Ram Weinberger

New K-Ar ages of basalts from the Harrat Ash Shaam volcanic field in Jordan: Implications for the span and duration of the upper-mantle upwelling beneath the western Arabian plate

The volcanism in the western Arabian plate extends from the Red Sea through the Harrat Ash Shaam system to western Syria, as far north as the Bitlis suture in the Taurides. The Harrat Ash Shaam volcanic system in Jordan consists of northwest-trending dikes and a volcanic field that together encompass a width of 220 km. In terms of width, direction, and age of the main volcanic phases, the system is similar to the Red Sea dike belt. About 130 new K-Ar age determinations show that the ages of the Harrat Ash Shaam system (dikes and flows) range from Oligocene to Quaternary. However, there is a distinct gap in the ages between ∼22 and 13 Ma. This gap coincides with an apparent decrease in volcanism in the Red Sea region from around 20 to 12 Ma. We interpret this 9 m.y. gap as a quiescent period interrupting the volcanic activity in the region and suggest that from ∼22 to 13 Ma, tectonic activity in the Arabian plate was mainly restricted to the Red Sea region. A renewal of volcanism along the western margins of the Arabian plate at 13 Ma was very likely associated with the sinistral movement along the north-trending Dead Sea transform. This renewal of volcanism and tectonic activity may reflect the emergence of upper-mantle upwelling beneath the western Arabian plate at that time.

Mechanical modeling and InSAR measurements of Mount Sedom uplift, Dead Sea basin: Implications for effective viscosity of rock salt

We present a mechanical model for the growth of an emerging salt diapir in a tectonically active basin. The analytical model is applied to and serves to constrain the effective viscosity of rock salt and strain rates during diapirism of the wall-shaped Mount Sedom rock salt diapir, Dead Sea basin. The model is based on one-dimensional flow of Newtonian viscous fluid (salt) in a vertical channel that has been driven by the load of the overburden and affected by shear along the channel walls. Because the Poiseuille (channel) flow profile is parabolic and the Couette (shear) flow profile is linear, a one-dimensional model provides three sets of predicted profiles: topography, uplift rate, and shear strain. The present topography of Mount Sedom represents the shape of the Sedom diapir, and hence the effective viscosity of rock salt can be constrained by a model that best fits the present topography of the mountain. The resulting Sedom rock salt viscosity is determined to be between 2 and 3 × 1018 Pa s, and the associated strain rate is between 5 and 6 × 10−13 s−1. Geological structures indicate strain rates of 9 × 10−13 s−1 and 3 × 10−14 s−1 during the Holocene emerging stage and at the Plio-Pleistocene pre-emergent stage of the Sedom diapir, respectively. The uplift history of Mount Sedom predicted by the model and the current topography are compared to Interferometric Synthetic Aperture Radar (InSAR) measurements of salt uplift. The maximum uplift rates of Mount Sedom are 8.3 and 5.5 mm/yr for its northern and southern parts, respectively. The InSAR uplift profiles resemble topographic profiles obtained along the same traverses, implying that the uplift history during the last 14,000 years is stable. Steep uplift gradients observed by InSAR along the western margin of the diapir are higher than predicted by modeling of Newtonian viscous flow. This could imply that flow of power law viscous fluid may be more suitable than that of Newtonian viscous fluid for the Sedom rock salt at high strain rates above 8 × 10−13 s−1.

Evolving deformation along a transform plate boundary: Example from the Dead Sea Fault in northern Israel

[1] We analyzed geologic structures adjacent to the Dead Sea Fault (DSF) along the margins of the Sinai and Arabian plates in northern Israel in order to investigate the style and sequence of deformation associated with a transform plate boundary. The field area, located between the Hula basin in northern Israel andtheLebaneserestrainingbendinsouthernLebanon, is divided into distinct structural blocks by a series of distributed faults that comprise this approximately N-S trending section of the DSF. Cretaceous and Tertiary rocks within and adjacent to the structural blocks are folded into broad anticlines and synclines, with more intense localized shortening manifested by tight folds and thrust duplexes. Kinematic analyses of folds, faults, and veins provide evidence for two directions of regional shortening: (1) NW-SE shortening responsible for the formation of NE-SW trending fold axes and left-lateral strike-slip motion along N-S trending faults and (2) E-W shortening as indicated by N-S trending fold axes, N-S striking thrust faults, and extensional calcite-filled veins that strike E-W. Crosscutting relations and U-Th ages of the vein material suggest that the E-W phase of transform-normal shortening represents the most recent and presently active phase of deformation. The structural analysis provides evidence for the transition from an early (Miocene–lower Pliocene) phase of pure strike-slip motion to a late (Pleistocene to Recent) phase of convergent strike slip. The latter phase is characterized by strain partitioning, which is manifested by discrete left-lateral strike-slip motion across weak N-S faults and the development of a fold-thrust belt in response to transform-normal shortening. Analogous to the strain partitioning observed in southern California, we suggest that blind thrust faults adjacent to the DSF in the study area may pose a seismic risk to populations in northern Israel and southern Lebanon. Citation: Weinberger, R., M. R. Gross, and A. Sneh (2009), Evolving deformation along a transform plate boundary: Example from the Dead Sea Fault in northern Israel, Tectonics, 28, TC5005, doi:10.1029/2008TC002316.

