroded. fault~angle depression.

NWPARANÁ BAStN

=l!..=!=J.oo,ion sol'

oueee- Senonlon

Bouru Group

C:zlPOltlOZOiC

I f lhe eonttnimtal borâer of the Bflnlol'J Bu!n,FtJ:. 2 - Po •• lble tectono.maamRtlc uolut 00 n:l rlU a.nd df:poatted a lreal thickn~'lll 0'( eV8)'lOrltc8 over lhe1) Durlng lho ApUan, lhe eee noodeC! tbe onel to the northwetJl tht!le lav{l8 cover pateczotc p('dlments.Lower CrelaeeouI bua1tl~ lavlUl. ln lh.o PAraM :a~~n, prol'TefllfIIlvé IlUbeidcnc(', nceumulated detrtc Mdimentl'l2) Durlng lho Lower.genonfnn, the 8nnlo' BIl3I, fnated dunni; lho UPP(lr Oretaeeoua, ncxt to th(l Bantoefrom the er'OlloD of lhe Serrll do Mar, Tha latter orte the elen.ted bor<1er of the 8QnloB Bn.8ln.fautt, The fI"t pbate af atkAl'ne 'VOlcanltmt oee'=e o:Oleanl.in procreHed to the Interior or lhe CQIIUnent.:1) Between the campanlan an4 PaleoceM. lhe depo.tted In the Parant Dallln.At the end of tbe &enontan, lhe Bauna Q1'Ot1P.'WU.. aeUvtty ln tho regton, thé derormattou er tbe Japi4) Durlng the 011coeen~. atter lhe end of th~ 'O'oleanlc I ff~"" uparched. whHe lhe PArolba ~r:\b!"nerasl" eurft\Ce commeneed (")" In Flg. 24). The JQ.pl eT08tonto::o lllke evolved tn thé interior or thp. ~rnh •.•n.began U•• downthrow ." lhe ufal cone. At uw,moment. 8. :et dIn' wbleh .edlmmta of lhe Tremembó :Alnnatlon ac:~~:t o~ 'UH~:graben "ubllided markedly. pl\rll)' ('.olllp~n-5) DUrtng lhe PUoeene and the Qua.tematY. lhe bu Fonnntlon, The upw8rjllng or lhe nelghborlnJ.'( mnuntalllRMted by lhe pUoeene Ic<:umulet\!on of lhe 8&0 Paulo M.anUquellil o.nd the Serrn. da. Boeatns. ln 'ta prG·contlnued slmultaneoulty, repreeenled by !e dS:t"~a:(\ a.bandoned lhe allroHne tnlruRloo8 or lhe 1~IAnd afwes81ve retreat due to eroRlon,. the Se .! I hcd detrltnl sedlm('nt!; to lhe Santos Br\!!II1.sao SebnstlAo and 'Olher n(!lghb<lrlng cOMlal 1810n(19. ,m(\ um !I

22FERNANDO F. 111. DE ALMEIDA

GUlmar~es ~]947), observing the aJignmentof the alknhne Il1lrusions afon thís .sugge.'lted tha.t 't g depresstnn,

I Jl111y correspond to a faultzune, whkh Images or the ERTS-l salscern to ccrrobors.ts. ellite

Thc Guanabal';t Rift extends nt J tJOl1 I~etwcen the Barr-a de Sáo João :a:d 200Scpetlbn Ba.y. 1C the Ilh G theb . a rnnda Bay were toe JOcluded i n lhe calculn tton, lhe Rift wouJd

:each 280 km. TlJe wtdth of lhe Guanabara B1.~ betwecn 2!l t., 10 ay. th' . km. Il llal'l'Ows howeveJ'm . e RIO BOJlito merfdlnn A tOpographícaIJ 'depL esscd region, not flooded by lhe ;Ypr'('Rcnts fi n~Jjr;>fof low büts ( d seu. Ibr-inn rocka Th . . orme or precam.de . e HII{flhne Intrusions ot the Serra

.rv;endn~hH. of Rio Bonito and or the Morrode Sfio Jorro stfllld out in the rclief.

