Intra-Arc Basins: Structure, Stratigraphy, and Resource Potential

Gary A. Smith

Convergent-margin tectonic models feature forearc and back-arc basins and generally portray the arc itself as structurally static. However, intra-arc tectonics not only control distribution and petrology of extrusives and plutons, but also generate basins along the magmatic axis. Magma withdrawal and crustal loading by volcanic edifices contribute to subsidence, but most intra-arc basins are grabens or half-grabens indicative of extension. Grabens are isolated or continuous along long segments of the arc. Basin development may alternate with periods of arc uplift. No unique set of conditions causes intra-arc extension; numerous scenarios may initiate extension and subsidence of thermally weakened arc crust. Transtension related to oblique convergence contributed to the fo mation of most modern intra-arc basins. Andean basins may result from gravitational spreading of an unusually high-standing arc.

Intra-arc basin sediment traps may starve arc-adjacent basins from coarse volcaniclastic detritus. Terrestrial intra-arc basins accommodate thick volcanic and volcaniclastic sediment sections, including lacustrine sequences. Marine intra-arc basins include bounding carbonate shelves, marginal and local intrabasinal submarine fans and aprons, and basin plains receiving pelagic and hemipelagic sediments.

Structural patterns are appropriate for trapping hydrocarbons, source rocks are commonly present, and high heat flow favors early maturation. Reservoir quality is typically poor because of volcaniclastic diagenesis, but secondary porosity from dissolution of framework feldspars and carbonate or laumontite cements, and the known productivity of some volcanic reservoirs, suggest the potential for hydrocarbon accumulations. Geothermal resources and modest coal potential have also been recognized.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.