Signature of Transpressional Tectonics in the Holocene Stratigraphy of
Lake Azuei, Haiti: Preliminary Results From a High-Resolution Subbottom
Profiling Survey
Abstract
The left-lateral Enriquillo-Plantain Garden Fault (EPGF) is one of two
transform systems that define the Northern Caribbean plate boundary
zone. Relative motion across its trace (~ 10 mm/yr)
evolves from nearly pure strike-slip in western Haiti to transpressional
in eastern Haiti, where the fault system may terminate against a
south-dipping oblique reverse fault. Lake Azuei is a large (10 km x 25
km) and shallow (< 30 m deep) lake that lies in the direct
extension of the EPGF in eastern Haiti. A single core previously
collected in the lake suggests high sedimentation rates at its
depocenter (~6 mm/yr). The shallow lake stratigraphy is
therefore expected to faithfully record any tectonic deformation that
occurred within the past few thousand years. In January 2017, we
acquired a grid of high-resolution (~10 cm), shallow
penetration (~4 to 5 m) subbottom seismic (CHIRP)
profiles spaced 1.2 km apart across the entire lake. A new bathymetric
map compiled from these CHIRP data and some prior echosounder survey
reveals a flat lake floor (<0.01°) surrounded by steep
(~5°) shoreline slopes. The CHIRP profiles highlight
several gentle folds that protrude from the flat lakebed near the
southern shore, an area where transpressional deformation is presumably
focused. Thin (< 20 cm) horizontal strata from the lakebed can
be traced onto the flanks of these gentle folds and pinch out in an
upward curve. They also often pinch upward onto the base of the
shoreline slopes, indicating that young sediments on the lakebed
bypassed the folds as well as the shoreline slopes. We interpret this
feature as diagnostic of sediments deposited by turbidity currents. The
fact that young turbidites pinch out in upward curves suggests that the
folds are actively growing, and that active contractional structures
(folds and/or blind thrust faults) control much of the periphery of the
lake. A few sediment cores were strategically located where beds are
pinching out in order to maximize stratigraphic records. Two of these
cores successfully penetrated strata imaged by the CHIRP profiles.
On-going Pb210 dating of sediment samples from the cores should
constrain sedimentation rates and thus help quantify the rates of the
tectonic deformation.