d 1'he ~C(!llmulnt:ed scdiments are very Hmitedan thfn 111 the mtes-lm- or the rift. In addTto Quater-na I'y fltlVi;t! ~edimeniatlon thi 1 IOn

linental layers OCCUrarouJld lhe Guan n con-pos~ibly related to til S. abat-a Bay,The I e ao Paulo FormationC::thn .;,mn I. fHlJlt~an,;'e Jacustrine Basln oieu' ~Oç . south of Nlterói, was filled by a hundred

, meters 01: calcfl1'cous Lhí k .(rom thcl'maJ aouroas 11" lC ness, onginatingremnins t Ccu t.o 105·31 ..•emz, :938), Marnmalian

• õJ~ contamed j threou,s cnvilies indiente an Upper P ~ e calca-The calcareous Hre pl'Obabl Lo a ,eocene age.01' L,lte Cret Y wel PaJeocenealk ,: n~eo1l8, {'Ol1tempol'lmeous with the~he :t~lIe. volcanJsm in the ncighborhood, to which

a8m seenl~ to he genetically tied.The Guana ba l'it Rift is bordered b .

moun tains, especialJy t tI' y h1ghSerra do Mar i~ locnll ~ le north where theOJ'gãos. The ;ntter's \ . nown as lhe Serra dos226:.l m which resul;s il~frhest. altitude ('eacheswith the d . n dlfference in levei

, epression af over 2000 ' .pa,'Uy includes lhe \Vem'in m. l.hl!) valueSena dos O - g by eroslOn. The

rgam: W:I,., inlcrpf'cted b R 1111944) to be . . y ue anCall1pOs f n block ongmally.leveled by theWNW. SUr see, fAUltcd NNE nnd in~lined

The elevated southel'n bnrder .lJar-ely J'eaches 1000 1l1. Jt is v of lhe Rlfterosion, transformed in th : ery worn by(Serra da. Carioc~ N't ó' e LlttoraJ ma~sifs

'<. I er I massif, ete.).Faults Jike the Rio B ·t

]065), RI'e found' t1 '. ~nJa fault (Rosler,telJite imnges III le mterlOr af the Rlft. Sn-this fault f:'O,~~em to indicnte lhe extension af)\.,(. _ the Gunnab'l.ra Bay to th

on o de SJt.o João. 'rhe f' etinues to th '. ault probably con-

. e c0ntll1cntnl ~helf on the S:- TFllUlt which .Petrnbnh; .' . ao OJn6C •. lc('hllJCIaIlS (Asmus 19-0.

alllpn!-! f·t 11., 1!)7'I' P , (..••, , onte :md Asmus, 1976)

.411. Arlltl. õr(lJ;. (';(1",." (l!Jifil. 4,~. (SlIpl!'l11('nt.o)

identified next to the edge of the Parafba do.Sul River delta.

The Campos surface's probabJe Eocemar-ka the maximum a e Hm ne agebara Rlft. The maestrl!hthm lt tor the Guana-cani.sm and th to Pateocene vot,

e small calcareous C b 'associated wlth the Rfft I t a ,uçu BBSIn,Iatter since it ' ex 5 ed prevtous to themations is' was not fiIJed by lava, Infor-th . Incomplete for an interpretatIon orP

e evolution of this structure, Similar to tharaiba do Sul G b e

have j ra en, the Rift alao aeema toneogeni~ t:~e~n morphologicaJ features to Bwith orne pulsntion, possibly eeaocíated

a prolonged eroston.

MAGMATIS~r

The fissural basaltic magmsUsm of thraná Bastn (Serra Geral b e Pa-ticnlly tholeiitic thou h with asalts), Characte.ris_variatlons (RU g important chernIcalLate J egg, 197::i), occurred between tha

urassic and Lower Cet aI. J966) Th' retaceous (Amaratexcee~ing 2· miJlil~nmagmatis~ affected an area

square kllometers ín SAmerica, particularJy in the central d outhzones south ot the 14" an eaaternextended up to lSOO k parn.llel. In BraziJ, ttlhen active Ponta G m from the consto Theby swal'ms af d' b rOSsa arch was penetrated

