Elsevier

Geomorphology

Volume 97, Issues 3–4, 15 May 2008, Pages 321-355
Geomorphology

Late Cenozoic uplift of the Amanos Mountains and incision of the Middle Ceyhan river gorge, southern Turkey; Ar–Ar dating of the Düziçi basalt

https://doi.org/10.1016/j.geomorph.2007.08.014Get rights and content

Abstract

Using the Ar–Ar technique, we have obtained the first numerical dates for the Pleistocene volcanism along the valley of the River Ceyhan in the Düziçi area of southern Turkey, in the western foothills of the Amanos Mountains. Our six dates indicate a single abrupt episode of volcanism at ∼ 270 ka. We have identified a staircase of 7 fluvial terraces, at altitudes of up to ∼ 230 m above the present level of the Ceyhan. Using the disposition of the basalt as an age constraint, we assign these terraces to cold-climate stages between marine oxygen isotope stages 16 and 2, indicating rates of fluvial incision, equated to surface uplift, that increase upstream through the western foothills of this mountain range at between 0.25 and 0.4 mm a 1. Extrapolation of these uplift rates into the axis of the range suggests that the entire ∼ 2300 m of present-day relief has developed since the Mid-Pliocene, a view that we confirm using numerical modelling. Since ∼ 3.7 Ma the Amanos Mountains have formed a transpressive stepover along the northern part of the Dead Sea Fault Zone, where crustal shortening is required by the geometry. Using a physics-based technique, we have modelled the overall isostatic response to the combination of processes occurring, including crustal thickening caused by the shortening, erosion caused by orographic precipitation, and the resulting outward flow of mobile lower-crustal material, in order to predict the resulting history of surface uplift. This modelling suggests that the effective viscosity of the lower crust in this region is in the range ∼ 1–2 × 1019 Pa s, consistent with a Moho temperature of ∼ 590 ± 10 °C, the latter value being in agreement with heat flow data. This modelling shows that the nature of the active crustal deformation is now understood, to first order at least, in this key locality within the boundary zone between the African and Arabian plates, the structure and geomorphology of which have been fundamentally misunderstood in the past.

Introduction

The documentation of vertical crustal motion, separating regional uplift from local components caused by active faulting, is important within Earth science, relating (for instance) to the understanding of feedback mechanisms between crustal deformation and climate. Use of dated long-timescale river terrace staircases is emerging as an important method for investigating this topic, given that fluvial incision can often serve as a proxy for surface uplift (e.g., Bridgland and Westaway, in press). K–Ar or Ar–Ar dating of basalt flows that cap fluvial terrace deposits is a robust technique for dating river terraces that has been widely used in volcanic regions worldwide, for instance, in the French Massif Central (e.g., Pastre, 2004), the Colorado Plateau of the western USA (e.g., Pederson et al., 2002), and in Turkey (e.g., Westaway et al., 2004, Westaway et al., 2006b, Bridgland et al., 2007) and adjoining parts of Syria (Demir et al., 2007). It is applied here to the terraces of the River Ceyhan within the Amanos Mountains in southern Turkey; these mountains form part of the boundary zone between the African and Arabian plates (Fig. 1).

The Ceyhan (Fig. 2) is the largest river flowing to the Mediterranean coast of Turkey, with a ∼ 300 km length and a 20,670 km2 catchment. In its natural state it was a major source of sediment; Russell (1954) described the dramatic seasonal coastal progradation around its mouth near Karataş (Fig. 1), resulting from sediment transported each spring by the peak discharge following nival melting upstream. Çetin et al. (1999) estimated that this process typically increased the land area by ∼ 7.5 ha annually. Since the Aslantaş Dam (Fig. 2, Fig. 3) was completed in 1984, the river downstream of it has become virtually sediment-free, with coastal erosion around its mouth now a significant problem (e.g., Çetin et al., 1999). The United Nations database (http://www.fao.org/ag/agl/aglw/sediment/) reports the annual sediment load of the Ceyhan as 5.5 million tonnes, a value presumably representative of its pre-dam state. Assuming a sediment density of 2000 kg m 3, this indicates a spatial average erosion rate of ∼ 0.13 mm a 1 throughout the Ceyhan catchment.

A major factor governing this erosion rate is high rainfall, due to orographic precipitation as moist air from the Mediterranean Sea flows eastward over the Amanos Mountains (Fig. 1, Fig. 2). Annual rainfall is ∼ 1000 mm in their western foothills (e.g., Yüce, 2001), and approaches ∼ 1500 mm at higher altitudes, making this one of the wettest parts of Turkey. The high rainfall results in profuse vegetation, restricting outcrop exposure and thus making stratigraphic relationships difficult to determine.

The Upper Ceyhan (Fig. 2) can be regarded as the ∼ 120 km reach within the Taurus Mountains, upstream of Kahramanmaraş. The Middle Ceyhan is thus the ∼ 80 km reach through the Amanos Mountains to Cevdetiye (Fig. 3); the ∼ 100 km long Lower Ceyhan crosses the Cilician Plain (Fig. 2). The construction of dams (Fig. 2) means that the ∼ 15 km reach between the Aslantaş Dam and Cevdetiye (Fig. 3) is the only part of the Middle Ceyhan where the natural valley remains intact.

