Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEpu

Anguita, Francisco

Castilla Cañamero, Gabriel

Fairén, Alberto G.

García, Antonio

Hacar González, Álvaro

Pablo Hernández, Miguel Ángel de

Rodríguez, Sergio

Ruiz Pérez, Javier

Toloba, Elisa

Docta Complutense

THREE AGES FOR THE MARTIAN LITOSPHERE. Alberto G. Fairén, Miguel A. de Pablo, Gabriel Castilla, Javier Ruiz,  Jorge Anguita, Álvaro Hácar, Elisa Toloba, Antonio García, Sergio Rodríguez. Seminar on Planetary Sciences, Universidad Complutense, 28040 Madrid, Spain. Fairen.ag@terra.es   INTRODUCTION.  The magnetic lineations detected in Mars were originally explained as a sea-floor spreading effect [1]. Here we argue instead that the linea-tions could have been formed at a convergent plate margin, through both the accretion of ter-ranes and back-arc spreading. This would imply a differential process in the origin of the Martian litosphere, which would determine the formation of three extended areas in different times.  SCIENTIFIC BACKGROUND.  The discovery of la rge expanses of Mar-tian crust with remanent magnetism [2] required an in-depth revision of the previous ideas on Mar-tian dynamics. As Connerney et al. (1999) put it [1], “Plausible alternatives to this interpretation [the Mars highlands as remnants of an early “oce-anic” crust] are few”. They call to intrusives, folding, and hydrochemical alteration. Since the Earth’s oceanic crust is created by sheet dike in-trusions, the first one is not really different from sea floor spreading. The folding alternative would only explain the lineations geometry, but would pose an almost impossible geologic problem in ex-change. On the contrary, the chemical magnetiza-tion would be at odds to explain a linear geometry. Barring the ultimate alternative (namely, that a physical or chemical process unknown on Earth is the responsible of a feature which would only apparently resemble Earth’s magnetic linea-tions), we propose that the Martian magnetic bands could be related to a past period of plate tectonics, but to a process of lithospheric destruc-tion instead of generation. If the Martian boundary is, as proposed by Sleep (1994) [3], a former con-vergent plate border, the area southwards of it would be a zone where fragments of crust would accrete, the geometry of which would be defined, as in Earth, by the angle of collision between the two plates. The five clearly linear bands of Terra Sirenum-Terra Cimmeria (110º-200ºW, 40º-80ºS, Fig. 1) would thus be sheared terranes, and their transition to more round-shaped magnetic features (110º-220ºW, 10ºN-40ºS) would indicate face-on collision conditions.  An earthly parallel with the process pro-posed can be seen in the aeromagnetic map of Alaska (Fig. 2) published by Saltus et al. (1999) [4], where the southern linear anomalies, with similar sizes (∼ 100 km) and geometry as the Mar-tian lineations, are interpreted as sheared terranes. There is a debate between the continental [5, 6] or oceanic [7] interpretations of the interior region, a broad expanse of neutral magnetic character. In what concerns the Martian magnetism, the inter-esting point is that this neutral zone steps la terally into a magnetic high, just the same geometry we see at Terra Cimmeria (200ºW, 80ºS; Fig. 1) and Terra Sirenum (120ºW, 45ºS; Fig. 1). Mars mag-netized crust could thus be interpreted as the Earth’s: a jigsaw of continental and oceanic pieces. Alternatively, some of the magnetic highs could be mafic intrusions (related or not to rift epi-sodes), as in the interpretation of the deep mag-netic high in Alaska northern region.  THE AGE OF THE MARTIAN CRUST. Following the previous ideas, the magnet-ized Martian highlands would be the same age as the Martian paleomagnetic field [8][9]. And the northern lowlands would have the same sea-floor spreading origin the highlands have, instead some time after; in fact, the lowlands seems to be a landscape derivated from the own ocean presence [10]. So, it seems that the magnetization process would have affected them in the same way. Nev-ertheless, no significant anomalies have been de-tected on the Martian lowlands and, in any case, no linear bands have been described. It is likely that this area has never presented magnetic anomalies, because the possible plate tectonics would have called for a melted nucleus to operate; and the inner self-induced dynamo would have required, furthermore, this nucleus to be in con-vection to generate a magnetic field. Lunar and Planetary Science XXXIII (2002) 1131.pdfPLATE TECTONICS IN MARS. Fairén, et al.  Once more, the Earth provides support for this proposal. A planetary dynamo needs a great energetic contribution, as high as 6x1022 J for the terrestrial magnetic field; whereas the absolute superficial geothermic flux energy emission is only 7,5x1020 J for the Earth. Following these data, we can imagine an evolutive scenario where the Mar-tian core would have had progressively dissipated its energy (faster than Earth’s, due to the smaller size of the planet and its high relation sur-face/volume), driving the magnetic field to cease its activity before the litosphere dynamism did. This delay would have been long enough for plate tectonics to generate new, unmagnetized crust in the lowlands, until the disappearance of the sea floor spreading effect on its own. As this process would have been gradual, the weaker and local-ized anomalies detected in some areas in the younger crust of the lowlands [11], as the ones at Arabia Terra are, could be taken as the final stages of  the magnetic field activity. In this model, the area between ~60º S and Chasma Australe, which also lacks magnetic lineations, would be the only remainder of the Mars’ primitive crust, constituted even before the nucleus formation and the inner dynamo began to operate.  References. [1] Connerney et al., Science, 284, 794 (1999). [2] Acuña, M. H., et al. Science, 284, 790 (1999). [3] Sleep, N.H. J. Geophys. Res., 99, 5639 (1994). [4]Saltus et al, GSA Today, 9, 3. (1999). [5] Gemuts et al., in Proceedings of the 1982 Symposium on Western Alaska Geology and Resource Potential: Alaska Geological Society, 3, 67, (1983). [6] Dover, J. H. In Plafker & Berg, eds., The geology of Alaska, G-1, 285, (1994). [7] Plafker, G. & Berg, H. C. Ibid., 989, (1994). [8]Nimmo, F. & Stevenson, D. J. Geophys. Res., 105, 11969 (2000). [9] Schubert, G., et al. Na-ture, 408, 666 (2000). [10] Fairén, A.G. & de Pablo, M.A. Lunar Planet. Sci. Conf., XXXIII #1013 (2002). [11] Purucker, M., et al. Geophys. Res. Lett., 27, 2449 (2000).                 Fig. 1: Map of the Martian magnetic anomalies (from ref [2]). Fig. 2: Aeromagnetic map of Alaska, after ref [4]. Lunar and Planetary Science XXXIII (2002) 1131.pdf

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Three ages for the Martian lithosphere

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Three ages for the Martian lithosphere

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