Surprisingly young Rb/Sr ages from the Simav extensional detachment fault zone, northern Menderes Massif, Turkey

Abstract

The present paper demonstrates that exposed semi-brittle–brittle detachment fault zones, in addition to footwall mylonites and syn-extensional granitoids, can be used to date the timing of faulting and constrain the history and evolution of metamorphic core complexes. We employed Rb–Sr geochronology on micas, sampled directly from a part of the Simav detachment fault (SDF) zone in the northern Menderes Massif (western Turkey). The exposed part of the fault zone is marked by ∼3-m thick zone of low-grade mylonites/foliated cataclasites, in which mylonitic fabrics in orthogneisses are overprinted by fabrics of semi-brittle deformation. The low-grade mylonites/foliated cataclasites are characterized by coexistence of brown and green biotites. Rb–Sr ages on muscovite and brown and green biotite from the low-grade mylonites/foliated cataclasites are ca. 30 Ma, 17–13 Ma and 12–10 Ma, respectively; green biotite ages are interpreted as dating fluid-assisted deformation-induced dynamic recrystallization and suggest that a part of the SDF was active during a 12–10 Ma interval. The ca. 30 Ma muscovite ages date dynamic crystallization and probably beginning of extensional exhumation of the northern Menderes Massif. The coexistence of brown and green biotites in the same sample indicates retrogressive processes associated within a detachment faulting during which green biotites have recrystallized from primary brown biotites with an age of 19 ± 2 Ma in this area. This further means that the isotopic system became opened during faulting and that the green biotite ages therefore record the activity of the SDF. We have also dated an orthogneiss sample exposed well away from the detachment fault zone (devoid of any retrogressive processes); muscovites and biotites from this sample yield Rb–Sr ages of 45.7 ± 0.6 and 18.17 ± 0.18 Ma, respectively. The biotite age is in accord with regional biotite ages (19 ± 2 Ma) and record cooling of the footwall rocks of the detachment fault. We argue that the ca. 46 Ma muscovite age record synkinematic recrystallization during, and lend credibility to interpretation of, the prograde Barrovian-type regional Main Menderes Metamorphism as an Eocene event and that, prior to the onset of extensional detachment formation, the northern Menderes Massif (or at least the rocks we measured) experienced Eocene temperatures in excess of 500 °C to get this age. New brown biotite and secondary green biotite ages together with previously published age data argues that the footwall rocks reached to biotite cooling temperature conditions by ca. 18 Ma and stayed at similar conditions for a longer period until ca. 12 Ma. The ca. 18–12 Ma time interval corresponds to a period of tectonic quiescence during the Neogene and is attributed to the locking of SDF activity by the intrusion and rapid cooling of I-type syn-extensional granites. Secondary green biotite Rb–Sr ages indicate that the fault then become active during 12–10 Ma. Published low-temperature apatite–zircon fission track and apatite (U–Th)/He ages further suggest that displacement along the entire fault zone has continued until 8.0 ± 0.5 Ma. Combined with previous radiometric and low-temperature thermochronologic ages, the new ages argue for episodic core-complex formation and the Simav detachment fault activity between ca. 30 and 8 Ma. The data therefore argues for a complex history of core-complex formation in the northern Menderes Massif; it occurs in pulses and involves periods of alternating rapid and slow cooling/denudation rates.