Seismic refraction data permit the southern Lesser Antilles arc and surrounding regions to be divided by the velocity of their basement. We propose that high-velocity basement of the arc platform beneath the Grenadine islands and below a part of the Tobago Trough forearc basin is oceanic and continuous and was originally connected with oceanic crust of the Grenada Basin. Low-velocity basements of the Tobago terrane and arc platform from St. Vincent north lie south and north, respectively, of the high-velocity basement of the arc platform. An oceanic origin of this high-velocity crust in the Grenadines is argued to be more plausible than an origin as unroofed lower arc crust.

The segment of probable oceanic crust in the arc platform was greatly uplifted during development of the present island arc, mainly in late Neogene time, relative to the Grenada Basin and Tobago Trough. Accepting the proposition of shallow oceanic crust in the Grenadines, early middle Eocene and possibly older pillow basalts of Mayreau, the oldest rock unit of the southern Lesser Antilles arc platform, may be an exposure of such basement. Major and minor element compositions of Mayreau Basalt are indicative of a spreading rather than arc origin. The stratigraphy of the pillow basalts indicates extrusion in an open marine environment, distant or shielded from sources of arc of continental sediment, followed by a period of pelagic sedimentation above the carbonate compensation depth. The Eocene basalt and pelagic cover formed a relatively deep floor of a marine basin in which arc-derived turbidites and pelagic sediments accumulated over the succeeding 25--30 ma. Such basalts thus indicate a probable spreading origin of the Grenada Basin and an age of cessation of spreading in the region of Mayreau in Eocene time. The configuration of the Eocene basin and the direction of spreading, however, are unknowns. Regional structural relationships imply the spreading was probably backarc, an origin also hinted at by Ce/Zr ratios of Mayreau Basalt. Although the Avese Ridge probably was the remnant arc, the existence and position of an early middle Eocene frontal arc are unresolved. The early Paleogene tectonic architecture of the southeastern Caribbean differed substantially from that of the Neogene. ¿ American Geophysical Union 1991