Anisotropia da susceptibilidade magnética: aplicação ao estudo da deformação de um depósito de talco, Vale da Porca-Macedo de Cavaleiros

Abstract

0 talco da regiio de Vale da Porca (Macedo de Cavaleiros), encontra-se associado com urn alinhamento estrutural maior - o Cavalgamento de Vale da Porca. A origem e forma sigmoidal dos corpos de talcoxisto esta relacionada com a evolugio da deformagio introduzida pela falha. Assim, do ponto de vista do potential metalogen&ico, torna-se importante caracterizar a deforma~iore lativamente aintensidade e cinematica. As variagces de forma (deformagio) e aspectos da hist6ria cinematica das rochas, especialmente as tectonicamente deformadas, podem ser inferidas a partir do fabric do grio e mineral. Estes procedimentos sio diiceis e de grande consumo de tempo, requerendo a presenga de texturas e estruturas especiais. Utilizando a Anisotropia da Susceptibilidade Magnetica (ASM), 6 possivel caracterizar o petrofabric de rochas e, consequentemente, a sua origem e evolugio estmtural, de forma rapida e precisa (o elipsoide de susceptibilidade magnetica pode ser determinado em poucos rninutos com uma precisio, entre eixos, de 1:10000). Estes metodos podem ser aplicados a grandes escalas e a urn grande numero de problemas geologicos, comparativamente corn os metodos convencionais. Podem, obviamente, surgir problemas corn a interpretagio, particularmente com a intensidade da deformqio, mas que So sio signijicativamente maiores que os encontrados pelos outros metodos, pois todos dependem do conhecimento dos mecanismos causadores da deformagio. A Anisotropia da Susceptibilidade Magnetica e uma propriedade petrofisica que pode ser expressa por um tensor simetrico de segunda ordern e geometricamente representada por um elipsoide, cujos eixos principais, km, 2 k,, 2 k-, podem ser correlacionados com as direcg6es principais que definem a deformagso iinita: X, Y e Z. Contudo, a calibraqio da forma do elipsoide de ASM contra a intensidade dos processos fisicos e dificil. Esta dificuldade deve-se ao facto da forma do elipsoide combinar contribuig6es de virios minerais, cujo elipsoide individual tem diferentes formas (Borradaile e Henry, 1997). Assim, o principal objectivo deste trabalho foi estudar o simcado estrutural dos dados direccionais da ASM no deposit0 de talco de Vale da Porca, o que passa por medir variaq6es de intensidade e orientaqiio do fabric magnetico em diferentes dominios estruturais. 0s parhetros de anisotropia utilizados, mostraram-se bons marcadores da deformaqiio das rochas estudadas e as suas variaq6es reflectem a variaqiio da deformaqiio. 0s dados de orientaqiio do fabric magnetico, conjuntamente com OS dados geologicos permitiram aferir um mode10 cinematic0 transpressivo esquerdo para o cavalgamento de Vale da Porca. 0s estudos sobre o tema desta dissertagiio encontram-se numa fase inicial em Portugal, no entanto, noutros paises assiste-se j& especialmente nas ultimas duas decadas, a um increment0 dos estudos de Anisotropia da Susceptibilidade Magnhtica com aplicaq50 geologica: Borradaile, 1981, 88, 91 e Borradaile et al., 1981, 84, 87, 88, 90, 97; Hrouda, 1976, 82, 87, 91 e Hrouda et al., 1988, 89, 90; Jelinek, 1981; Aranguren et al., 1996; Riller, et al., 1996, entre outros. Em Tarling e Hrouda (1993), pode encontrar-se uma sintese de conhecimentos. A apresentagiio foi dividida em tr&s partes. A este resumo segue-se o capitulo 1, onde siio apresentadas as equaq8es hndamentais do magnetismo, OS diferentes tipos de ordem magnaca, a estrutura de dominio dos griios magneticos, OS principios gerais da magnetizaqiio das rochas e a importincia da susceptibilidade magnetica e respectiva anisotropia. A descriqiio do conjunto de tecnicas wlgarmente utilizadas em estudos de magnetismo de rochas e OS metodos de interpretaqiio usados, sublinhando-se as vantagens e 1imita~Sesd e cada urn, encontra-se compilada no capitulo 2. No capitulo 3, e apresentado o exemplo de aplicaqiio - o Deposito de Talco de Vale da Porca e e feita a sintese de todos OS resultados obtidos. Finalmente, no capitulo 4, siio tiradas as conclusSes possiveis.

