Uri Schattner

The lowest place on Earth is subsiding—An InSAR (interferometric synthetic aperture radar) perspective

Since the early 1990s, sinkholes and wide, shallow subsidence features (WSSFs) have become major problems along the Dead Sea shores in Israel and Jordan. Sinkholes are readily observed in the field, but their locations and timing are unpredictable. WSSFs are often difficult to observe in the field. However, once identified, they delineate zones of instability and increasing hazard. In this study we identify, characterize, and measure rates of subsidence along the Dead Sea shores by the interferometric synthetic aperture radar (InSAR) technique. We analyze 16 SAR scenes acquired during the years 1992 to 1999 by the European Remote Sensing ERS-1 and ERS- 2 satellites. The interferograms span periods of between 2 and 71 months. WSSFs are observed in the Lisan Peninsula and along the Dead Sea shores, in a variety of appearances, including circular and elongate coastal depressions (a few hundred meters to a few kilometers in length), depressions in ancient alluvial fans, and depressions along salt-diapir margins. Phase differences measured in our interferograms correspond to subsidence rates generally in the range of 0–20 mm/yr within the studied period, with exceptional high rates that exceed 60 mm/yr in two specific regions. During the study period, the level of the Dead Sea and of the associated ground water has dropped by ∼6 m. This water-level drop within an aquifer overlying fine-grained, marly layers, would be expected to have caused aquifer-system consolidation, resulting in gradual subsidence. Comparison of our InSAR observations with calculations of the expected consolidation shows that in areas where marl layers are known to compose part of the upper 30 m of the profile, estimated consolidation settlements are of the order of the measured subsidence. Our observations also show that in certain locations, subsidence appears to be structurally controlled by faults, seaward landslides, and salt domes. Gradual subsidence is unlikely to be directly related to the sinkholes, excluding the use of the WSSFs features as predictable precursors to sinkhole formation.

A mid‐Pleistocene deformation transition in the Hula basin, northern Israel: Implications for the tectonic evolution of the Dead Sea Fault

The Dead Sea fault (DSF) plate boundary has accommodated relative sinistral motion between the Sinai and Arabian plates since the Neogene. Geologically based models point to a long-term (5–0 ma) ∼N-trending relative motion roughly parallel to the central sector of the DSF. GPS-based calculations of present-day relative motion between the Sinai and Arabian plates indicate a northward increasing convergent component across the DSF, suggesting a kinematic change over the Plio-Pleistocene period. We study the evolving deformation of the Hula rhomb-shaped graben situated along the central sector of the DSF in order to examine the possible kinematic change. The Hula graben is widely accepted as a pull-apart basin, part of a series of basins extending southward. We use a 3-D approach, combining mapped surface structures with subsurface seismic reflection profiles (∼173 km) and borehole data (38 boreholes), gathered here into a single geographic information systems database. Results indicate that during the mid-Pleistocene a major tectonic transition modified the structure of the basin fill. A subvertical NNW-trending left-lateral throughgoing strike-slip fault developed diagonally across the basin. Consequently, the basin entered a new geodynamic phase where subsidence was controlled by both the basin bordering faults and the diagonal fault, while the vertical displacement across the transverse faults is minor. Synchronous structural changes recorded in additional localities along the DSF attests to a mid-Pleistocene (∼1 ma) regional tectonic transition that was associated with a northward increasing convergent component across the central and northern sectors of the DSF.

Pockmark asymmetry and seafloor currents in the Santos Basin offshore Brazil

Pockmarks form by gas/fluid expulsion into the ocean and are preserved under conditions of negligible sedimentation. Ideally, they are circular at the seafloor and symmetrical in profile. Elliptical pockmarks are more enigmatic. They are associated with seafloor currents while asymmetry is connected to sedimentation patterns. This study examines these associations through morphological analysis of new multibeam data collected across the Santos continental slope offshore Brazil in 2011 (353–865 mbsl). Of 984 pockmarks, 78% are both elliptical and asymmetric. Geometric criteria divide the pockmarks into three depth ranges that correlate with a transition between two currents: the Brazil Current transfers Tropical Water and South Atlantic Central Water southwestwards while the Intermediate Western Boundary Current transfers Antarctic Intermediate Water northeastwards. It is suggested that the velocity of seafloor currents and their persistence dictate pockmark ellipticity, orientation and profile asymmetry. Fast currents (>20 cm/s) are capable of maintaining pockmark flank steepness close to the angle of repose. These morphological expressions present direct evidence for an edge effect of the South Atlantic Subtropical Gyre and, in general, provide a correlation between pockmark geometry and seafloor currents that can be applied at other locations worldwide.