Geology of the Metulla quadrangle, northern Israel: Implications for the offset along the Dead Sea Rift

A stratigraphic analysis of Jurassic to Eocene rock units in the Metulla quadrangle provides ample evidence for a left-lateral offset based on the differences between the two sides of the Dead Sea Rift (DSR). The stratigraphic evidence for this offset is as follows: (1) The Jurassic Kidod shales of Mount Hermon face a limestone domain on the west side of the DSR throughout all of the Galilee; (2) the Neocomian volcanic sequence east of the DSR at the base of the Hatira sandstones in Mount Hermon is equivalent to the Tayasir Volcanics to its west in northern Samaria, and is different from the volcanic sequence of the Naftali Mountains and of Gebel Niha, which occur higher in the stratigraphic section; (3) sandstones of the Aptian Hidra Formation exposed in Mount Hermon are correlated with sandstones from the same stratigraphic unit in Samaria, while the Hidra Formation in the Naftali Mountains lacks sandstones; (4) the Albian Mas’ada Formation of Mount Hermon comprises limestone in the lower part and marl above it, while the equivalent Rama Formation in the Naftali Mountains is basically a marl sequence; (5) the Turonian Bina Formation exposed in the Shamir “windows” is divided into three units comparable to the Derorim, Shivta, and Nezer formations in the Gilboa Mountains, 90 km to the south and west of the DSR; (6) the Paleocene Taqiye marls in the Hula 3 borehole, north of Kefar Gil’adi and less than 2 km to the east of the Qiryat Shemona fault (and west of the Tel Hay fault) is about 360 m thick, which is comparable with the 370-m section exposed in Nahal Bezeq 100 km to the south, only several kilometers west of the western fault of the DSR. The Taqiye Formation of the Naftali Mountains is much thinner, and it appears in a marl and chert facies. Based on the last evidence, we suggest that the Qiryat Shemona fault forms the boundary between the African and Arabian plates in northern Israel.

Damage zones around en echelon dike segments in porous sandstone

We investigate arrays of en echelon dike segments and their associated deformation in porous sandstone to infer the segmentation mechanism and the state of stress during dike emplacement. The en echelon arrays are interpreted as breakdown segments of planar parent dikes that propagated from greater depth under mixed-mode conditions. Typically, an array consists of either continuous nonoverlapping stepped segments (offset smaller than segment thickness) or overlapping connected segments (offset larger than segment thickness). The deformation associated with the nonoverlapping stepped segment arrays consists of newly documented fan-like patterns of deformation bands (lamellae of crushed detrital quartz grains), whereas the overlapping connected segment arrays consists of net-like patterns of deformation bands. Thus the patterns of deformation are related to offset geometry and are likely to be diagnostic of the states of stress. We simulated the stress and deformation fields around interacting breakdown segments by applying a continuum damage mechanics model. The simulation results mainly illustrate the stress dependence of the damage distribution and the sensitivity of the damage distribution to the geometry of the segment offset and the mutuality of segment propagation. By changing the applied stress and by controlling the segment tip growth, symmetric and asymmetric distributions of damage were produced. We describe which aspects of the generated damage zones satisfactorily correlate with field observations. Damage mechanics simulations are useful tools for studying the state of stress during dike emplacement.

Paleomagnetic reconstruction of a diapir emplacement: A case study from Sedom diapir, the Dead Sea Rift

Reconstructing the emplacement path of a diapiric sequence is relatively complex, in part because the stratigraphic (top versus bottom) sense of the sequence can be obscured by the complex structure. Paleomagnetic study of the diapiric sequence may reveal simultaneously three characteristics of it: (1) the stratigraphic sense, (2) the three-dimensional rotation path of the tilted sequence, and (3) the magnetic polarity of the sequence. We apply paleomagnetism to reconstruct the emplacement path of the Sedom diapir in the Dead Sea Rift, Israel. This application resolves the ambiguity regarding the geometrical orientation of the thickest intrarift stratigraphic section (2 km) exposed in the Dead Sea region. We here report the natural remanent magnetization for 172 oriented samples (mainly dolomite and siltstone) from seven localities in the steeply inclined beds of the Sedom diapir. A fold test, as well as other field and laboratory tests, shows that the characteristic remanent magnetization (ChRM) component of the rocks was acquired before the tilting of the Sedom sequence, whereas the low-coercivity magnetic component of probably chemical remanent magnetization origin was acquired after the tilting. This low-coercivity (secondary) component, likely acquired in the Brunhes Normal chron, records up to 40° young counterclockwise rotation about a vertical axis. The ChRM and secondary components (together with the bedding plane) were restored to their expected (original) position by rotating them about horizontal, vertical, or inclined axes to get the complete three-dimensional rotation path. The resulting structure of the southern Sedom diapir is composed of a thin western “salt wall” that is juxtaposed against a thick eastern salt wall. In the northern part of the mountain apparently only the thick eastern salt wall is exposed.

