Yehuda Eyal

The tectonic development of the western margin of the Gulf of Elat (Aqaba) rift

Abstract Along the western coast of the Gulf of Elat, a 30 km wide shear belt of subparallel faults trending N-S to NE-SW is developed. This shear belt is observed mainly within the Precambrian basement terrain. Sinistral movements on these faults have been recognized based on offsets of magmatic bodies and lithological contacts in rocks of Precambrian age. The cumulative displacement, measured independently at several localities across the belt, attains a total of 24 km. Early Neogene volcanism, in the form of long dikes trending NW-SE is known from the eastern desert of Egypt, of the Gulf of Suez and along the Red Sea coast of Saudi Arabia. Volcanic bodies of a similar pattern have been identified in eastern Sinai. These bodies, mainly dolerites and basalts, intruding Mesozoic sediments, are parallel to each other and to the Gulf of Suez. They are tens to hundreds of kilometers long and spaced several tens of kilometers apart. Dating (K-Ar) of bodies in Sinai, Saudi Arabia and in the eastern desert of Egypt suggest an age of 20–22 m.y. for this extensive volcanism. All the dikes are sinistrally displaced by the individual faults of the eastern Sinai shear belt. The amount of movement recorded using the Precambrian markers is identical to movement recorded by offsets of the Early Neogene dikes. This proves that the total 24 km shear of this fault system postdates the 20–22 m.y. volcanic phase. It is assumed that the recorded movements are part of the 105–110 km shear persumed to exist along the Gulf of Elat which is also younger than 20–22 m.y. Preliminary results suggest that a similar amount of movement is observable along a shear zone developed on the eastern coast of the Gulf of Elat. Thus only some 60 km movement are taken up by faults in the Gulf itself. The evolving model suggests that: 1. (a) The intrusion of very long basaltic dikes in a NW-SE direction, on both sides of the Red Sea, accompanied the initial stage of development of the Red Sea—Gulf of Suez rift valley. 2. (b) A basic change in the geotectonic regime occurs after the intrusion of this volcanic suite: large scale horizontal movements are initiated, being recorded as a 105–110 km sinistral movement along the Arava—Jordan rift. This movement implies a synchronous opening of the Red Sea.

Early Pan-African evolution of the basement around Elat, Israel,and the Sinai Peninsula revealed by single-zircon evaporation dating, and implications for crustal accretion rates

We report 2<r7Pb/206Pb single-zircon evaporation ages for early Pan-African rocks from southern Israel and the northeastern Sinai Peninsula, the northernmost extension of the Arabian-Nubian shield. The oldest rocks are metamorphic schists of presumed island-arc derivation; detrital zircons date the source terrain at ca. 800-820 Ma. A major phase of tonalite-trondhjemite plutonism occurred at ca. 760-780 Ma; more evolved granitic rocks were emplaced at about 745 Ma. A metagabbro-met adiorite complex reflects the youngest igneous phase at ca. 640 Ma. We Find no evidence for pre-Pan-African crust, and our data document important crust-forming events that correlate with similar episodes elsewhere in the shield. The widespread presence of early Pan-African juvenile rocks (i.e., ca. 760-850 Ma) in many parts of the Arabian-Nubian shield makes this period the most important in the magmatic history of the shield and supports earlier suggestions for unusually high crust-production rates.

