Deeply incised river networks are generally regarded as robust features
that are not easily modified by erosion or tectonics. Although the
reorganization of deeply incised drainage systems has been documented,
the corresponding importance with regard to the overall landscape
evolution of mountain ranges and the factors that permit such
reorganizations are poorly understood. To address this problem, we have
explored the rapid drainage reorganization that affected the Cahabon
River in Guatemala during the Quaternary. Sediment-provenance analysis,
field mapping, and electrical resistivity tomography (ERT) imaging are
used to reconstruct the geometry of the valley before the river was
captured. Dating of the abandoned valley sediments by the Be-10-Al-26
burial method and geomagnetic polarity analysis allow us to determine
the age of the capture events and then to quantify several processes,
such as the rate of tectonic deformation of the paleovalley, the rate of
propagation of post-capture drainage reversal, and the rate at which
canyons that formed at the capture sites have propagated along the
paleovalley. Transtensional faulting started 1 to 3 million years ago,
produced ground tilting and ground faulting along the Cahabon River, and
thus generated differential uplift rate of 0.3 +/- 0.1 up to 0.7 +/- 0.4
mm . y(-1) along the river's course. The river responded to faulting by
incising the areas of relative uplift and depositing a few tens of
meters of sediment above the areas of relative subsidence. Then, the
river experienced two captures and one avulsion between 700 ky and 100
ky. The captures breached high-standing ridges that separate the Cahabon
River from its captors. Captures occurred at specific points where
ridges are made permeable by fault damage zones and/or soluble rocks.
Groundwater flow from the Cahabon River down to its captors likely
increased the erosive power of the captors thus promoting focused
erosion of the ridges. Valley-fill formation and capture occurred in
close temporal succession, suggesting a genetic link between the two. We
suggest that the aquifers accumulated within the valley-fills, increased
the head along the subterraneous system connecting the Cahabon River to
its captors, and promoted their development. Upon capture, the breached
valley experienced widespread drainage reversal toward the capture
sites. We attribute the generalized reversal to combined effects of
groundwater sapping in the valley-fill, axial drainage obstruction by
lateral fans, and tectonic tilting. Drainage reversal increased the size
of the captured areas by a factor of 4 to 6. At the capture sites, 500 m
deep canyons have been incised into the bedrock and are propagating
upstream at a rate of 3 to 11 mm . y(-1) deepening at a rate of 0.7 to 1
5 mm . y(-1). At this rate, 1 to 2 million years will be necessary for
headward erosion to completely erase the topographic expression of the
paleovalley. It is concluded that the rapid reorganization of this
drainage system was made possible by the way the river adjusted to the
new tectonic strain field, which involved transient sedimentation along
the river's course. If the river had escaped its early reorganization
and had been given the time necessary to reach a new dynamic
equilibrium, then the transient conditions that promoted capture would
have vanished and its vulnerability to capture would have been strongly
reduced.