The Enriquillo-Plantain Garden Fault (EPGF), one of two left-lateral transforms that define the Caribbean-North American plate boundary in Haiti, plunges beneath Lake Azuei in the eastern part of the country. In 2017, we acquired 220 km of sub-bottom and multichannel seismic reflection profiles (CHIRP and MCS) in a 1.2 km grid pattern across the lake. The two seismic methods achieved subbottom penetration of up to 15 m and 200 m, respectively. CHIRP and MCS data reveal folded turbidites across the expected extension of the EPGF fault zone at the south end of the lake, although direct evidence for faulting is lacking. Along the west side of the lake, however, MCS data image a broad NW-SE monoclinal fold whose geometry is compatible with an underlying SW-dipping blind thrust fault. CHIRP profiles image patches of soft- sediment deformations above the monoclinal fold; they also image a distinctive 11 m-deep paleoshoreline all around the lake that is uplifted by 1-2 m above that fold. These results are compatible with a scenario where some large slip event(s) on the presumed blind thrust occurred after formation of the paleoshoreline, locally uplifting the lakebed and causing liquefaction. Two short sediment cores sampled a layer correlatable to a reflection in the CHIRP profiles. That reflection extends laterally below the 11m shoreline, and thus predates its formation. On-going 14C dating of material from the two cores are expected to provide a maximum age for the shoreline, and thus for some hypothetical slip event(s) on the presumed blind thrust fault. The prominent character of the paleoshoreline suggests that it was stable, something best achieved if the lake level was controlled by the sill depth of an outlet. Presently, no such outlet exists and the lake level fluctuates. Pending results from radiometric dating, we propose that, earlier in the Holocene, the lake overflowed eastward into adjacent Lake Enriquillo along the narrow valley marking the extension of the EPGF fault zone - a valley that is presently blocked by an alluvial fan. Regardless of the relevance of that model, the uplifted shoreline implies a significant uplift rate on a structure extending up to 10 km north of the EPGF fault zone and striking oblique to it, confirming that transpressional tectonics is partitioned over an area at least as broad as Lake Azuei.

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.