Stress field fluctuations along the Dead Sea rift since the middle Miocene

The Dead Sea transform (DST) is the suture between the African and the Arabian plates, both of which subduct northward under the Eurasian plate. The collision of these plates with Eurasia and their relative motion are the source for their internal deformation, especially in the vicinity of the Dead Sea transform. Geological observations indicate that two distinct paleostrain regimes operated adjacent to the Dead Sea rift and in the Sinai-Israel subplate: (1) WNW shortening (less than 10%) and NNE extension, beginning in the Turonian, is associated with the development of the Syrian Arc fold belt and attributed to the Syrian Arc stress field (SAS); and (2) middle Miocene to Recent NNW shortening and ENE extension, associated with the 105-km sinistral displacement along the Dead Sea transform and the opening of the Red Sea, is attributed to the Dead Sea stress field (DSS). It was originally suggested that the SAS terminated during the middle Miocene with initiation of the DSS. However, trend and age analysis of many structures shows that formation of SAS-compatible structures continued after the middle Miocene up to the Recent. In some middle Miocene and younger rocks structures, such as faults, dikes, volcanic lineaments, and tectonic stylolites compatible with both stress fields were reported. In a few locations, structures compatible with both stress fields crosscut each other. It is suggested that the movements which resulted in the development of the Syrian Arc and other SAS-compatible structures are continuing to the Recent and that DSS movements are superimposed upon them. The overlapping of SAS and DSS structures adjacent to the DST results from spatial and temporal fluctuations in the overall stress state. We propose that such variations could be caused by the superposition of large earthquake stress drops, associated with movements along the DST, on a steady background, plate scale, stress regime associated with the SAS. Thus DSS-compatible structures should form in preseismic activity periods, when DST-related elastic strain is high. On the other hand, the SAS-compatible structures should form during interseismic activity times, subsequent to large local stress release along the DST.

The Sa'al volcano-sedimentary complex (Sinai, Egypt): A latest Mesoproterozoic volcanic arc in the northern Arabian Nubian Shield

New zircon U-Pb age data and geochemistry for the Sa9al metamorphic complex (SMC) in Sinai (Egypt) provide the first robust evidence of latest Mesoproterozoic island arc rocks at the northernmost Arabian-Nubian Shield, possibly indicating that formation of the shield commenced prior to 870 Ma. An older series of calc-alkaline volcanic and intrusive rocks yielded ages of ca. 1.03–1.02 Ga. Zircon xenocrysts within these rocks attest to arc magmatism predating the SMC by ∼80 m.y., as well as the minor contribution of Paleoproterozoic crust. Detrital zircons of the SMC pelites exhibit textural and U-Pb age patterns supporting their derivation from the volcanic rocks as arc detritus. A ca. 820 Ma gneissic pluton intruding the SMC indicates that by Cryogenian time, the SMC was already incorporated within the evolving Arabian-Nubian Shield. The 1.0–1.1 Ga SMC rocks provide a possible connection between latest Mesoproterozoic ocean closure during the assembly of Rodinia and the later buildup of Gondwana. There is growing indication, including the findings of this study, that 1.0–1.1 Ga crust composed a more significant component in northernmost Gondwana than hitherto recognized.

Faulted joints: kinematics, displacement-length scaling relations and criteria for their identification

Structural geometries and kinematics based on two sets of joints, pinnate joints and fault striations, reveal that some mesoscale faults at Split Mountain, Utah, originated as joints. Unlike many other types of faults, displacements (D) across faulted joints do not scale with lengths (L) and therefore do not adhere to published fault scaling laws. Rather, fault size corresponds initially to original joint length, which in turn is controlled by bed thickness for bed-confined joints. Although faulted joints will grow in length with increasing slip, the total change in length is negligible compared to the original length, leading to an independence of D from L during early stages of joint reactivation. Therefore, attempts to predict fault length, gouge thickness, or hydrologic properties based solely upon D‐Lscaling laws could yield misleading results for faulted joints. Pinnate joints, distinguishable from wing cracks, developed within the dilational quadrants along faulted joints and help to constrain the kinematics of joint reactivation. q 2001 Elsevier Science Ltd. All rights reserved.

