Active fault motion in a coastal wetland: Matagorda, Texas
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Active growth faults contribute to the subsidence of deltaic basins around the world and can significantly augment local rates of relative sea level rise, which can inundate and drown wetlands. Little information exists on the temporal frequency and spatial magnitude of motion along these faults. Our objective was to quantify fault activity and displacement at multiple time scales in a salt marsh wetland on the East Matagorda Peninsula, Texas. We present evidence of this activity that includes remotely sensed aerial imagery, LiDAR data, ground penetrating radar data, shallow seismic data, lithostratigraphic and biostratigraphic evidence from core data, and sub-centimeter GPS and survey monitoring. The results support the interpretation that the Matagorda fault is currently active, and has been active in the past. Subsurface data to depths of ~. 150. m displays evidence of disrupted strata at the fault plane, as well as thicker strata on the downthrown side of the fault. In the shallow subsurface down to ~. 3. m of depth, vertical displacement in stratigraphic markers and surface deformation exhibits a sinusoidal pattern that runs perpendicular to the fault plane, which we interpret to represent fault-propagation folding. Maximum throw on this fault is estimated at ~. 0.75. m over the last ~. 40-50. years. We also recorded a vertical drop of -. 0.208. m in the span of a single year, at a location close to the fault on its downthrown side. Records of yearly elevation change also show evidence of ongoing fault-propagation folding. We conclude that whereas the surface at the Matagorda fault moves intermittently up and down at the yearly time scale, fault displacement manifests as a sinusoidal pattern of displacement and deformation which has been integrated into the stratigraphic record at decadal-to-millennial time scales. This study presents a rare look into active motion and details its effects on the modern evolution of unconsolidated sedimentary surfaces. 2012 Elsevier B.V.