IR ase dykes whJch trended NWparalleUng their axls Th 'with . ese were associatedsame some alkaline intrusion centers, having thed age a~ the basalts (Fig. 1). The Serra

o Mar regIon was at that ti .tenslon fractures, Jnto which a . me sub]ect toof d' n Immense numberr nd I~~~e dykes penetrated. These trended NE

, wlthout having been associated .thcontemporaneous, central_t . \VIEions. On the othcr hand ype alkahlle intru-aCCur rare Iam h ,however, there did1966). prop yre dykcs (Damasceno,

A.rter the Turonian at~ok plnce. The m8g~ati:w \magmatic Ph~setically a central t vas charactens_lh ype, marltedly alkaline 'th

e r..bsencc of tholeiitic basalts. ' WIthe few available Potassium_Ar onTo jUd~e fromthat two different sub g ages, lt seemsprocesso The oJdest phases occurred in thisLower Senonian, repre;::te:eveloped during theexposed on the Sã p by intrusive centera

o auJo coast (e énearby islands (Sã S b' anan Ia) ando e astJão MontA d ..

Buzio., Vitória). Thls sub- h ' o e Tngo,to the reglon next to th f P ase V/as not Iimited~he Santos Ba.sin, but :at~ult .flexures borderingmtensely in the . er lt WIlS manlfestedfnulted on th reglOn similarly f1exured nndraná Basin ine northeastern border of the Pa-

the States or Minal'3 Gerais nnd

I

IIf-

i

THE SYSTEM OF CONTINENTAL RIFTS BORDERING THE SANTOS BASIN. BRAZIL 23

Goíâe, where the magmatism of this phase Isnotable for its abundance of melnnoeratic auca-Jine rocks.

The second aub-phase evolved during theCampanian and Paleocene (or Lower Eocene,should the 53 m . y. daUng of the Cabo Frio rocksbe confirrned) . This phase seems· to be charac-teristlc of the erea occupied by the tectonlcsystem of Serra do Mar. Its intruslve centeraa t-e located in the interior ai the rifts or onlhe bordering mountalns. Leucocrntic and me-socratic rocks, such as nephellne syenites, foyat-

. tes, tjotítes. pulaskltes, ttnguaítes, umptekites,phonoütes. nordmarkttes, ete., predomlnate abso-Iutely ín these centere. Even peralka1ine granlteaoccur In Itatlaía (Ribeiro Filho, 1967).

Because of the Iittle knowledge now avat-lable. It 18 dlrflcult to clarlry the relattonantpthnt may nave exletild between lhe alkaUnenmg mauam and tectoníca In thia reglon. TbereIH crenrty a geograpblc_assoclatlon. partlçularlywlth respect to the second magmatlc 8ub:phas •.The Itattata, Paolia,Quatro and Morro ~ondocenters are 8ltuated wbere tbe Ja.pt· 8urtaceroee hlgheat In Slo Paulo. TIl.·enUre magmaUcproco.. preceeded· til•. development of the riftsnow expolled on,~ cQnUnent. An IntoMa! otprolonged tecton1é caím ,eparat.ed lhe ma.gma-t1"1n and tlle rlrt.ng, nece80llry for the IlIghlydeveloped elaboraUon ar the J apt .roolon surtaeeand the almo.t tolal de.tructlon or th. etu.lvoproducts of that msgtnatl8in by eroslon.

.,!II,

The facUI arld' tnteren<:ce dlscu88ed abovelend to the tolrowlni InterpretnUon of thetcctonle evoluUon ar the reglob, Belween lheLnle Jurasslc and Lower Cretaceous, a pheno-menon In the upper mantle, possibly of a therma.lorlgln. resulted In the elevatlon of the modernSerra do Mar regton anel the Ponta Grossa arch,with lhe subsequent fracturlng provoked bytenslon and nbundant lholeUtlc flB8ural volca-nlsm. The colneldence of thls proces! with thewidcly admlUed South Atta.ntic opening slncethe Late Jurnsstc ar Early Cretaceous (Maxwellct a1., 1970) leada one to cansider these pheno-meno. related to the taphrogenesis with whtch t.hesepRratlon or the contlncnts was lnl tlated. Atlhe westcrn border of the original rlft, a systemof (oulta and tlexures exlsled, or whlch lheSantos fn\llt would becomc a part. To .lhe eastof the rlft bordar, the SR.lltos BaRin was down-fa\llted, havlng basalt.tc lavas of Lower Creta-ceou!>nge, pnssibly lntercalnted with continentn.l

