This abstract was published in the Transactions of the Geological Society of America Penrose Meeting, in Puerto Plata, Dominican Republic, Copyright 1999 by Geological Society of America.

Tectonic Activity in the Mona Passage Area during the three Cretaceous-Holocene tectonic phases of the Puerto Rico-Virgin Islands platform.

Jean-Paul Van Gestel  Department of Geological Sciences and Institute for Geophysics, The University of Texas at Austin, Austin, TX 78712, USA, Paul Mann  Institute for Geophysics, The University of Texas at Austin, 4412 Spicewood Springs Road, Bldg. 600, Austin, TX 78759-8500, USA, James F. Dolan  Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA, Nancy R. Grindlay  Department of Earth Sciences, University of North Carolina at Wilmington, Wilmington, NC 28403-3297, USA.

Introduction

In the summer of 1996 single-channel seismic reflection, sidescan, bathymetry, gravity, and magnetic data were collected during cruise EW 96-05 on board of R/V Maurice Ewing in which all authors participated. These newly acquired data have been integrated with and correlated to onland outcrops and well information and have been used to re-examine the proposed regional geologic and tectonic models. The depositional history of the different sequences of the platform North of Puerto Rico has been studied and three different tectonic phases have been distinguished.

Setting

Puerto Rico occupies the northeastern segment of a lower Cretaceous to Holocene island arc chain that extends from western Cuba to the north coast of South America. The core of the island is formed by Cretaceous arc basement rocks. Both in the north and in the south Oligocene-Early Pliocene carbonate platform sediments can be found. In the north the platform covers a small portion of the island but extents under a smooth continuous 4 degree dip offshore to a depth of 4 km deep. In the south the platform has a steeper dip and it extends less far offshore. The whole shallow submarine area in between the islands of Hispaniola, Puerto Rico and the Virgin Islands is also covered with Oligocene-Pliocene carbonate rocks. The Puerto Rico-Virgin Islands platform overlies the Puerto Rico-Virgin Islands microplate that has been defined on the basis of earthquake data and marine geophysical sidescan surveys of offshore areas. To the north, the microplate is bounded by the Puerto Rico trench. To the east of Puerto Rico, the platform trends to the east-northeast and covers most of the area of the Virgin Islands. The eastern and southeastern edge of the platform and the Puerto Rico-Virgin Islands microplate are sharply bounded by the Anegada fault zone. This fault zone runs trough the deepwater Anegada Passage between Puerto Rico and St. Croix and connects the Sombrero and Virgin Islands basins. These two fault-bounded deeps are pull-apart basins along the right-lateral Anegada fault zone. The southern edge of the Puerto Rico-Virgin Islands microplate is defined by the Muertos Trough where the Caribbean plate is subducted beneath the microplate. To the west of Puerto Rico, the carbonate platform extends across the Mona Passage and onto the island of Hispaniola. Bathymetric deeps in the Mona Passage correspond to rifts that locally extend and fragment the platform between Puerto Rico and Hispaniola. The origin and age of this rifting in the Mona Passage area is unclear and will be the focus of this presentation.

Three tectonic phases of the northern margin of Puerto Rico

In a regional study of the offshore seismic reflection profiles surrounding Puerto Rico, we have documented three distinct, Cenozoic tectonic phases that occurred during development of the northern margin of Puerto Rico.
Tectonic phase 1: Late Cretaceous to Middle Eocene formation and sedimentary infilling from the south of a forearc basin. This basin was formed between down-to-the-north normal faults near the present day coast of Puerto Rico and an outer-arc ridge near the present day shelf break. Tectonic phase 1 contained the last volcanic arc activity in Puerto Rico that was produced by the subduction of oceanic crust of the North America plate beneath the Caribbean arc system. The end of tectonic phase 1 is related to an initial collision between the Caribbean arc and the Bahama carbonate platform.
Tectonic phase 2: Latest Eocene to early Pliocene formation of a 1578 m thick, northward-thickening Puerto Rico-Virgin Islands platform, predominately formed of carbonate sediments. Depositional thicknesses of sedimentary layers deposited during phase 2 are controlled by two large arches: 1) The NNW-trending Guajataca arch appears to have formed as the elevated flank of the north-south striking Mona rift, which forms the western boundary of the present day Puerto Rico-Virgin Islands microplate, 2) the northeast-trending San Juan arch can not be related to any adjacent structure or plate boundary feature. The results of paleomagnetic studies of the carbonate sediments show that Puerto Rico experienced a counterclockwise rotation during tectonic phase 2.
Tectonic phase 3: Early Pliocene to Holocene northward tilting of the Puerto Rico-Virgin Islands platform, submerged the northern edge of the platform to a depth of 4 km and elevated the southern edge of the platform to several hundred meters above sea level on Puerto Rico. Northward tilting of this area occurred on the northern limb of a large arch or anticline formed parallel to the long axis of the island of Puerto Rico and its shelf areas. The arch probably formed in response to a post early Pliocene convergence between the North America and Caribbean plates.

Reconstruction measurements in the Mona Passage

Three lines with seismic reflection data have been collected in the Mona Passage area during cruise EW96-05. Two longer NE-SW lines that cross the Mona and Yuma Rift areas and one east-west line that only crosses the Mona Rift. These three lines have been interpreted and reduced to their original length using reconstruction software packages in an effort to calculate their extension. For the East-West trending line 31 the extension has been estimated to be 6.35 km. In the two other lines crossing the Mona Rift, being line 32 and 35, the estimate is only 4.5 and 3.1 km. However one has to take into consideration that the orientation of these two lines is not parallel to the estimated main direction of extension which will cause the calculated amount to be smaller than the real extension. One has to take into consideration that these numbers contain a fairly large interpretation uncertainty. Extension of 6.35 km between the two islands of Hispaniola and Puerto Rico combined with the 2 mm/yr of difference in the eastward movement of the two islands observed in the GPS measurements would lead to a 3.2 million year old Mona and Yuma Rift extension.

Comparisons between the carbonate platform on both sides of the Mona Rift

In our study of the well, seismic reflection and outcrop data there was a considerable thickness difference noted in the two sequences of the carbonate platform on both sides of the Mona Rift. In the north coast basin the maximum thickness of the platform is 1578 m. However towards the Mona Rift it appears that the platform is thinning to a minimum of only 600 m on the east-slope of the Mona Rift. When studying the isochron maps of the different sequences within the platform this thinning can be found to appear in the lower and older sequences, while the upper and youngest sections of the platform have a more uniform distribution. The thinning of the lower sequences onto the edge of the Mona Rift can be explained by activity of the NNW-trending Guajataca Arch. This arch appears to have formed as the elevated flank of the north-south striking Mona rift, which forms the western boundary of the present day Puerto Rico-Virgin Islands microplate.

Conclusions

In conclusion we believe that the Mona Rift has evolved in two different periods. The rifting was first active in the second tectonic phase, when the area in between Hispaniola and Puerto Rico functioned as the pivoting point of the counterclockwise rotation of Puerto Rico. Compared to the homogeneous, constant thickness platform in the north coast basin of Puerto Rico the isochron map of the carbonate platform and the seismic reflection profiles show a large amount of variation in the Mona Passage area. This is the result of seismic activity during the formation of the carbonate platform. After the formation of the platform in tectonic phase 1, the rotation stopped and the arching phase began. During this third tectonic phase, the Mona Rift started opening, which seems to agree well with the calculated age of the origin of the extension.

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