Abstract: Extensional Tectonics and Related Structures in the South Flank of San Jorge Basin, Argentina

Figari, Eduardo; Conforto, Gustavo; Cid de la Paz, Maria and Cevallos, Martin - YPF

The intracratonic East- West trending San Jorge basin is located in the central part of Patagonia Terrain, between the North Patagonian and Deseado Massifs. The mainly continental sedimentary record reaches more than 8000m thickness and evolved since Jurassic to Tertiary times. According to structural style, the Golfo San Jorge basin is divided into three domains. The Eastern domain is characterized by an extensional style, with WNW-ESE trending basement involved normal faults. They develop a complex antithetic fault system and rollovers in upper sedimentary levels of the hanging wall. In the more abrupt North Flank, main faults are dipping to the SW while in the gentle South Flank they are dipping to the NE. In the Central domain reactivated NNW-SSE trending normal faults originated N-S box shape folds in the hanging wall of high angle reverse faults. Finally, in the Western domain, despite a closer position to the Andean Belt, the extensional style with a WNW-ESE trend is preserved but, contrary to Eastern area, the more abrupt border is southward and the majority of the main faults dip to NE. Comparing both original WNW-ENE "vs." NNW-SSE trends through structural maps of the pre-rift sequences, it is clear that the last trend is longer and deeper and could be older than the first one. This suggests that more than one stress field have been placed in the San Jorge Basin. The first rifting, NNW-SSE trend requires a stress field where s1 was close to the vertical, s2 was horizontal, and s3 was horizontal in ENE-WSW direction. The second rifting pulse, WNW-ESE, suggests a s1 close to the vertical, s2 horizontal, and s3 horizontal in NNE-SSW direction. Finally, a Tertiary compression with a s1 horizontal in W-E direction, a s2 horizontal in N-S trend and s3 vertical satisfies the presence of tectonic inversion anticlines. The different behavior of these two rift systems related to Tertiary contraction can be explained by their original orientation with respect to the maximum W-E stress. Whereas on transverse or high angle oblique direction (NNW-SSE) originated reverse faults, on parallel or low angle oblique direction (WNW-ESE) originated only strike slip displacement or did not affect them.

Data available from 3D seismic interpretation in the South Flank of San Jorge Basin showed that WNW-ESE trending regional extensional faults are planar to softly curved in map view, 5 km long, en echelon arrangement with soft linkage. In cross section, faults are listric in depth, with a maximum vertical displacement of 500 m, but upward these faults are dipping up to 65 loosing displacement. In the hanging wall rollover folds and planar antithetic faults are developed. Borehole images showed an associated damage zone in each of these regional faults and confirmed the same stress field orientation figured out from megascale analysis belonging to the second rifting pulse.

To conclude, the Golfo San Jorge is an intracratonic basin where at least two different rifting pulses have been occurred during late Jurassic- Early Cretaceous times. The two principal structural trends had different behavior during a Tertiary contraction.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil