Cobucci, M.L.P.

Correa, D.R.

Correa-Gomes, L C.

Espineira, A.R.

Falcáo, A.C.S.

Jshioka, K. J.

Moraes, J.W.O.

Silva, I.R.

Souza, M.N. de

English

Cadernos Espinosanos (E-Journal)

BoI.IG-USP, Publ.Esp., 10:1-8, 1991 MAFIC DVKES: RELATIONSHIPS AMONG GEOMETRV, INTERNAL FRACTURES ANO FISSURAL TECTONIC PATTERNS INTRODUCTlON L. C. Corrêa-Gomes 1 M.N. de Souza2 D.R. Corrêa2 I.R. Silva3 A.C.S. Falcã03 M.L.P.CobucdJ K. T.lshioka 4 A. R.Espineira3 J. W.O.Moraes3 The understanding of systerns associated with the genesis and evolution of mafic dykes presumes comprehension of the phenomena that control (I) the formation and development of the fracture-conduit, (11) the associated stress field, and (Ui) the tectonic regime prevalent at a particular time. Several experiments on material deformation (e.g. on rocks, hallte and c1ay) indicate that fractures begin as locaIly concentrated small tension nuclei (RECHES, 1983, 1988; RECHES & DIETRICH, 1983). The length of a fracture depends on its deformation-propagation velocity, low velocit1es favouring short length and coalescence phenomena (OLSON & POLLARD, 1989; POLLARD, 1Eacola T6cnica Federal da Bahia, Universidade Federal da Bahia, Rua Caetano Moura, 123,40210 Salvador, BA, Brazi!. 2Universldade Federal da Bahia, Rua Caetano Moura, 123,40210 Salvador, BA, Brazii. 3Eaco1a T6cnica Federal da Bahia, 40210 Salvador, BA, Brazi!. ·Univeraidade Católica, 40210 Salvador, BA, Brazi!. 1973); thls Is specially true when linear and parallel concurrent fracture palrs are formed (SEGALL & POLLARD, 1980). On the other hand, the filllng or not of a fissure Is determined by relations among shear stress (Ss), tenslon stress (Ts), fluid pressure (Fp) and lithostatlc pressure (Lp). When Ss > Ts + Fp, the flssure will not be fillOO; if Ss < Ts + Fp, filling will occur; and under the special condition of Fp > Ss + Lp the emplacement wlll take place under conditlons of hydraullc fracturing (POLLARD & HOLZHAUZEN,1979; BEACH,1980; COX & ETHERIDGE, 1989). The distensive tectonic situation appropriate for dyke formation is obtained rnainly under three condltlons: a) domai upflift (BHATTACHARY & KOIDE, 1987; WILSON, 1973); b) linear extenslon by pure shear (RUPELL et ai., 1988); c) transtenslon by simple shear (CHOROWICZ et aI., 1987; ZALAN, 1986; DENG et ai., 1986). Each case can be revealed at different scales by the dlstrlbutlon, shape and structures associated with mafic dykes. Good examples of well-preservOO mafic dykes along the coast of the city of Salvador, Bahia, permit successful analysis of such relationships. LOCATlON ANO METHODOLOGY The mafic dykes of the Salvador region are intrusive into 2.0 Ga granulite and amphlbollte-grade metamorphic rocks. Dykes are c. 1.0 Ga old (D'AGRELLA-FILHO et ai., 1989) and range from a few centimeters to 47 m thick. They strike N 130 0 to N 160 0 with subvertical NE dips. The maln dyke occurrences are found on the beaches of Vitória, ltapoan, Pituba, Ondina, Barra, Rio Vermelho (RV) and Amaralina districts (Fig. 1). The last three have been chosen for this study because of the diversity of inforrnation gatherOO and their accessibility. A large amount of data (2636 measurements altogether) on internai coollng joints, contact planes, enelaves, country rock follation as well as planar and linear e1ements related to the dyke hlstory of emplacement and chill has been collected. OEVELOPMENT OF OYKE FORMSANO ASSOCIATEO STRUCTURES The great variety of cooling joint positions and dyke forms reveals a complex rheologic-tectonic situation and sequence of formation. Some of the most common geometries observOO are shown in Figure 2. They are: 1) bifurcate, with an angle of 30 o between the principal 2 1 2 V v i v ri V V " ., V v " 11 ;' ""-./ ./ " " ~--------------~ ;' ./ ./ ;' ' ....... _- ---------' Figure 1 - LocatIon of the study ar... See text for deta/la. ~ 3 y 4 v v v .. .. lf! v y \I \ ., v 11 " ., V y ~" v ~ I .., v v lo t i ~FLUX I~I Figure 2 - PrIncipal forma of rnafic dykea from the Salvador 0088II1.,.. 1) Tabular, Barra. 2) L.entlcular and "zig-zag", Amaralina. 3) Blfuroated, wfth narrow branche&, RIo Vermelho. 4) Blfuroated, wfth large branch, Amaralln .. 