M. 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.

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.

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.

Thermal history of the eastern margin of the Gulf of Suez, I. reconstruction from borehole temperature and organic maturity measurements

Abstract The Gulf of Suez is a Tertiary continental rift associated with prominent flank uplift. Despite numerous studies which focused mainly on the western and central parts of the graben, the thermo-mechanics controlling the tectonic evolution of the Gulf of Suez is still enigmatic. We have integrated borehole temperatures and organic maturity measurements in the eastern margin of the Gulf of Suez, in order to study rift-related paleothermometry and the present-day thermal regime. The data obtained suggest that the present thermal regime represents the maximum heat flow and temperatures for the sedimentary section in the basin. Furthermore, lateral distributions of geothermal gradient and heat flow in the Gulf of Suez do not correlate, mainly because of extensive variability in lithology and thermal conductivity. Rift-related heat flow increases systematically and subparallel to the rift axis, from about 60 mW/m2 in the Darag subbasin in the north to about 80 mW/m2 in the Ras Garra area in the south. Both values are higher than 45 mW/m2, the average heat flow assumed for the pre-rift stage and the characteristic level for the present-day heat flow away from the rift. The north to south increase in heat flow probably reflects the southward increase of extension as well as lateral transfer of heat flow from the Red Sea. This latter conclusion is supported by the fact that heat flow in the southern Gulf of Suez recorded by the paleothermometric reconstructions and borehole temperature data is somewhat higher than that estimated by model calculations for the extension derived from structural and subsidence reconstruction.

A major Late Devonian-Early Carboniferous (Hercynian) Thermotectonic event at the NW Margin of the Arabian-Nubian Shield: Evidence from zircon fission track dating

Zircon fission track (ZFT) ages of 17 Precambrian samples from deep boreholes and outcrops in southern Israel and Sinai fall within the range 328–373 Ma (Late Devonian-Early Carboniferous). Single zircon grain age distributions are unimodal with a high chi-square probability. The age data indicate total resetting of the ZFT clocks but only partial resetting of coexisting sphenes, constraining the temperatures attained to about 225° ± 50°C, followed by relatively rapid regional cooling at the times indicated. In the study area, the lower Paleozoic section presently overlying Precambrian rocks is limited to Cambrian strata, up to 300 m thick. Further, stratigraphic evidence for sub-Carboniferous erosion is preserved only in SW Sinai. To the east, in southern Jordan, a 2–2.5 km thick lower Paleozoic succession is reported. Despite the lack of stratigraphic evidence in the study area, the ZFT data (1) strongly suggest that an equivalent or thicker section also existed, (2) constrain the timing of the erosion to Late Devonian-Early Carboniferous (Hercynian), and (3) indicate that thermal gradients may have reached ≥50°C/km prior to the uplift/erosion event. Regional stratigraphic evidence indicates that the Late Devonian-Early Carboniferous event was confined to a relatively narrow belt extending from the Gulf of Suez area to the vicinity of NE Syria and SE Turkey.

Thermal history of the eastern Gulf of Suez, II. Reconstruction from apatite fission track and 40Ar39Ar K-feldspar measurements

Abstract Apatite fission track (AFT) measurements from Miocene graben-fill sediments of the eastern Gulf of Suez in three deep boreholes (Belayim 113 M-2/6, Ras Garra M-1 and Alma-2) yield a wide range of ages (from 125 to 320 Ma) and mean track-lengths (∼10.1–12.7 μm). This range is similar to that recorded from Precambrian crystalline basement flanking the graben which is the major source for the rift fill. Since the AFT ages exceed the age of the host strata, which are presently at their highest post-depositional temperatures, the rift-related thermal regime is one of only moderate heating. Downhole AFT data in the Belayim 113 M-2/6 borehole attest to a higher rift temperature than in the Ras Garra M-1 borehole, some 85 km to the south, where little or no thermal overprinting is evident. These findings are consistent with previously reported bottom hole temperatures and vitrinite reflectance data in the study area. Despite the higher syn-rift thermal regime indicated for Belayim 113 M-2/6, apatite provenance ages in Ras Garra M-1 are considerably younger. Thus, the AFT data in the Ras Garra M-1 area do not record significant rift-related thermal effects, but rather, they mainly retain a pre-rift provenance signature which reflects the order and depth of erosion at the uplifted flanks. The younger AFT ages in Ras Garra M-1, despite the weaker rift-related thermal effect, suggest a deeper level proportional to an additional ∼5–10°C of exhumation of the uplifted crystalline basement southwards along the eastern rift flank by Mid-Miocene time. This result is consistent with earlier findings which indicate both increased extension and heat flow southwards in the Gulf of Suez, and earlier exhumation of the rift flank. Immediately preceding extension and opening of the Gulf of Suez, the most deeply exhumed basement rocks presently exposed on the rift flanks were heated to temperatures ≥110°C (total annealing of apatite), but 40 Ar 39 Ar data and non-resetting of zircon FT clocks in sinai, 40 Ar 39 Ar data from granite penetrated in graben drilling at ∼3.89 km further corroborates pre-rift palaeotemperatures

