Dating the exhumation of a metamorphic dome: geological evidence for pre-Eocene unroofing of the Nigde Massif (Central Anatolia, Turkey)

Abstract

The timing of exhumation of metamorphic rocks and granitoids of the Niğde metamorphic dome, at the southern tip of the Central Anatolian Crystalline Complex, is a matter of debate. According to some authors, the metamorphic rocks are overlain nonconformably by a sedimentary sequence of late Maastrichtian to Late Palaeocene age. In contrast, other authors recently argued that the Niğde dome represents an extensional core complex of Oligocene–Early Miocene age, finally unroofed during late Miocene times. On the one hand, the results of our study contradict the latter interpretation. A sedimentary sequence of earliest Eocene to early Middle Eocene age nonconformably overlies the high-grade rocks of the Niğde dome on its southeastern flank. Pebbles from the metamorphic rocks are ubiquitous in the conglomerates of this sequence. As a result, the Niğde metamorphic rocks must have reached the surface before Eocene times, or at the very beginning of the Eocene at the latest. The Üçkapılı granite, whose crystallization age has been inferred to be Early Miocene, has intruded the metamorphic complex during exhumation. The granite is also found as pebbles within the conglomerates of the Eocene sedimentary sequence and, thus, is actually older than the Eocene. Apatite fission track dates of 12–11 Ma across the Niğde dome do not indicate that the metamorphic rocks were still on their way to the surface at that time; instead, they must reflect a later event, which is most probably heating during late Neogene magmatism. Lastly, there is no ductile-then-brittle extensional detachment in the two areas where it has been invoked, that is, on the western and southern flanks of the dome. An extensional detachment nevertheless exists at the top of the Niğde dome, best documented in its northern part, where the detachment fault superposes a superficial unit made up of massive ophiolitic rocks onto the high-grade metamorphic sequence. Field evidence indicates that this detachment developed before Eocene times. On the other hand, our observations do not confirm the nonconformity of the sedimentary sequence dated as late Maastrichtian–Late Palaeocene onto the Niğde high-grade rocks. Field relations show either a tectonic contact between the two, or the direct nonconformity of the Eocene sediments onto the metamorphic rocks. The lack of coarse clasts originating from the Niğde high-grade rocks within the Maastrichtian–Palaeocene sequence further suggests that the metamorphic dome did not reach the surface before Late Palaeocene times. These results compare well with available data from the north-western part of the Central Anatolian Crystalline Complex, suggesting that exhumation has been broadly synchronous on the scale of the massif, as a result of an episode of high magnitude extension that affected the region in Campanian to Palaeocene times.

A long-lived Late Cretaceous–early Eocene extensional province in Anatolia? Structural evidence from the Ivriz Detachment, southern central Turkey

“…Structural analysis of the Kır¸sehir Block identified several extensional detachments (Gautier et al, 2002;Lefebvre et al, 2011) and retrograde extensional shear zones (Isik et al, 2008). When corrected for Cenozoic vertical axis rotations obtained from granitoids postdating metamorphism (Lefebvre et al, 2013), these data show that extensional exhumation occurred under ∼E-W-directed extension since at least ∼80-75 Ma.…”

“…The Afyon Zone contains a coherent stratigraphic sequence of Pan-African basement unconformably covered by a Paleozoic continental clastic sequence. This is, in turn, overlain by alkaline volcanic rocks (zircon U/Pb age of 229 ± 2 Ma; Özdamar et al, 2013) and unconformably covered by Early Triassic clastic sediments grading into an Late Triassic to Cretaceous carbonate platform sequence (Okay, 1984) and Maastrichtian continental clastics (Göncüoglu et al, 1992). Incipient blueschist-facies conditions have been reported both in glaucophane-bearing Pan-African metagabbro and in its sedimentary cover (Fe-Mg carpholite-bearing rocks).…”

“…(1) The Afyon Zone contains a stratigraphical column that reaches the Maastrichtian (Göncüoglu et al, 1992) Paleocene sediments unconformably cover the Afyon Zone metamorphic rocks (Candan et al, 2005), suggesting that there the exhumation started slightly earlier than in Central Anatolia. Our field and thin-section observations show that ductile deformation related to the detachment largely occurred under lower blueschist to sub-greenschist facies conditions.…”

“…The size is comparable to the Mediterranean provinces, but in Central Anatolia exhumation occurred much earlier, in the Cretaceous to Paleogene. Although recent studies have identified isolated occurrences of extensional detachments that aided the exhumation of the Kır¸sehir Block (Gautier et al, 2002;Lefebvre et al, 2011), it was unclear how exhumation was accommodated in case of the Afyon Zone, which can be traced from western to central Turkey over a distance of ∼800 km, fringing the Kır¸sehir Block in the south (Figs. 1, 2).…”

Paleomagnetic constraints on the timing and distribution of Cenozoic rotations in Central and Eastern Anatolia

“…This was followed by the latest Cretaceous to Eocene accretion of the Tauride fold-andthrust belt that largely consists of carbonate nappes (Demirtasli et al, 1984;Gutnic et al, 1979;Monod, 1977;Özgül, 1984). The Taurides mostly escaped metamorphism and accreted, while the Kırşehir Block and Afyon Zone were exhumed by extension in the Late Cretaceous to early Eocene time (Gautier et al, 2002(Gautier et al, , 2008Isik, 2009;Isik et al, 2008;Lefebvre et al, 2011Lefebvre et al, , 2015.…”