We present a study of older rocks exposed on Grenada and the Grenadine Islands of the southern Lesser Antilles arc platform (SLAAP) with a view toward understanding the structural and stratigraphic evolution of the platform that led to the modern (and Neogene) magmatic arc. The modern SLAAP is an east-tilted half horst that probably developed early in Miocene time by uplift along a zone of north-striking normal faults relative to both sea level and the crust of the adjacent Grenada Basin. The uplift was completed before Neogene magmas erupted at the surface of the SLAAP beginning at about 12 Ma.
Twenty-six units of older rocks, defined as those that preceded local Neogene magmatism, have been identified by us and previous workers in the SLAAP. Our contributions are new stratigraphic and structural analyses and microfossil and radiometric dating. The age range of older rocks is early middle Eocene to middle Miocene. The most stratigraphic information comes from Carriacou, which contains newly defined formations of Eocene and Oligocene age that are thrust above markedly different late Eocene facies. The thrust is covered by Miocene, possibly late Oligocene, strata below which there is a lacuna possibly as long as 10 m.y. The oldest rocks of the SLAAP are the middle Eocene Mayreau Basalt. Earlier claims of a Cretaceous age of rocks on Union Island are in error; we show them to be Eocene. Assuming they were all deposited contiguously, the older rocks units can be grouped and interpreted as follows: I, middle Eocene pillow basalt of spreading origin and pelagic cover; II, late middle Eocene–middle Miocene deep basinal sediments comprising arc-volcanigenic turbidite and hemipelagite, together with minor intrusive basalt; III, early(?) and middle Miocene local carbonate platform. Group I basalts are correlated with crust of the Grenada Basin, and the Eocene and Oligocene sediments of I and II are equivalent to the deep strata of the Grenada Basin. No magmatic arc rocks of Paleogene age are recognized in the SLAAP.
Older rocks of the SLAAP are deformed by folding, thrusting, and foliation development, and by Neogene normal faulting and intrusion. The principal deformation was a horizontal contraction of northerly bearing in late Oligocene and (or) early Miocene time before the uplift of the half horst. A later horizontal contraction of westerly bearing occurred in the middle Miocene after the horst had developed. The northerly contraction in the SLAAP is interpreted to have arisen by accretion of Grenada Basin cover and slices of shallow basement during a brief episode of subduction of the oceanic crust of the Grenada Basin relatively southward below the (?) Tobago terrane. The accretionary prism of the SLAAP is inferred to have been continuous with the accretionary belt of the Southern Grenada Basin Deformation Front that extends west-southwest to a point west of Margarita.
A Paleogene magmatic arc was the source of copious volcanigenic and carbonate clastic sediment to the SLAAP basin in late Middle Eocene and Oligocene times. This arc was not in the SLAAP and has not been directly located. We infer its locus is south of the Grenada Basin, striking west-southwest from Grenada; its original orientation is uncertain, but scanty paleomagnetic data provide no evidence for large rotation.
The Neogene magmatic arc of the southern Lesser Antilles is not a clone of the Paleogene arc. It developed transverse to the Paleogene arc and SLAAP accretionary prism, presumably by a major reconfiguration within the Caribbean-American plate boundary zone. The large change in arc trend is thought to be due to collision between the Paleogene arc system and continental South America.
