Late Cenozoic uplift along the northern Dead Sea transform in Lebanon and Syria

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Abstract

Evidence of long-term, late Cenozoic uplift, as well as strike-slip faulting, is revealed by topographic and geological features along the northern 500 km of the Dead Sea fault system (DSFS)—the transform boundary between the Arabian and African plates in the eastern Mediterranean region. Macro-geomorphic features are studied using a new, high-resolution (20 m pixel) digital elevation model (DEM) produced by radar interferometry (InSAR). This DEM provides a spatially continuous view of topography at an unprecedented resolution along this continental transform from 32.5° to 38° N. This section of the left-lateral transform can be subdivided into a 200 km long Lebanese restraining bend (mostly in Lebanon), and the section to the north (northwest Syria). Spatial variations in Cenozoic bedrock uplift are inferred through mapping of topographic residuals from the DEM. Additionally, high altitude, low-relief surfaces are mapped and classified in the Mount Lebanon and Anti Lebanon ranges that also provide references for assessing net uplift. These results demonstrate an asymmetric distribution of post-Miocene uplift between the Mt. Lebanon and Anti Lebanon ranges. Antecedent drainages also imply that a major episode of uplift in the Palmyride fold belt post-dates the uplift of the Anti Lebanon region. North of the restraining bend, the Late Miocene surface is preserved beneath spatially extensive lava flows. Hilltop remnants of this paleosurface demonstrate Pliocene-Quaternary uplift and tilting of the Syrian Coastal Range, adjacent to the DSFS north of the restraining bend. This late Cenozoic uplift is contemporaneous with strike-slip along the DSFS. Geometrical relationships between folds and strike-slip features suggest that regional strain partitioning may accommodate a convergent component of motion between the Arabian and African plates. This interpretation is consistent with regional plate tectonic models that predict 10–25° of obliquity between the relative plate motion and the strike of the DSFS north of the restraining bend. We suggest that this convergent component of plate motion is responsible for uplift along and adjacent to the DSFS in the Syrian Coastal Range, as well as within the Lebanese restraining bend.

Introduction

The Dead Sea fault system (DSFS) is a globally prominent, continental transform that comprises a prominent tectonic element in the eastern Mediterranean region (Fig. 1). Recent plate tectonic models have begun to suggest that the northern portion of this transform may be characterized as transpressional in nature. Recent studies have identified clear evidence of active tectonism along the DSFS, including paleoseismic indicators, for the main fault branches along the northern 500 km of the DSFS (i.e., north of approximately 32.5° N) [1], [2], [3], [4], [5]. Results of these studies firmly refute recent assertions that the northern DSFS and the strike-slip faults in the Bekaa Valley are presently inactive [6], [7].

The availability of new databases allows the refinement and improvement of the knowledge and understanding of this tectonic system. To assess Pliocene-Quaternary tectonic signatures, this study constructed a high-resolution (20 m pixel) digital elevation model (DEM) based on Synthetic Aperture Radar (SAR) interferometry, and incorporated this DEM with other remotely sensed imagery (e.g., Landsat TM, SPOT, ERS 1/2 SAR). The highest resolution DEM previously available spanning the entire region was the 90 m pixel Shuttle Radar Terrain Mapper (SRTM) data set released in 2004. Hence, this DEM provides a continuous view of the topography along the northern DSFS at unprecedented resolution. With 20 m pixels, detailed neotectonic features, such as fault scarps, can be clearly identified, and thus permit a more accurate mapping of fault zones and associated landforms. Furthermore, it has been demonstrated that morphometric parameters such as topographic slopes are underestimated by low resolution DEMs. Consequently, the new, higher resolution DEM may be more suitable for morphometric analyses that attempt to identify neotectonic signatures in the landscape.

