AAPG/GSTT
HEDBERG CONFERENCE
“Mobile
Shale Basins – Genesis, Evolution and Hydrocarbon Systems”
June 4-7, 2006 – Port of Spain, Trinidad & Tobago
Episodic Slab “Roll-Back” and its Effects on the
Caribbean-Atlantic Accretionary Prism: A regional trigger mechanism for the
diapirism of Eastern offshore Trinidad
and Tobago.
N.G. Evans1, C.J. van Brummen1, T.P. Buerkert1,
P. Eisner1, D. Widdoes1,
L. Jennings1, R.K. Sawyer1, K.Tindale1
and R. Butler2
1BHP Billiton Petroleum (Americas)
Inc., Houston, USA
2University of Leeds,
United Kingdom
Episodically
shifted forearc magmatic complexes and adjacent sedimentary basins, significant
regional unconformities indicative of major structural readjustment, and sedimentary
basins locally encountered on a sporadically advancing foreland propagating
thrust system together characterize the Barbados-Trinidad Accretionary prism. All
of these features, as they are encountered, are not easily explained by a
steady state deformation model of progressive plate subduction and classic
plate accretion. Rather, they exhibit clear features of a system that has
deformed in pulses: active structural deformation and uplift followed by
periods of quiescence and rapid subsidence. Localised shale diapirism is also a
key component of the system. This contribution will focus on the eastern
offshore of Trinidad, a prolific hydrocarbon
province, where understanding of the diapiric belt is generally poor despite
recent industry activity, and will provide a discussion of the overprint of the
regional crustal-scale tectonics on its development.
Many authors describe the eastward retreat of the Lesser Antilles Trench
and the growth of the Caribbean Sea floor at the expense of the Atlantic. The distinct relocation of Caribbean
arc volcanism indicates a pulsed history of roll-back, perhaps linked to a
pulsed propagation of the southern lithospheric tear fault located close to the
northern margin of the South American continent (the STEP model of Govers and
Wortel, 2005, EPSL). The tectonic adjustments at the plate boundary are
recorded by regional unconformities that bound and explain the distribution of
distinct depositional megasequences. Specific observations that support this
model in the greater Trinidad area are as
follows:
1) Thick sedimentary
accumulations of the Barbados Trough and Columbus
Basin are located between the inner
fold and thrust belt (the continuation of the Interior Range)
and outer fold and thrust belt (present day maximum eastern extent of the
Trinidad-Barbados accretionary prism).
2) Thick accumulations such
as these are indicative of the creation of vast amounts of accommodation space
(consistent with a subsidence anomaly).
3) Observed seismic
geometries indicate rapid development of the Tobago basin, and time the jump of
magmatism from Bonnaire-Aves to the Lesser Antilles. This large-scale shift in the position of
intra arc magmatism is not consistent with progressive subduction and fixed arc
location.
4) The age, distribution and
thickness variation in foredeep sedimentation coupled with clearly defined,
significant regional unconformities suggest that the thrust front established
during the Miocene rapidly shifts to the east.
The sequence
consisting of the creation of accommodation space and fill, regional uplift, relocation
of volcanism and the shift in the location of the frontal deformation belt is
consistent with pulsed eastward rollback of the Atlantic plate.
The foredeep
ahead of the Paleogene/Early Neogene Interior Range fold belt of the Eastern
Venezuela Basin (EVB) is characterised by a northeast oriented mud diaper belt
which continues through Trinidad and swings northwards to Barbados. This mobile shale zone
runs roughly parallel to the Lesser Antilles Arc and the continuation of the Interior Range fold belt. The diapiric belt in
the Eastern Offshore of Trinidad widens to a maximum of 200 km, north and east
of the Columbus Basin. The
shale, thought to have been derived from the Miocene Foredeep in front of the Interior Range fold belt, exhibits a lateral variation
in composition and, more significantly, thickness. In the EVB, onshore and
coastal Trinidad, the foredeep basin is
restricted and consequently moveable shale was scarce. Here, little evidence exists
of significant diapirism other than in response to backthrusting at the frontal
edge of the underlying Paleogene/Early Neogene thrust front. In the eastern
offshore of Trinidad where the foredeep basin
is more extensive, moveable shale is widely distributed and was initially
thicker. The shales have undergone several episodes of movement from Miocene
through Plio-Pleistocene times, and complex diapiric structures predominate. In this region, poor seismic imaging
complicates efforts to fully understand the link to regional tectonics. The number and complexity of diapiric
structures, which are genetically related to the volume of shale, configuration
of the foredeep basin and overburden load, present an extremely challenging
geophysical terrane.
Within a
geodynamic model of pulsed slab roll-back, a trigger mechanism exists for the
development of the diapiric zone in eastern offshore Trinidad. It is proposed that the diapiric belt was
created in the following manner. The initial production of a foredeep basin in
the Miocene accumulated significant thicknesses of ‘mother’ shale. Subsequent creation
of a subsidence anomaly by pulsed slab roll-back, as indicated by the sudden
eastward relocation of the thrust front to its present day location, resulted
in the formation of a deep intra-prism basin (Barbados Trough). Finally, the
accumulation of a thick Plio-Pleistocene overburden initiated shale
overpressuring, mobilisation, and diapirism.