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