Toward an integrated understanding of Holocene fault activity in western Puerto Rico: Constraints from high-resolution seismic and sidescan sonar data

Abstract

It has been postulated that the western boundary of the Puerto Rico-Virgin Islands microplate lies within the Mona Passage and extends onland into southwestern PuertoRico. This region is seismically active, averaging one event of magnitude 2.0 or larger per day, and over 150 events of magnitude 3.0 or greater occurred during the past fi ve years. Moreover, there have been at least 13 historical events of intensity VI (MM) or greater in the past 500 years. We conducted a high-resolution seismic and sidescan sonar survey of the insular shelf of western and southern Puerto Rico during May 2000 in an effort to identify Holocene faults and to further assess the seismic hazard in the region. We focus on an ~175 km 2 part of the surveyed area offshore of western Puerto Rico, extending from Punta Higuero to Boquerón Bay. This area was targeted as a likely place to image recent faults, because multi-channel seismic profi les offshore western Puerto Rico show numerous WNW-trending normal and strike-slip faults that offset Oligocene-Pliocene age carbonate rocks and underlying Cretaceous basement rocks. Analyses of these data identify three zones of active deformation within the survey area: (1) the Cerro Goden fault zone; (2) the Punta Algarrobo/ Mayagüez fault zone that lies offshore the city of Mayagüez; and; (3) the Punta Guanajibo/Punta Arenas fault zone. Two of the offshore fault zones, the Cerro Goden and Punta Algarrobo, show strong correlation with fault zones onland, Cerro Goden and Cordillera, respectively. Many of the mapped faults offshore appear to reactivate older WNW-trending basement structures and show evidence of some component of right-lateral motion that is consistent with geodetic measurements. The offshore deformation zones are also associated with headlands and linear NW-SE magnetization lows (serpentinite dikes?) mapped offshore. Elongate outcrops of serpentinite in western Puerto Rico are colinear with the fault zones we have mapped offshore, Grindlay, N.R., Abrams, L.J., Del Greco, L., and Mann, P., 2005, Toward an integrated understanding of Holocene fault activity in western Puerto Rico: Constraints from high-resolution seismic and sidescan sonar data, in Mann, P.

Current block motions and strain accumulation on active faults in the Caribbean

“…We find no evidence for internal deformation of the Puerto Rico‐Virgin Island block at the ∼0.5 mm/yr level (RMS of residual velocities), consistent with the absence of significant active faulting [ Frankel et al , ]. Holocene faulting within Puerto Rico reported, in particular, on the Cerro Goden‐Great Southern fault zone [ Grindlay et al , ] therefore has to be occurring on very slow slipping faults. The Caribbean‐North America plate motion along that segment of the plate boundary is accommodated by oblique slip on the plate interface, consistent with slip vector directions of instrumental earthquakes (Figure ), though a portion of the plate boundary‐parallel slip could be accommodated by the shallow Bowin and Bunce faults on the inner wall of the Puerto Rico trench [ Grindlay et al , ; ten Brink and Lin , ].…”

“…The Caribbean-North America plate motion along that segment of the plate boundary is accommodated by oblique slip on the plate interface, consistent with slip vector directions of instrumental earthquakes (Figure 1) portion of the plate boundary-parallel slip could be accommodated by the shallow Bowin and Bunce faults on the inner wall of the Puerto Rico trench [Grindlay et al, 2005b;ten Brink and Lin, 2004]. The partitioning of the oblique convergence between the Caribbean and North American plates into plate interface convergence and intra-arc strike-slip faulting, therefore, ceases as the plate boundary transitions from the oblique collision of the Bahamas platform with Hispaniola to the oblique subduction of old (> 100 Ma) lithosphere under Puerto Rico Mann et al, 2002;Grindlay et al, 2005a;Mondziel et al, 2010].…”

Extension in Mona Passage, Northeast Caribbean

“…Furthermore, there is little geophysical or seismological evidence to suggest the Another proposed single-phase extension model attributes a 25 km-wide zone of normal faults in Mona Passage and Puerto Rico to extension at the crest of a 230-km-wide crustal arch or fold (bending-moment faulting; Fig. 15D; van Gestel et al, 1998van Gestel et al, , 1999Grindlay et al, 2005;Hippolyte et al, 2005;Mann et al, 2005a,b), driven by N-S compressive forces. The primary evidence used for the interpretation of the Puerto Rico-Virgin Islands island arc as a crustal arch is the variation in dip of the carbonate platform surrounding Puerto Rico and the Virgin Islands from N-dipping along the northern margin of the platform to S-dipping along the southern margin.…”

“…7), based on offset of acoustic basement on the timemigrated R/V Pelican seismic lines and assuming an approximate average velocity of 2000 m/s for overlying sediment and 2750 m/s for carbonate platform units (van Gestel et al, 1998), is estimated to be approximately 540 m. These faults appear to extend from the Puerto Rico insular shelf to south of Isla Desecheo where their bathymetric expression disappears and seismic reflection coverage is insufficient to determine if they are present in the subsurface. The eastern ends of some of these Mayagüez basin faults may connect with faults identified nearshore by Grindlay et al (2005) between Punta Higuero and Punta Guanajibo (Fig. 6).…”

Morphotectonics of the central Muertos thrust belt and Muertos Trough (northeastern Caribbean)

“…7a, b). These normal faults would be related to the extensional regime documented in the carbonate platform (van Gestel et al, 1998;Mann et al, 2005;Grindlay et al, 2005).…”