TIMING AND NATURE OF SURFACE UPLIFT OF THE NORTHERN RANGE, TRINIDAD: AN ANALYSIS WITH KNICKPOINT INDICES

Fluvial geomorphology can be a useful record of tectonic activity. This study analyzes the spatial and temporal distribution of knickpoints from streams that drain the Northern Range, on the island of Trinidad. The Northern Range is bound to the north by the Caribbean Sea and is located ~30 km north of the active southeast Caribbean-South American dextral plate boundary. Recent studies suggest that the Northern Range has been tectonically active in the Quaternary and is titled east-side up. To better constrain the distribution, timing, and rates of recent surface uplift and subsidence, stream profiles and knickpoints were analyzed from 44 catchments. Preliminary results yield knickpoints that are consistently located at ~50 and ~200 meters in elevation across the Northern Range. Knickpoints are more prevalent in the east, yet normalized steepness indices (k_sn) are higher in the west. Two representative catchments, one from the west and one from the east, were analyzed for lithology. We identified and mapped constructional (tufa) knickzones, and then focus our analysis on erosional knickpoints. Within the two western and eastern catchments, tributary knickpoints that are not associated with lithologic control are localized at similar elevations around the trunk stream. In the west, the stream profile is concave and the 200 m elevation knickpoints separate steep upper channel reaches (k_sn > 250) from shallow lower channel reaches (k_sn < 45). In contrast, the eastern stream profile is convex and the 200 m elevation knickpoints separate upper and lower channel reaches with similarly low steepness (k_sn < 20–10). We interpret these spatial patterns of knickpoints as transient landforms caused by a drop in base level either due to range-wide tectonic tilting or sea-level fall. The timing of the base-level change is estimated with paleochannel profile reconstructions and catchment-scale erosion rates converted to volumetric estimates. Our estimates indicate that these knickpoints formed since at least the last 2.8 Ma and possibly as recently as ~600 ka. Continued work will further constrain the timing and cause of the varied geomorphology of the stream channels in the Northern Range.