Honduras Geology  (12/1/98)

Neotectonics Along the North American-Caribbean Plate Boundary, Roatan Island, Honduras

Cox, R T, Arkansas State University Dept. of Chemistry and Physics P.O. Drawer 419 State University, AR 72467 United States rtcox@navajo.astate.edu

Uplifted areas of beachrock, beach sediment terraces, and fringing coral reefs give evidence of neotectonic activity on the island of Roatan, Honduras, an elongate island that lies along the south side of the ENE-striking Swan Islands/Motagua fault system. This fault system, the principal North American-Caribbean plate boundary in this region, has produced a number of large historic earthquakes including the Feb 4, 1976 Guatemalan earthquake (ML= 7.5), and deformation on Roatan is occurring along NE-striking subsidiary faults of this system.

Field investigation of active tectonism was conducted at West End Point at the southwest tip of the island. The southeast coast of Roatan from West End Point to Coxen Hole is a highly linear margin (strike= 052) that drops nearly vertically 400m subsea. This linear margin is interpreted to be the southeastern fault of a 0.5km-wide horst block. An active fringing reef north of West End Point is truncated abruptly on the south by a 9m scarp of uplifted reef comprising the northwestern fault of this horst. At West End Point within the horst block, a beach terrace and an adjacent area of beachrock are uplifted 1.8m, and the beachrock cement yields an AMS 14C age of 1490+50 y.b.p., indicating a significant Late Holocene seismic event.

The uplifted beachrock is developed on an uplifted, tilted coral reef (045,35S) that yields an AMS 14C age of 34,300+480 y.b.p. Community structure of the uplifted reef was surveyed to 5m above sea level. Head corals are dominant (Montastrea spp., Diploria spp.), and Acroporaspp. are absent, suggesting this portion of the uplifted reef occupied a fore-reef position (15m to 25m depth) before faulting. Karst cavities developed within the uplifted reef contain vertical speleothems post-dating tilting. The magnitude of reef uplift and evidence of multiple deformation episodes suggest an extended late Quaternary history of significant tectonism and possibly strong seismicity for Roatan.

Reference:

Cox, R T, 1998, Neotectonics Along the North American-Caribbean Plate Boundary, Roatan Island, Honduras [abstr.]: in 1998 abstracts; the American Geophysical Union 1998 Fall meeting, San Francisco, CA Dec 6-10 1998. (final citation forthcoming)

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