Throughgoing fractures in layered carbonate rocks

Fracture surveys were conducted across a broad anticline in southern Israel in order to investigate the development and geometries of throughgoing fractures in layered carbonate rocks. At Halukim anticline, throughgoing fractures form by the linkage and coalescence of preexisting, bed-confined joints. Thus the internal morphology of these structures is highly segmented, often consisting of vertically aligned zones of subparallel fractures and bed partings linked together across the stratigraphy. The large population (n = 132) of throughgoing fractures represents a continuum of structures at various stages of development that can be classified into three main geometric categories corresponding to increasing levels of brittle strain: incipient, linked, and linked with aperture. Despite the wide variety of internal morphologies and geometries, the throughgoing fractures display a consistent east-northeast–west-southwest orientation, parallel to a set of bed- confined cross joints. The spatial distribution of throughgoing fractures varies as a function of structural position, the highest frequency and estimated strain intensity being located at the fold crest. Results suggest that throughgoing fractures develop only after a critical level of strain is achieved, as quantified by the density of bed-confined joints. Throughgoing fractures are multilayer structures that may greatly enhance the connectivity of a fracture network; therefore, understanding their formation, geometry, and distribution may contribute to efforts of flow modeling in fractured carbonate rocks.

Miocene magmatism of Sinai related to the opening of the Red Sea

Extension associated with the initial (early Miocene) stages of opening of the Red Sea resulted in intrusion of a widespread system of dikes and smaller bodies, primarily along the northeastern margin of the Red Sea/Gulf of Suez axis. Dikes, some up to more than 800 km in length, were emplaced dominantly parallel to the Red Sea/Gulf of Suez from the Sinai Peninsula to Yemen. Intrusive rocks are mainly basaltic in composition, but minor granophyres are also present. In the Sinai, this magmatic event is about 20 m.y. old. Dikes of the southern and southeastern Sinai crop out in Precambrian terrane. In the central and northern Sinai, they occur in Phanerozoic terrane, where they may be associated with linear grabens and cryptovolcanic features. Basaltic dikes of the Sinai underwent variable degrees of alteration. Alteration phases include seriate, quartz, smectite, chlorite, and serpentine. X-ray diffraction and petrographic analysis indicate that, in spite of extensive alteration, primary minerals still control the major-element compositions of the rocks. Basalts are tholeiitic in composition, with SiO2 = 48.5 to 50%, and Na2O + K2O = 3 to 3.5%. Rb and Sr concentrations are probably dominated by alteration and by introduction of secondary material (primarily carbonate). Zr/TiO2, which is relatively unchanged during alteration, substantiates that the basaltic rocks are subalkaline. Initial 87Sr/86Sr, determined after leaching of samples, is approximately 0.7043–0.7063, much higher than Red Sea axial trough basalts ( < 0.703). We infer that these Sinai basalts equilibrated at depths of 35–50 km, possibly within an enriched mantle. Whether the main trough of the Red Sea is underlain by extended and modified continental crust or by oceanic crust is controversial. Since Miocene dikes were intruded along both coasts (but primarily the Saudi Arabian coast) of the Red Sea/Gulf of Suez axis in a zone up to 150 km wide, they may also underlie the main trough of the Red Sea and result, at least partially, in the discontinuous magnetic and gravity anomalies observed there. Possibly the abundance of dikes beneath the main trough is much greater than adjacent to the Red Sea, giving rise to properties intermediate between those of continental and oceanic crust. Downwarping and widespread intrusion of dikes along the Red Sea 18–26 m.y. ago probably signified crustal dilation and the initiation of a continental rift. Because tholeiitic basalts of the Sinai are higher in 87Sr/86Sr than Red Sea axial trough basalts, we infer that the conditions of their origin must also be very different. We suggest that the Sinai basalts may have been generated within but near the base of subcrustal continental lithosphere , perhaps indicating thermal thinning of lithospheric mantle above upwelled asthenosphere associated with continental rifting. This inference, if correct, would indicate that the lithosphere was at least 35–50 km thick during the early Miocene phase of rifting but possibly not much thicker. Since dikes farthest from the Red Sea are apparently identical in composition to those closest, asthenospheric upwarping must have been broad enough that all magmas were derived from approximately the same depth. In the Yemen-Ethiopia areas, volcanism began earlier (about 30 m.y. ago) and was much more voluminous than elsewhere along the Red Sea/Gulf of Suez axis, suggesting the presence of underlying mantle upwelling accompanying rifting of the Red Sea. In spite of differences in age and volume, the presence of large volumes of tholeiites among the Miocene basalts of the Yemen-Ethiopia areas suggests that the lithosphere was similarly thinned along the entire length of the Red Sea.

