Continental Microplate Research Articles

Distribution and tectonic setting of plutonic rocks of the Arabian Shield

Although it is still not possible to model in detail the tectonic evolution of the Arabian Shield, its evolution can be interpreted in terms of the classic pattern of Phanerozoic plate tectonics (the ‘Wilson cycle’). During the first stage of evolution (900-630 Ma), plutonism was dominated by intermediate plutonic rocks (diorite, quartz diorite, tonalite and trondhjemite) and involved a progressive evolution from primitive tholeiitic series are rocks to mature calc-alkaline series rocks. These rocks formed in both ensimatic island arc and continental—marginal arc environments. The magmatic-arc stage was terminated by two collisions (680-630 Ma); between an accreted ensimatic arc terrane and a continental microplate and between the microplate and a continental(?) plate. These collisions resulted in a shift from arc magmatism dominated by intermediate plutonic rocks to collision-related granitic (granodiorite to monzogranite) magmatism (660-610 Ma). The final phase of plutonism within the Shield (610-510 Ma) was the formation of widespread postorogenic intracratonic evolved peraluminous to peralkaline alkali-feldspar granites. Such granites typically form during the terminal relaxation phase of continental collisions. Although minor amounts of evolved peraluminous granites are present in the eastern part of the Shield, well-developed S-type granites (tourmaline- or cordierite-bearing granite) appear to be lacking. Minor amounts of syenitic plutonic rocks were also emplaced throughout the Shield from about 620 to 550 Ma. The plutonic rock assemblage of the Arabian Shield is composed of approximately 37% granite, 19% granodiorite, 17% tonalite and trondhjemite, 1.3% dioritic rocks, 7% alkali-feldspar granite (including 2.3% alkali granite and 1.3% aluminous granite), 6% gabbro, and 1% syenitic rocks. Plutonic rocks compose approximately 55% of the outcrop area of the Shield.

U-Pb Isotopic evidence for the accretion of a continental microplate in the Zalm region of the Saudi Arabian Shield

The area covered by this work includes three of the main tectonic units of the Arabian Shield: the Afif continental terrain, the Nabitah suture with its associated mobile belt, and the Asir ensimatic arc terrain. The geology of the Zalm area is well understood, and this geochronologic and isotopic study confirms that the southern Afif terrain was a continental microplate in the late Proterozoic. The study also provides a time frame for the crustal evolution of this part of the Shield. The Kabid formation is the oldest in the region and comprises pelitic, arkosic and felsic high-grade paragneisses. U-Pb zircon data from a pelitic garnet-sillimanite gneiss show that this part of the continental basement in the southern Afif terrain may be as old as 1770 Ma. Isotopic nalyses indicate that lead from the Kabid gneiss resided in the upper continental crust for a long period before 1770 Ma, and consequently Archaean source rocks may be present within the southern Afif terrain. Pb and Rb-Sr isotopic data in the Zalm region reveal a change in the nature of the underlying crust, from continental basement in the northeast, to less radiogenic marginal arc rocks in the southwest. This change is coincident with both aeromagnetic data, and a facies change within a pre-collision marginal basin. Miogeosynclinal continental shelf facies of the Siham group lie unconformably over the Kabid formation, and are in the area of continental lead signatures. Eugeosynclinal deep water sediments and volcanics, in association with ultramafic rocks, occur in the area of marginal arc signatures. U-Pb zircon age determinations show that this 'Andean' continental margin developed before about 720 Ma, and emplacement of calc-alkaline plutonic rocks continued until about 690 Ma. During the period 685-640 Ma, the continental Afif microplate collided with the Asir terrain as part of the Nabitah orogeny. At approximately 640 Ma ago, the Najd strike-slip faulting commenced with a dextral phase that controlled the emplacement of granitic plutons as well as the development of a series of large pull-apart grabens. Some of these grabens were floored by new oceanic crust and were filled with volcanosedimentary rocks of the Bani Ghayy group. Subsequently, the Najd fault system changed to sinistral strike-slip motion at about 620 Ma ago.

