Published seismic reflection records show that the north coast of Puerto Rico subsided in three stages, each dominated by different physical mechanisms. In the late mid-Eocene to mid-Oligocene, tensional thinning of hot island-arc crust and sediment loading caused uniform subsidence of 1 to 2 km. During the Oligocene and Miocene the area sank uniformly almost 2 km because of sediment loading and thermal contraction from the previous stage of magmatism. In the Pliocene, flexural isostasy led to mild uplift on land and over 4 km of sinking along the south side of the Puerto Rico Trench. The flexure was driven mainly by tectonic erosion of island-arc crust by the newly underthrust North American plate.

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