3 bodyand lateral branch; 2) tabular, with uniform thickness; 3) lenticular, with acute terminations, emplaced within "en echelon" fractures; 4) ·zig-zag", where coaIescencecf 'fracture pairs are fraquent; 5) "bent forms"; and 6) "L-shapecl" forms, having 90 o angles between principal and lateral branches whlch termlnate abrupUy. Form 6 incUcates an important action cf Fp in the formation of orthogonal ramlflcatlons. Form 5 can be explained by fissure refractlon batore or durlng magma intrusion with a slgnlflcaot role played by Fp in the latter situatlon (MOTOKl et ai., 1988; MANDL, 1987; POLLARD, 1973). Dlfferent clrcumstances, probablya transtenslonal system are raquired to explaln forms 4 and 3. Unear extenslon can aecount for the geometry of forms 2 and 1 by transtensional or domai uplift systems. Forms 1-4 can define local or regional situations, whereas 5 and 6 represent local behaviour only. In the case cf cooUng jolnts, two sets, both vertical to subvertical, deserve special emphasis: longitudinal fractures (L), which parallel the dyke strlke, and transversa fractures (Tr) which parallel the dlp directlon and are often orthogonal to L. The geometric behavlour cf the jolnts is also varlable within the conduits: 1) parallel (L) and orthogonal (Tr) (Fig. 3.1); 2) parallel and orthogonal on dyke marglns, passing abruptly to diagonal in the centre (Fig. 3.2); 3) parallel and orthogonal on margins, progresslvely curving until the become diagonal in the centre (Flg. 3.3) and 4) both diagonal (Fig. 3.4). Â= t. FIgure 3 - OrIentatIon 01 cooling joints in dykes. 1) Parallel and orthogonal to conduit margins. 2) Parallel and orthogonal at margina, and · diagonal in the centre. 3) Progressively curving inwards. 4) Diagonal. Arrows indicate the principal tensor poaItIon. 4 The conduit form and the tensor positions during magma soIidification are the most Important factors that control the oriented formation of Internai joints in mafic dykes. For example, the orlentatlon In Figure 3.1 indicates parallellsm between the main tensor (sigma1) and the dyke geometry, whereas In the other lIIustrated examples this condltlon holds only In the Inltial phases of conduit openlng, after whlch, the tensor g09S to a diagonal posltlon. Two aspects are worth mentlonlng here: flrstly, example 3.1 Indlcates an undisturbed tectonlc settlng, whereas the others reflect an unstable regime (CORR~ GOMES et ai., 1988). Secondly, example3.2 Is observed In dykes 1 to 3 m thick; 3.3 In Intrusions thlcker than 3 m; and 3.4 In dykes a few centimetres thick. Good exposures of cooIing jolnts in dykes OCCur on the beach near the Merldlen hotel, Rio Vermelho dlstrlct. Jolnt types shown I Figures 3.2, 3.3 and 3.4 can be found there in dykes wlth thlcknesses 1.5 m, 26.0 m and 4-8 cm, respectively. Possibly ali dykes have solldified under oblique ten8l0n cOndltlons, and owing to a delay in total magma solldification (Ionger times for thicker dykes) , the different fracture patterns appear to reflect differlng histories of rotation of the principal tensor. Thls observatíon is an accordance wlth resuJts of FABRE et ai. (1989) that indicate a total consolldatlon timeof 4 to 6 days for a dyke 1 m thick, emplaced in shallow crustal conelltions. A speciaJ case in the Rio Vermelho area elucidates at the same time both the system of dyke formatlon and the Importance of the tensors on the orlentation of coollng jolnts (Fig. 4). A parallel pair cf vertical veins, 8.0 cm thick, is interconnected by a third vein cf sigmoidal shape. The planes occupled by the pair (N135 O) represent two sinistral shear planes. With shear attenuation, Fp became more Important than S8, anel the magma fUled these planes, whUe the weak Ss caused rotation cf the L jolnts. Later, the principal . tensor became parallel to the pair cf veins, leading to internai fracturlng cf the central veln (N95 O) that was formed under hydraulic fracturlng condltions. CONCLUSIONS Some Important conclusions can be drawn from the study cf the maflc dykes of Salvador. The cooIlng jolnt posltions in the veins are diagonal, principally in their central portlons, anel not paralleI or orthogonal as expected. This happened because the position of the principal tensor Influenced the system durlng solidification. Olfferent thicknesses and times cf consolidation are the most Important factors durlng the distlnct geometrlc evolutlon cf cooling joints, uneler conelitions of principal tensor rotation. 