The goal of this study is to document evidence for Late Cenozoic (post Miocene) uplift and provide additional constraints of long-term strike-slip displacements, along the northern DSFS. Part of this portion of the transform includes a large restraining bend where transpressional uplift is expected. We further suggest that uplift of the northwestern Syrian Coastal Range, a linear mountain chain parallel to the transform and located outside of the “Lebanese” Restraining Bend, also results from oblique plate convergence. We interpret that plate motions oblique to the transform result in regional strain partitioning between strike-slip faulting and folds parallel to the strike-slip faults.

Section snippets

Tectonic setting

The DSFS consists of three main sections (Fig. 1) [8], [9], [10]: A ∼400 km long southern section from the Gulf of Aqaba through the Araba, Dead Sea, and Jordan River Valleys; a ∼200 km long northeast–southwest striking restraining bend through the Mt. Lebanon and Anti Lebanon ranges, and a ∼250 km long, north–south striking section in northwestern Syria and southern Turkey. This study focuses on the latter two sections of the transform, herein referred to as the northern DSFS.

The present-day

DEM production

One of the basic data sets for this study is a new, 20 m pixel digital elevation model (DEM) constructed using interferometric synthetic aperture radar (InSAR). 14 pairs of SAR images (i.e., 28 scenes) were used to construct the DEM (Fig. 2A). Raw SAR data from the ERS 1 and ERS 2 satellites were selected with perpendicular baselines between 50 and 350 m. All but one of the pair were tandem pair (i.e., 1 day between image acquisitions); the other image pair had a 70 day period between images.

Topographic residuals and relief

This study also used the DEM to provide a first-order view of spatial variations in topographic relief along the DSFS. Fundamental morphometric parameters of topography, (including the topographic slope gradient (Fig. 2B), the slope azimuth, and the curvatures) were calculated from the DEM. These properties are typically determined from the DEM by attempting to fit a polynomial function to the elevation values within a relatively small window (3 pixels by 3 pixels, in this case).

Hydrographic

Neogene paleosurface and uplift along the restraining bend

Low-relief, high altitude surfaces may represent elements of relict landscapes that remain relatively unaffected by the present-day base level conditions. When they can be regionally correlated and reconstructed, these surfaces constrain magnitudes and spatial patterns of long-term uplift (e.g., [32], [33]).

Based on the DEM we identified and mapped high-altitude, low-relief surfaces, which may indicate late Cenozoic uplift along the DSFS. The recognition of these high-level relict surfaces in

Constraints on strike-slip displacements

Prior studies have remarked on strike-slip landforms in the LRB (e.g., [10]) and along the DSFS in northwestern Syria (e.g., [5]). In this study, we discuss several specific observations from our mapping that provide additional constraints on the timing and magnitudes of late Neogene and Quaternary strike-slip displacements.

Contemporaneous uplift and strike-slip in a regional context

The above observations indicate contemporaneous, Late Cenozoic uplift and strike-slip tectonics for the northern DSFS. It appears that kinematic variations along the northern DSFS probably reflect geometrical variations in the structure of the plate boundary and do not necessarily require temporal changes in plate motions as suggested by some [28]. The geometrical changes correspond with two factors: (1) restraining (e.g., Lebanese Restraining Bend) and releasing (e.g., the Ghab Valley) bends

Conclusions

Integrating a new, high resolution DEM with other high resolution satellite imagery has provided a new view of the large-scale neotectonic features along the northern Dead Sea fault system (DSFS) in Lebanon and Syria. These results indicate strike-slip movements along the northern DSFS during the Pliocene and Quaternary, as well as Late Cenozoic uplift. These kinematics demonstrate regional strain partitioning of oblique plate motions within the Lebanese Restraining Bend, and along the

Acknowledgements

We benefited from helpful discussions with Greg Hoke, Bryan Isacks, and Eric Sandvol. ERS 1 and ERS 2 data were provided by the European Space Agency under data grant AO3-168. Jennifer Yu and Douglas Alsdorf provided assistance with initial InSAR data processing. This research was partially supported by NSF grant EAR-0106238. The field work associated with this study benefited from significant logistical support provided by the Lebanese National Center for Remote Sensing and the Syrian Higher

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