Using the Messinian and Plio-Quaternary deposits as markers of the vertical motions in the tectonically inverted Algerian margin

The Algerian margin represents one of the rare examples of tectonically inverted margins worldwide. The inversion is characterized by blind reverse thrust faults dipping landwards the below the continental slope, and by uplift of the coastal domain. The inversion process is active as evidenced by the offshore seismicity of Algeria (2003 Boumerdes earthquake, Mw 6.9); however the onset of inversion is poorly constrained. The morphology of the Algerian margin results from the interplay between tectonic and sedimentation together with periodic sea level changes during Quaternary. A major sea level fall at the end of Miocene marks the onset of the Messinian Salinity Crisis (MSC). In this study, we focus on a part of the Algerian margin located west of Algiers: the Khair al Din Bank (KADB). The KADB is a major structure interpreted as a tilted block originated during the opening of the Algerian basin in early Miocene. Because of its weak (low) slope, the sedimentary record all along the continental slope is better preserved than in the rest of the Algerian margin. The aim of this study is to constrain deformation and vertical motion of several sedimentary units in order to quantify the amount of uplift and the timing of inversion. We selected for uplift calculations 6 sedimentary units as follows. Sedimentary units onshore are: the highest marine terrace, the marine-continental transition and the MIS 5.5 uplifted beach. Sedimentary units offshore are: the boundary of the upper Unit of the MSC, and the erosion surfaces of the MIS 6 and MIS 2 sea level lowstand. The age of these units ranges between 5.6 Ma and 20 ka BP and permits to estimate the uplift and uplift rates all along the Plio-Quaternary time. Our interpretation is based on geophysical data acquired in 2003 (Maradja 1 cruise). We describe the main characteristics of the MSC erosion surface, i.e. the initial stage, and the sedimentary evolution during the Plio-Quaternary time. During the MSC sea level lowstand, the KADB area formed a deeply incised subaerial area continued in the marine domain by detrital chaotic deposits and evaporitic series. The subaerial domain exhibits a peculiar shape related to the inherited morphology of the passive margin as the river valley flows to the ENE and turns abruptly to the WNW on the mid-slope. In the inner shelf, the sedimentary evolution during Plio-Quaternary time is mostly characterized by the deposition of foresets and the absence of sediment aggradation, suggesting that the sedimentary evolution is dominated by a long term tectonic uplift of the KADB. Plio-Quaternary anticlines are present both onshore and offshore. As these structures do not correspond with crests and valleys of the MCS drainage network, we suggest that folding occurred after the MSC. We estimate maximum uplift values of 400 m for the oldest sedimentary marker suggesting that the inversion of the margin started at least 3.5 Ma ago.