sedíments as a basement. During the Aptian.an epicontinental sea coming trom the south(Maack, 1969) penetrated the original rtrt. Inthe desert cUmate of the time, t.hts sea preclpt-tated the g-reat volume of evaporítes or lhelower pnrt ot the Baatn'a sedimentary t1tting.The Serra do Mar did not extat at that time asa hlgh rellef bordering the Bnairt.

The ancestra.l Serra do Mar W8$ fnrmcddurlng the Upper Cretaceous probably beforethe Senontan: only st ra ttg-rnphtc atudles of lheSantos Basin wUl define the aetual date uccut-e-tely. e Serm do 'Mar. einerged In a place not

-tO<> tu h'Om the Santos fault, and aínce thenhaa retrated to ItJJ present poslUon as • result

;:;otLert!!!lon.fFlg. 2).

The Senoniah and the Paleocene were mo-merit,s ot great tectontc acttvf ty ín the Serra doMar fegton, whlch was rtsing by that time. TheSêrra .co Mar' suppltcd sedímente to the ParanáBasln in the interior of the conttnent, whet'e utthe end of the Senontan the Bauru Oroup beganto aceumulate. The Santos Basín alao beganto aecumulate Uttoral and mnrine aedimenta otUpper Cretaceous age, as confirmed by core logdata. The aJkaU.ne volcanlsm that A.ccompaniedthliJ tectonlc pulsaUon was more inten~e in t11C.reglan durlng the Lower Senonlan and bclwecnthe CA.mpanlan and Paleocene. The Jutler tookplace more ln the Interior. in the region wherelhe cenozoic ritts begnn to develop. A tcntaUverclatlonshlp may be eslabUshed between lheCompan1an-Paleocene tectonic-magmatic epísode.nnd the deposlUon of the sedlmentnry unit B{Butler, 1970). It i5 interesting to keep inmind that I.JC Pichon and Hayes 11971) a.ttribu-ted an important varJation in the ocean floor~preadlng of the Atlantic to the ::;tlmc timetnterva!.

The Japi erosion surface represcnls the cndof a long erosive processo probably rcachingback to lhe Eocene. It leveled the ~cnonianalkaline intrusions. but. those of ItntiHia undPassa Quatro, formed between the Mncsll'ichtianand Danian. remained as residual relie(s.

The down-faulting of the Parníba do SulGraben began during the Oligocene. togethcrwith the sirnultaneous deformation of t.he Japlsurface and its upwarping nt the edgc!'l of thedepression. The formatíon of the GtmnabnraRift as well as the movement of impl")f't:wt faultbiocks In the Serra do Mar, In an nn'n.. tndaycavered by the continental shelf, prohnhly tonkplace nt lhe same time. This tectonic n.divitvwhich. seems to hnve eoincided with :"tn IIl1lple

Ali. Acori. brr'8, Ci~"c., (1976). 411. ISuplt"'TI1l'ut,»)

24 FERNANDO F. M. DE ALMEIDA

regresaíon of tne sea, already documenled inthe Esplrito Smto Baeln (Asmus, 1972), IR POS-terIor to any k.rrown volcanlsm in the regIon.'I'hls acttvíty should be reflected ln the SantosBastn, po8sib~ to bc reune in the layer ofseismic velocity 2.4 km/sec., (seeBaccar, 1970.Fig. 5), whhh directly covers the crystatlínebasement of the continental shelf. Le Pichonnlld Hnyes (!!)71) 3 tfrtbuted another occastonof rmportant va r-rnnon ín lhe procese or theAtlantic sprelding to lhe Middle Tertlary.

