D. Frizon de Lamotte

Magnetic fabric and petrographic investigation of hematite-bearing sandstones within ramp-related folds: examples from the South Atlas Front (Morocco)

An investigation of the anisotropy of magnetic susceptibility (AMS) of hematite-bearing sandstones has been carried out along two fault-propagation folds located at the front of the High Atlas Belt (Goulmima area, Morocco). It reveals two types of unusual magnetic fabrics. The first one is always located in the forelimb of the anticlines and is characterised by an oblique magnetic foliation with respect to bedding. The relationship between bedding and magnetic foliation indicates a sense of shear, which is consistent with the one expected from kinematic modelling. It is proposed that such an oblique magnetic foliation is the result of a weak strain that develops during folding. The other striking feature of the AMS results is that 40% of the magnetic lineations are parallel to the regional shortening direction. From structural and petrographical data, it is suggested that this magnetic lineation corresponds to the intersection lineation between two planar elements that are, respectively, the bedding and iron oxides mineralised veins. The latter result from iron-rich fluid circulations through a fracturing network.

Kinematics of large scale tip line folds from the High Atlas thrust belt, Morocco

Abstract The kinematics of several tip line folds situated along the South Atlas Front near Goulmima (Morocco) is established using both forward modelling and analysis of syn-folding deformation. From the Sahara foreland to the Atlas, the two successive major structures forming the mountain front in this area fit geometrically with a ‘generalised fault propagation fold’ model and a model of ‘fault propagation fold’ altered by late transport on the upper flat, respectively. Field evidence shows that the two folds are developed more or less synchronously. The analysis of minor structures helps address the question of migrating hinges vs fixed hinges during the fold’s growth.

Comment on ''The ultimate arc: Differential displacements, oroclinal bending, and vertical axis rotation in the External Betic-Rif arc'' by J. P. Platt et al.

