Grindlay, N. R.¹, Mondziel, S. A.², Mann, P.³ and Escalona, A. 3

¹Center for Marine Science, University of North Carolina Wilmington, 5600 Marvin K. Moss Lane, Wilmington, NC 28409 United States
²Center for Marine Science, University of North Carolina Wilmington, 5600 Marvin K. Moss Lane, Wilmington, NC 28409 United States
³Jackson School of Geosciences, Institute for Geophysics, University of Texas Austin, Austin, TX 78759-8500 United States
E-mail corresponding author: grindlayn@uncw.edu

The integration of multiple, high-quality marine geophysical data sets including multibeam bathymetry, sidescan sonar imagery, and single- and multi-channel seismic reflection profiles document two phases of east-west Middle Oligocene to Recent opening of the Mona rift on the northern Puerto Rico margin. Extension of the southern and central rift occurs along a west-dipping, listric normal fault on the east side of the rift, and east-dipping antithetic normal faults on the west side of the rift. Stratigraphic thickness changes across the rift indicate that extension initiated in the Middle Oligocene, approximately 30 Ma. This is much older than the previous estimate of 1.2 Ma that was predicted based on extrapolating the present-day rate of opening using GPS measurements. A total minimum extension of 6.05 km is calculated. The extension is proposed to have occurred in at least two phases. Phase I with 1.66 km of opening, occurred from the Middle Oligocene to Late Miocene at a very slow rate (0.087 mm/year). A possible cause of initial rifting may have been oblique subduction along the Puerto Rico trench. Phase II occurred from the Late Miocene to the Recent and resulted in at least 4.39 km of extension. The oblique collision of the Bahamas platform with the northern margin of Puerto Rico and resulting counter-clockwise rotation of Puerto Rico is proposed to have initiated the later, more rapid (0.4 mm/year) stage of extension. The dramatically different trend and axial gradient, and the minimal fill in the northern portion of the Mona rift suggest that it may have experienced different structural controls. The possibility that the northern rift formed as an extensional zone at the trailing edge of a forearc sliver is explored.