Coincidence or not? Interconnected gas/fluid migration and ocean–atmosphere oscillations in the Levant Basin

A growing number of studies on shallow marine gas/fluid systems from across the globe indicate their abundance throughout geological epochs. However, these episodic events have not been fully integrated into the fundamental concepts of continental margin development, which are thought to be dictated by three elements: tectonics, sedimentation and eustasy. The current study focuses on the passive sector of the Levant Basin on the eastern Mediterranean continental margin where these elements are well constrained, in order to isolate the contribution of gas/fluid systems. Single-channel, multichannel and 3D seismic reflection data are interpreted in terms of variance, chaos, envelope and sweetness attributes. Correlation with the Romi-1 borehole and sequence boundaries constrains interpretation of seismic stratigraphy. Results show a variety of fluid- or gas-related features such as seafloor and subsurface pockmarks, volumes of acoustic blanking, bright spots, conic pinnacle mounds, gas chimneys and high sweetness zones that represent possible secondary reservoirs. It is suggested that gas/fluid migrate upwards along lithological conduits such as falling-stage systems tracts and sequence boundaries during both highstands and lowstands. In all, 13 mid-late Pleistocene sequence boundaries are accompanied by independent evidence of 13 eustatic sea-level drops. Whether this connection is coincidental or not requires further research. These findings fill gaps between previously reported sporadic appearances throughout the Levant Basin and margin and throughout geological time from the Messinian until the present day, and create a unified framework for understanding the system as a whole. Repetitive appearance of these features suggests that their role in the morphodynamics of continental margins is more important than previously thought and thus may constitute one of the key elements of continental margin development.

An extensive pockmark field on the upper Atlantic margin of Southeast Brazil: spatial analysis and its relationship with salt diapirism

We present new evidence for the existence of a large pockmark field on the continental slope of the Santos Basin, offshore southeast Brazil. A recent high-resolution multibeam bathymetric survey revealed 984 pockmarks across a smooth seabed at water depths of 300–700 m. Four patterns of pockmark arrays were identified in the data: linear, network, concentric, and radial. Interpretation of Two-dimensional multi-channel seismic reflection profiles that crosscut the surveyed area shows numerous salt diapirs in various stages of development (e.g. salt domes, walls, and anticlines). Some diapirs were exposed on the seafloor, whereas the tops of others (diapir heads) were situated several hundreds of meters below the surface. Extensional faults typically cap these diapirs and reach shallow depths beneath the seafloor. Our analysis suggests that these pockmark patterns are linked to stages in the development of underlying diapirs and their related faults. The latter may extend above salt walls, take the form of polygonal extensional faults along higher-level salt anticlines, or concentric faults above diapir heads that reach close to the seafloor. Seismic data also revealed buried pockmark fields that had repeatedly developed since the Middle Miocene. The close spatio-temporal connection between pockmark and diapir distribution identified here suggests that the pockmark field extends further across the Campos and Espírito Santo Basins, offshore Brazil. Spatial overlap between the pockmark field topping a large diapir field and a proliferous hydrocarbon basin is believed to have facilitated the escape of fluid/gas from the subsurface to the water column, which was enhanced by halokinesis. This provides a possible control on fossil gas contribution to the marine system over geological time.

Convergent strike-slip across the Dead Sea Fault in northern Israel, imaged by high- resolution seismic reflection data

Weinberger, R., Schattner, U., Medvedev, B., Frieslander, U., Sneh, A., Harlavan, Y., and Gross, M.R. 2009/2010. Convergent strike–slip across the Dead Sea Fault in northern Israel, imaged by high-resolution seismic reflection data. Isr. J. Earth Sci. 58: 203–216. We combine geological and geophysical observations made along the margin between the Arabian plate and Sinai sub-plate to investigate the style and sequence of deformation associated with motion along the Dead Sea Fault (DSF). Our analysis focuses on one of the youngest rock units—the Pleistocene Hazbani Basalt. Integration of field mapping, K-Ar dating, and interpretation of high-resolution seismic reflection profiles yields a map of the top surface of the Hazbani Basalt, which highlights the architecture of faulting and folding. Results attest to a dominance of both contractional structures and strike–slip faulting along the northwestern rim of the Hula basin. Our new find ings show how a series of faults extend from within the boundaries of an extensional basin and beyond its margins, and are associated with the formation of positive flower structures. The structural analysis provides evidence for a transition from an early (pre-Pleistocene) phase of almost pure strike–slip to a late (Pleistocene) phase of convergent strike–slip faulting. Many of the faults investigated in this study displace the Pleistocene Hazbani Basalt and the overlying sediments and should thus be considered as potential active faults for seismic hazard assessments.