Talc occurring in the area of Vale & Porca (Macedo a2 Cavaleiros) has been found linked to a larger structural alignment - the Vale 02 Porca thrust. The origin and shape of the talc-schist units can be related to the history of the deformation produced by the fault. Thus, the characterisation of the deformation, according to intensity and kinematics, becomes very important for the assessment of its metalgenetic potential. Variations in shape (deformation) and some aspects of the kinematic history of rocks, particularly those that have been tectonically deformed, can be inferred 5om grain and mineral fabrics. These procedures are complex and time-consuming, and ask for the natural occurrence of special textures and structures. Through studies of the Anisotropy of Magnetic Susceptibility (AMS), it has become possible to characterise the petrofabric of a rock, and, consequently, to trace down its origin and structural evolution, in a rapid and accurate way (the magnetic susceptibility ellipsoid can be determined in a few minutes, with aprecision of 1:10000, between axes). These methods surpass the conventional ones in which they can be applied to quite a number of geological problems, and be extended to larger scales. Data interpretation may, obviously, be hindered by difficulties, particularly when attempting to infer the degree of deformation. However, these difficulties are not significantly worse than those offered by other methods, as they all depend on the knowledge of the mechanisms that have produced the deformation. The Anisotropy of Magnetic Susceptibility is a petrophysical property, whose measurement along distinct directions gives a second order symmetrical tensor, geometrically represented by an ellipsoid; its principal axes, km& ? ki, 2 k- can be related to the principal directions characterising the finite deformation, X, Y and 2. However, the calibration of the shape of the ASM ellipsoid, against the magnitude of the physical phenomena, is no simple matter. The limitations arise from the fact that the shape of this ellipsoid sums up contributions from various minerals, with diversely shaped individual ellipsoids (Borradaile & Henry, 1997). This work has intended, as a prime target, to study the structural sigmficance of the direction data provided by analyses of ASM performed on the Vale da Povca talc deposit. For that purpose, variations in magnitude and orientation of the magnetic fabric from different structural domains were measured. The anisotropy parameters adopted turned out to be excellent deformation markers for the rocks under study, their variations effectively reflecting the differences in deformation recognised on-site. As a consequence, magnetic fabric orientational data, together with geologic information, led us to put forth the hypothesis of the occurrence of a sinistral strike-slip movement at the Vale da Porca thrust fault. Studies on subjects similar to the one at question are just beginring in Portugal, yet, in other countries, and for the last two decades, the number of studies of the Anisotropy of Magnetic Susceptibility, with geological applications, has been increasing: Borradaile, 1981, 88, 91 andBorradaileetal., 1981, 84, 87, 88, 90, 97; Hrouda, 1976, 82, 87, 91 and Hrouda et al., 1988, 89, 90; Jelinek, 1981; Aranguren et al., 1996; Riller, et al., 1996, among others. In Tarling & Hrouda (1993), a synthesis of the state-of-the-art can be found. The presentation of this thesis has been divided in three parts. Chapter 1 recalls the hndamental equations of Magnetism, the diierent types of magnetic order, the structure of the magnetic grain domain, the general principles underlying the procedures for the magnetisation of rocks, and the importance of magnetic susceptibility and respective anisotropy. In Chapter 2, the description of the set of techniques commonly used in Rock Magnetism, and of the related interpretation methods was made, and the advantages and limitations for each method were pointed out. Chapter 3 deals with a specific example of geological application -the Vale da Porca talc deposit - and presents the synthesis of all the results obtained. Finally, the conclusions we found possible to withdraw are shown in Chapter 4.