Radiocarbon Chronology Of The Holocene Dead Sea: Attempting A Regional Correlation

Holocene sedimentary and geomorphic sequences from the Dead Sea region, Israel, are compared by correlation of more than 50 radiocarbon dates. The 14 C dates provided the chronological basis that enabled us to detect basin-scale events that are hard to ascertain in single-site records. This paper is the first attempt to compare different Holocene records from several sites along the Dead Sea, based on their chrono-stratigraphy. Included is the first publication of the paleoclimatic record of the Nahal Darga ephemeral stream valley. Such a regional compilation is needed, because only the integration and comparative evaluation of several records can produce a reliable climatic history by establishing the height of former Dead Sea levels that may be complicated by tectonics and the rise of Mount Sedom. A relatively high level of the Holocene Dead Sea occurred during the mid-Holocene around 4400 BP or about 3000 cal BCE after calibration. The lake level fell sharply around 4000 BP, i.e. 2500 cal BCE, and later fluctuated close to early 20th century levels. The 14 C-based correlation is also used to estimate the rising rates of the Mount Sedom salt diapir that are apparently smaller than 10 mm per year.

Joint development during fluctuation of the regional stress field in southern Israel

Four trends of joint sets (WNW‐ESE, NW‐SE, NNW‐SSE and NE‐SW) are found in upper Turonian carbonate rocks within the Neqarot syncline of south-central Israel. The two most predominant sets strike parallel to the trend of maximum compressive stress directions (SH) associated with the plate-related Syrian Arc stress field (SAS; WNW‐ESE) active during the Cretaceous to present and the perturbed regional stress field (NNW‐SSE) related to stress accumulation on the Dead Sea Transform during the Miocene to the present. Eighty-two percent of the beds in this study contain joints parallel with the latter trend, whereas 42% contain joints parallel to the former trend. All beds with layer thickness to spacing ratio (FSR) . 1.5 have NNW‐SSE joint sets compatible with the Dead Sea Transform stress field (DSS), whereas all joints sets that are not compatible with the DSS stress field fall beneath this value for FSR. Considering lithology, joints in five of six chalky limestone beds and all marly limestone beds are compatible with the DSS, whereas joints compatible with the SAS do not develop in these marly and chalky limestone beds. In the study area, the joint sets lack a consistent formation sequence where more than one set is found in a single bed. We use these observations to conclude that all studied joints are Miocene or younger, that the regional stress field from the Miocene to the present fluctuated, between DSS and SAS states, and that the higher FSRs correspond to a greater amount of joint-normal strain in response to the DSS. q 2001 Elsevier Science Ltd. All rights reserved.

Alpha-Recoil Damage in Monazite: Preferential Dissolution of the Radiogenic Actinide Isotopes

A preferential dissolution by a factor of 1.1 to about 10 of the radiogenic nuclides /sup 234/U, /sup 230/Th, and /sup 228/Th relative to their corresponding structurally incorporated isotopes /sup 238/U and /sup 232/Th has been observed upon leaching of natural monazite samples in a bicarbonate-carbonate solution. This isotopic fractionation may be attributed to radiation damage caused by alpha recoil atoms. The observations have implications for the storage of crystalline nuclear waste forms in deep geological formations. The damage may endanger the integrity of any crystalline phase that contains alpha-emitting nuclides in groundwater environments. It is inferred that in monazite-like phases the overall alpha-recoil damage may increase nearly in proportion to the alpha-particle dose over the long time range ( about 10/sup 5/yr) required for the isolation of actinide wastes.