Stratigraphic, Gravimetric, and Radiometric Evidence for the Oligocene Emergence of the Nascent Lesser Antilles Volcanic Arc between the Grenada and Tobago Basins, Southeastern Caribbean Sea



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Using a single, 645-km long, deep-penetration seismic reflection and collinear refraction line, previous workers proposed that the southern segment of the Lesser Antilles volcanic arc (LAVA) migrated 250 km eastward from its extinct Late Cretaceous location along the Aves Ridge to its Oligocene to Recent location that now separates the Grenada Basin (GB) from the Tobago Basin (TB). A second group of previous workers proposed an alternative hypothesis that the Lesser Antilles arc developed above a rifted arc fragment from the Aves remnant arc. In order to test these differing tectonic hypotheses, I integrate an extensive data set from the two basins that include: 9,320 line-km of 2D industry seismic data; a 1,050-km-long 2D gravity model using satellite gravity data; and a compilation of published 22 radiometric ages from the volcanic islands of the southern LAVA. I integrate these three, different data types to better constrain the crustal structure and tectonic events affecting the broad, forearc area of the two basins where the nascent LAVA emerged as a linear ridge during the early Oligocene. The main results of this thesis include the following: 1) Using the seismic grid, I mapped five, north-south-trending half-grabens within both the Grenada and Tobago Basin; the north-south trend of the half-grabens with both the Grenada and Tobago Basin constrains their east-west opening direction and resolves a long controversy on the basin-opening direction for both basins; 2) Using a tie to previous wells along the Venezuelan margin, I constrained the age of the half-grabens as Middle Eocene; several half-grabens fringe the western end of the Grenada Basin and the western edge of the Tobago Basin and overlie rifted arc crust that is 12-17 km in thickness; the thinned-arc crustal provinces transition into zones of oceanic crust of 4-8 km in thickness that underlie the deepest areas of both basins; 3) The oceanic crust in the Grenada Basin is 4-8-km thick as imaged seismically from a reflective Moho as seen on the industry seismic and as supported by 2D gravity transect across both basins; this narrow 340-300-km-wide strip of oceanic crust is inferred to represent an extensional forearc basin that formed seaward of the Late Cretaceous-Paleocene intraoceanic volcanic arc (Aves Ridge); 4) Heat flow estimates for the two basins are based on measured depths to the gas hydrate horizon, averages taken from global studies of other areas underlain by oceanic crust, and local heat probe measurements in the Grenada Basin; the three methods yield a present-day heat flow range in the Grenada and Tobago Basins of 64 to 58 mW/m2, which is used as a constraint for basin modeling; 5) Onlap of Oligocene to Middle Miocene deep-marine strata against the inverted Eocene sediment and present-day bathymetric ridge of the LAVA as seen on seismic reflection lines constrains the age of intrusion of the nascent LAVA as Oligocene (35–23.5 Ma); the magmatic body of the LAVA penetrated upwards through the Eocene oceanic forearc basin crust that is now exposed in outcrops in the Grenadines; 6) the uplifted and deformed oceanic crust has been biostratigraphically and radiometrically dated as Early Oligocene (34-30 Ma). Based on the integration of all of this information, I infer the LAVA was established in its new location 250 km east of the Aves remnant arc during Early Oligocene time. The emergence and eastward shift of the LAVA volcanic arc is attributed to seaward (eastward) slab rollback of the Cretaceous age Atlantic oceanic crust. Basin modeling reveals a depth to the gas window at 3.5 km in these 13-8.5-km-thick and underexplored basins.



Lesser Antilles volcanic arc, Grenada Basin, Tobago Basin, nascent volcanic arc