Heat flow at Cajon Pass, California, revisited

Abstract

In recent studies of a 3½km borehole near Cajon Pass we showed that the observed high heat flow and its sharp decrease with depth are predictable effects of independently determined erosion history, topography, 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 analysis that lowers the predicted surface heat flow from the upper end (∼100 mW/m2) to the lower end (∼90 mW/m2) of the range of measurement uncertainty at this complex site; it permits, but does not require, a source increment of up to 10 mW/m2 not accounted 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 would be too small to attribute to conventional high‐strength faulting models, and the basic conclusion of the original study is unchanged. The most likely cause of the relatively small discrepancy between predicted and observed heat flow (if it exists) is preferential three‐dimensional flow into the higher‐thermal conductivity rock that occupies the upper part of the borehole.