Major Late Cretaceous Mass Flows in Central Turkey Recording the Disruption of the Mesozoic Continental Margin

Abstract

The newly recognized Upper Cretaceous (~87 Ma) olistostrome belt in central Turkey west of Ankara extends for more than 112 km subparallel to the Izmir-Ankara suture with a width of 10 km. The Alacaatlı Olistostromes are stratigraphically underlain by a Triassic basement, and are up to 2 km thick. Over 80% of the blocks in the olistostromes consist of pelagic limestones, which reach up to 300 m in size; other blocks include basalt, chert, serpentinite, tuff, and sandstone. The limestone blocks are Jurassic and Cretaceous in age with micropaleontology documenting the presence of Callovian-Oxfordian, Tithonian, Berriasian, Aptian, Albian, Cenomanian, and Turonian stages. The flows are separated by intrabasinal sediments of shale, siltstone, and volcaniclastic sandstone with Albian (108-101 Ma) detrital zircons. The olistostromes show minor tectonic deformation, and are unconformably overlain by Santonian pelagic limestones. The deposition of the Alacaatlı Olistostromes was followed by arc magmatism, which started in the Campanian (~78 Ma) after a period of shortening and uplift, and the region became a fore-arc basin with deposition of shale and volcaniclastic sandstone with Campanian (78-72 Ma) detrital zircons. A number of peculiar features of these olistostromes including rapid uplift and erosion before the creation of a deep, short-lived (89-86 Ma) ephemeral basin, dominance of deep marine limestone blocks, and inception of arc magmatism approximately 9 Myr after their deposition indicate a major tectonic event involving the disruption of the continental margin prior to the onset of arc magmatism. This event is interpreted as a change from transform margin to subduction.

Uplift of Anatolia

“…The Beypazarı Granite is a coarse-grained granodiorite of quartz, plagioclase, K-feldspar, biotite, and hornblende (Öztürk et al, 2012). Zircon U-Pb ages from the Beypazarı Granite are 74.0 ± 1.0 Ma (Helvacı et al, 2014;Okay et al, 2019Okay et al, , 2020 and indicate crystallization in the Late Cretaceous. The Beypazarı Granite intrudes into metamorphic rocks (Figure 8).…”

İzmir‐Ankara Suture as a Triassic to Cretaceous Plate Boundary—Data From Central Anatolia

“…A similar scenario of continuous subduction from Middle Jurassic to Neogene with episodic accretion is inferred for the generation of the Franciscan Complex (Wakabayashi, 2015). Although northward subduction of the İzmir‐Ankara ocean was continuous, accretion was episodic with major accretion episodes in the Late Triassic‐Jurassic and Late Cretaceous interspersed with episodes of subduction‐erosion possibly during the Early Cretaceous and transform‐type boundary during the early Late Cretaceous (Okay et al, 2019). Thus, the İzmir‐Ankara suture represents the trace of a Tethyan ocean, which was in existence at least from the Late Triassic to the latest Cretaceous.…”

“…Helvacı et al (2014) report zircon Pb‐Pb evaporation ages from four samples of the Beypazarı Granite ranging from 79 Ma down to 73 Ma, and Okay et al (2019) provided U‐Pb laser ablation ICP‐MS ages from two samples (8823 and 9456) of 73.7 ± 0.4 Ma and 74.8 ± 0.4 Ma, respectively. In the region studied the outcrops of the granite are divided in two patches through an intervening Eocene cover (Figure 7).…”

Apatite fission track dating of the Beypazarı Granitoid: insight for the inception of collision along the Northern Neotethys, Turkey

“…In this sense, the Beypazarı granitoid intrudes into the basement metamorphic rocks of the Pontide block (sensu Şengor & Yilmaz, 1981) and nonconformably overlain by Palaeocene-Eocene Neotethyan sedimentary sequences comprising basal conglomerates with pebbles of the Beypazarı Granitoid (Helvacı & Bozkurt, 1994;Okay et al, 2020) (Figure 1). In this respect, relative age of the granitoid is proposed as older than Palaeocene which is also quantitatively proven by 95.4 ± 4.2-70.5 ± 3.4 Ma; U-Pb zircon ages (Speciale et al, 2012;Helvacı et al, 2014;Okay et al, 2019). Geochemically, the intrusion is defined as a calcalkaline, I-type, metaluminous arc-related magmatic body (Helvacı & Bozkurt, 1994;Yücel-Öztürk et al, 2012).…”

“…Eurasian margin in Turkey comprises various tectonic terranes (e.g Istanbul zone and Sakarya Zone - Okay & Tüysüz, 1999). These terranes were amalgamated prior to Palaeocene (Okay & Tüysüz, 1999;Okay et al, 2019) and the amalgamated terranes are defined as a single tectonic block, the Pontide block (Şengor & Yilmaz, 1981). In this sense, the Beypazarı granitoid intrudes into the basement metamorphic rocks of the Pontide block (sensu Şengor & Yilmaz, 1981) and nonconformably overlain by Palaeocene-Eocene Neotethyan sedimentary sequences comprising basal conglomerates with pebbles of the Beypazarı Granitoid (Helvacı & Bozkurt, 1994;Okay et al, 2020) (Figure 1).…”