[1] Platt et al. [2003a] have made a major contribution to our knowledge of the Gibraltar arc by documenting the kinematics of the Betic-Rif External zones through structural and palaeomagnetic measurements. They suggest that the ‘‘External Betic-Rif arc’’ is a thin-skinned fold-andthrust belt formed in Miocene time as a result of two causes, i.e., first, the westward component of motion of the ‘‘indenting Alboran Domain’’ (Betic-Rif Internal zones) relative to the convergent Iberian and African plates; and second, the extensional collapse of the Alboran Domain. [2] The concept of an ‘‘indenting Alboran Domain’’ goes back to the work of Andrieux et al. [1971], who referred to an ‘‘Alboran microplate.’’ We argue hereafter that the ‘‘Alboran Domain’’ did not act as an indenter, and that both its collapse and the external Betic and Rif fold belts are consequences of a slab retreat process. We also emphasize that defining the Betic and Rif external fold-and-thrust belts as a single, thin-skinned arc is misleading. [3] The crucial role of the late orogenic collapse in the building of the Betic-Rif chain goes back to the work of Platt and Vissers [1989], who assumed that it was an intracontinental collisional orogen, formed at the expense of the neighboring Iberian and African margins, without any oceanic domain nor exotic terrane in between. However, relics of lost oceans do occur (1) along the African margin, i.e., the Beni Malek serpentinites and basalts at the very edge of the margin [Michard et al., 1992] and the spilites and radiolarites at the bottom of the exotic Flyschs [Durand-Delga et al., 2000], which altogether document a former Maghrebian ocean, and (2) on top of the NevadoFilabrides units of central eastern Betics, i.e., the metaserpentinites and metabasalts of the Mulhacen complex, which document a former Betic ocean [Puga et al., 1999, 2002a, 2002b]. The occurrence of former oceanic domains makes necessary to consider subduction processes there. Some authors even assume that two subduction zones operated, first the SE dipping Alpine-Betic subduction, then the NW dipping Apenninic-Maghrebian subduction [Rehault et al., 1984; Torres-Roldan et al., 1986; Andrieux et al., 1989; Guerrera et al., 1993; Doglioni et al., 1998, 1999; Frizon de Lamotte et al., 2000; Chalouan et al., 2001; Michard et al., 2002; Chalouan and Michard, 2004]. Consistently, east dipping active subduction beneath the Gibraltar arc is currently evidenced [Morales et al., 1999; Gutscher et al., 2002]. [4] Platt et al. [2003a] accept the concept of an easterly, allochthonous ‘‘Alboran Domain.’’ In fact, the latter includes two different tectonic systems, the Nevado-Filabrides (NF) at the bottom, and the ‘‘Alboran Terrane’’ on top, which in turn consists of the metamorphic Alpujarrides-Sebtides (AS) overlain by the Malaguide-Ghomarides and Dorsale Calcaire units [Chalouan et al., 2001; Michard et al., 2002, and references therein]. The NF consists of the HT-LP Veleta schists overlain by the HP-LT Mulhacen Complex, which has the characters of a subduction tectonic complex [Puga et al., 1999, 2002a, 2002b]. Only the Alboran Terrane does represent an exotic domain of easterly origin [Bouillin et al., 1986; Michard et al., 1991, 2002]. As the Alboran Terrane formed at the onset of Neogene times an orogenic prism made of young, ductile metamorphic units, it can hardly be regarded as an ‘‘indenter,’’ which would be the initial cause of the ‘‘External Arc’’ building. We rather suggest considering that the Alboran Terrane operated as a backstop related to the subduction of the Maghrebian Tethys (see section 6). [5] Platt et al. [2003a] argue that ‘‘the present dramatic geometry of the arc is a result of extensional collapse of the Alboran Domain during the Miocene.’’ They agree that the subduction rollback hypothesis [Frizon de Lamotte et al., 1991; Lonergan and White, 1997] could be ‘‘reasonably’’ proposed, but prefer the collapse mechanism that they also consider as the cause of the opening of the Algerian basin TECTONICS, VOL. 24, TC1005, doi:10.1029/2003TC001603, 2005

Magnetic fabrics and oblique ramp-related folding: A case study from the western Taurus (Turkey)

Abstract An analysis of magnetic fabric has been performed in weakly deformed Paleocene-Eocene limestone adjacent to two subparallel ramp-related folds (Akseki and Ormana folds) from the western Taurus (Turkey). The magnetic fabric of tectonic origin records two trends of well-defined magnetic lineation: N160 ° ± 7 ° and N130 ° ± 14 ° in front of the Akseki and Ormana folds, respectively. The trend of the magnetic lineation is oblique to the Akseki fold, whereas in the Ormana fold, magnetic lineation and fold axis are subparallel. The observed change in the trend of magnetic lineation is probably linked to a change of the shortening direction that occurred during the development of the thrust system. This shows that the two subparallel folds result from two different translation vectors.

Conclusion: Continental Evolution in Western Maghreb

In this conclusive chapter, our aim is to summarize the lessons which can be drawn from the preceding chapters with regard to continental evolution. Obviously, only part of the vast geophysical/geochemical/historical problem of continental evolution can be enlightened through the Moroccan case study, as Morocco extends only on a very restricted, marginal part of Africa. Moreover, Morocco is located in a particular area of the continent, at its northwest “corner” (Fig. 10.1). This restricted region of the large African continent is bounded by the Atlantic passive margin to the west, as old as Early Jurassic, and by the Oligocene-Neogene West Mediterranean Sea to the north, a young thinned continental crust/oceanic basin born within the Alpine collisional domain. The geological evolution of the Moroccan lithosphere is deeply marked by this situation. Second, this chapter aims at evoking the most important avenues of research, which are currently opened or should be traced in the near future in order to get a best and fruitful knowledge of the Moroccan subsurface geology and deep lithosphere structure. In this field, the interpretation of topography still occupies a significant place.