Stratigraphy and palaeogeography of the Siham group:direct evidence for a late Proterozoic continental microplate and active continental margin in the Saudi Arabian Shield

The presence of ancient continental crust in the Afif terrane of the Arabian Shield has been indicated by radiometric data. Detailed mapping in the central Shield reveals a volcano-sedimentary succession older than one of the final cratonizing events of the Shield and unconformable upon high grade paragneisses, providing direct evidence for a western continental margin to the Afif terrane in late Proterozoic times. The basement gneisses were metamorphosed to almandine-sillimanite amphibolite facies and preserve evidence of a deformation not seen in any other rocks of the area. The Siham group, unconformable upon this basement, was metamorphosed to upper greenschist facies. It has been divided into six formations which provide an E–W section through the group. The lower members are volcanic and are absent in the easternmost formation. Their clastic content increases eastward. A carbonaceous shale with exhalative chert and marble, tuffs and associated volcanogenic massive sulphides is associated with the volcanics. It is absent in the east where shallow marine marbles and quartzites lie unconformably upon basement. Rhyolite and conglomerate complete the succession. In the west, the base of the group is not seen but the shale is associated with numerous sheet-like bodies of mafic and ultramafic rock emplaced in an extensional tectonic setting. Other plutonic rocks are calc-alkaline and are emplaced mainly in the west. Deep water oceanic conditions prevailed in the west and a shallow marine continental shelf environment existed in the east, with deposition of the group across this continental margin. The stratigraphy of the Siham group is particularly akin to the Mesozoic of Peru. Magmatic arc and marginal basin were coincident in space and time, and the marginal basin was probably extensional. There is some limited evldence that it was ensialic. The Siham group is bounded to the west by a N-S suture zone formed in one of the final collisional events in the cratonization of the Shield. This event caused the deformation of the Siham group and marked the termination of the magmatic are.

Development of the Pannonian basin basement through the Cretaceous-Cenozoic collision: A new synthesis

A geodynamic analysis was carried out going back in time from the present. The collision took place during the Cretaceous and Cenozoic. Based on the subduction origin of the Neogene calc-alkalic volcanites three microplates are delineated. The southern and the northern ones are approximately the same as in previous plate tectonic syntheses but the third one is outlined here for the first time as the basement of a Miocene island arc. Restoring the former position of microplates a basin of oceanic crust is outlined on the site of the present Pannonian basin of continental crust. At the beginning of the Neogene, the present northern microplate was located somewhere west of this basin and the present southern one was located on its southern shelf. The reconstructed Cretaceous-Paleogene histories of these microplates were different. During the whole Barremian to Oligocene time interval the southern microplate was located on the passive margin of an oceanic basin which was coming into being through continental rifting of the European plate margin in the Valanginian to Hauterivian. The northern microplate, located on the African plate margin, was undergoing collision with an island arc in the Early Cretaceous. The oceanic basin of marginal-see type behind this island arc was closed along a southerly directed subduction zone in the Eocene. This resulted in the appearance of a calc-alkalic volcano-chain along this microplate and in a new basin opening behind. This marginal sea basin divided the present northern microplate from the African plate and joined the oceanic basin north of the present southern microplate. The present Pannonian basin was assembled as a lithospheric mosaic through convergent and transcurrent movements of the defined microplates during the Neogene. Sediments of the former oceanic basin now strike along the contacts of continental microplates as flysch belts.

Shelf-Slope Boundary at Active Subduction Zones: ABSTRACT

About 30 subduction-zone segments are identified worldwide by intermediate-focus earthquakes, calc-alkalic volcanic arcs, and lines of rapid mountain uplift. The total length of these active convergent plate boundaries is approximately 58,000 km. Of this length, 46% is of the Japan type, where the upper plate is relatively stable horizontally, 30% is of the Andes type, where the upper plate actively overrides the trench, and 24% is of the Himalaya type, where continental plates or microplates collide with continents. Subduction zones of both the Japan and Andes types are marked by basement highs at the shelf break. Uplift of the crust and upper mantle at the edge of the upper plate causes these highs when a relatively low-density prism of accreted trench sediment is emplaced below. The accretionary prism usually forms within 5 m.y. after a new subduction zone is established, during the time that the megathrust at its sea-floor outcrop is evolving from an initial dip of 30 to 45° to a steady-state dip of about 10°. Except during this relatively brief initial period of accretion, trench sediment at subduction zones normally is carried deeply into the lithosphere. Elongate sedimentary basins form on both sides of the shelf-break uplift, but compression generally removes the fluids from sediment on the lower trench slope before thermal maturation of oil precursors can occur. Elsewhere at the active margin, although the low geothermal gradient caused by subduction of cold oceanic crust delays hydrocarbon maturation, it can resume after continental collision or after subduction-zone realignment. End_of_Article - Last_Page 959------------