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By Samarjit Chakraborty, Ewing-Worzel Fellowship.

Abstract:
I have interpreted the shallow (seafloor to 0.5-1.5 second TWT) structure of four 3D seismic volumes to establish the pattern of major faults, folds, and mud diapirism in the eastern offshore area of Trinidad. Rapid sedimentation related to Neogene deltaic sedimentation of the Orinoco River delta means that most of the structures observed at these levels are Pliocene or younger in age and are therefore useful for inferring the recent tectonic events that have affected the area. The westernmost block, 2AB, occupies a 1424 km2 area of the shelf, is located 8-64 from the eastern coast of Trinidad, and has water depths ranging from 70 to 460 m. A central block, 4B, occupies a 2258 km2 area of the shelf edge and slope with depths ranging from 160 to 1275 m. The two easternmost blocks, 25AB and 26, are continguous and cover a combined area of 5038 km2 of the deep basin. Water depths range from 743 m to 1300 m.

Faults in the combined area of the four blocks consist of at least four, subparallel zones of ENE and E-striking, right-lateral strike-slip faults in the northern part of the study area. The most prominent of these faults is the seaward extension of the Central Range fault zone of onland Trinidad. Recent GPS work onland in Trinidad suggests that this fault accommodates most of the active Caribbean-South America plate motion (about 21 mm/yr). On the shelf block (2AB), the fault is marked by a linear scarp in lithified shelf sedimentary rocks of Pleistocene age. In more thickly sedimented slope and basin blocks, this fault and parallel faults to the south are marked by ENE alignments of elongate and shear mud diapirs or continuous, narrow mud walls.

Offshore folds consist of large, open synclines and anticlines with axial traces parallel to ENE-striking strike-slip faults. Limbs of folds in lithified Pleistocene shelf sedimentary are present on the seafloor on the shelf block (2AB) and at shallow levels on the shelf edge and slope block (4B). The dimensions and style of these open folds at or near the seafloor is similar to mapped fold structures onland in the Southern Range of Trinidad. The Darien ridge, a narrow ridge-like bathymetric feature on the shelf, is a complexly folded and thrusted structure with no obvious correlation to a mapped surface structure the Southern Range of Trinidad. In the deeper water blocks (25AB, 26), folds are more deeply buried and appear less active. Surfacial mud diapirism above these buried folds appears more random and lacks the strong ENE alignment of diapirs localized along the ENE strike-slip faults to the north and northwest.

An anomalous rift trending WNW and near the seaward projection of the Darien ridge is present on the seafloor in the slope block (4B). The rift appears to terminate or transfer strike-slip motion between the EW Darien ridge and ENE strike-slip faults in the deeper water area. Rift faults cross cut fold trends; the recency of seafloor scarps and these crosscutting relations indicate that rifting is younger than folding. The rift acts as an important conduit for shelf sediments to move from the shelf off of eastern Trinidad to deepwater area. The alignment of the Darien ridge and the rift may divert young deltaic sediments to the deepwater area or trap sediments on the shelf south of the Darien Ridge.

The suite of structures present in all four blocks is consistent with strain partitioning of ENE plate motion: ENE strike-slip faults accommodate the strike-slip component of Caribbean-South America motion while ENE folds accommodate a convergent component of motion. Diapirism appears to occur as a result of ENE strike-slip motion, sediment loading, or a combination of both processes.