CHANGES IN EROSION RATE AND EROSION PROCESSES INDUCED BY THE PASSAGE OF HEADWARD MIGRATING KNICKPOINTS IN THE LUQUILLO CZO: INSIGHTS FROM DETRITAL COSMOGENIC 10BE

The modern island of Puerto Rico started to lift up from near-sea level some 4 Ma ago. Pervasive planation surfaces, interpreted as either ancient wave-cut platforms or peneplains are found over most of its mountainous highlands and are progressively dissected by erosion. As a result, the relief of Puerto Rico is dominated by transient landscapes where relict platform remnants undergo limited erosion. River knickpoints, migrating headward along the main streams propagate into this relict landscape a wave of renewed erosion.

In the Luquillo Mountains, the most prominent knickpoints are found along streams that drain the Rio Blanco batholith, an Eocene stock of quartz diorite. The lips of these knickpoint all stand at the same elevation (~600m), which is also the elevation of one of the uplifted wave-cut platform. They do not correlate with lithological changes and likely nucleated at the coast when uplift of the wave-cut platform started. They separate a slowly-eroding relict upland region from faster eroding lower slopes. We used in situ ¹⁰Be concentrations in river-borne quartz to compare erosion rates above and below the knickpoints. We find that over timescales of 10⁴-10⁵ years the lower slopes are eroding three times faster.

The change in erosion rate is also associated with a change in the dominant erosional processes, as reflected by the grain-size dependency of ¹⁰Be concentration in quartz sediments and the hillslope morphology revealed by analysis of a recently acquired LiDAR DEM. The uplands exhibit a strong variation in the ¹⁰Be concentration as a function of sediment grain size, which reveals a deepening landscape with faster erosion of slopes than the intervening ridge tops. Erosion is focused into deep coves were seepage and mass-wasting progressively dismantle a deep saprolite. Downstream of the knickpoint lips, such coves are absent, hillslopes are straight, ridge crests are narrow and the ¹⁰Be dependency with grain size is less pronounced, indicating a better coupling between ridge, slope and river erosion rates. Collectively, these observations indicate a shift from chemical weathering dominated erosion upstream of the knickpoints to mechanical weathering dominated landscape below the knickpoints.