A thlrd tcc tontc pufaatlon affected theSerra do Mar s yss t.em, probably during Pliocene.lhe se me Urre in whlch the São P<\ulo Basinorlginated and wa.s fllled witb sedlments. Sfmul-taneousry, thcsubsidence or the Taubaté and Re-

sende Baslna )ecame accentuated. These basínswere atso flI'ed wtth clastíe continental sedí-

ments, This process was accompnnied by anaccentuation)f the relief of the netghbonngmountains am l'ifts whlch at that time assumedtheil' modern Ispect~. Presumably, it was at thattime thnt the em an, conttnentsl unfosstliferous.snud-cln y depH'lits nccur-red, represented by theAlexandra (farnná.) and Pariquera-Açú (SãoPaulo) FOrnlltions, together with those thatexist a round :he Guanabara Bay. The Pltocenewas a time cf genen:\lized epeirogenic elevationof the entírr central anel eastern regfon ofBrazil, whicll resulted in a.n intense erosiveprocess and çonsequent sedimentation in theCORsta) regiOls, represented by the BarreirasGroup. The latter may be tentatively reJatedto the more recent layers of the continental~helf with sdsrnic ví'locitics of 1.85 km/sec.,whosc deposifion !'StllJ proceeds. A low seismlcarlivity of Ue tectonlc system of the Serra doMar seems b indicnte thet lt ls not cntirelyinaC'tive.

ORWI)I" O," THI~ JUFT SYSTF:~t

The avalable knowJedge 00 the SantosBasin and tte Sermo do Mar rlfl"i is evldentlyinsu(ficient fa-the prcsentatbnof a sOtisfactorygenetic mOdl1 expIainlng the!r origin. Anyhypothesis, h)wever, should take the followingfl1cts lnto eOT.sideration:

1 - TJ1C systcln of rifts developed late onUle rising ba-der of the continent, adjacent toa basjn subsding since the .Aptinn time. Thenppos,ite verlc;al rnovcmcnt reached a minlmumor 11 km in L distnnce of 200km,

Ali. Aend. bn.'l.(Snplcmento)

. 2 - TIle alkallne magmatlsm occurred dur-ring the ttrst part of that rnovement, Indícattnga genetic connotatlon between the two pheno-mena. However, the rJfting or the continentalblock developed after the Eocene when the mag-matlc activity ín .the area had ceased.

3 - The system of rltts developed In fi

region where an intense taultlng took placedurlng- the Cambro-Ordovlcian, together wíth thereacUvatJon or major transcurrent faults appro-xirnateJy paraJ1eJ1ng the northwestem border of

the Santos Basín and the nelghborlng coast.Some ot these rautts represent ancient deepUneaments of Precambrian age.

4 - The layers of the Santo. Baaín do notreveal folds other than those due to halokineslsar local movernents next to the faults, maklng itimprobable that compression stresses caused theelevation or the continental block with tts con-eequent division ínto rlfts.

The causes of the Jarge opposíte verticalmovements between the Santos Basín and thenelghboring continental area are dlfflcult toexplaín, posslbly having been a consequence ofmovements ín the upper mantle. However, theexiatlng difference in leveI between the elevatedcontinental border and the floor of the Basincould have been a Cause of the late rlfting ofthe continental block.

IlIIes in 1970 suggested a proces. of gra-vltational sliding towards the continental margin1n order to expIaln the origin of some of thelarge. neighboring, continental grabens, Such aoexplnnatlon would best flt the example of theSerra do .Mar rift system rather than the othershe cites, not only because of the great proxlmityaf the Santos Bastn, but also because af thegreat differcnce in level established in the region.The cambro·ordovlcian faulta could have tn someway facllltated the process, whlch would alsoexplain the tlJtlng of the faulted blocks towardsthe interior of the contInent.

ACKNOWLEDGF:ME~~S

The n.uthor thanks the Instituto de PeB~guisas EspecIais, the ceasion of MSS images ofthe ERTS-l satcUite, the Professors Y. Hasuland G. R. Sadowski for provlding sugge.stlonsfor the improvement of the manuscrfpt andMarta Morria for her ald in 1t8 t1'anslatlon.

THE SYSTEM OF CONTINENTAL RIFTS BORDERING THE SANTOS BABIN, BRAZlJJ 25

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