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

The late Proterozoic terrane of the Arabian Shield uniquely exposes the records of magmatic processes and deformation patterns that shaped the Earth over half a billion years ago. This Precambrian to Cambrian terrane is composed of supracrustal rocks underlain and intruded by igneous rocks. The plutonic shedding and accretion of the crust has been studied in detail. Satellite 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 interpretation with detailed ground studies are discussed. The supracrustal rocks display complex fold interference patterns, associated with the emplacement of the basic plutons (Jarshah and Mifsah) and mantled gneiss-domes (Qardath and Miktaa) which took place coeval with crustal shortening by regional deformation. 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). Finally, a subvolcanic complex (Mahala) was emplaced, which includes a 2.5-km radius ringdike of several hundred meters thickness. The geologic features of our study area can be explained by a simple model of island arc evolution in six stages: (I+II) Pre-tectonic and pre-plutonic, paleogeographic situation (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 relationships are compatible with the geochronologic ages of rock units previously dated by radiometric methods only. The accretion rate of the Arabian Shield region is here estimated at 0.1 km3 yr−1 or 10% of the total global accretion rate at the present-day. The average Arabian arc addition rate is 33 km3 km−1 Ma−1 for the late Proterozoic, which is rather close to estimates of modern island-arc accretion rates of 30 km3 km−1 Ma−1. We conclude that the process and rate of island-arc accretion onto active continental margins of Arabia in the late Precambrian eon evolved in a fashion similar to that of island-arc accretion in the Phanerozoic eon. This conclusion 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 rates.