STRUCTURE AND DEFORMATION HISTORY OF THE NORTHERN RANGE OF TRINIDAD AND ADJACENT AREAS

Conflicting models have been proposed for both the evolution of Northe rn South America and the neotectonics of the south Caribbean plate bou ndary zone. The Trinidadian portion of the margin is particularly cont roversial, but surprisingly it has been little studied. We present a s tructural analysis of Trinidad's Northern Range, pertinent updates of the island's stratigraphy and sedimentology, and new zircon fission tr ack age determinations, and use them to constrain Trinidad's geologic history, and to better understand the controlling tectonic processes. In our interpretation Trinidad's three E-ENE striking ranges, which ar e separated by late Neogene-Recent depocenters, expose (1) the Norther n Range Group, generally greenschist-metamorphosed Upper Jurassic to C retaceous north facing continental slope sediments of the Northern Ran ge, deposited on the northern South American passive margin 200-400 km to the WNW, and (2) the Trinidad Group, Cretaceous-Paleogene shelf sl ope sediments of the central and southern Trinidad deposited less than 100 km WNW of their present location. A small allochthon composing th e Sans Souci Group Cretaceous tholeiitic volcaniclastic, basaltic, and gabbroic rocks (Sans Souci Formation) and sediments (Toco Formation) now in the northeastern Northern Range, has been transported hundreds of kilometers from the west with the Carribean Plate. Despite earlier references to Cretaceous orogenesis, all deformation in Trinidad is of Cenozoic age. The first deformation in the Northern Range (D1) formed north vergent nappes and induced greenschist metamorphism, probably i n the Late Eocene or Oligocene. The developed either by the underthrus ting of the Proto-Caribbean crust beneath South America due to converg ence between North and South America, or as gravity slides caused by o versteepening induced by this convergence and/or the passage of the Ca ribbean Plate's peripheral bulge and arrival of its foredeep. Northern Range D2 deformation is south vergent and represents the incorporatio n of Northern Range metasediments into the Caribbean accretionary pris m. The transition to D3 brittle transpressive right-lateral strike-sli p faulting is interpreted to be due to the uplift and east-southeastwa rd transpressive emplacement of Northern Range/Caribbean Prism rocks o nto the South American stepped shelf. This emplacement formed the Mioc ene transpressive thrust belts and foreland basin in central and south ern Trinidad. In the final phase of Northern Range deformation (D4) ap proximately E-W normal faults and shear zones and conjugate NNW-SSE an d NE-SW normal faults developed, and displacement on preexisting appro ximately E-W right-lateral strike-slip faults continued. The 11 Ma Nor thern Range zircon fission track ages suggest rapid uplift from the La te Miocene to Recent. Late Miocene subsidence of the Tobago platform i mmediately to the north of the Northern Range, and greater than 3 lan of normal, down to the north, displacement indicated for the North Coa st Fault Zone separating the Northern Range and Tobago platform, leads us to postulate that the rapid uplift of the Northern Range was in re sponse to the northward detachment of the Tobago from above the Northe rn Range, along the north-dipping transtensional North Coast Fault Zon e. This Late Miocene change in deformation style can be explained by a change from Caribbean/South American right-lateral transpression to r ight-lateral strike-slip generally striking 080-degrees. This has gene rally induced a component of extension on pre-existing faults striking at greater than 080-degrees, and a component of compression on faults striking at less than 080-degrees.