Mid-crustal dynamics and island-arc accretion in the Arabian Shield: insight from the Earth's natural laboratory

The late Proterozoic terrane of the Arabian Shield uniquely exposes the rec ords of magmatic processes and deformation patterns that shaped the Earth o ver half a billion pears ago. This Precambrian to Cambrian terrane is compo sed of supracrustal rocks underlain and intruded by igneous rocks. The plut onic shedding and accretion of the crust has been studied in detail. Satell ite images (TM Landsat and SPOT) were processed to enhance both lithologic content and structural patterns, using spectral transformation and spatial filtering techniques. Results obtained by integrating the image interpretat ion with detailed ground studies are discussed. The supracrustal rocks disp lay complex fold interference patterns, associated with the emplacement of the basic plutons (Jarshah and Mifsah) and mantled gneiss-domes (Qardath an d Miktaa) which took place coeval with crustal shortening by regional defor mation. The intrusion of a younger pluton (Arafadi) caused the collision of two pre-existing plutons (Jarshah and Mifsah), involved extensive sideway shedding of supracrustal rocks and formed a major shear zone (Qurayn). Fina lly, a subvolcanic complex (Mahala) was emplaced, which includes a 2.5-km r adius ringdike of several hundred meters thickness. The geologic features o f our study area can be explained by a simple model of island are evolution in six stages: (I + II) Pre-tectonic and pre-plutonic, paleogeographic sit uation (0.9 to 0.66 Ga); (III) Syntectonic, basic plutonism (0.65 Ga); (IV) Syntectonic gneiss-domes (0.62 Ga); (V) Post-tectonic, acidic plutonism (0 .59 to 0.54 Ga); and (VI) Post-orogenic volcanism (0.54 to 0.5 Ga). In our scenario, the relative ages of events inferred on the basis of field relati onships are compatible with the geochronologic ages of rock units previousl y dated by radiometric methods only. The accretion rate of the Arabian Shie ld region is here estimated at 0.1 km(3) yr(-1) or 10% of the total global accretion rate at the present-day. The average Arabian are addition rate is 33 km(3) km(-1) Ma(-1) for the late Proterozoic, which is rather close to estimates of modem island-are accretion rates of 30 km(3) km(-1) Ma(-1). We conclude that the process and rate of island-are accretion onto active con tinental margins of Arabia in the late Precambrian eon evolved in a fashion similar to that of island-are accretion in the Phanerozoic eon, This concl usion is important because it counters the view that, based on unrealistic assumptions, accretion rates for the late Proterozoic crust of the Arabian craton were as much as 10 times faster than the modern crustal accretion ra tes. (C) 2000 Elsevier Science B.V. All rights reserved.