HEAT-FLOW AT CAJON-PASS, CALIFORNIA, REVISITED

In recent studies of a 3 1/2km borehole near Cajon Pass we showed that the observed high heat flow and its sharp decrease with depth are pre dictable effects of independently determined erosion history, topograp hy, and radioactivity, leaving little room for the large contribution from frictional heat required by conventional faulting models for the nearby San Andreas fault. We have since discovered an error in our ana lysis that lowers the predicted surface heat flow from the upper end ( similar to 100 mW/m(2)) to the lower end (similar to 90 mW/m(2)) of th e range of measurement uncertainty at this complex site; it permits, b ut does not require, a source increment of up to 10 mW/m(2) not accoun ted for in the prediction. Better agreement between the prediction and observations at depth confines the permissible extra heat flow to the upper part of the hole, making it difficult to attribute it to a deep frictional source. In any case, however, such a frictional source wou ld be too small to attribute to conventional high-strength faulting mo dels, and the basic conclusion of the original study is unchanged. The most likely cause of the relatively small discrepancy between predict ed and observed heat flow (if it exists) is preferential three-dimensi onal flow into the higher-thermal conductivity rock that occupies the upper part of the borehole.