Long-term landscape evolution of the Luquillo Mountains and its effects on today’s variability in erosion rates.

Project Description: 

Key Science Questions Involved:
1 – Large scale: When where the Luquillo Mountains uplifted to their present height?
2 – Mesoscale: The last 500-600 m of uplift generated a conspicuous wave of erosion still propagating toward the mountain tops. What is the magnitude of the spatial variability in erosion rate associated with this wave?
3 – Microscale : Upstream of the erosion wave, the relief is a mixture of a relict slowly eroding crest lines covered by an old grown Palo Colorado forest, and faster eroding corestone-chocked gullies occupied by the Palm forest . What is the variability in erosion rate in this landscape? Is the water-table seepage responsible for gully development or is it the competition between the two types of forests?

Initially a reef platform five millions of years, the island of Puerto Rico has since emerged incrementally. Extensive wave-cut platforms cover much of the highlands and witness several stages in the island growth. We mapped such a platform all around the Luquillo Mountains at 500-600 m asl. The platform surrounds higher grounds that once formed an island 10 km in diameter and 500 m high. Following the uplift of this platform, vigorous erosion has propagated headward along the main streams. Above the quartz diorite that forms the core of the Luquillo Mountains, the wave of incision has not propagated to the divide. The downstream reaches are deeply incised and separated from the unscathed uplands by dramatic waterfalls (Figure1). Early works (Brown et al., 1995; Brown et al., 1998) used the cosmogenic 10Be contained in the quartz of river sediments and soils to infer that the uplands are eroding very slowly (50-150 m/My). To address Question2, we have measured the in situ-produced detrital 10Be in quartz sediment in a larger number of streams, to obtain a more widespread an representative idea of the catchment-wide erosion rates over the uplands, and we undertook the first measurements from stream below the front of the incision wave. We find a X2 to X3 increase in erosion rate associated with the passage of the wave (Figure 2). The inner variability in erosion rates in the uplands (Question 2) is addressed using punctual measurements of erosion rates in the forest (20 sites), and in sub-catchments (10 sites). The erosion wave initiated when the mountain increased in elevation from its initial 600 m height to its modern 1100m elevation. This allowed the establishment of the altitudinal vegetation zoning we observe today. To determine the minimum age of this event (estimated at 3-5 Ma), we have sampled caves that formed below the wave-cut platform when they were uplifted. We date the case using the differential decay of cosmogenic 10Be-26Al in sediments carried into the caves by rivers that drained the crystalline core of the Puerto Rico.

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Contact Information
Person(s) Completing This Form: 
Gilles Brocard
Gilles Brocard, Jane Willenbring
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