⁴⁰Ar/³⁹Ar Age Constraints on HP/LT Metamorphism in Extensively Overprinted Units: The Example of the Alpujárride Subduction Complex (Betic Cordillera, Spain)

Abstract

How the dynamics of subducting lithospheric slabs interferes with crustal deformation in the upper plates of convergence zones is a major question in plate-kinematic and crustal-scale restorations. This question is particularly well illustrated in the Betic-Rif orogen (western Mediterranean) which results from complex interactions between the Africa and Iberia plates during their convergence and deep-seated slab dynamics (

Tectonic Evolution of the Nevado‐Filábride Complex (Sierra de Los Filábres, Southeastern Spain): Insights From New Structural and Geochronological Data

“…If Paleogene geochronological ages (Figure 9) are accounted for to reflect tectonic burial and metamorphism, there are two possible solutions. The first solution considers Paleogene burial followed by slow exhumation during the Miocene, hence relating Miocene ages to exhumation (Augier, Agard, et al., 2005; Bessière, 2019; Bessière et al., 2022; Li & Massonne, 2018; Monié et al., 1991). This interpretation correlates the NFC with the same paleogeographic continental unit as the AC and MC or the Upper Sebtides in the Moroccan Rif belt (AlKaPeCa unit, Bouillin et al., 1986), which record similar Paleogene ages of metamorphism (Bessière et al., 2022; Marrone, Monié, Rossetti, Aldega et al., 2021; Marrone, Monié, Rossetti, Lucci et al., 2021; Monié et al., 1994; Platt et al., 2005).…”

“…The first solution considers Paleogene burial followed by slow exhumation during the Miocene, hence relating Miocene ages to exhumation (Augier, Agard, et al., 2005; Bessière, 2019; Bessière et al., 2022; Li & Massonne, 2018; Monié et al., 1991). This interpretation correlates the NFC with the same paleogeographic continental unit as the AC and MC or the Upper Sebtides in the Moroccan Rif belt (AlKaPeCa unit, Bouillin et al., 1986), which record similar Paleogene ages of metamorphism (Bessière et al., 2022; Marrone, Monié, Rossetti, Aldega et al., 2021; Marrone, Monié, Rossetti, Lucci et al., 2021; Monié et al., 1994; Platt et al., 2005). This solution is consistent with currently available kinematic and P‐T data; however, it is difficult to reconcile with the very limited (<100 km) convergence in the Eocene, considering that all three of the AC, Ophiolite unit, and continental NFC must be stacked and subducted below the MC.…”

“…The base of the AC is defined by a zone of retrograde mylonites and cataclasites that represent a major extensional detachment known as the BMZ, separating the AC in the hanging‐wall from the NFC in the footwall across a significant tectonic omission (Martínez‐Martínez et al., 2002; Platt et al., 1984; Platt & Vissers, 1980). The AC in the eastern Betics reached low‐temperature blueschist facies metamorphic conditions with a peak temperature of 350–450°C (Booth‐Rea et al., 2002) in Eocene times, interpreted as subduction‐related metamorphism (Azañón, 1997; Bessière, 2019; Bessière et al., 2022; Goffé et al., 1989; Monié et al., 1994; Platt et al., 2005). Subduction of the AC was followed by its extensional exhumation reaching very low P/T gradients (within the andalusite stability field), resulting in a HT‐LP metamorphic overprint during the early Miocene (Azañón et al., 1998; Booth‐Rea et al., 2004; García‐Dueñas et al., 1992; Lonergan & Johnson, 2002; Platt, Whitehouse, et al., 2003; Rossetti et al., 2005).…”

“…The NFC is truncated at the top by a major extensional detachment system (Betic Movement Zone or BMZ, Platt & Vissers, 1980; Martínez‐Martínez et al., 2002) that separates it from the continental Alpujárride Complex (AC). The continental‐derived AC was buried to high pressure‐low temperature (HP‐LT) metamorphic conditions in Eocene times (Azañón, 1997; Bessière, 2019; Bessière et al., 2022; Booth‐Rea et al., 2002; Goffé et al., 1989; Monié et al., 1994; Platt et al., 2005) below rock units of the structurally highest, Maláguide Complex (MC). The AC and MC were interpreted to be derived from the AlKaPeCa microcontinental fragment that existed within the Alpine Tethys Ocean, together with tectonic units exposed in the Kabylides units of Northern Africa and units exposed on Sicily and Calabria (Bouillin et al., 1986; Handy et al., 2010; van Hinsbergen et al., 2014).…”

“…Two contrasting interpretations have been offered for the timing of burial in the NFC, based on different (or differently interpreted) geochronological data. One interpretation is based on Paleogene geochronological data derived from the continental units of the NFC in terms of 40 Ar/ 39 Ar ages on amphiboles (Monié et al., 1991) and white micas (Augier, Agard, et al., 2005; de Jong et al., 1992), and U‐Th‐Pb ages on monazites (Li & Massonne, 2018), interpreted as HP‐LT metamorphism in the continental NFC falling into the range of ∼48–30 Ma, simultaneously with the burial of the structurally higher AC (Bessière et al., 2022). This interpretation assumes Paleogene burial, while other Miocene geochronological ages are thought to reflect a stage of slow exhumation (e.g., Augier, Agard, et al., 2005; Bessière et al., 2021; Li & Massonne, 2018), resulting in a correlation of the NFC with the same paleogeographic continental unit as the AC and MC (AlKaPeCa unit, Bouillin et al., 1986).…”