New Thermochronologic, Paleomagnetic, and Fault-Slip Constraints on Pliocene Tectonics and Provenance, North Coast Marine area, Trinidad and Tobago

John Weber, Grand Valley State University
Jenny Arkle, University of Cincinnati
Scott Giorgis, SUNY-Geneseo
Jean-Claude Hippolyte, Aix-Marseille Université

Many important gas reservoir units in the North Coast Marine area are Pliocene-aged sandstones. The Pliocene is also a critical time in the transition from fossil Caribbean-South American plate oblique convergence to modern transform (strike-slip) tectonics.

New paleomagnetic data span key Plio-Pleistocene exposures in Trinidad and Tobago. Adjacent to the Central Range Fault in Trinidad, Plio-Pleistocene samples pass fold and reversals tests, and record 40±7˚ of clockwise rotation since the Pliocene. These data are compatible with modern transform tectonics being active as early as the Pliocene. Prior work in Tobago showed ~90˚ of clockwise rotation since the Cretaceous. New data from the mid-Pliocene Rockley Bay Formation give essentially modern paleomagnetic directions showing that Tobago has not experienced tectonic rotation since the mid-Pliocene. Thus, southeast-directed oblique plate motion of the Caribbean plate likely ceased by the Pliocene when modern eastward plate motion, together with a narrowing and a southward shift in the plate boundary, occurred.

Fault-slip data collected from 23 sites in Tobago from rocks ranging from Cretaceous (Albian) to Quaternary in age highlight four principal phases of onshore deformation: 1) Quaternary-Pliocene N-S extension, 2) *NNW-SSE compression associated with strike-slip and reverse faults, 3) *NE-SW extension that was tilted by Phase 2 faults, and 4) the oldest *NE-SW compression associated with foliated basement rocks (*provisionally given in current unrotated coordinates). Phase 1-type extensional faulting likely associated with strike-slip pull-apart steps, gravitational collapse of the obducted Tobago arc/forearc, and possibly also hinge rollback created the accommodation space offshore that was synchronously filled in by Pliocene clastic reservoir units. N-S extension continued into the Quaternary and continues even to today.

New thermochronology data include fission-track (FT) ages and time-temperature histories for a suite of 14 apatite fission-track (AP FT) samples from across the Northern Range (NR), Trinidad and in the Paria Peninsula, Venezuela. These data clearly show that the western NR (WNR) was being erosionally exhumed through ~110°C (=T blocking) in the Mio-Pliocene (ca. 6-5 Ma), at rapid rates of 0.6 mm/m.y. Whereas in the eastern NR (ENR) exhumation rates were slower (~0.2 mm/m.y.) and AP FT ages are older (ca. 13-18 Ma). New drainage-basin-wide 10Be TCN (terrestrial cosmogenic nuclide) data show slower and geographically reversed modern erosion rates, lower in the WNR (0.02-0.03 mm/m.y.) than in the ENR (0.05-0.1 mm/m.y.). AP FT samples from Tobago yield old ca. 45 Ma ages. Work in-progress using new detrital samples from onshore and offshore Pliocene reservoir units, and new samples from exposed Trinidad and Tobago basement rocks, will generate additional AP and ZR (zircon) FT data, and new AP (T blocking=70°C) and ZR (Tblocking=180°C) (U-Th)/He data. We aim to test whether the North Coast Marine Pliocene clastic fill was derived solely from the WNR or had additional sources, and if so, what and how significant these were.