DISCUSSION

From what we currently know of crustal stress and heat flow, neither is influenced by proximity to the San Andreas fault, the most conspicuous and best studied plate-boundary fault on the continents. The measured horizontal shear stress increases rapidly with depth (approx 8 MPa/km), essentially as would be predicted from laboratory measurements of friction and the assumption that crustal stress is limited by the frictional resistance of fractures forced together by the weight of overlying rocks. From this consistency of independent observations, two large "if's" lead to what seems to be a physical contradiction: (1) if these vertical stress gradients persist throughout the depth of the seismogenic faulting layer (approx 12-15 km), then the average of the maximum horizontal shear stresses throughout the layer is quite large (approx 50 MPa); and (2) if the direction of the San Andreas fault is aligned with this maximum-horizontal-shear-stress direction, then the frictional heat generated by such stress during the documented fault motion (tens of kilometers per million years) should cause the background heat flow to double as the fault is approached. In 100 heat-flow measurements over a 1,000-km span of the San Andreas fault, no such heatflow anomaly has been observed.

The contradiction stems from two separate lines of argument: (1) inplace and laboratory measurements of rock stress imply average fault stresses of about 50 MPa or more, and (2) the absence of a local heat-flow anomaly and the energy balance of the fault imply an average fault stress of about 15 MPa or less. At least one of these arguments must be wrong. We have outlined the major factors in each argument, and we shall now point out some possible loopholes and areas for further study.

The energy-balance argument leading to the heat-flow constraint on fault stress could be invalidated if the neglected energy sinks turn out to be important, or if the heat-conduction model is unrealistic or inappropriate. The general energy argument assumes that fault slip produces only seismic radiation and heat. It supposes that the energy consumed by the grinding of rocks into fault gouge (Lachenbruch and Sass, 1980) or the heat absorbed by possible phase changes or chemical reactions is negligible, and that the energy of seismic radiation does not grossly differ from the estimates made by seismologists. The heat-conduction model assumes that the frictional heat production occurs in a near-vertical fault zone (whose width is small relative to its depth) extending throughout the seismogenic layer. Systematic nonconductive removal of frictional heat by circulating ground water could invalidate this model (see O'Neill and Hanks, 1980; Williams and Narasimhan, 1989), as could a grossly different fault geometry - for example, a fault whose lower half was continually being rejuvenated because of migration of the upper half away from it along an upper-crustal detachment surface (Namson and Davis, 1988). All of these effects probably deserve further study.

The mechanical argument leading to large fault stress is based on observations of inplace stress (to maximum depths of approx 1 km in the San Andreas fault zone and of approx 4 km elsewhere on the continents), on laboratory measurements of rock friction and the efficiency of simulated earthquakes, and on downward extrapolation of these results through the seismic layer, on the assumption that fluid pressure is normal and frictional properties are uniform and isotropic. The consistency between the most frequently measured friction coefficients and the inplace determinations of the vertical gradient of maximum shear stress is reassuring (solid curves, fig. 10.9; fig. 10.12); however, the downward extrapolation of these results to depths of 10 or 15 km is an uncertain step, with loopholes that could invalidate the high-fault-stress conclusion.

There are at least three such loopholes. First, the fluid pressure might increase with depth, as it is known to do in some sedimentary basins, approaching the minimum principal stress (Berry, 1973). Second, the friction coefficient at depth might be lower than average laboratory values; such lower values have been reported in some studies of gouge and other clay-size aggregates (Wang and others, 1980). Each of these effects could substantially lower the maximum stress at depth. Third, the frictional strength properties might be anisotropic, with the main trace of the fault occupying a weak direction. If so, the maximum stress at depth might be high, as maintained in the mechanical argument, but the shear stress resolved on the fault might be very low, as maintained in the thermal argument. Under these circumstances, the fault must be nearly parallel to a principal axis, as suggested by Mount and Suppe (1987) and Zoback and others (1987). Such a condition could be consistent with the low friction on the main fault required by heat flow, while permitting high stresses to accommodate subsidiary faulting on more favorably situated planes with normal frictional properties.

As mentioned above in the section entitled "Introduction," geothermal studies of the San Andreas fault have provided evidence for a very weak fault for two decades; the meaning of this result depends heavily on the direction of principal stresses in the fault zone and the magnitude of the stress differences there. As we have shown, existing evidence is contradictory, especially in the Mojave Desert region. Many measurements of stress near the San Andreas fault suggest that the fault trace is inclined at an intermediate azimuth (approx. 450) to the principal-stress directions, and thereby imply that the fault coincides approximately with the direction of maximum shear stress and that the heat-flow constraint could not be satisfied unless horizontal shear stress (and stress differences) were low everywhere. In this case, both the San Andreas fault and active subsidiary faults with other orientations would have to be weak. If, however, the horizontal-stress differences are large, the weak fault required by heat flow is a "zero shear stress" boundary condition on the adjacent fault blocks that requires the fault to be almost normal to a principal-stress direction. In this case, the heat-flow constraint could be honored on an anomalously weak main trace, whereas subsidiary faults with other orientations and normal strength could also be active.

Thus, the occurrence of a weak direction may reconcile observations of rock mechanics with the longstanding implications of thermomechanical studies. It does, however, raise several questions:

1. Does the maximum-horizontal-principal-stress direction form an intermediate angle with the trace of the San Andreas fault (as was formerly accepted and as is required by isotropic frictional properties), or is the maximum compression nearly fault normal, as suggested by more recent observations (Mount and Suppe, 1987; Zoback and others, 1987)? As we have pointed out, there is conflicting observational evidence on this issue.

2. If the horizontal compressive stress is nearly fault normal, what is the physical mechanism that permits the fault to slip under the small shear stress resolved on its direction? We have shown that the mechanism commonly invoked to explain a weak fault - namely, locally elevated fluid pressure - is not likely; however, anomalously low coefficients of friction could account for slip under near fault-normal compression if the frictional fault model is valid.

3. How would such a weak plate-boundary fault evolve, and would its existence imply that the resistance to relative motion between the Pacific and North American plates is negligible? What is the role of decoupling and basal resistance? Lachenbruch and Sass (1973) pointed out that strong shear stresses in the far field which might drive dextral slip between the plates cannot be balanced by a weak fault without invoking unexpected strength in the lower crust and drag (and possible decoupling or "detachment") beneath the horizontal base of the faulting layers. If the faulted plate boundary should weaken as it evolves, then either such basal drag must develop near the fault, or the far-field stress must diminish to maintain the equilibrium condition. The best way to learn whether such basal tractions exist is to determine whether the shear stress resolved parallel to the fault diminishes as the fault is approached (Lachenbruch and Sass, 1973; McGarr and others, 1982). We have shown here that a transect normal to the San Andreas fault shows no such diminution, although the observations were much shallower (approx 1 km deep or above) than the depth of earthquakes; direct stress measurements at seismogenic depths (below 5 km) are needed. Whether or not such basal decoupling and drag exist near the San Andreas fault is fundamental to our understanding of its earthquakes and of the nature of continental transform plate boundaries and their resistance to plate motion.