GEOMORPHOLOGY AND QUATERNARY GEOLOGY OF THE NORTHERN RANGE, TRINIDAD: RECORDING QUATERNARY SUBSIDENCE AND UPLIFT ASSOCIATED WITH A PULL-APART BASIN

A suite of geomorphic features in the Northern Range that record Trinidad's long-term (10³-10⁵ years) vertical tectonics are used to constrain vertical motion in northern Trinidad in response to formation of the Gulf of Paria. The gulf represents a pull-apart basin resulting from dextral wrenching along the El Pilar Fault in Venezuela stepping across the gulf to the Central Range Fault in Trinidad. North of the Central Range Fault, the Northern Range, the range-bounding Arima Fault, and the Northern Basin form a quasi-rigid block sinking westward into the Gulf of Paria pull-apart basin. In response, the eastern portion of the block probably elastically rebounded upward a lesser amount.

Eleven adjacent drainage basins in the E-W trending Northern Range of roughly similar size and orientation, underlain by uniform geology, along a 60-km transect form the basis of this analysis. Mean elevation of the basins increases eastward, from 156 to 414 m, while elevation of the basin outlets, defined topographically by the mountainfront, increases from sea level in the west to 85 m in the east. The western-most basins are buried by as much as 110 m of sediment, with wide (> 1 km), flat valley floors. Central basins are characterized by inset valley fills (with cumulative thicknesses of 100 m) and more constricted valley floor widths, while eastern basins have v-shaped, bedrock valleys essentially free of sediment. These latter basins exhibit slight asymmetry, with steeper east-facing slopes. Mountainfront sinuosity decreases slightly but consistently from west to east. This is consistent with a block subsiding to the west and rebounding upward to the east with a fulcrum in the middle. Along the Arima Fault and in the Northern Basin, Pleistocene (?) fans associated with the easternmost basins are subaerially-exposed and dissected, but are largely buried by Caroni Swamp deposits in the west, with the exception of proximal remnants in the central portion. Proximal elevation of the oldest fan deposit (Kugler's 2c) increases eastward from 50 to 150 m in the central portion of the Range, tapering off to approximately 120 m adjacent to the eastern-most drainage basins. These observations suggest the Northern Range and Northern Basin are behaving as a coupled block in their response to the formation of the Gulf of Paria.