From plume head to continental lithosphere in the Arabian–Nubian shield

Structural and tectonic evolution of suture‐related belts and post‐accretionary systems in the Arabian‐Nubian Shield

Deforming belts in the Arabian‐Nubian Shield (ANS) are classified into (1) suture‐related belts, including arc–arc and arc‐continental, and (2) post‐accretionary systems, including N‐trending compression zones and NW‐trending strike‐slip faults. Terrane accretion took place in the ANS between 800 and 700 Ma, along arc–arc sutures. Such sutures are directed from E to NE in the northern part of the ANS, and from N to NE in the south, and are aligned in the north and east with N‐ or S‐verging ophiolitic nappes, or in the south with W‐verging nappes. The Asir, Hijaz, and Midyan terranes formed the Western Arabian shield by 715 Ma. The Afif terrane collided with the Hijaz and Asir terranes between 680 and 640 Ma, terminating the subduction along the Nabitah suture. Subduction began west of the Al Amar arc near the margin of the Ar Rayn terrane at 670 Ma. Afif and Ar Rayn terranes collided along the Al Amar‐Idsas suture about 640 Ma, producing the Idsas orogeny that initiated the major faulting and folding. Strike‐slip faults and upright folds related to oblique convergence between terranes and/or post‐accretionary systems deform the southern sutures. The eastern and western boundaries of the ANS are marked by arc‐continental sutures and characterized by N‐trending deformation belts that formed at 750–650 Ma when the ANS collided with East and West Gondwana.

Weakening of the South Asian summer monsoon linked to interhemispheric ice-sheet growth since 12 Ma

The evolution and driving mechanism of the South Asian summer monsoon (SASM) are still poorly understood. We here present a 12-Myr long SASM record by analyzing the strontium and neodymium isotopic composition of detrital components at IODP Exp. 359 Site U1467 from the northern Indian Ocean. The provenance investigation demonstrates that more dust enriched in εNd from northeastern Africa and the Arabian Peninsula was transported to the study site by monsoonal and Shamal winds during the summer monsoon season. A two-step weakening of the SASM wind since ~12 Ma is proposed based on the εNd record. This observational phenomenon is supported by climate modeling results, demonstrating that the SASM evolution was mainly controlled by variations in the gradient between the Mascarene High and the Indian Low, associated with meridional shifts of the Hadley Cell and the Intertropical Convergence Zone, which were caused by interhemispheric ice-sheet growth since the Middle Miocene.

The Central Asian Orogenic Belt and growth of the continental crust in the Phanerozoic

Asia is the world’s largest composite continent, comprising numerous old cratonic blocks and young mobile belts. During the Phanerozoic it was enlarged by successive accretion of dispersed Gondwana-derived terranes. The opening and closing of palaeo-oceans would have inevitably produced a certain amount of fresh mantle-derived juvenile crust. The Central Asian Orogenic Belt (CAOB), otherwise known as the Altaid tectonic collage, is now celebrated for its accretionary tectonics and massive juvenile crustal production in the Phanerozoic. It is composed of a variety of tectonic units, including Precambrian microcontinental blocks, ancient island arcs, ocean island, accretionary complexes, ophiolites and passive continental margins. Yet, the most outstanding feature is the vast expanse of granitic intrusions and their volcanic equivalents. Since granitoids are generated in lower-to-middle crustal conditions, they are used to probe the nature of their crustal sources, and to evaluate the relative contribution of juvenile v. recycled crust in the orogenic belts. Using the Nd-Sr isotope tracer technique, the majority of granitoids from the CAOB can be shown to contain high proportions (60 to 100%) of the mantle component in their generation. This implies an important crustal growth in continental scale during the period of 500–100 Ma. The evolution of the CAOB undoubtedly involved both lateral and vertical accretion of juvenile material. The lateral accretion implies stacking of arc complexes, accompanied by amalgamation of old microcontinental blocks. Parts of the accreted arc assemblages were later converted into granitoids via underplating of basaltic magmas. The emplacement of large volumes of post-accretionary alkaline and peralkaline granites was most likely achieved by vertical accretion through a series of processes, including underplating of basaltic magma, mixing of basaltic liquid with lower-crustal rocks, partial melting of the mixed lithologies leading to generation of granitic liquids, and followed by fractional crystallization. The recognition of vast juvenile terranes in the Canadian Cordillera, the western US, the Appalachians and the Central Asian Orogenic Belt has considerably changed our view on the growth rate of the continental crust in the Phanerozoic.

Neoproterozoic dextral faulting on the Najd Fault System, Saudi Arabia, preceded sinistral faulting and escape tectonics related to closure of the Mozambique Ocean

The Neoproterozoic Najd Fault System extends for 2000km across the East African Orogen, yet its history of motion and tectonic significance are widely debated. The Halaban-Zarghat Fault is the northeastern-most of the major NW-striking Najd faults in the Arabian Shield. Several sedimentary basins of the Neoproterozoic Jibalah Group are bounded by strands of the Halaban-Zarghat Fault and other Najd faults, particularly along right steps in the fault trace. Among the largest of the basins is the Jifn. The geometry of the Jifn Basin and the sedimentary facies of Jibalah Group indicate that it is a dextral pull-apart basin between strands of the Halaban-Zarghat Fault. A zone of high-grade mylonitic gneiss is located along a left step in the fault zone and may be a deeply eroded pop-up structure related to dextral transpression. Analysis of structural data from around and within the Jifn Basin, the position of other pull-apart basins and high-grade mylonite zones along the Halaban-Zarghat Fault are all consistent with early dextral movement along the Halaban-Zarghat Fault. Offsets of distinctive older rock units and transection of the Jifn Basin by sinistral faults, however, show that the latest and most significant sense of offset on the Halaban-Zarghat Fault and other Najd faults was sinistral.

A U-Pb zircon date of 624.9 ± 4.2 Ma from rhyolitic basement of the Jifn Basin gives a lower limit for the formation of the basin and initiation of dextral movement along the Halaban-Zarghat Fault. This age is interpreted as the earliest age for the collision of East and West Gondwana. A 621 ± 7 Ma pluton is offset 10 km dextrally along the Halaban-Zarghat Fault, showing that dextral motions continued for some time past 621 Ma, before switching to sinistral motions, and accreted terranes caught between the two continents were forced toward an oceanic-free face to the north. A 576.6 ± 5.3 Ma U-Pb zircon date from an undeformed felsite dyke that intrudes the Jibalah Group gives an upper time limit for movement along the Halaban-Zarghat Fault. This may mark the time that collision and escape tectonics ended, or it may reflect the time that displacements were transferred to other Najd faults in more interior parts of the East African Orogen.