Tectonic and structural controls on Neogene fluid release in the Patagonian Continental Margin

Abstract

Analysis of high-resolution multi-beam bathymetry, 2D multi-channel seismic, and high-resolution seismic sub-bottom profiles revealed the presence of widespread fluid escape features in the middle slope of the Patagonian Continental Margin. On the sea-bottom, these features correspond to pockmarks and mud volcanoes, whereas in the sub-surface they are represented in the seismic records by several acoustic anomalies such as chimneys, acoustic blanking, enhanced reflectors, and reverse-phase enhanced reflectors among others. Some of these acoustic signatures can be traced to syn-rift deposits that fill a deep re-activated and inverted graben. Analysis of pockmarks elongation and pockmarks alignments show a good correlation with inverted normal faults, suggesting that faults and fractures have influenced the pockmark's shape and might have acted as pathways for upward fluid migration. Some of the acoustic anomalies associated with pockmarks are interpreted as evidence of gas. The gas observed in the seismic data seems to be thermogenic, and the seismic data suggests a deep origin associated with over-pressured syn-rift deposits. Two morphometrically different sets of pockmarks were identified: an older set hosted by Miocene aged rocks, and a younger set hosted by Quaternary deposits. Different potential triggers are discussed for the genesis of the Miocene pockmarks in relation to the seismostratigraphy, structural geology and regional tectonics. It is concluded that, tectonic activity associated with the Neogene inversion of the graben faults, due to Andean compression, is the most likely cause for the formation these pockmarks. The presence of gas charged sediment and young pockmarks also suggest that after the Middle Miocene tectonic climax, a more recent pulse of release of fluids occurred.