U-Th dating of striated fault planes

Direct dating of brittle fault activity is of fundamental importance to tectonic reconstructions and paleoseismic studies. One way to address this issue is by constraining the timing of fault striations, but this requires a better understanding of the striation formation mechanism and associated mineralization. We present results from a microstructural, geochemical, and geochronological study of calcite precipitates associated with striated fault planes from the Dead Sea fault zone in northern Israel. We recognize four types of coexisting calcite precipitates, including calcite cement in dilation breccia, calcite in striated groove morphology, calcite gouge associated with hydraulic fracturing and pressure solution, and calcite coating of the fault surface. Carbon-oxygen isotopes, 87Sr/86Sr ratios, and rare earth element and yttrium (REY) patterns indicate various precipitation mechanisms associated with formation of syntectonic (calcite cement and striations), coseismic (calcite gouge), and interseismic (calcite coating) precipitates in the fault zone. Using U-Th dating of samples from three adjacent fault planes, we delineate four welldefined deformation ages in the period from 220 to 60 ka. We conclude that these ages constrain the timing of activity along the Dead Sea fault zone in northern Israel, and argue that a similar methodological approach could potentially shed light on the timing of deformation in other brittle fault zones.

Coseismic horizontal slip revealed by sheared clastic dikes in the Dead Sea Basin

Despite the hazard caused by near-surface destructive horizontal displacements during earthquakes, field evidence for coseismic slip along horizontal discontinuities is exceptionally rare, mainly due to the lack of adequate exposure and markers. However, within the seismically active Dead Sea Basin, the late Pleistocene Lisan Formation contains vertical clastic dikes that are sheared laterally at maximum depths of 15 m, and thereby provide unique profiles of such horizontal displacement. In order to investigate how coseismic horizontal shearing is distributed near the surface, we document an ∼1-m-thick brittle shear zone, consisting of up to 11 slip surfaces that can be traced for tens of meters in the Lisan Formation. Displacements along individual slip surfaces are up to 0.6 m, and the total displacement across the shear zone is up to 2.0 m. Displacement profiles and gradients indicate that the brittle shear zone formed by simple shear, and deformation was associated with slip partitioning and transfer between primary and secondary slip surfaces. Evidence for concurrent displacement along slip surfaces during a single event indicates that the brittle shear zone was formed during a coseismic event subsequent to 30 ka. We consider the mechanical effect of seismic-wave–related transient stress, which, when added to the initial static effective stress, may result in concurrent horizontal shear failure along detrital-rich layers in the Lisan Formation. The exceptional quality of exposures and markers enables us to document, for the first time, the details of near-surface horizontal shearing, and indicates that displacement along horizontal bedding planes is a viable mechanism to absorb coseismic deformation in well-bedded near-surface strata.

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.

Improving the method of low‐temperature anisotropy of magnetic susceptibility (LT‐AMS) measurements in air

This study examines the limitations of the method of low-temperature anisotropy of magnetic susceptibility (LT-AMS) measurements in air and presents technical improvements that significantly reduce the instrumental drift and measurement errors. We analyzed the temperature profile of porous chalk core after cooling in liquid nitrogen and found that the average temperature of the sample during the LT-AMS measurement in air is higher than 77K and close to 92K. This analysis indicates that the susceptibility of the paramagnetic minerals are amplified by a factor !3.2 relative to that of room temperature AMS (RT-AMS). In addition, it was found that liquid nitrogen was absorbed in the samples during immersing and contributed diamagnetic component of !29 3 10 SI to the total mean susceptibility. We showed that silicone sheet placed around and at the bottom of the measuring coil is an effective thermal protection, preventing instrument drift by the cold sample. In this way, the measuring errors of LT-AMS reduced to the level of RT-AMS, allowing accurate comparison with standard AMS measurements. We examined the applicability of the LTAMS measurements on chalk samples that consist <5% (weight) of paramagnetic minerals and showed that it helps to efficiently enhance the paramagnetic fabric. The present study offers a practical approach, which can be applied to various types of rocks to better delineate the paramagnetic phase using conventional equipment.