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


Gautier P., Bozkurt E. , Hallot E., Dirik K.

GEOLOGICAL MAGAZINE, cilt.139, ss.559-576, 2002 (SCI İndekslerine Giren Dergi) identifier identifier

  • Cilt numarası: 139 Konu: 5
  • Basım Tarihi: 2002
  • Doi Numarası: 10.1017/s0016756802006751
  • Dergi Adı: GEOLOGICAL MAGAZINE
  • Sayfa Sayıları: ss.559-576

Özet

The timing of exhumation of metamorphic rocks and granitoids of the Nigde 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 Nigde 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 Nigde dome on its southeastern flank. Pebbles from the metamorphic rocks are ubiquitous in the conglomerates of this sequence. As a result, the Nigde metamorphic rocks must have reached the surface before Eocene times. or at the very beginning of the Eocene at the latest. The Uckapth 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 Nigde 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 Nidge 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. Oil the other hand, our observations do not confirm the nonconformity of the sedimentary sequence dated as late Maastrichtian-Late Palaeocene onto the Nigde 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 Nigde high-grade rocks within the Maastrichtian-Palacocene 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 northwestern 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.