5 R:SINTn~ RIEDEL Figure 4 - Parallel mafic veins joined by sigmoidal vein, as the result of sinistral shear and posterior filling. Note that cooling jointa of the parallel pair of vein (N135·) and sigmoidal vein (N95·) are diagonal (See text for details). TR = Transverse fractures. L = Longitudinal fractures. The isotopic dating of these dykes represents not only the emplacement age but a1so the age of the transtensionaJ tectonism associated with this processo Combining ali information about shapes and internai fractures of the dykes, it is possible to suggest the most adequate tectonic setting that affected this area approximately 1.0 Ga ago. Unear tenslon due to pure shear, associated or not wIth domai uplift, would be one of the tectonic regimes chosen; but the principal regime was, without a doubt, transtensional as evldenced in a1mast ali the dykes studled. Furthermore, the abundance of blfurcated, bent and orthogonaJ forms reveaJs the Important influence cf Fp on the dyke geometries in paraJleI and/or wedge zones. The result of this comparison between emplacement planes and fracture retation systems is that the majortty cf dykes were emplaced under dextral reverse shear regimes. REFERENCES BEACH, A (1980) Numerical models cf hydraulic fracturing and the interpretation of syntectonic veins. 6 Journal of Structural Geology, 2(4):425-438. BHATTACHARY, S. & KOIDE, H. (1987) Th~retical and experimentai studies of mantle upwelling, penetrative magmatlsm and development of rifts In continental and oceanic crusts. Tectonophyslca, 143: 13-30. CHOROWICZ, J.; La FOURNIER, J.; VIDAL, G. (1987) A medel for rIft development in Eastem Atrica. Geologlcal Journal, 22:495-513. CORR~ GOMES, LC.; TANNER de OUVEIRA, MAF.; LEAL. LR.B. (1988) Feições estruturais dos diques máflcos - relações entre o magmatlsmo flssuraJ e as condições geológicas do meio. In: CONGRESSO BRASILEIRO DE GEOLOGIA, 35., Belém, 1988. Anais. V.5, p.2217-2226. COX, S.1. & ETHERIDGE, MA (1988) Coupled grain scale dilatancy and mass transfer during deformational hlgh fluid pressures: examples trom Mont, Lyell, Tasmania. Journal of Structural Geology, 11(1/2):147-162. D'AGREUA-FILHO, M.S.; PACCA, I.G.; ONSTOTT, T.C.: RENNE, P.R.; TEIXEIRA, W. (1989) O estado atual da cooperação USP /Prlnceton Universlty: resultados paleomagnéticos e geocronológicos em diques máflcos das regiões de Salvador, Olivença e Uauá, Cráton do São Francisco. In: WORKSHOP DIQUES MÁFICOS PRECAMBRIANOS DO BRASIL São Paulo, 1989. Boletim especial trabalhos apresentados. São Paulo, IUGS, UNESCO (Projeto PICG-257). DENG, Q.; WU, O.; ZHANG, P.; CHEN, S. (1986) Structure and deformatlon character cf strlke-slip fault zones. pageoph., 124(1/2):203-223. FABRE, P.; KAST, Y.; GIROD, M. (1989) Estlmatlon of flow duratlon cf basaltlc magma in flssures. Journal of VoIcanology and Geolhwmal Reaearch, 37:167-186. MANDL, G. (1987) Discontinuous faul zones. Journal of Structural Geology, 9(1):105-110. MOTOKl, A; VILA, CA; ROIG, H.L (1988) Estudos Iltológicos e geológicos dos corpos intrusivos tabulares no município de Arraial do Cabo-RJ. In: CONGRESSO BRASILEIRO DE GEOLOGIA, 7 35., Belém, 1988. Anais. Belém, SBG. V.6, p.2727-2739. OLSON, J. & POUAAO, 0.0. (1989) Inferring paleostresses trom natural fracture pattems: a new method. Geology, 17:345-348. POUAAO, 0.0. (1973) Oerivation and evaluation of mechanical model for sheet intrusion. Tectonophyslcs, 19:233-269. POUAAO, 0.0. & HOLZHANSEN, G. (1979) On the mechanical interaction between a fluid fillec:l fracture and earth's surface. Tectonophyslcs, 53:27-57. RECHES, Z. (1983) Faulting of rocks in three-dlmenslonaJ stráln flelds. /I - theoretical analysis. Tectonophysics, 95: 133-156. RECHES, Z. (1988) Evolution offault pattems in clayexperlments. Tectonophysics, 145:141-156. RECHES, Z. & OIETRICH, H.J. (1983) Faulting of rocks in three-dimenslonal strain fields. I-Failure of rocks in poIiaxial servo-control experiments. Tectonophysics, 95:111-132. RUPELL, C.; ROYOEN, L; HOOGES, KV. (1988) Therrnal modeling of extensional tectonics: application to pressure-temperature-time histories of metamorphic rocks. Tectonlcs, 7(5):947-957. SEGALL, P. & POUAAO, P. (1980) Mechanism of discontinuous faults. Journal of Geophyslcsl Reaearch, 85(138):4337-4350. WILSON, T. (1973) Mantle plumes and plate motions. Tectonophyslcs, 19:149-164. ZALAN, P.V. (1986) A tectônica transcorrente na exploração de petróleo, urna revisão. Revista Brasileira de Geociincias, 16(3):245-257. 8 

Mafic dykes: relationships among geometry, internal fractures and fissural tectonic patterns

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Mafic dykes: relationships among geometry, internal fractures and fissural tectonic patterns

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