The Ceyhan–Osmaniye volcanic field is a zone of Quaternary basaltic volcanism, ∼ 60 km long (NE–SW) and ∼ 15 km wide, adjoining the Middle and Lower Ceyhan (Fig. 2, Fig. 3). Basalt flanks the Middle Ceyhan in the Düziçi area (e.g., Bilgin and Ercan, 1981, Arger et al., 2000), in the western foothills of the Amanos Mountains (Fig. 3). We will use its disposition, capping terraces of the Ceyhan, to constrain local surface uplift. This analysis will be based on our own fieldwork, supplemented by unpublished documents prepared by the Devlet Su İşleri (DSİ) Genel Müdürlüğü (General Directorate of State Water Works), which show the basalt and fluvial sediments in the vicinity of the Aslantaş Dam before this dam was built. Recent investigations of other parts of this volcanic field, farther west (and thus excluded from the present study), include those by Parlak et al. (1997), Polat et al. (1997), Yurtmen et al. (2000) and Gürsoy et al. (2003).

The aims of the present study are thus to describe the geomorphology of the study region in relation to its geology, to discuss the disposition of Pleistocene basalt flows within the landscape, to date these basalts, and thus to use their heights to determine uplift rates, enabling modelling of the regional geomorphic development using a quantitative, physics-based technique, to provide a first-order indication of the combination of processes responsible for the dramatic uplift and fluvial incision that are evident. We thus show that the Amanos mountain range (Fig. 1), now ∼ 2300 m high, is very young; having developed since the Mid-Pliocene in what was formerly a lowland area. Moreover, we show via the modelling that this topographic development is entirely consistent with the combination of processes known independently to be occurring in the region: erosion, and crustal thickening caused by active transpression along the northern Dead Sea Fault Zone (DSFZ), the boundary between the African and Arabian plates. Other aspects of our work, not directly relevant to the geomorphology, including discussion of bedrock geology, palaeomagnetic data and geochemical analyses, as well as full documentation of our Ar–Ar dating, are provided in the online supplement.

Section snippets

Regional background

We now review background details, concerning the geology of the study region and its active crustal deformation, required to demonstrate relationships between structure and geomorphology and to enable estimation of parameter values required for the quantitative analysis of the landscape evolution to be presented later (in Section 6).

Disposition of Pleistocene basalt

Quaternary basalt has been reported in the study area at six localities (Fig. 4): on the east side of the Middle Ceyhan valley around Düziçi, Karagedik, Cevdetiye, and Pınarözü, and on its west side around and downstream of Aslantaş Dam. The extensive basalt west of Düziçi (1 in Fig. 3) has flowed northward down tributary valleys (notably that of the Sabun Suyu River; Fig. 4) towards, but not reaching, the present course of the Ceyhan (Fig. 6a). By ‘damming’ these tributaries it has created an

The Aslantaş Dam area

The DSİ survey work at the Aslantaş Dam site revealed deposits assigned to two Ceyhan terraces (Fig. 7, Fig. 8b). Sand and gravel of terrace 1 were widely observed to reach ∼ 5 m above present river level, being overlain by up to ∼ 10 m of fine-grained temperate-stage floodplain deposits. Deposits of terrace 2, now inundated, were reported on the left bank, ∼ 100–500 m upstream of the dam, reaching ∼ 120 m a.s.l. or ∼ 35 m above present tiver level, with a maximum thickness of ∼ 10 m (Fig. 8b).

About

Ar–Ar dating of basalt

Preparation for Ar–Ar dating involved initial screening by inspection of hand specimens and petrographic thin sections. Samples were then crushed, washed in deionized water and dilute hydrochloric acid, sieved to a 60–80 μm size fraction, and phenocrysts and xenocrysts were removed by magnetic separation and hand picking. Following irradiation, argon isotopes were measured in the microcrystalline groundmass at the Laboratory for Noble Gas Geochronology, Massachusetts Institute of Technology,

Numerical modelling of surface uplift

Seyrek et al. (2007) have suggested that the surface uplift within the Amanos Mountains is being sustained by the transpression along the northern DSFZ. We will now test this suggestion by physics-based numerical modelling, after Westaway (2002). Before presenting the modelling results, we first summarise the evidence that provides input to this modelling. Previous work by others (e.g., Meigs et al., 2003) has investigated the interaction between distributed crustal shortening, surface

Conclusions

We have used the Ar–Ar technique to obtain the first numerical dates for the Pleistocene volcanism along the valley of the River Ceyhan in the Düziçi area, in the western foothills of the Amanos Mountains. Our six dates indicate a single abrupt episode of volcanism at c. 270 ka. In part through our own fieldwork and in part by utilising old documents relating to the damming of the Ceyhan, we have identified a staircase of 7 fluvial terraces, at altitudes of up to ∼ 230 m above present river

Acknowledgments

This work was supported in part by HÜBAK (Harran University Scientific Research Council) grant 428 (A.S.). We thank the personnel at the DSİ regional headquarters in Adana and Kahramanmaraş for granting access to, and permission to publish, their documents relating to the construction of the Aslantaş Dam. We are also grateful to the manager of the Aslantaş hydro-electric power station, now operated by EÜAŞ (the Turkish state electricity generation company), for his hospitality and for fieldwork

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  • Cited by (0)

    1

    Present address: Laboratory for Noble Gas Geochronology, Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 54-1013, Cambridge, MA 02139-4307, USA.

    2

    Present address: 41 Kingsway East, Westlands, Newcastle-under-Lyme, Staffordshire, ST5 5PY, UK.

    3

    Present address: School of Computing, University of Leeds, Leeds LS2 9JT, UK.

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