Significance of Combined Vitrinite Reflectance and Fission-Track Studies in Evaluating Thermal History of Sedimentary Basins: An Example from Southern Israel

Vitrinite reflectance and fission-track age determinations (apatite, zircon, and sphene) from three deep boreholes in the Har HaNegev area, southern Israel, have been integrated for reconstruction of the thermal history. Drill holes, Ramon 1, Makhtesh Qatan 2, and Kurnub 1, each sited in the core of a separate breached anticline (“makhtesh”), penetrated a discontinuous Early Permian to Mesozoic sedimentary succession unconformably overlying a Precambrian arkosic and igneous complex. Post-Early Permian burial history was reconstructed from borehole data and regional stratigraphy.

Cretaceous to present paleothermal gradients, central Negev, Israel: Constraints from fission track dating

Abstract Apatite and zircon fission track ages (FTA), vitrinite reflectance (VR) data and burial history curves were integrated for reconstruction of Early Cretaceous to present maximum thermal gradients in four deep boreholes in the central Negev, Israel. The most complete data set is available from the Ramon 1 borehole. Supplementary data were obtained from Hameishar 1, Makhtesh Qatan 2, and Kurnub 1 boreholes. Between ca. 122-90 Ma the constraints on thermal gradient obtained from apatite FTA overlap with those derived from zircon FT and VR data, restricting them to Thermal constraints derived from apatite FTA and VR data in Makhtesh Qatan 2 and Kurnub 1 boreholes are consistent with those obtained post-56 Ma for Ramon 1. For pre-56 Ma, only VR data are available and these indicate considerably lower maximum gradients than those obtained for the same time period from Ramon 1. This dichotomy reflects different Early Cretaceous-Early Tertiary thermal regimes between the northern and southern parts of the study area.

The Katherina Ring Complex (Sinai Peninsula, Egypt): Sequence of emplacement and petrogenesis

The Katherina Ring Complex (KRC) in the central Sinai Peninsula, Egypt, was formed in three consecutive stages: volcanic, subvolcanic, and plutonic. The outer Katherina ring dike, about 30 km in diameter, marks the contour of a paleocaldera. Volcanic ignimbrite extrusions representing the earliest stage of the KRC were followed by emplacement of subvolcanic peralkaline microgranite bodies. The ring dikes are composed mainly of porphyritic quartz monzonite and plagioclase-rich quartz syenite, with less abundant alkali feldspar quartz syenite and peralkaline granite. The central alkaline granite pluton (ca. 210 km2) was emplaced at ∼595 Ma. The quartz monzonite–syenite group is characterized by positive Eu anomalies (Eu/Eu* = 1.1-1.6), which is consistent with its enrichment in accumulated plagioclase crystals (xenocrysts). These features, along with the positive εNd(T) values in quartz monzonite (up to +5.6) suggest that the initial silicic magma was hybridized by plagiclase-rich mafic magma. Mineral geothermometry and melt inclusion studies point to the formation of the silicic magmas at high temperatures, up to 900 to 1000 °C. Oxygen and Sr-Nd isotope data suggest that the source of the magmas was moderately depleted mantle or young juvenile crust. The trend of compositional change from quartz alkali feldspar syenite to alkali feldspar and peralkaline granite is consistent with a fractional crystallization model. A specific feature of the magma differentiation process is that the residual melt separation could occur when the magma was ∼55 percent crystallized (“rigid percolation threshold”) and clusters of crystals formed a rigid skeleton in the magma. At this stage, residual melt flowed pervasively through the pore space. The suggested model of melt separation alleviates the problem of a differentiation process since it does not require crystal settling that seems unrealistic in highly viscous silicic magmas at shallow depth. Chemical and Nd isotopic distinctions between the leucocratic volcanic–subvolcanic rocks (εNd(T) = 4.2-4.6) and the Katherina pluton granite (2.6-3.9) suggest that the silicic magmas of the volcanic-subvolcanic and the plutonic stages were probably produced from different mantle-derived sources.