Detrital Zircon Geochronology Indicates Synchronous Evolution Of Western Anatolian Supradetachment Basins

Western Turkey underwent episodic N-S extension following a Late Cretaceous–Paleogene compressional regime. This extension resulted in the formation of the N-S-oriented Selendi and Gördes basins, as well as the E-W-trending Alaşehir Basin. However, the timing of basin formation is debated, hampering geodynamic model development or links to causal mechanisms. Here, we test whether N-S- and E-W-trending basins formed synchronously by determining maximum depositional ages from detrital zircon or stratigraphic ages from zircon-bearing tuffs in the basin fill of the Gördes and Alaşehir Basins.

Existing basin chronology for the Gördes Basin is inferential and inconsistent. Previous research suggests the onset of sediment accumulation in the Gördes Basin occurred between 24.1 and 21.7 Ma. However, the older age is based on K-Ar dating of dikes that cross-cut the basement of the Gördes Basin and are lithologically correlated to clasts in the lower basin fill. The younger age comes from the tuffaceous uppermost formation and, therefore, represents a minimum age for the basin fill. K-Ar ages from volcanic domes underlying the oldest stratigraphy in the center of the Gördes Basin range from 18.4 ± 0.8 Ma to 16.3 ± 0.5 Ma, implausibly implying they erupted after the surrounding basin fill was deposited. This discrepancy suggests that either the age of the basin fill or the conclusion that the igneous rocks are volcanic is incorrect.

The age of the Alaşehir Basin is based primarily on palynological biostratigraphy and magnetostratigraphy. Both yield middle Miocene ages (~16.4–14.4 Ma), but it is unclear whether these represent the oldest stratigraphy in the basin. Detrital zircon provenance data indicate that the earliest basin-filling sediments in the Gördes Basin were derived from a mixture of sources with affinities to the Tauride and Anatolide belts. Sediment provenance changes rapidly upsection, and within 50 meters, the Anatolide source is absent. In the Alaşehir Basin, the Anatolide source is never present, and sediment provenance is dominated by Tauride sources from the onset of basin filling.

Preliminary chronostratigraphic data indicate that the onset of sediment accumulation in the Alaşehir and Gördes basins may be synchronous but also highlight significant problems with the stratigraphic model for the Gördes Basin. Data from a sandstone in what is considered the lowermost formation of the Gördes Basin yield a maximum depositional age of 17.5 ± 0.2 Ma, younger than the oldest reported K-Ar ages of 21.7–20.5 Ma. A stratigraphically higher tuffaceous sample from the same formation yields an upward-younging age of 16.9 ± 1.7 Ma. However, two ignimbrite samples from what is considered a younger formation yield ages of 18.2 ± 2.8 Ma. These age inversions and stratigraphic inconsistencies indicate significant issues with the stratigraphic model for the basin. In comparison, a sandstone sample from the lowermost formation of the Alaşehir Basin yields an age of 19.0 ± 2.9 Ma. We conclude that the onset of sedimentation in the two basins is synchronous within the resolution of our methods, but significant work is needed to determine more precise basin chronologies and resolve apparent age inversions in the Gördes Basin.