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ABSTRACT: Thermal Evolution of "Hot" Accretionary Prisms in California and Japan

M.B. Underwood, M.M. Laughland

Accretionary prisms represent poor targets for petroleum exploration because intense polyphase deformation reduces the size and permeability of potential reservoirs. However, high geothermal gradients will promote petroleum generation at relatively shallow depths. High geothermal gradients are caused by (1) close proximity of a spreading ridge to a subduction front (e.g., Cascadia margin), (2) subduction of a spreading ridge (e.g., Nankai Trough), or (3) migration of a trench-transform-transform triple junction (e.g., Mendocino triple junction). Each condition produces high heat flow on a regional scale, plus local hydrothermal activity and/or off-axis igneous activity. The principal cause is heat conduction from young/hot oceanic lithosphere into the accretionary prism; imilarly, attenuation of lithosphere along the trailing edge of a migrating triple junction allows heat conduction from the asthenosphere. The three-dimensional thermal structure is complicated further by advective heat transfer. Hot fluids can become focused along major structural discontinuities (e.g., the basal decollement), but they are also distributed more evenly through fracture porosity. Thermal overprints have affected the King Range terrane of northern California, the Big Sur terrane of central California, and the upper Shimanto belt of Japan. In each case, peak paleotemperatures (typically 180-300°C) were attained at relatively shallow depths and towards the end of, or following, the second stage of deformation. If sequences such as these are heated after burial beneath f re-arc/basin deposits (such as the Eel River basin in northern California), then hydrocarbons can migrate from the accreted source beds into overlying reservoirs.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990