The Puerto Rico Trench is parallel to and north of Puerto Rico and the Virgin Islands and reaches depths of more than 8000 meters. Puerto Rico, the closest land area, has a central longitudinal core of Cretaceous and early Tertiary volcanic rocks, some serpentinite of undetermined age, and numerous small intrusions. These rocks were folded into an anticlinorium and intensely faulted into hundreds of fault blocks in an orogeny that lasted from Late Cretaceous until middle Eocene time. Most fold axes and the major fault zones trend approximately west-northwest at an angle of about 15° to the general trend of the island and to the Puerto Rico Trench. Near the north coast, late Oligocene rocks rest on a truncated surface of Cretaceous and early Tertiary volcanic rocks. The Oligocene and overlying Miocene sedimentary rocks strike approximately east, parallel to the Puerto Rico Trench. Apparently the trench is not related to the tectonic movements that folded and faulted the Cretaceous and early Tertiary rocks, but it is closely related tectonically to the Oligocene and Miocene rocks. The trench probably first appeared in Oligocene or possibly late Eocene time and may have reached its maximum depth in late Miocene or Pliocene time.

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