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Abstract: The Arabian Plate originated ∼25Ma ago by rifting of NE Africa to form the Gulf of Aden and Red Sea. It is one of the smaller and younger of the Earth''s lithospheric plates. The upper part of its crust consists of crystalline Precambrian basement, Phanerozoic sedimentary cover as much as 10km thick, and Cenozoic flood basalt (harrat). The distribution of these rocks and variations in elevation across the Plate cause a pronounced geologic and topographic asymmetry, with extensive basement exposures (the Arabian Shield) and elevations of as much as 3000m in the west, and a Phanerozoic succession (Arabian Platform) that thickens, and a surface that descends to sea level, eastward between the Shield and the northeastern margin of the Plate. This tilt in the Plate is partly the result of marginal uplift during rifting in the south and west, and loading during collision with, and subduction beneath, the Eurasian Plate in the northeast. But a variety of evidence suggests that the asymmetry also reflects a fundamental crustal and mantle heterogeneity in the Plate that dates from Neoproterozoic time when the crust formed. The bulk of the Plate''s upper crystalline crust is Neoproterozoic in age (1000–540Ma) reflecting, in the west, a 300-million year process of continental crustal growth between ∼850 and 550Ma represented by amalgamated juvenile magmatic arcs, post-amalgamation sedimentary and volcanic basins, and granitoid intrusions that make up as much as 50% of the Shield''s surface. Locally, Archean and Paleoproterozoic rocks are structurally intercalated with the juvenile Neoproterozoic rocks in the southern and eastern parts of the Shield. The geologic dataset for the age, composition, and origin of the upper crust of the Plate in the east is smaller than the database for the Shield, and conclusions made about the crust in the east are correspondingly less definitive. In the absence of exposures, furthermore, nothing is known by direct observation about the composition of the crust north of the Shield. Nonetheless, available data indicate a geologic history for eastern Arabian crust different to that in the west. The Neoproterozic crust (∼815–785Ma) is somewhat older than in the bulk of the Arabian Shield, and igneous and metamorphic activity was largely finished by 750Ma. Thereafter, the eastern part of the Plate became the site of virtually continuous sedimentation from 725Ma on and into the Phanerozoic. This implies that a relatively strong lithosphere was in place beneath eastern Arabia by 700Ma in contrast to a lithospheric instability that persisted to ∼550Ma in the west. Lithospheric differentiation is further indicated by the Phanerozoic depositional history with steady subsidence and accumulation of a sedimentary succession 5–14km thick in the east and a consistent high-stand and thin to no Phanerozoic accumulation over the Shield. Geophysical data likewise indicate east–west lithospheric differentiation. Overall, the crustal thickness of the Plate (depth to the Moho) is ∼40km, but there is a tendency for the crust to thicken eastward by as much as 10% from 35–40km beneath the Shield to 40–45km beneath eastern Arabia. The crust also becomes structurally more complex with as many as 5 seismically recognized layers in the east compared to 3 layers in the west. A coincident increase in velocity is noted in the upper-crust layers. Complementary changes are evidenced in some models of the Arabian Plate continental upper mantle, indicating eastward thickening of the lithospheric mantle from ∼80km beneath the Shield to ∼120km beneath the Platform, which corresponds to an overall lithospheric thickening (crust and upper mantle) from ∼120km to ∼160km eastward. The locus of these changes coincides with a prominent magnetic anomaly (Central Arabian Magnetic Anomaly, CAMA) in the extreme eastern part of the Arabian Shield that extends north across the north-central part of the Arabian Plate. The CAMA also coincides with a major structural boundary separating a region of northerly and northwesterly basement trends in the west from a region of northerly and northeasterly trends in the northeastern part of the Plate, and with the transition from high-stand buoyant Shield to subsided Platform. Its coincidence with geophysically indicated changes in the lower crust and mantle structure suggests that a fundamental lithospheric boundary is present in the central part of the Arabian Plate. The ages and isotopic characteristics of xenoliths brought to the surface in Cenozoic basalt eruptions indicate that the lower crust and upper mantle are largely juvenile Neoproterozoic additions, meaning that the lower crust and upper mantle formed about the same time as the upper crust. This implies that the lithospheric boundary in the central part of the Arabian Plate dates from Neoproterozoic time. We conclude that lithospheric differentiation across the Arabian Plate is long lived and has controlled much of the Phanerozoic sedimentary history of the Plate. [Copyright &y& Elsevier]