Neotectonics
Different researchers have proposed that different
azimuths of relative motion between the Caribbean and South American plates,
ranging from 45 o to 135o. Perliminary GPS data (Dixon
and others, personal communication) suggest that west of Margarita Island the
azimuth is greater than 90o, and east of the island less
than 90
o. Near Trinidad the azimuth is 85+/-2 o. However, as the
relative motion vector is small, ~2.0 cm/yr, and distributed across a 300-500
km wide plate boundary zone, many more years of GPS deployments are necessary
to constrain these motions. The duration of measurements required cannot be
completed in the short span of this proposal.
Seismicity along SE Caribbean plate boundary
zone. Seismicity along most of the Venezuelan Caribbean margin is
moderate and related to shallow earthquakes (Pennington et al., 1990; Bosch and
Rodriguez, 1992). However, east of
mainland Venezuela, in the area of the Gulf of Paria and Trinidad (Figs. 1-3 ),
the seismicity abruptly changes. Seismicity maps (Kafka and Weidner, 1981;
Perez and Aggarwal, 1981; Van der Hilst, 1990; Pennington et al., 1990; Russo
et al., 1992, 1993) clearly indicate an important NW-SE-trending boundary
beneath the Gulf of Paria, east of which both shallow and intermediate depth
earthquakes occur (Russo et al., 1992).
The seismicity defines a NW-dipping plane that has been interpreted as
the Wadati-Benioff zone between the Atlantic and Caribbean plate (Speed, 1985),
and is also likely the location of the plate tear occurring between the
Atlantic and South America as the Atlantic oceanic crust subducts and South
America does not. In the west,
where only shallow earthquakes occur, the Caribbean plate is being subducted to
the southeast benath South America (Fig. 6).
Earthquake-focal mechanisms and megascopic faults:
Western segment. Focal-mechanism of earthquakes in
western Venezuela (Kafka and Weidner, 1981; Pennington, 1981; Deng and Sykes,
1995) are consistent with displacements along EW-trending right-lateral
strike-slip faults (Figs. 2-3) of which the Mor—n is the most important
one (Audemard et al., 1989; Bosch and Rodriguez, 1992). These strike-slip faults are located
approximately in the middle of the Caribbean-South American plate boundary
zone.
The Leeward Antilles arc, and the South Caribbean
Deformed Belt occur north of these faults (Figs 2-3; Case et al., 1984). The
South Caribbean Deformed Belt, which is the surface expression of subduction
developing beneath South America, is the northern limit of deformation in the
Caribbean-South American boundary zone. It is well developed in the west (Figs
2-3). Seismic activity is low, but seismic reflection lines (Silver et al.,
1975; Biju-Duval et al., 1982; Ladd et al., 1984; Holcombe, 1990) strongly
suggest that the South Caribbean Deformed Belt is an accretionary complex
related to south directed Tertiary subduction of the Caribbean plate beneath
South America.
The southern limit of active tectonism is in the
Serran’a del Interior fold and thrust belt as indicated by geomorphic
studies (Audemard et al., 1989), seismicity (Bosch and Rodriguez, 1992), and
the occurrence of hot springs.
The simultaneous activity on EW-trending right
lateral strike-slip faults and thrust faults indicate that the relative
convergence rate vector between the Caribbean and South American plates
trending approximately WNW-ESE, is partitioned into a S component (thrusting)
and a EW right lateral strike-slip component. The orientation and magnitude of this vector are not
well-known (Molnar and Sykes, 1969; Minster and Jordan, 1978; Stein et al.,
1988; Deng and Sykes, 1995), but recent, GPS data (Dixon and DeMets, 1997;
Dixon and Mao, 1997) suggest that the convergence vector trends about
ESE-WNW.
Central segment. In eastern Venezuela, focal mechanism studies (Perez
and Aggarwal, 1981; Russo et al., 1993) are consistent with right lateral
displacements along EW-trending strike-slip faults, of which the El Pilar fault
is the most important one, and also on NW-SE-trending right lateral strike-slip
faults such as the Urica and San Francisco faults. The latter also likely have
a large normal component of slip (Munro and Smith, 1984). These faults (Figs.
2-3) are inherited structures that formed as transform faults during the
breakup of Pangea and the formation of the Proto-Caribbean sea floor (Figs 4,
5: George and Sams, 1993; Pindell et al., 1998), and control the shape of
physiography and thus the basins along the modern margin. In the offshore
region, near La Blanquilla Island, incipient south-directed subduction is
indicated by seismic reflection lines (Biju-Duval et al., 1982). Focal mechanisms of shallow earthquake
in the Carœpano Basin and to the south in the Serran’a del Interior
fold and thrust belt (Figs. 2-3) are consistent with the mapped
ENE-WSW-trending,
south-directed thrust faults. Thus, displacement partitioning is occurring here
as well.
Eastern segment.
The southeast corner of the Caribbean plate has a well developed Wadati-Benioff
zone as the Lesser Antilles abruptly gives way westwards to the tranpressional
strike-slip tectonics along the northern margin of the South American plate
(Kafka and Weidner, 1981; Perez and Aggarwal, 1981; Pennington et al., 1990;
Russo et al., 1992, 1993). The change occurs along a NW-SE-trending
structure in
the Gulf of Paria, between Trinidad and mainland Venezuela, site of the ~225
mgal Bouguer gravity low (Fig 3). Russo and Speed (1992, 1994) interpreted it
as the boundary between an area of northwestward subduction and continental
wedging in the east and an area of detached or detaching oceanic
lithosphereä to the west.
Across the entire margin on the mainland of Venezuela, in the Gulf of Paria, and on Trinidad
many ENE-trending thrust faults are mapped or recognized on seismic reflection
lines and are active as indicated by seismic activity (Russo et al., 1993). The
NW-SE-trending, NE-dipping faults in the Gulf of Paria are identified as normal
faults on seismic reflection lines and the edge of the continental margin
(Robertson and Burke, 1989; DiCroce, 1995; Ysaccis, 1998; Babb and Mann, 1999),
showing that the Gulf of Paria is underlain by a major modern pull-apart basin.
A few earthquake focal-mechanisms may be related to motion on these faults
(Russo et al., 1993). The details
of how the pull-apart structure (and hence the gravity anomaly) is related to
Atlantic subduction beneath the Caribbean and to the foreland basin of the fold
and thrust belt are unclear.