DEFINITION OF HISTORY OF TECTONIC MODELS

The Arabian shield includes an assemblage of Middle to Late

Proterozoic rocks exposed in the western and central parts of

the Arabian Peninsula and overlapped to the north, east, and

south by Phanerozoic sedimentary cover rocks. Several parts

of the shield are covered by Tertiary and Quaternary lava

flows that were extruded concurrently with rifting of the Red

Sea. Rocks of the Arabian shield may be divided into assemblages

of Middle to Late Proterozoic stratotectonic units, volcano-

sedimentary, and associated mafic to intermediate

intrusive rocks. These rocks are divided into two major categories,

the layered rocks and the intrusive rocks. Researchers

variously interpret these assemblages as a result of volcanism

and magmatism in ensialic basins or above subduction zones.

More recent workers suggested that many of these assemblages

belong to late Proterozoic volcanic-arc systems that

comprise distinct tectonic units or terranes, recognized following

definitions established in the North America cordillera.

Efforts in suggesting models for the evolution of the Arabian

shield started in the 1960s. Early workers suggested that

the Arabian shield experienced three major orogenies in the

Late Proterozoic Era. They also delineated four classes of plutonic

rocks that evolved in chemistry from calc-alkaline to

peralkaline through time. In the 1970s a great deal of

research emerged concerning models of the tectonic evolution

of the Arabian shield. Two major models emerged from this

work, including mobilistic plate-tectonic models, and a nonmobilistic

basement-tectonic model.

The main tenet of the plate-tectonic model is that the evolution

of the Arabian shield started and took place in an

oceanic environment, with the formation of island arcs over

subduction zones in a huge oceanic basin. On the contrary, the

basement-tectonic model considers that the evolution of the

Arabian shield started by the rifting of an older craton or continent

to form intraoceanic basins that became the sites of

island arc systems. In both models, late stages of the formation

of the Arabian-Nubian shield are marked by the sweeping

together and collision of the island arcs systems, obduction of

the ophiolites, and cratonization of the entire orogen, forming

one craton attached to the African craton. Most subsequent

investigators in the 1970s supported one of these two models

and tried to gather evidence to support that model.

As more investigations, mapping, and research were carried

out in the 1980s and 1990s, a third model invoking

microplates and terrane accretion was suggested. This model

suggests the existence of an early to mid-Proterozoic

(2,000–1,630-million-year-old) craton that was extended, rifted,

then dispersed causing the development of basement fragments

that were incorporated as allochthonous microplates

into younger tectonostratigraphic units. The tectonostratigraphic

units included volcanic complexes, ophiolite complexes,

and marginal-basin and fore-arc stratotectonic units that

accumulated in the intraoceanic to continental-marginal environments

that resulted from rifting of the preexisting craton.

These rocks, including the older continental fragments, constituted

five large and five small tectonostratigraphic terranes

that were accreted and swept together between 770 million

and 620 million years ago to form a neo-craton on which

younger volcano-sedimentary and sedimentary rocks were

deposited. Most models developed in the period since the

early 1990s represent varieties of these three main classical

models, along with a greater appreciation of the role that the

formation of the supercontinent of Gondwana played in the

formation of the Arabian-Nubian shield.

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