Deformation processes in mantle peridotites : with emphasis on the Ronda peridotite of SW Spain

Abstract

This thesis aims to study upper mantle deformation processes in a case study of the Ronda peridotite in the Alpine Betic Cordilleras of SW Spain. The choice for this massif is motivated by its following characteristics: (i) pioneering geochemical and geological studies have shown that the W Mediterranean peridotites are among the most heterogeneous mantle fragments exposed at the Earth's surface (e.g. Kornprobst 1969, Dickey 1970, Obata 1980, Zindler etal. 1983, Freyet aL 1985, Suenand Frey 1987, Saddiqi etal. 1988, Pearson et al. 1989, Reisberg et aL 1991). This heterogeneity is particularly evident in the Ronda massif of Southern Spain. Obata (1980) was the first to show that the Ronda peridotite has preserved all three peridotite fades, Le. garnet-, spinel- and plagioclaselherzolite facies, inan essentiallycoherent outcrop of some 300 km2• (ii) The isotopic (e.g. Reisberg and Zindler 1986/7) and elemental (Frey et al. 1985, Suen and Frey 1987) heterogeneityofthe Ronda peridotite suggestsa lithospheric origin for the Ronda peridotite (Menzies andDupuy1991) as opposedtoanasthenospheric origin. In addition, the recent recovery of graphite pseudomorphs after diamond documented from the Beni Bousera peridotite in N Morocco (Pearson et ai, 1989, Tabit et al. 1990) as well as from Ronda(G.R. Davies, pers. comm.) point to an origin at deep lithospheric or asthenospheric levels. In anycase, it follows thatthe Ronda andotherW Mediterranean peridotites have preserved imprints of successive stages in their history of uplift and eventual emplacement in the crust. (iii) A third reason motivating the present choice for the Ronda peridotite is its geologically young emplacement age (-22 Ma; Priem et al. 1979, Zindler et aI. 1983). Marked gravity highs underneath the W Mediterranean (Bonini et aI. 1973) indicate that tectonic processes in the region are still active. Theonshore peridotite exposures can thus be expected to contain (micro)struetures and metamorphiC assemblages related to a still active geodynamic setting that can also be studied independently by geophysical observationsuch as to provide tests for geology-basedworking hypotheses and vice versa. This study aims to obtain detailed information on the geometry and kinematics of upper mantle deformation preserved in the Ronda peridotite, on the thermal history of the peridotitebodyand onthe rheological significance of the various structures observed, and to evaluate these data in the context of an orogenic process in a still active tectonic environment.