Obando-Orrego, Sebastián, Contreras-Reyes, Eduardo, Tréhu, Anne M. and Bialas, Joerg (2021) Shallow seismic investigations of the accretionary complex offshore Central Chile. Marine Geology, 434 . Art.Nr. 106437. DOI 10.1016/j.margeo.2021.106437.

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Abstract

Highlights

• We obtain shallow two-dimensional and three-dimensional tomographic Vp models at the landward edge of the Maule accretionary prism (Chile at 35°-36°S).
• The Maule accretionary prism is characterized by thrust ridges and shallow cold seep activity caused by the vertical migration of warm methane-rich fluids into the GHSZ.
• Thrust ridges and associated splay fault systems play an important role in the upward fluid migration during the dewatering process of the accretionary prism.

Abstract

Thrust ridges are accretionary structures often associated with local uplift along splay faults and cold seep activity. We study the influence of a NS-trending thrust ridge system on the transition between the accretionary prism and the continental framework (shelf break) offshore the Maule Region (central Chile at 35°–36°S) by examining its 2-D and 3-D seismic velocity structure. The experiment comprises five densely spaced seismic refraction lines running subparallel to the trench and recorded at nine OBH/S (ocean bottom hydrophone/seismometers) deployed along the central line. Results show a narrow margin-parallel volume (approximately 6 × 50 × 5 km3) whose velocity distribution is consistent with sedimentary rocks. The shallow sedimentary unit is characterized by the presence of very low velocity hydrate-bearing sediments (<1.7 km/s), which are interpreted as highly porous sedimentary rocks (>50% porosity) within the Gas Hydrate Stability Zone (GHSZ) suggesting low hydrate content. These zones spatially correlate with fluid activity in the vicinity of the NS trending thrust ridges based on local high heat flow values (>40 mWm−2) and seepage mapping. On the other hand, the splay faults that crop out on the flanks of the thrust ridge structures might be responsible for tectonically induced vertical fluid migration.

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